the influence of rainfall on the mechanincs of soil
TRANSCRIPT
The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non-commercial research purposes only.
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Univers
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Univers
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THE INFLUENCE OF RAINFALL
ON THE 1 E C HAN I C S 0 F S 0 I L E R 0 S ION ,
with particular reference to
SOUTHERN RHODESIA
A Thesis presented by
N •• HUDSON
For the Degree of aster of Science
in the Faculty of Engineering of
THE UNIVERSITY OF CAPE TO, J
I The copyri:':~;t r;.i . .': .. t:.I'~:'; Ii held by the I . - _. 1 LJili''''~ : ',I'" .:: I :"' " ; .: r". I .
F..c~ ; ~ .. " Ie t ~ ; .... ·:-r any part
m "J b .. , n" ',> r,. ' ~""'4'J p:..r'l:;v;~$ only, and t .... , .' ....... ~ I
not for fJ\!~Hc:Jtion.
J
THE INFLUENCE OF RAINFALL
ON THE ltECHANICS OF SOIL EROSION,
with particular reference to
SOUTHERN RHODESIA
A Thesis presented by
N.W. HUDSON
For the Degree of aster of Science
in the Faculty of Engineering of
THE UNIVERSITY OF CAPE TO,
The copy ri:?:~~t d tl: .. r:.r.!:~ L; held by the . 1 Unj·!<'!!, 'il'" (:' - :. --; .. ;: :". I '
IXci;'~ " It \ i. . . , c-r any part . m~t b,:: n-: .. ..:I,? 1\:' :r. ....... 'j p ... rpv .. '~s only. and
not for pu~llc=ition .
Univers
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Abstract
Index of Acknowledgements
vu.uu~ 1
Publications - Supervisor's
Chapter 1. THE HISTORICAL BACKGROUND OF EROSION AND EROSION CONTHOL.
1.1. 1.2. Extent 1.3. Growth of Erosion Research
2. M:Er.rHODS OF EROSION RESEARCH
3. THE SCOPE OF THE
3.1. opment and Location 3.2. Field 3.3. 3. 3
4 • RAINFALL VUJ"' ... ...,,'v AND OF METHODS OF MEASUREMENT.
4.1. 4.2.
3. 4.3.1.
3.2. 4.3.3. 4. 4.4.1. 4.4.2.
4.3. 4.4.
4.4.5. 4.4.6. 4.4·7.
of
Calculation Early Experimental Later Studies
and Drop ze Direct Collection and Radar Photographic Methods Calculation of
4.4.8. The Pellet 4.5. Kinetic or Momentum
Chapter 5. CALIBRATION OF THE FLOUR PELLET METHOD.
1. red for 5·1.1. ducing Water Drops 5.1.2. Measurement of 5.1.3. Mechanics of Drop 5·1.4. The Effect 'of of
Mass upon
on
i if
ix x
xi xii
1 2 3
7
11 12 16
20 21
30
35 36
38
41 45
48
I Abstract
s
"'T\~."'''''' 1. THE BACKGROUND OF OW,I.\ ... ' ....... ,",'
EROSION CONTIOL
OF EIOSION RESEARCH
n.<>nLOT' 3. THE SCOPE
3. • 3.2. 3.3. 3. 3
n'D'I':'Ctc",crm EXPERIMENTS
METHOD.
i if
xii
1 2 3
7
11 12
20 21
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5.1.5· I 5.1.6. 5.1.1. 5.1.8. Relationship between and
Mass - Mass 56 5.1.9. at Treatment 5.1.10. Due to 5.1.11. of Velocity
5.2. Cali bration 5.2.1. Conditions 5.2. of
5.3.
6. EFFECT OF WIND ON
6.1 • of Wind on of Quan:tity Intensity of
6.2. of Fall 6.3. of
of Rain 81 of Wind Measuroments of Inclination
88 ribution
Chapter 1. DIRECT MEASUREMENTS OF AND KINETIC
1.l. and Energy 91 1.2. 1 96 1.4. Diaphragm 7.5. Results from Acoustic Recorder 99
I I [ ,
Chapter 6.
Chapter 7.
Chapter 8.
5.1.5· 5.1.6. 5.1.7. 5.1.8.
5.1.9. 5.1.10. 5.1.11.
5.2. 5.2.1. 5.2.2.
5.3.
DU,8 to of Velocity
Cali bration Control Conditions
of
and
EFFECT OF WIND ON
6.2. 6.3.
6.4. 6.4.1.
2.
of Wind on and Intensity of
of Rain
of Quantity
and Inclination
of Wind Measuroments of Inclination
Di rection of
DIRECT MEASUREMENTS OF "'''''''A''A;.;J'''' .AIm KINErI C
and Energy 7 .l. 7.2. 7.3. 7.4. 7.5. Results from Acoustic Recorder
SIZE, EN1:!.IRGY AND
8.l. 8.1.1.
ew of Early of Drop Size and
Distri. bution 8.1.2. 8.1.3. 8.1.
8.2.1. 8.2.2. 8.2.3. 8.2.4. 8.2.5. 8.2.6.
Median
8.3. C~lculation 8.3.1. Pellet
8.3.2.
8.5. 8.5.l. 8.5.2. 8.5.3. 8.5.4.
and Intensity of Distribution
of Intensity rture and Time of
of Flour Pell
Groups and Conversion to
Results in B form of rain)
I' unit time) I' unit
56
66
81 85
88
91
96
99
106
107 108 110 111 112 118
119 125
127
1 132
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--8.6. of Earlier
9. MEASURES OF EROSIVITY AND ERODIBILITY
9.1. 9.2. 9.3.
of Measures of Erodibility
• APPARATUS ANALYTICAL EROSION EXPERIMENTS
10.l. .1.1.
10.1.2. .2. 10.2.1. 10.2.2. 3.
10.4. 10.4.1. 10.4.2.
VOLUME 3.
Chapter 11. RESULTS OF ANALYTICAL
.1. 11.2. Sand Splash Cups 11.3.
11.3.1. 11.3.2. Soil Loss from
11.4. 11.4.l. 11
11·5. 11.6.
PART IV.
• REVIEW OF
12.1. Development
158
162
163 161
168
111
112 114 116 118 183
191 198
211
12.2. with Individual Drop Formation 219 12.2.1. 12.2.2.
12.3. Spray Simulators 12.4. . of
13. DESIGN OF A NEW RAINFALL
.1 • • 2.
13.3. .3.1. Method of
13.3.2. Type of .4. 13.4.1.
cation
Conditions
225
228
230
I
I'll.
8.6.
.1. 11.2. 11.3.
IV.
12.1. 12.2.
of
OF
of Simulators -
230
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PARr V.
... ,
.4.2 • • 4.3 • • 4.4. and Conclusions
13.5.' Assessment of Final .6. and Field Testing
BRIEF REVIE1il OF METHODS AND J.J.cJ,-'VJ.\.L.L
PRESENT E.,'<PERIMENTS
14.1 • • 2.
14.3 • • 3.1.
14.3.2. 14.3.3. .4.
and Recorders Sampling Devices Frequency of
of Field
AND FIELD
OF
15.1. Determining Erosivity from 1 Records 15.1.1. Annual Variation of Erosivity
245
255
259
15.1.2. of 269 .2. Some Effects of 276 .~. The Effect of 278
15 between Soil Type, 280
15.5. .6. 284
SUMMARY OF lifEW AND FOR FURTHER RESEARCH
APPENDICES
PARr V.
BRIEF REVIEW OF METHODS AND l.I£J .... v~u_r
15
PRESENT
.1 • • 2 • • 3 •
• 3.1.
.2 • • :3.
15.5 • • 6.
APPENDICES
AND FIELD
Soil
AND
OF
1 Records
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INDEX OF FIGURES
4.1. Sketch of intensity recorder
4.2. Four recordings of intensity
5.1. Variation of average drop size in groups of twenty drops
5.2. Variation of drop mass with change in time of formation
5.3. Variation of drop mass with change in time of formation
5.4. Least time of formation required for production of drops of uniform size
5.5. Variation of average pellet mass in groups of twenty pellets
5.6. Effect on mass ratio of length and temperature of drying period
5.7. Effect on mass ratio of varying the standing time before drying for twelve hours
5.8. Effect on mass ratio of allowing a prepared flour pan
5·9· 5.10.
5.11. 5.12. 5·13. 5·14 5.15. 6.l.
6.2. 6.3. 7.l.
7.2.
7.4.
7·5· 7.6.
7·7. 8.l.
8.2. 8.3. 8.4. 8.5. 8.6.
8.7. 8".8.
to stand before exposure to rain
Effect of drop velocity on mass ratio
Relation between drop size and height of fall required to reach 95% of terminal velocity
Relation between drop mass and pellet mass ,
Relation between drop mass and pellet mass
Relation between mass ratio and pellet mass
Comparison of several mass ratio calibrations
Blanchard's Calibration Data
Effect of inclined and vertical rain gauges
Angle of inclination of falling rain
Direction of inclination of rain
Simultaneous records of momentum and intensity
Circuit diagram for acoustic recorder
Example of acoustic record
Example of acoustic record
Example of acoustic record
Synchronised acoustic and intensity records
Relation between intensity and acoustic recorder
Circuits for rain sampling machine
Intensity charts for some typical thunderstorms
Sequence of rainfall sampling
Intensity charts for raindrop sampling machine
Sketch of section through sampling aperture
Construction of volume distribution curve
Variation of median drop diameter within storms
Variation of median drop diameter between storms
~ I 25 26
44
50 51
52
57
59
60
62 65
67 69 70 71 74 77 80 87 89 95
100 101 102 103 103 104 115 117 117 124 126 135 136 137
5.2. 5·3.
5·4.
5·6.
5.8.
5·9· 5.10.
5.11.
5· 5·13.
5·14 5.15. 6.l.
6.2.
6.3.
7.l.
7.2.
7.4.
7·5· 7.6. 7·7. 8.1. 8.2.
8.3. 8.4. 8.5. 8.6.
8.7. 8':8.
INDEX OF FIGURES
Sketch of intensity recorder
Four recordings of intensity
Variation of average drop size in groups of twenty
Variation of drop mass with change in time of formation
Variation of drop mass with change in time of formation
Least time of formation required for production of drops of uniform size
Variation of average pellet mass in groups of twenty pellets
Effect on mass ratio of length and temperature of drying period
Effect on mass ratio of varying the standing time before drying for twelve hours
Effect on mass ratio of allowing a flour pan to stand before exposure to rain
Effect of drop velocity on mass ratio
Relation between drop size and height of fall required to reach 95% of terminal velocity
Relation between drop mass and et mass ,
Relation between drop mass and pellet mass
Relation between mass ratio and pellet mass
Comparison of several mass ratio calibrations
Blanchard's Calibration Data
Effect of inclined and vertical rain gauges
Angle of inclination of falling rain
Direction of inclination of rain
Simultaneous records of momentum and intensity
Circuit diagram for
Example of acoustic record
Example of acoustic record
Example of acoustic record
c recorder
Synchronised acoustic and intensity records
Relation between intensity and acoustic recorder
Circui ts for sampling machine
Intensity charts for some typical thunderstorms
Sequence of rainfall sampling
Intensity charts for raindrop sampli~~ machine
Sketch of section through sampling aperture
Construction of vol~~e distribution curve
Variation of median drop diameter within storms
Variation of median drop diameter between storms
25 26
44 50 51
52
57
.59
60
62
65
67
69 70
71
14 77 80
87 89
95 100
101
102
103
103
104 115 117 117 124
126
135 136
137
8.10.
8.11.
8.12.
8.13.
8.14.
8.15. 8.16.
8.11.
8.18.
8.19. 8.20.
8.21.
9.1. 10.1.
10.2.
10.3.
10.4.
10.5. 10.6.
10.1. 10.8.
10.9. 11.1.
11.2.
11.3.
11.4.
11.5.
11.6.
11.1.
11.8.
11·9. 11.10.
11.11. 11.12.
1l.13.
1l.14.
vii
General relation between median drop diameter and intensity
Drop size distribution at low and medium intensities
Drop size distribution at high intensities
Drop size distribution at all intensities
Drop size distribution by Shakori
Prel~mina.ry plot of momentum and intensity
Relation between momentum (A form) and intensity
Use ·of intensity record to compute momentum
Rolation b:tuoon mooontu:"1 (13 forl1) and intensity
Kinetic energy at high and low levels of intensity
Relations between kinetic energy and intensity
Comparisons of momentum and energy
Comparison of energy/intensity relationships
Detennination of the "Dosign Storm"
Sketch of Free's soil pans
Sketch of rainfall laboratory
Bisal 's calibration
Extension of Bisal's Correction
Effect of size of splash cups
Splash loss correction for edge effect only
Splas~ from cups full and part full
Correction curve for splash loss
Correction chart for sand splash losses up to 30 gm.
Critical intensity of erosive rain
Relation between splash erosion and (K.E.) 1)
Relation between run-off from soil tr~s and rainfall
Relation between erosion from soil tr~s and (K.E.) 1)
Depletion curve for antecedent soil moisture index (A.S.M.)
Soil moisture index 1960/61
Predicted and actual run-off 1960/61
Soil moisture index 1961/62
Predicted and actual run-off 1961/62
Relation between soil loss from field plot and
(K.E .... 1)
Soil loss correction for run-off rate
Soil 10S6 correction for run-off amount
Relation between soil loss from field plot (with
corrections for run-off) and (K.E. > 1)
Intenaity for greatest erosivity
140
141 142
143
145 150
153
154
156
151 159 160
161A
166
110
115
119 181
182
184
185
181 188
193 196
199
202
205 206
201 208
209
211
212
212
214 216
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1. 13.2.
13.3.
13.5 • • 6. .. 7 .. .8.
14.1. 14.2.
14.4. 15.1. 15.2.
15.8.
15.
.10 ..
nozzles
Sketch of prototype rain
radial
positions pressure 2 p.s.i.
ze at various pressures -
average over whole test aroa
of size of test area
Sketch of
Site
for
model rain
rain
I
ries II
III
IV
cator
Site
Annual and soil loss from bare
Annual erosivity and annual
in annual
Average monthly
and
three stations in
of cover season
effect of slope on soil
Testing effect of soil
Interaction of effects of soil and
The of successive crops on
land
of crops after
231
236
250
253
263 264
270 271
272
279 281 282
286
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1.
8.1. 8.2 ..
8
11.1.
.2.
13.1. .2.
.5 • • 6.
.1.
Evaporation from water drops fall
of or smooth flour on mass
of flour ts on mass
pellets calibration resul
of of
of exposure time of flour pans
Correction for in exposure time
CT""""c,n groups of
between mesh size of and mean
size
of used to
ze, momentum and kinetic energy
of distribution data
volume of rain for all
storms of and 1959/60 seasons
of c
energy
summar..y
from
differs
of mvmc;u
momentum
cups whose
wi
loss from
rainfall characteristics
of run-off from
of
and
rainfall
d plot
) test data
Simulator test series)
of
with
of intensity and erosivity
from
intensity
in
of degradation of bare
soil on 4t% loam
64 68 86
120
122
130
133
139
148
186
191
200
200
238
244 248
268
273
1.
8.1. 8 ..
8. 8
from
or smooth flour on mass
ts on mass
of exposure time of flour pans
in exposure
0'-;'''''''''1'1 groups of
between mesh size of
size
of
and mean
volume of rain for all
storms of seasons
8
8. 10.1.
11 .. 1.
.2.
of cal
energy
summar:y
from
differs
of
momentum
cups whose
wi
loss from
rainfall characteristics
.. ). of from
.1. of
.1 •
• 2.
test data
r test data (
of
rainfall
) )
on of bare
with
in
loam
122
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Fronti
3.1.
3.2.
3.3.
3.4.
3·5.
Aerial view of experimental sites
I
II site
III expcrinental
of rainfall
Inside of
4.1. Intensity recorder as normally
4.2.
5 • 5·2.
Intensity recorders in r
process of formation
laboratory
6.1. The three-gauge instrument for
7·3. 7.4. 8.1.
) 8.2.
8.3. 8
8.5. .1.
10.2.
3. 11.1.
.1.
2.
.3. 14.1 ..
14.2. .3.
14.4. 14.5.
of inclination of rainfall
recorder in
Momentum outside laboratory
Microphone for acoustic recorder
acoustic recorder inside
Early variation of the flour sampling machine
The table
of sieved ts
cups after exposure to a
tr~s in position outside the rainfall
laboratory
Two cups
bare roil
experiment
exposure to the same storm
of the mosquito
distribution of rain
of
rig
The collecting tanks
colle tP.J1k.s
and IV
Tractor cultivation
collecting
on
of
nozzles
Series I
on Series II
tanks used
of the field
on Series III
rain
on
14
24
40
84
93 98 98
113
114
128
173 177
215 234
234
260
260
260
261
1.
4.2.
5 •
1·3. 1.4. 8.1.
) 8.2.
8.3. 8
8.5.
3. 11.1.
.1.
2.
.2.
.3.
process of formation
instrument for
of inclination of rainft~ll
recorder in
outside 1 abo
14
40
J:"~"C"~ for acoustic recorder 98
acoustic recorder inside
and
variation of the flour
table
of sieved ts
cups after exposure to a
in outside the
machine
exposure to the same storm
of t
distribution of
IV
of
tP.J:1ks on
nozzles
es I
II
tanks used on
rain
rain
of the field eXDAJ~rrle
on III
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Grateful acknowledgement is to the
of
for
some of
has been
from so many
be
my
field
advice on
the
T.
and ~d
on to use
of Agriculture
the
of this thesis, for six months study allowing
work to be of Town.
the years in which this research programme
assistance been received
and individuals it would
them all. I wish to thank of
Research
is also due to
.,.."-TnOYIT, Uni verei ty of
of the
of
and
ss I.M.
Town, for much
from
, and the aeri photographs
on pages 13 and eee are by of
re. J. & Co.,
Grateful acknowledgement is
of Conservation and Extension, ~d
for on to use results of
to the
of Agriculture
the
prepar,<,tion of this thGsis~ and for six months study allowing
some of the work to be done at the of Town.
Durir..g the eight years in which this research programme
has been conducted~ help, ad"vice, and assistance been received
from so many Government and individuals it would
be impractical to acknowledge them all. I wish to thank of
rr~ who assisted in thc and
field experiments at Henderson Research
Mr. D.C. Jackson.
Acknowledgement is also due to
Mathematics Department, University of
advice on analysis of the
photographs on page
Seeing the Unseen" by of
ss I.M.
Town, for much
from
• T. Branford, Newton; , and the aeri photographs
on pages 13 and 14 and the frontispiece are by of
re. J. & Co., Salisbury.
Univers
ity of
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from a number of previous publications by
author is in the
this takes the form of short quotations reference
is made in to cases
the has been included with new data in
chapters indi below.
1) of Field on /I
of Engineering
VoL No.
2)
at Henderson Station,
, Vol.
Erosion and Tobacco II
• 54, No.6, 1
4) "Results Achieved in the Measurement of Erosion and II to the
aba -
November, 1 15. Kinetic
of Sub-Tropical Rainfall." Paper No. 94 to the
C.C.T.A. , 1961. 1 and 8.
6) "An Introduction to the Mechanics of Soil Erosion Under.
of
Address to the
Februar.y 1961.
ons of
Vol.
Published in
Sci
, 1961.
I that material from
been included my
II . and
99 10 and 11.
publi ions has
I is
the
1
)
5)
)
I
been
Vol.
of
Address
Vol.
number of
below.
of Field
Achieved in the
that
my
from
with new in
"
3 and
and II
of Erosion and
the
aba -
, 8
II .
ions has
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ity of
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as a
existence,
outside Europe
Butitis
soi~ erosion isaunost
to s if 110 t to his very
this is sl10wn by the fact most GoveI"r'.ments
active to progranrnes of soil COlW,eX"'Y
as of
of to consider the ~ .... ~u"""~ which was
a~most years ago, and no\'T enjoys YlOrl.d "'Tide atte~1.tion ..
Bennett (1) and Lovvderrnilk (2) have both studies of
the effect of erosion on civilisations, and have SllO'Wn that
a Cause of the doYmfall of many was soil
de g,r adat ion., Although this is evident throughout seven
thousand years of an awareness" of the
vcry very We ::h8.Ve few records the
J1astcrn civilisations, but in such as are available there is no
Old
up,
A
with
of fuly conscious erosion ar;..d the
of these civilisations the
tr.reats and
but few exhortations about
i'iI'i tors mention la.'1.d
to rest the soil, and
mf'1''Y¥'1,~rt:''rn'''·,"''t.; Hamer advocates
associates floods and erosion
o!l fore The Romans had a better
reC01";lmerii \,.,118. t wroM be
oalled conservation farming, but right up to the c:::Jd of the
nineteenth the basically
were a f ow lone in the wilderne ss
but no awarenes s of the
af the
of erosion may stem from the fact civilisations
I
soi~ erosion is~~st
as a to S if 110 t to his very
existence, ard this is s.!'c.vn by the fact [{lost Goverr.!IT~ents
outside active to progranrncs of' soil conservation ..
Butitis before
to oonsider the
any as sessJnent of
of of 'tl:1is
almost yeal~s <".go, and n01!Q" enjoys viOrld vlide
Ben."'l.ett (1) and Loviiermilk (2) lk'we both studies of'
the effeot of erosion 011 ch"'ilisations, ar .. d have SllO\''tn that
a oause of the dovmf all of rna.'1,Y was soil
this is evident tl>xoughout seven
ttcusand years of an awareness' of the
We fhave few records of the vcry very
J1astcrn oi but in such as are aV'llA..ilable there is no
evidence of any conscious erosion a."'ld the
of those civilis&tions the
Old contains tr.reats aJld
up, and btlt few ex.l1ortations about
A v'!ri ters mention la."'ld Homer advocates
to rest tile soil~ and associates floods arrl erosion
v.ri th o!l fore The Romml,s had a
reco17unenii what today vrooM be
up to the c~1d of the
nineteenth cen:t:\lrY the pcsition
were a few lone in tll.e ,rlldern.e 8S
awarenes s of tho
Part of the
of erosion may stem from the fact civilisations
all. arose on irrigated alluvial plains, anct 'V7ere frequently
dependent upon flood deposits of' silt for continued fertility.
The civilisations of the valleys of the l':file, the Tigris and
1;;he Euphrates, which owed their existence to erosion in the head
waters, could hardly be expected to seeerozion in the s~ne light
as a modern agricultural cornrnuni ty.
1.2. TFDIi EXT.r!11·:T OF TIlE ]:'ROE~il2.1.
When in 1798 I'ial thus put fOI"Vvard his theory that the
population of tbe world woulct .outstrip available food resources,
he Yl8.S dismissed as a crank, ariL indeed at that time the resouces
and po:;:mlatiml \vere in stable equilibrium. However ,~the' rorld
populat:Lon dO'L:bJ.ed betYTCen 1840 and 1940, and at the same time
the industrial revolution put in l:lan l s hand the tools, machines,
and transport, which made possible the large scale exploitation
W1d exportation of natural resouroes. The reduction in world
resouroes due to this exploitation, 8.nc1 the increase in demand
caused by the population explosion led to the unstable equilibrium
of today, ,/hen the w::>rld ' s population is only being fed a:.'ld clothed
by the exploitive use of non-renewable resources (Vogt 3, Jacks
and Whyte 4). The extent of the destruction of land resources
may be judged in the lisht o£ the only detailed quanti taU ve
survey, that oi' the United States in 1934 (1) which shovlEld that
out of a total of 411\- million acres of arable lard,
50 million acres were ruined
50 million acres v,rere almost i""Llined
100 million acres had lost more than hali' the topsoil
100 million acres :had lost more than a quarter of the topsoil,
i.e. nearly 75~~ was seriously dama[;ed. The:ce is little doubt
that equally devastating results ,muld emerge from surveys in
most countries of' the "vorld where agricultural development has
been recent.-
StalliUEs (5) points out t]:12,t in America a big increase
in proG.uctivi ty should have arisen from the introduction of
iuproved crop management, bett0l' s0ed, better tillage implerrents,
and greatly increased use of fertiliser. In fact the benefits
of these have been so rrruch offset by the decline in fertility due
to erosion that average yields have hardly changed in sixty yeeJ..~s.
The problem in African is particularly serious. Bosazza
(6) has shown that geological erosion has always been particularly
Univers
ity of
Cap
e Tow
n
the rcsi stance to erosion of
soils in th(;ir natural
vulnerable to reduction in
e.rii erosion ..
,
of'
carried out by the soil scientist (8), who from
1877 - cal~ried out detailed studios, small to
r:1C&sure a Vlide range of effects such as effect of
aID mulches on of rainfall 2l1d ol'l. the
reduction o:i. deterioration of soil structure, and the effect of
soil type on run-off am erosion.. this
the lead in erosion research has come aLnost
from tr~e United of
farmers of mechanical
the's, until in 1907 tl;e United
Agricul ture an offictal
The first
Forest Sel~ce in
in , w;,ich leu in
1)rotection.
¥}Bre laid down
those of
to the Cil~s t
of
the
results of 1'iol(;. (9) .. Other similar
the same n:.ethod, am ,vere
ti~e c.l10CG.tion o~· funo,s by in
, Y[hj.C~l cl1.::;,:Jlcd ':}ennett n;Dd J'oneG -co establish bctv;(:en 1928
stations. the
l')rogc&rnme
, a,lcJ i:l.cluded er08ion
control and run-off from tl'lin
the "Jork \faS limiJ.:;cd to
Ulricr field Dondi tiorlS; and it had beer;. :(rom
the earliest o,f cover ,laS of '\tital
pro~~l'amme on the
of the processes of erosion. Pion;;;er 'work ir:
this field Vias carried out in the s individuals like
(lOY, Borst ana YJoodbLu1 n (11) I and (12), arl& lcd
to the first detailed of 116_tL;ral rain Laws in (13)
and the first of' the actio:n of'
I the rcsi stance to erosion of
soils in th(;ir natural
vulnerable to reduction in
e.rii erosion ..
,
of'
carried out by the soil scientist (8), who from
1877 - cal~ried out detailed studios, small to
r:1C&sure a Vlide range of effects such as effect of
aID mulches on of rainfall 2l1d ol'l. the
reduction o:i. deterioration of soil structure, and the effect of
soil type on run-off am erosion.. this
the lead in erosion research has come aLnost
from tr~e United of
farmers of mechanical
the's, until in 1907 tl;e United
Agricul ture an offictal
The first
Forest Sel~ce in
in , w;,ich leu in
1)rotection.
¥}Bre laid down
those of
to the Cil~s t
of
the
results of 1'iol(;. (9) .. Other similar
the same n:.ethod, am ,vere
ti~e c.l10CG.tion o~· funo,s by in
, Y[hj.C~l cl1.::;,:Jlcd ':}ennett n;Dd J'oneG -co establish bctv;(:en 1928
stations. the
l')rogc&rnme
, a,lcJ i:l.cluded er08ion
control and run-off from tl'lin
the "Jork \faS limiJ.:;cd to
Ulricr field Dondi tiorlS; and it had beer;. :(rom
the earliest o,f cover ,laS of '\tital
pro~~l'amme on the
of the processes of erosion. Pion;;;er 'work ir:
this field Vias carried out in the s individuals like
(lOY, Borst ana YJoodbLu1 n (11) I and (12), arl& lcd
to the first detailed of 116_tL;ral rain Laws in (13)
and the first of' the actio:n of'
I
severe in Africa 0,;.1.1l2, to a corabination 01' extremes of climate
aD..rl stoep ri vcr grwicnts. Eden (-;) Cla::LllS that in spite of
the relatively high resistance to c..rosion of sub-tropical run tropical soils in H:.dr 11.atUl~al ;.ta.te, they are particularly
vulnerable to reduction in fcrtili ty a.nd consequent degrailation
<::.nD. erosion ..
1.3 TEE: GRmrrE OF I~qg.QL RJaSEl<.RCH.
The first scientific investigations of erosion ':.'Bre
carried out by the Gcrmfui soil scientist Wolin,: (8), .. mo from
1877 - 4.895 ca:cried ou-(; detailed studies, using small plots to
measure a wide rfu"lge of effects such as the effect of v~getation
am surface mulches on interception of rainfall D.ni on~he .
reduction oJ. deterioration of soil structure t and the effect of
soil type am slope on rLm-off and erosion. .AJ?art from tbis
pioneer vrork, the lead in erosion research has come aLnost entirely
from tho United States of America. Isolated cases of practical
application by farmers of' mechanical cOEservation Ylorks increased
from thc 1850 t s, until in 1907 t:i.e United States Department of
Agicul ture declared an official policy oi' lana.~)rotection.
The firs t .ArnE:riCall qua...'1ti tati ve e~(pcr u'1cnts V'/!;:;re laid dovrn by the
Forest Service in 1915 in Utah, closely i"ollO'aed by those of
Miller in Iv.iissouri in 1917, W~'lich ll.u in 1923 to the i'irst
l:)ublishOO results of field plot experiments (9). Other similar
expcrincnts follovl(:;Cl., usi11::;, essentially the same mothod, and were
given added impetus tho ecl10C[;ction oJ.: funds by Congross in
arii. 19~~3 i:' .. :.ldLyrc:,:l~ 0::' te;" field cxperilhcnt stations. During the
liOXt U8.;.:::.co tic:!::- :;;,rogcC'Jnme c),-,paxdcd until i'Ol:u.,ty-fou: .. ~ stations
,;ere OlA)rat:;,l1L" and. i~lcluded cxperir.lents Oli. ;n0chanicc:Q erosion
control and run-o:t'f from small catc~Jtlonts .. 'l'lxour;l1out thiG
period the ,lOrk nas limit;cd to applied :cesoarch, studying p::.~oblems
miler field condi tiorlS; and o.lthou:)1 it had bceE apparent from
the earliest of Wollny thc.t vegetative cover .tas of vi tal
importa...'1ce, tl-lc:.ce ;:,:a8 :no co-ordinato~l rcsearch pro~~rallune on the
al1al;ytical study of the processes of erosion. Pioneer v,"ork in
tl>.is field was carl'ied out in the 1930 l s by individuals like
Baver (10)', Borst alld YTood.bLU'l!. (11), a:n.Cl. Muserave (12), axi:. lod
to the first detailed study of ns.tuJ'al rain by Laws in 19~·0 (13)
and the first analysis of the mochD.:nical action ofrairrlrops on
-4-
the soil by Ellison in 19h4 (14). The implications of thi s £l.re
best described by :::.tallings (5) who says
"The discovery t:Vl.t raindrop splash is a major
facl;cr i.n the Y:atcr erosion process n18.l1ks the end of one
era in man t s stru_ggle ,,-r.i. th soil erosion, and ushers in another
vv:hich, for the time holds out rlOpe for a successful
solution to the problem. exact nature of the effects
of rail1l:lrop splash is the phase of the water-erosion process
that escaped detection during the 7,000 years of
civilisation. It explains 7!hy tho e.,'forts at protecting
the lancl against scour erosion these 7 ,000 ~eal1s have failed.
It explainz Yll::.y there is or no erosion on land ,;i tl1
ample plant cover. It explains ma..'1.y thing;:; that have'
puzzled agricultural leat;lers r.:i1C~ practitioners throughout
this long and troublesome period. 1I ••••••••••••
••••• "It remained for Ellisen to recognise the_ true role
of the i'. . .JJ.il1g raindrop in:.:hu ','later-erosion process. He
was -t.;...') first to
cOlI!Plete erosive
that the fallil1t, raindrop was a
'<Ii tmn i tseli' allfl that little or no
erosion occurred vllOn -::he
ample cover. He shoY.'Cd.
cover Vias clue to the
of its kinetic energy.
sl.Tfacc was protected by
the protecti vo effect of plant
that it 'robbed the i'allin.::; rail1drop
1:11ison I s discovery opened a new
of Boil erosion sciunce."
'1i ho developL".ent of' this new lire of investigation was,
howcvc:c, :;Lot stn:.if.;;htf'oI";rara.. Some. of tho pioneer -;rorkers ,".'Bre
ur4;,,\blo to t~le new ideas -.;;i th scientific impartiality. For
exa.1:plc Dr. H.l1. Bennett, the :;:'irst Chief of' trJe Soil Conservation
Servi(;e, aru:1 vinose contribution to the k..YJD'.;ledge and <rP2lication
of conservation practices is still unequalled, was mietaken "hen
he wrote in 1951,
"Some recently published statenerits yrith respect
to the effects of raindrop splash have left the impression
that tins is the most ~ortant factor to do vtith
the erosion process. It is &"1. important factor, but,
as alreac1y pointed out, it is only one of' scvel'al factors
having to do vn. th erosion on :('arr:l lands. As a matter of
fact, the cutting and abrashe effcct of :rtll"l!o-Qff from
rains and tho mel ti:n.g of sr.ow a:ce of far more irru?ortance
than raindrop splash, Ylhich makes its principal contri bu tion
by hurlinr; soil particles iEto ;::;uspension il': the run-off". - (15)
Howev(;r" the need f'or detailed 11eseaL~ch on splash erosion
-5-
to support field experinentation VJaS realised by most rorkcrs ~ and
the majority of research in this neVI phase is now taking place
in the United 8tates of America, like the earlier field work.
Analytical research was directed onto more specific
objecti vaS by the setting up in 1954 of a national study, which
used modern techniques of data analysis and electronic con:q;mtors
to correlate the results of all tho field experiments, aild
pinpointed the main features in the: erosion process (WischneiGr 16).
The vade publicity given to the results of the field
experiments of the 1920' s led to interest in many other countries.
In this Africa has been vJell to the fore. In the Republic of
South Africa, Professor D.G. Haylett established the first run.-off ·4
plots in Africa at the University of Pretoria in 1929. A·
second. group was adcled in 1936" al'jd similar installations
follmved at Grootfontein College of Agricul turc, Cape Province ~
and Estcourt Research Station, HataJ.., in 1938, and Glen College
of' Agriculture, Orange Free State in 194].. Similar plots to
study the effect::;. of '.ie .. ttle management ,vere built by S~rry (17)
of the Y/attlo Rcseartilh Institute in 1;a.tal in 1951.
Dlso'l'nerc in Africa, Staple.s laid dovm plots in Tanganyika
in 1933 (18)!\ ' Farbrother at the 5mpi'le Cotton Grovang Corporation
research station bot NamulongYic, Uganda, developed an unconventior.al
circular run-off plot, since used at Samaru, Nigeria from 1956
onwards. In the French ov~rseas terri':::orics in Africa, field
experiments vrere established from 1953 to 1956 in f\iadagascar,
Senegal, l"rench Guinea, Upper Volta, and Ivory Coast.
In SouthomRhodesia, run-off experiments using large
plots of several acres were started in 1934, but i'lorf.. abalnoned
s.fter a few yOa.l~S ,men gold mining started on the site. In 1953
field experiments were started by the autbor on the HOlnerson
Research Station, aJ.1d have eXflaxliled until they today constitute
the most comprehensive installation in Africa.
Apart froLl these field plot studiGS , lit tle work has been
done in africa on the mechanics of orosion., Rose (19) at
Makerere College, Uganda, axld van :'Icerdcn (20) at the University
of Pretoria, have studied rain drop splash using artificial rain.
The only known study of thc subject uncler conditions of natural
rain in Africa is that presently reported by the author, and has
been ca.l'ricd out together vrl th the field erosion studies at
Henicrson Research Station.
Univers
ity of
Cap
e Tow
n
Puc:rto Rico, and the
tile basic research und",r
is that of :,Iihar3. (21) in
in
To summarise, the "~!hole history of
extends back yec'XS, and the vast
field research has
or "basic research
the
started some
ten :yoars, but
of or
Japan, ana. Ker (22)
research
of the
years.
years ago,
or
and very
or urrle:t
of' this has been done outside the United
and
Puc:rto Rico, and the
tile basic research und",r
is that of :,Iihar3. (21) in
in
To summarise, the "~!hole history of
extends back yec'XS, and the vast
field research has
or "basic research
the
started some
ten :yoars, but
of or
Japan, ana. Ker (22)
research
of the
years.
years ago,
or
and very
or urrle:t
of' this has been done outside the United
and
Univers
ity of
Cap
e Tow
n
Soil erosion is a man;y
factors, and the amount of soil lost upon such
as the soil , the , the length of the , the
crops grovm and the mal'lner of their the
ai.1D. so on almost .. Jl'U1~th(;rmore there a.re interaction
effects bctvT8en many of the factors, so that it is
to arri VB at a solution to the
A numbl.:r of have boen used to introduce some
mathematical basis into 0i ther of soil loss, or ro-
COd·.j\J11.dat::'on~~ '::'01' l:::,ni uSt..' 'Jluch v411 lirnit erosion to a chosen
vwue .. ,An. of tins knoym formerly as
tl'iC and more as tho
Soil Loss is sholm in tho
(23) from an carliE;r model (24) in vlhioh
A '1' .. R .. -, . .1 6 1 .. 4 := J.' •
F is a factor the arnount
of fertiliser or i;~anure
factors,ll and
as the soil
is a
amount of
,II the , the ffierner of t;ht;;ir
many
upon suerl
, the
the
.. I\'tli~th~;rrnore thoro 8.re interaction
effccts many of the factors, so that it is
to arri V(; at El. solution to the
A number have been uscd to intl~oduce some
mathematical basis :Lnto c;i ther of soil loss, or ro-
Ie·xii usc' ,-"ilicL • .rill limit erosion to a chosen
of tIns
,:uri more
is shown in
from an earlier
knoym
in ·which
A '1'"
p
T
loss
a factor
, on tho conservation
e .. g .. 1 I'or maize in ro-us
up 0 .. 5 for
relative
, R is a factor for various stat!LLacd rotations,
actor
of f'ertiliscr or '·,:anure
the G:imount
Univers
ity of
Cap
e Tow
n
L is thG
S is the
units are very
the
of
thus S is in , L
are
various conditions are ellose1'1 so thd in combination
they values of A \~'U.,~~~.soil loss in which
To
measured values.
soil loss under a
of all the
armual soil
set of cOl1.di tions the
~~'vAA~~"aG to these oonditions
if a level
of soil loss is
fixed conili. tion:::; such as
ler~th of slope, cGrtain
A so be
conbinations A tJ.J.81~ 5 "v7ould l1.0t be
Obviously a Elet:-~od has valuable in erosion control
land use '.Jut the the
of' the various 0:11 the 11&1(1. side of tne
is
evidence of -;;he vc,lu(:s of the
, or the 'll8;rmcr in v-rhich should be or
of the metl-'od is th;;\_t if have been
it is to use the
guess at
the e,UeSses evidence as it accuracy
beco::nes • is ;nore or in the
(25) there is
for all the
ani in the la to st
some evidence for the choice of
v;rill no tloubt:Jo improved almost
T hore are tYro main to erosion
first is method of using nola.
plots to Iaf"asu::ce erosion vrhen the are set up
in a limi ted number of cOi;J.binations. l'he uses
m.ore or under
controlled
in
used to study
G tudies Il1ay be used to
type.
.of cou.::cse; field
sucn as land
the
, a11li
b etv-re en
may be
L is thG of :J a:i1.d
S is
are very thus S is in
the are
various condi. tions arc G11oso1'1 so in combination
values of A \~'U.,~~~.soil loss in which
measured values.
To soil loss under a sct of cOl1.di tions the
of all the
annual soil
~~'vAA~~"'AG to these ccnditicns
if a level
of soil loss is
fixed conro tion;} such as
so be
conbinations A
a met:'~od has valuable in erosion control
land use '.Jut the
of' the various 0:"1 the 11&lCl side of tne
is
evidence of ';;he of the
, or the 'll8;rmcr in v-rhich should be or
'"hich i'acto:cs should
of the metl"'od is th;;\.t if
it
guess at
accuracy
beco::nes
some
to use
unmeasured values, and
the e',uesses
.. is ;nore
in latest
evidence for the choice of
will rlO doubt
have bcen
evidence as it
the
is
for all the
Thore tYro main to erosion
first is method of
to mf;'.asu::ce erosion v-Then the are sct up
in a limi ted number of cOi-:lbinations. l'he uses
,nore or under
controlled
in of cou.::csc; field may be
used to sucn as land
G tudies Il1h.y be used to the
, and.
behrecn and
Univers
ity of
Cap
e Tow
n
The of field is the,t provide
answers to locM ...,J. OLio-'-'- icant dHf erellces
ru.'e
the
these
cor:nnuni ty. or , an.8vrers :nt:l.Y be
su:f'ficient for , such as yihich of tvro
'allows the lesser amount of' erosion under the condi tions in a
tlrca. Thc dis:.ldva.:.ttc,gcl:.> of' field are that
to and
f'luctuo.te year to y(;c;.r,
erosion must contirlUc for ma.ny ,yoars
to
, conclusions mc~ be cases v,'hore ten
years of results ho.ve cut ard
years results have shovm U:.at these conclusions
had been influenced by term varis,t:i.ons
C(;,o,,"non to mEu.'1y branches of and
Ylhich to erosion is the :tremendous
number of variable f ccctors .. .An like the
amount of soil lost from a maize crop can only bc solved by
if ant; a soil ty~e, ,
of of mo.izc, of r&:',nfall, ,rlan!lCr of
amount of arrl so on. may ha:-ve
or o.nd values, the number of
arKl pE~tations is so vast that field can
cover a mirJU te of
is
be the; research
may
of close
of otht'-r , When the individual have been studied of the results ...
in isolation, the r:la~r '1)0 exter.ded to g;rOltpS of variables
betvreen If studies of a vo.riable such as the
of soil
the oollectiO~l of is made very I!1Uch
and
act the collection of
lead to U:s o:C' a mach:~ne yrhi ch could rainfall,. and
so be used':' 0:1.' :.o'.lch studies either in thE) or in the
:,s in ai'fect the , one factor'
at a HIne, to field
The tvJO methods are thus to each
The evaluation of each varia01e by the
are
the
o.f 1'::I..(;1d.
answer:;;, to lOGcl
for
the le sseI' a:mount
orca.. The
fluetuQte
erosion , Gonclusio:ns i:nc':y be
years of results hQve
to
, suuh as '.'lhiGh of t\'vo
erosion un~ler the condi tiOl:'£ in a
of field
are
Gut aId
al1swers" but tho year~:; resul t3 i~ave shovm tLat these conclusions
had ovon influenced by term varis.t:ions
and.
J..::; the :tremendous
of variable:: f' like tho
amount of soil lost i'rorn a mc.1Zic; erol:) CEm only be solved by
OYle soil
of' of mQizc,
amount .::rii. so on ..
or and values, the 1111l!lDel' of
nne!. permutatiol'..B is [30 vast that fiel{:l a all
is that each
be the.: rcsce.rch of close
. DX!.d.
of the results '" When the individual factors have been studied
in X:laJ T)e c:xter,ded to gruu:ps of va.riables
between If st;udics of a vD.:ciable such as the
of soil
the oollectiO:l of tion is made very L1UCh
in I).ct the 0011cotion of
lead to 02 a mach:~nc yrLl. ell eould r cD. nf' all., . a.:nd.
or in the
in oi'fect fI one faetol'"
to field
The tVlO to eucil
The evaluation of CEJ.::h v<?J.ri;o,i.11e by the
Univers
ity of
Cap
e Tow
n
method more to be from results of
the results of field which
study_ A good of this
inter-rel&tion is the
shovm
Borst and
the reason"
in a
the
(26)
, Dnd
f
of erosion as was
around wilich modern research
f oum in field experirr.en ts of
reduced
simil&r field Ellison (27)
to 0-0 serve and to the
on covered and uncovered His
of "che mechanics of erosion was then taken back
to tho field experiments of Osborne (28) and others for
of
the
V1B:yS of the new iclee.s .. This is ortc case ,mere
CL"'.,,,,~. well bet,'J(;)en the and the but
the more has ooen research workers
have limited themselves to ei ther onc or tho other ]'ield
carried out Dln
answers to .. Lack of
and lack of limit the extent
to V'lhich results f) rom different f'ield st[!,tiOl1S me:v be to
evcluate, for e x:::um:) J.e , differences .. On the oth(;r hand
soil scientists vall be "~'~'~j of
academic interest if are not linked to field studies ..
V~W"~~'V' severcl indioes of For
based on determinations of soil
soil been
such as
composi tion t'.JJ.Cl (Lutz 29 and Middleton
30), but r-tEl.ve not been used to any extent because vrere not
related to field
A balanced reseaxch progranme must -ooth
moves forrtard Can bc to come
fro::l1 each than from bot;l
v:ork was for the first
or five ycr:;,xs
When sui'ficient
continued but the
research for three
liinited to
vms
and tho
~[J}J..L..LG'-'- field.
these
to detailed
information then
to make more effective use of the field
method T"J.ore to be from results of
, and the results of fit:::ld show which
study_ of this
intcr-rel.s.tion is the of EtS \vaS
the 2U'ound wluch modern research
Borst al"ld (26) found in field of
reduced
the reason .. , [.no. similar field , ( ) , leq Ellison ,27 :L:Ct a to observe the
on covered and uncovered His
erosion was then taken back
to tho f1 icl(1 of Osborne (28) and others for
of the new i&e2.5 .. T}us is are case \",here
tho well bet,';6en the and the hu.t
the more has been. research ,Yorkers
have limited themselves to ei thor one or the other :B'ield
carried out 311.0.
ansvrers to .. Lack of
aril lack of linlit the extent
to which results from f'icld Ste.tiOl1S lnc.:v De to
ev&luate, for ~ climatic differences .. On the othc:r hand
Boil sciontists vall be of
academic interest if are not linked to field studies"
For , sever&l indioes of soil been
based on dcterndnutio:ns 01~ .soil such as
29 and Middleton
30); but r.t[ive not been used to a:ll.y extent because "Jere not
related to J'ield
balDl1ced reser,;rch progl:'eu:rne must -oath and
When sui'ficient
continued ~Jt the
research for three
-. . t - t .l:lIn:L eu 0
alld tho
to corne
1':ork was for the first
tl1GSe
to detailed
information then
to make more effective elSt: of the field
Univers
ity of
Cap
e Tow
n
-11-
In 1951 the Conservation Engineer of the Department
of and Extens ion :re oommende d that a programme of
:research on erosion control should be on ........... ,J.v»
similar to projects he seen a study tour of
States. The very
(Cormack ) the not changed materiaJ.ly
the ten years since then. The author was CLv ... n,,,.,-,,
in to start this research programme and has
of it since then.
It was that :re first be concentrated
on was thought to be the most serious aspect of erosion in
RhodeSia, namely water erosion from land in the high
areas of the central watershed. of field
was to be s
of erosion. The which will
00 on, will to use the
to extend into other ~.~~~ with
other use patterns. The three phases are not
and there is a
The first two
both in
have been completed
subject matter.
the last eight years,
and are reported in account. 'l'he third of "",..,,"" ... ,
extension
The at Research
a large agriculturaJ. research station of the Department of
Research and for types
of reseexch, and a wide range of soils
most of the country.
The Department of Services
I
-11-
In the
of and Extens ion re oommende d that a n",,,,n-.<l
research on
he a.
) the
the ten years since author was CUJ".""'.'-"
in to start research prograrr~e and has
of since then.
It was
on was
areas of the
that re
to be the mos t
erosion from
watershed.
was to be
of erosion.
0000, will to use the
to extend into
use The three
first be nnn",,'r
of e in
field
The which will
other
, and there is a ma tter.
first two have been years,
and are account. 1he third
extension
'-f'he Research
research of
of research, and a wide range of soils
Univers
ity of
Cap
e Tow
n
made
research
Salisbury
about
-12-
land, workshop and
an elevation of 4,500
acre s, lying between two
.. North-east of
'Ihe present area
of of .... v,""" .... <;;;
so all
are to be found from flat red soils to broken
in a
is 34 inches,
summer rainfall season
Out of seaSon shovlers are very
effect upon or crop production.
There are four main groups of
on a distinct soil In
all of which
October and ,",,,.1, LL
and have no
, each
• known as
I, was
(Plate 3.1.
on clay loam g)
.) The
of the Tatagura series.
(Project )
consists of sixteen
, where run-off
maize/green manure
of 1/20th acre
soil losses are
maize/ley
on three different
from maize,
Previous
publications
recording and
of the
) .. uU.' .... i:>U'H 32) and
On the same rotations
site Project
surface flow,
separate effects of r
three plots of ~lOOth acre
splash
different
of cover.
Other "'...,.'"'.,~....; in this Series are to practical
not directly
effect of varying
the pr'3sent
between contour
, such
on
of long
methods of
(Project I/4) the
I/5) •
1954 the
clay
were extended to
the Mazoe , Project
the pattern of the on Series I,
of 1/20th acre on two slopes to measure the
rotations maize production.
1955 similar AX1'Ar"'lm,9r were si ted on
s
soil.
acre on a i:>U(~~.V"
III/IO has
sand, (Plate 3.
Project IV/12 has four similar plots on
as the
1/2),
effect of
follows
of
made
about
in a
-12-
,workshop and
approximately 20
an elevation of 4,500
facilities " of
'!he present area
acres, lying between tvro ranges of
formation so
be fotmd from flat red soils to broken
is inches, all of which
summer rainfall season
Out of seaSon shovlers are very
effect
on a
or crop
'!here are four main groups of
In
, each
, mown as
on clay loam 8:) of the Tatagura series •
of sixteen
;:;; .... ,.JjJ""." where run-off
maize/green manure
. ) of 1/20th acre
soil losses are
maize/ley
(Project )
on three different
from maize,
Previous
publications of the
the
site ) "
J.j.UCA."'U'H 32) and
On the same
the s effects of r splash
different surface flow, three plots of l/loOth acre
of protective cover.
Other in this are
not directly related to the pr'3sent
to practical
:t such as the
effect of varying bet'Heen contour 1/2) ,
on
the
of long term (Project I/4)
methods of '\ I·
1954 the were extended to s
of the Mazoe , Project
pattern of the on Series I,
of 1/20th acre on to measure the
rotations maize production.
1955 similar A'1/"1"''''1''''''1 were sited on two of
soil. On III/IO has
acre on a i:>U'~"~V" (Plate 3. ) and
IV/12 has four similar on
of
.).
Univers
ity of
Cap
e Tow
n
Univers
ity of
Cap
e Tow
n
P.J.ate 3.2 !Series II Exp eriment cl 2.ite ( Photo J. i'i.lu) . P.late 3.2 Series II llicpl!rimcn.tcl 3i te (Photo J. huu) .
Univers
ity of
Cap
e Tow
n
----Plate 3.}. Series III Experimental Site. P1ate 3.3. Series III Experimental Site.
flute .5.4. Outside of Hainf'all Laborator:;r.
Univers
ity of
Cap
e Tow
n
Splash erosion and wash-off e rosion were measured simul
taneously in the r ainfall l abora tory using miniature run-off plots, and
on a field scale run-off plot 8 . short distance awa::l. The r ainfall I
l aboratory studies were carried out over three rainy sea sons from
1958 to 1961.
3.4. QBJECTIVES OF THE EXPERIMENTS.
The purpose of the analytiQo.l part of the studies may be
expresse d by asking the que s tion tlVlha t re the features or
characte ristics of sub-tropica l r ainfall which de t e rmine its ability
to CaUse soil erosion?" D Before conside ring this question as a
whole , an answer is r e quired to the first peJ't - !T1fIhat are the
characteristics of sub-tropica l r a infall?" . It was shown in
Chapter 1 that the importance of r a indrop splash as an erosion agent
h as only b een appreci ate d in recent years. Deta iled studie s of
rainfall properties in rela tion to erosion, arising from t his
apprecia tion , are the r efore few, and moreove r have b e en chiefly
concerned with types of r a infall othe r thcn the high intensity
convective thunde rstorm r a i n of the tropics. Since the necessary
informa tion is not available, the first obje ct of th present
studie s is to fill t h is g ap . The experiments on r ainfall, and the
ir£ormation ob t a ined, are de scribed in Part II (Chapte rs 4 - 8).
The next objective is to establish the r e l ation between
rainfall charaGteris t ics and e rosive po vel', and to define the
erosive power in qUBntitativc t erms. In 8.ddition to r ainfall,
erosion is influenced by many properties r e l a ting to the roil or to
tre a tment impo sed on the so:Ll, but the approach will be first to
hold cons tant all soil propertie s while relating erosive power to
physical properties of r ain, and subsequently to test the r e l a tion
Qnder various soil conditions. The potential ability of r a in to
Cause erosion is dcfined as "Eros ivity", and the soil's vulnerability
to erosion i s define d as "Erodibility". A value on the scale of
e rosivity is based solely on r ainfall propertie s, and to this
extent it i s independent of the soil. But a quantitative measure
ment may only be made "men e rosion occurs , and t his involve s the
erodibility of the eroded materials. Similarly relative values of
erodibility 8-~ not influence d by r a in, but may only be measured
unde r r ain which has e rosivity. Thus ne ither is independently
quantita tive , but each Jay be studie d quantita tively while the other
is held constant.
There is also the possibility of interaction betvreen the
Splash erosion and wash-off erosion were measured simul
taneously in the rainfall l aboratory using mi niature run-off plots, and
on a ' field scale run-off plot a short distance away . The r ainfall I
l aboratory studies were carried out over three rainy seasons from
1958 to 1961.
3.4. OBJECTIVES OF THE EXPERIMENTS.
The purpose of the analytiou 1 part of the studies may be
expressed by asking the question I1Ylhat ar e thc features or
characteristics of sub-tropical r ainfall which de t ermine its ability
to CaUse soil erosion?!!. fufore considering this question as a
whole , an answer is r equired to the first part - l1 y;rhat are the
characteristics of sub-tropical r ainfall?" . It was shown in
Chapte r 1 that the importance of r aindrop spl ash as an erosion agent
has only been appreciated in recent years. Detailed studies of
rainfall properties in relation to erosion , arising from t his
apprecia tion , are ther efore few, and moreover have been chiefly
concerned with types of r ainfall othcr th~n the high intensity
convective thunderstorm r ain of the tropics. Since the neces sary
information is not availabl e , the first obje ct of the present
studies is to fill t his gap . The experiments on r ainfall, and the
information obtained , are described in Part II (Chapters 4 - 8) .
The next objective is to establish the r e l ation between
rainfall characteris t ics and e rosive po 'ler, and to define the
erosive power in quantitative t erms . In addition to r ainfall,
erosion is influenced by many properties r e l a ting to the roil or to
treatment imposed on the soil, but the approach will be first to
hold cons tant all soil properties while relating erosive power to
physical properties of r ain, and subsequently to test the r el a tion
under various soil conditions. The potential ability of rain to
Cause erosion is defined as "Erosivity", and the soil's vulnerability
to e rosion is define d as "Erodibility". A value on the scale of
erosivity is based solely on rainfall properties, and to this
extent it is independent of the soil. But a quantitative measure
ment may only be made when erosion occurs, and t his involves the
erodibility of the eroded materials . Similarly relative values of
erodibility ~~ not influence d by r ain, but may only be measured
under r ain which has erosivity. Thus neither is independently
quantitutive, but each may be studied quantitatively whilc the other
is held constant.
There is also the possibility of interaction between the
Univers
ity of
Cap
e Tow
n
-28-
two effects, i.e. when two dif ferent storms fallon s and soil, the
mathematical re.tio of tf ' ~ ir erosive po'wers may not be the same as
if they f ell on clay soil. Since by definition erosivity depends
solely on physical properties of r a in, and erodibility on physical
soil condi hans, such inte r action is unlikely to be of any consequence.
Before considering the complicated interplay between soil effects and
rain effects the information on rair~all properties must be r e lated to
erosive power, and this is done in Part III (Chapters 9 - 11).
305" APPLICATIONS.
The under2,tanding of the rela tions betvl"e en erosion and
rainfall allovrs two practical applications; one of which le ads to
further advances in the analytical appro ach; the other increase s
the informa tion obtainable from the f i eld experiments.
The analytical application of a quantita tive measure of
erosivity is in the design of critical experiments to evaluate
individual factors in the e rosion equat:Lon. For example, to measure
the effect of s oil slope or gradient, t es t conta iners of soil may b e
placed on a t abl e v{hich c en be adjusted to provide various slopes ,
and the soil then subjected to sim'Jl a t c d rninfall from some type of
spray apparat"lJs. This is a very convenient t e chnique, but the r esults
ar e lif.tble to be me aningless if the erosive power of this simulated
r a in bears little r e l a tion to tha t of n a tural rain. A measure of
the orosi ve power of natural r a in is thus a primary r equirements in
the usc of artificia l rain. Similar critical oxperiments using
naturc.l r a in ar c equally handicapped by t.he absence of a measure of
the r e in's erosi vi ty •
The information obta ined on the n a ture of sUb-tropical r a lll
and its e rosion effects, made it possible to s t ert in Dece mber 1960
on exploratory designs for an 8rtificial rain-making device. These
were developed in the winter of 1961 at the Hydraulic Laboratories of
the University of Cape Town to the stage of a practica l working
prototype suitable for fie ld testing. The requirements and the de sign
of this instrument are discussed in Part IV (Chapters 12 and 13).
In the field experiments, v~~ich involve crop rotations and
crop management, the ability to asse S3 quanti to.ti vely the e rosive
powe r of r a infall means th8.t much unde sire.ble variation can be
elimina t e d. For exanrple, a n experiment mi ght be designed to t es t
the effectiveness of stubble mulching, and to s ce how this Elff'ective
ness differs at the b e ginning and end of the growing season. It
would be remarkc.bly fortun['te if exactly equal storms f e ll at the
beginning :>nd end of the season and so allowed a direct compa rison.
-28-
two effects, i.e. when two diLferent storms fallon sand soil, the
mathematical ratio of tt 1ir erosive powers may not be the same as
if they f ell on clay soil. Since by definition erosivity depends
solely on physical properties of r a in, and erodibility on physical
soil condi hans, such interaction is unlikely to be of any consequence.
Before considering the complicated interplay betv,een soil effects and
rain effects the information on rair~.ill properties must be r elated to
erosive power, and this is done in Part III (Chapters 9 - 11).
3 ·5 ' APPLICATIONS.
The underst~mding of the rela tions between erosion and
rainfall allovfs two practical applications; one of which le ads to
further advanccs in the analytical approach; the other increases
the information obtainable from the field experiments .
The analytical application of a quantitative me asure of
erosivity is in the design of critical experiments to evaluate
individu8~ factors in the erosion equati.on. For example , to measure
the effect of soil slope or gradient, test containers of soil may be
placed on a te.blo -which c n be adjusted to provide various slopes ,
and the soil then subjected to sim1J.l atcd rainfall from some type of
spray apparat1..l.s. This is a very convenient t echnique, but the results
Dre liable to be meaningless if the erosive power of this simulated
rain bears little r elation to tl1at of n p-tural rain. A measure of
the erosive power of natural r ain is thus a primary requirements in
the use of artificial rain. Simil<- r critical experiments using
natural rain are equally handicapped by the absence of a measure of
the rain's erosivity .
The information obtained on the nature of sub-tropical rain
and its erosion effects, made it possible to start in December 1960
on exploratory designs for an artificial rain-making device. These
were developed in the winter of 1961 at the Hydraulic Laboratories of
the University of Cape To~m to the stage of a practical working
prototype suitable for field testing. The requirements and the design
of this instrument are discussed in Part IV (Chapters 12 and 13).
In the field experiments , vmich involve crop rotations and
crop management, the ability to asse ss quantitatively the erosive
power of r a infall means the.t much undcsire.ble varia tion can be
eliminated. For examplo, Em cxpc: rimcnt might be designed to test
the effectiveness of stubble mulching, and to see how this Eri'fective
ness differs at the beginning and end of the growing sea son. It
would be remark bly fortunete if exactly equal storms fell at the
beginning .~d end of the season and so allowed a direct comparison.
Univers
ity of
Cap
e Tow
n
In fact the most noticeable feature of sub-tropical is the
infinite variety of patterns. A quantitative
measure of would the reduction of storms
to a common so an assessment of the effectiveness
of the season. A occurs in
, such as one to compare the annual
sucoessive crops of , i.e. to measure
deterioration of the soil. This Can only be aocurately
if the succe ssi ve seasons are similar, (or of mi'lar
erosi vi ty ) , also is an in Can tral
If the can be to
a , then the seasons can be reduced to a common
of annual losses.
to these two
There are innumerable
(Hudson, 34) because in
base for
applications
any field on erosion the number of is so
of or Latin design are
seldom from
the results of storms or seasons of s of erosivity is
therefore of the utmost Some s of
tion are in V (Chapters and ).
In fact the most noticeable feature of sub-tropical is the
infinite variety of patterns. A quantitative
measure of would the reduction of storms
to a common so an assessment of the effectiveness
of the season. A occurs in
, such as one to compare the annual
sucoessive crops of , i.e. to measure
deterioration of the soil. This Can only be aocurately
if the succe ssi ve seasons are similar, (or of mi'lar
erosi vi ty ) , also is an in Can tral
If the can be to
a , then the seasons can be reduced to a common
of annual losses.
to these two
There are innumerable
(Hudson, 34) because in
base for
applications
any field on erosion the number of is so
of or Latin design are
seldom from
the results of storms or seasons of s of erosivity is
therefore of the utmost Some s of
tion are in V (Chapters and ).
Univers
ity of
Cap
e Tow
n
-20-
has been obvious from the very
research that the of soil erosion
very poor (in fact it is an
days of
amount of
and
measures are to describe in its
to erosion. The features or characteristics of rain which
may affect i~s erosivit.y are total quantity,
various properties to and the size, size
drops or groups.of drops •. The size,
or mass, and
kinetic energy.
may be combined and as momentum or
Each of these must be to see
whether
•
where
be
I or
is not so, to
to
accurate, is
methods ca'
the information.
lIDy measure of amount is a sample, and so associated
with the inevitable problems of sampling, namely, the sample
of the whole?" and the sample accurately?".
of the main factors , i.e.
of the voluminous literatUl~
on the subject are outside the scope of the
lID excellent review of raingauges and recorders is given
(35) a most detailed study with illustrations and 1097 references.
For erosion is sufficient to a
accepted, in the
used are those of the Department of Meteorological
of and • The standard
gauges used are five inch diameter (
to prevent splash), set
, and free from surrounding
more than 3<$ with the hori20 n tal.
each if the sites are more than 300
funnel
a turf
of
for
-20-
has been obvious from the very
research that the of soil erosion
very poor (in fact it is an
days of
amount of
and
measures are to describe in its
to erosion. The features or characteristics of rain which
may affect i~s erosivit.y are total quantity,
various properties to and the size, size
drops or groups.of drops •. The size,
or mass, and
kinetic energy.
may be combined and as momentum or
Each of these must be to see
whether
•
where
be
I or
is not so, to
to
accurate, is
methods ca'
the information.
lIDy measure of amount is a sample, and so associated
with the inevitable problems of sampling, namely, the sample
of the whole?" and the sample accurately?".
of the main factors , i.e.
of the voluminous literatUl~
on the subject are outside the scope of the
lID excellent review of raingauges and recorders is given
(35) a most detailed study with illustrations and 1097 references.
For erosion is sufficient to a
accepted, in the
used are those of the Department of Meteorological
of and • The standard
gauges used are five inch diameter (
to prevent splash), set
, and free from surrounding
more than 3<$ with the hori20 n tal.
each if the sites are more than 300
funnel
a turf
of
for
Univers
ity of
Cap
e Tow
n
In addition eaoh has an arrto~~E~b~o recording gauge. The
of the Meteorological Department is
Cassella natural siphon, with siphoning
CJ.()cJ!::W'o,rlt driven 24 hour chart 12 inches every one on a
.. may be measured to some extent from the rate
of the chart trace, but the of short
high intensities is not I and s
were used.
The of wind on rain measurements were ron~idcred,
used to measure the of ~~~k.~'~~~~.~'
direction in azimuth are discussed separately in 6.
There is evidence of a
and intensity particularly important as a
potential of erosivity because it is the only of
rainfall which, in addition to amount, is frequently recorded at
conventional stations. Data on OCCUI'l'<::ilt:C
of recorded on most
and no erosion ...... ~ ..... y. be "',....".,."" .... -1
form of recorder. Jrny
on some function of intensity has thus a
than an on E!nY other
on, for ____ MX-__ , ki.netic energy INULL.l.. .....
useful even if new measure
some
, or
scope fro:
Jrn
energy I but would only be applicable
and so is less than an
collection of new data,
on in tensity.
TYro of
difference between them must
the
the cloud
with rain in relation to
used are
air amouniB of rain as
volume of c.c.) volume of (one
. cubic metre). The corresponding of the rate
of of this volume of water with respect to dis ""............ in
time. This means that the total intensity is a of a
function of and own fall velocity. This definition is
particularly to of
Agriculturalists,
concerned with the rate of rainfall arriving
area of
dimensions, i.e.
and
intensi ty as t he volume of
in unit time. Both
x ~ (both ... "' ................. ...... T
, who are
of'
on unit
In eaoh has an autographic gauge. The
instrument of' 'f;;he Department is
the Cas~\ella siphon, with inch dit1JJleter rim, siphoning
ever.r one on a w.ockwork..driven 2h. hour chart 12 long .. may be measured to sOllie extent from the rate of
01.~ the chart but the of short at
high is not , and spocia.l intensi.ty recorders
were used.
The of' wind. o:n rain meas\.u'ements
~tnd the instruments used to llIeasUI"() the of
di:r.cction are in Chapter 6.
4.2. ~..±'!NSITY 9R RATE OF RAINF.AI.J:,.
There j .. s ev::i.J1ence of t\ close association between
aml intensity, and intensity
potential because it is the only of'
to amount,
stations.
of recorded on most
and no erosion would be pro}1€lrly without some
fom of' recorder. lrny
, on some f'lmction of'
solely, or
has thus a gl'eater scope f'ar
than an
on, for
usef'ul even if
eni~rs;tV J but would
so is less
of'
cubic metre).
of
The
of of' this
time. This me ans
of
concerned with
area of' in
x:& T
on any other
, ki.netic energy
are
own fall
as t he volume of'
time.
An
the
the cloud
to
as
the rate
in
of a
is
l!Ln.gl.l:le€~rS, who are
of'
Univers
ity of
Cap
e Tow
n
to convert from one to the if the drop size distribution
are known. 's concept of
used the study, i.e. the of
at the ground, but the IS is as it
the basis of intensity or drop size
which reference will be made
for meteorological
purposes are where
succe of raiht'all are recorded as a total
on a clock whose varies to the
fre . .f
the chart is to be ohanged, usually daily, -or
remote or areas, weekly or monthly. Three main of
mechanism are employed. direct , used v""'VV .. L ........
by the United , passes from the
funnel to a ve whose weight is recorded with a to
in inches (or mm.) of rain. The tilting bucket type
records in positive steps the quantity of rain a bucket
and Causes to pen a
Both these may have a mechanism to reverse the when the pen
limit to the reaches the of its travel, so there is no
may be recorded. The third , used in these experiments t
the collected rain water in a chamber a float linked
the pen arm. The pen records the increase of in
the with a s to the chamber and so return the pen
to zero increments of uaually one inch of rainfall. In all
the is of in the quantity of
i.e. the of in-
me as uremen t is suitable for averaging the over
periods, but when the time interval is short the method becomes
very ous, and is not very
A number of
constructed for purposes.
two models both based on
retained anyone
Both were
recording cumulative quantity of rain.
a funnel
of size. The fell onto
monostable a
have been
Lewis & Watkins (36) tested
of the weight of the rain
which the
to
Adkin (37)
machines
the from
which formed large drops
trigp:ered a
-n3c()rdler, the of d
giving a measure of intensity.
in
Ker (22)
number of
a
ve one
a time under the funnel. The in tensi ty was
from the in a known time, but
convert from or..e to the if the drop size
are known. The 's of used the i.e. the
at the but the 's as it the basis size
to which reference
purposes are where succe are recorded as a total
on a whose to the fre WH.L.LO!L the cp~rt is to be ,-or in remote or areas, weekly or monthly.
direct
funnel to a vessel ",hose
, passes
is recorded
L"1 inches (or mm.) of I' ain. The
Three main
, used
from
of
to
records in time the quantity of rain a bucket
Causes to the pen a
Both these may have a mechanism to reverse the when the
re.aches of its travel, so there is no upper
may- be recorded. , used in these
the collected rain water in a chamber a
the pen arm. The pen records the L"1crease of water in
the with a s to empty the chamber and so return the pen
to zero one inch of In these
the from the
recorded, i.e. the
is
but when the time interval
very "'..,UU'L .. ous, and is not very
A number of
constructed for
two models both based on
retained anyone
Both were
purposes.
cumulative quantity of
a
of
funnel
size. The fell onto
a
of
the
a measure of Ker (22)
in number of
in the quantity of
in-
the over
short the method becomes
have been
& Watkins ( ) tested
of the
which the
of' the
to indirect machines
from
a
coroer, the
a time under the funnel. The was
from the in a known time, but
Univers
ity of
Cap
e Tow
n
could only give
constructed a very i:>.,LlllIJJL.v
for short peri ods • Farbrother (38) and , which a variation
of the constant ..I.&"..u.J'~ method. Rain from the colle cting funnel
passes into a container with a ne..rrow up one
side • '!he level to such a point that the inflow from the funnel
through the slit, the at any
one time a measure of the rate of inflow. The mounted
on a cantilever spring and the
water in the cylinder is recorded
is calibrated directly by
the
outflow is •
caused by the
by a pen arm.
of
instrument
in rates,and
This same basic principle of
in the commercial ' J ardi '
re4:::o:r'aer of Messrs. Cassella, and in the ins 'truman ts
which were designed, built, and supplied, by the Federal of
Meteorological Services for the
is shown in its normal
rainfall laboratory in
., and in the
in
1. shows the operating
passes from the collecting
funnel A through B, to eliminate air locks and swirl
effects, to funnel C from where it is led to the bottom of float
D through vents designed to avoid swirl and eddies. The
water level rises, lifting float E, and the needle F, to such
a point that the gap around the needle where it passes through the
circular orifice G is enough for the outflow to equal the inflow.
As the rate of flow ( rises so does the float
pen arm, constantly """'<;;.<>. ...... a new
so that
of the
funnel (two feet
vertical movement of
with linear scale.
a
on F is
a direct
volume of water
to overcome any to
changes in water level in the float chamber is increased by at ~o., .......... e;
the pen arm support to a oounterweight via a pulley and light flexible
cable. Guides (not lOCate the float, taper needle and the rod
the pen arm. Any chart may be used to reoord
or as is 4.2 .. two
oh with
traces of the same storm, recorded at various
different purposes. These instruments
for the
are used to
of low
sent e , and in addition
may reoord
10 shows
for
instruments, by means
voltage rel~s which
operate the power circuits.
linked to
This gear is in Chapter B.
could only give
constructed a very i:>.,LlllIJJL.v
for short peri ods • Farbrother (38) and , which a variation
of the constant ..I.&"..u.J'~ method. Rain from the colle cting funnel
passes into a container with a ne..rrow up one
side • '!he level to such a point that the inflow from the funnel
through the slit, the at any
one time a measure of the rate of inflow. The mounted
on a cantilever spring and the
water in the cylinder is recorded
is calibrated directly by
the
outflow is •
caused by the
by a pen arm.
of
instrument
in rates,and
This same basic principle of
in the commercial ' J ardi '
re4:::o:r'aer of Messrs. Cassella, and in the ins 'truman ts
which were designed, built, and supplied, by the Federal of
Meteorological Services for the
is shown in its normal
rainfall laboratory in
., and in the
in
1. shows the operating
passes from the collecting
funnel A through B, to eliminate air locks and swirl
effects, to funnel C from where it is led to the bottom of float
D through vents designed to avoid swirl and eddies. The
water level rises, lifting float E, and the needle F, to such
a point that the gap around the needle where it passes through the
circular orifice G is enough for the outflow to equal the inflow.
As the rate of flow ( rises so does the float
pen arm, constantly """'<;;.<>. ...... a new
so that
of the
funnel (two feet
vertical movement of
with linear scale.
a
on F is
a direct
volume of water
to overcome any to
changes in water level in the float chamber is increased by at ~o., .......... e;
the pen arm support to a oounterweight via a pulley and light flexible
cable. Guides (not lOCate the float, taper needle and the rod
the pen arm. Any chart may be used to reoord
or as is 4.2 .. two
oh with
traces of the same storm, recorded at various
different purposes. These instruments
for the
are used to
of low
sent e , and in addition
may reoord
10 shows
for
instruments, by means
voltage rel~s which
operate the power circuits.
linked to
This gear is in Chapter B.
Univers
ity of
Cap
e Tow
n
-
Plate 4.1. Intensity Recorder as Normally Used.
Plate4.2. Intensity Recorders in Rainfall Laboratory.
-
Plate 4.1. Intensity Recorder as Normally Used.
Plate 4.2. Intensity Recorders in Rainfall Laboratory.
Univers
ity of
Cap
e Tow
nE
o
E
o
Univers
ity of
Cap
e Tow
n
-; -.~ .c: 0 .~
0 l1li d ." Z
I &aI ... )C l1li
II' 1ft u.t
~ IZ :> I&.
~ W 0 u.t
" "l~ ~ 0 ~
U CD
IX '''''i
~-~, ~"j l1li ...j ~ ..J -rE ... -z- f\l
OJ
:( E IX ):
u·· N
CD
0 N ....
I "" 2
. <if'
< b;)
,''''
"'~' '-.~-'-
-; -.~ .c: 0 .~
0 l1li d ." Z
I &aI ... )C l1li
II' 1ft u.t
~ IZ :> I&.
~ W 0 u.t
" "l~ ~ 0 ~
U CD
IX '''''i
~-~, ~"j l1li ...j ~ ..J -rE ... -z- f\l
OJ
:( E IX ):
u·· N
CD
0 N ....
I "" 2
. <if'
< b;)
,''''
"'~' '-.~-'-
Univers
ity of
Cap
e Tow
n
(Laws , and Gunn & Kinzer 40),
experimental methods, show very good agreement,
these values are considered acceptable f or use the
experiments without
very
... <1..1 ...... "' ..... treatments of the theory of.the velocity
of cu...I...'-U'6 drops or by Best (41) (42)
are the <:l!lM'>P .. ~; of Laws and Gunn.
and found a good agreement wi th Laws' experimental for small
up to 1 .. 6 mm. diame~r. Best includes a suggested calculation
covering the v in velocity due to the effect of increased
No is known, but the are
have been in the of
For the at which the experimental work was (4,500 feet), Best computes a correction factor for "summer tropical atmosphere tt
of 1 for of 0.3 - 6.0 rom., 1.033 for diameters
0.3 0.05 mm.,
(relative to 1.0 = 1.021 for smaller 0.05 mm.
terminal velocity at sea level).
4.3.2.
Mache (45) in 1904 used a
for the that it was the forerunner of that
used Laws years
Schmidt (44) in time for
drops to fall a fixed ~re mounted a fixed
on a at spee
a narrow on
disc which during the fall. amount
A
of
angular rotation could be from the position of the drop marks
on the lower disc, of fall and velocity were Calculated.
The results from measured ....... ,"'''''''' were used to create a
extended these
the results in a
method by
other drop sizes. (46) and J.J..I.,.3I',ouew:
but more than confirm
way. Flower (48) attempted a direct
the of of known
torsion
(Laws , and Gunn & Kinzer 40),
experimental methods, show very good agreement,
these values are considered acceptable f or use the
experiments without
very
... <1..1 ...... "' ..... treatments of the theory of.the velocity
of cu...I...'-U'6 drops or by Best (41) (42)
are the <:l!lM'>P .. ~; of Laws and Gunn.
and found a good agreement wi th Laws' experimental for small
up to 1 .. 6 mm. diame~r. Best includes a suggested calculation
covering the v in velocity due to the effect of increased
No is known, but the are
have been in the of
For the at which the experimental work was (4,500 feet), Best computes a correction factor for "summer tropical atmosphere tt
of 1 for of 0.3 - 6.0 rom., 1.033 for diameters
0.3 0.05 mm.,
(relative to 1.0 = 1.021 for smaller 0.05 mm.
terminal velocity at sea level).
4.3.2.
Mache (45) in 1904 used a
for the that it was the forerunner of that
used Laws years
Schmidt (44) in time for
drops to fall a fixed ~re mounted a fixed
on a at spee
a narrow on
disc which during the fall. amount
A
of
angular rotation could be from the position of the drop marks
on the lower disc, of fall and velocity were Calculated.
The results from measured ....... ,"'''''''' were used to create a
extended these
the results in a
method by
other drop sizes. (46) and J.J..I.,.3I',ouew:
but more than confirm
way. Flower (48) attempted a direct
the of of known
torsion
Univers
ity of
Cap
e Tow
n
and so when in 19,u (39 ) higher, using modern high a
very careful search was for errors.
none , and were later confirmed by
studies of Kinzer and Gunn(40). The method of
Gunn (49) was water crops were and
.l.WlfJu.J"i::)"" when the drop fell through were fed
through amplifiers to an so that the time to the
measured between the two '-'1..1,'''''''' be
Gunn's
Laws 3%, and been for ____ ,._ of
in the
present experiments •
•
As climates, or low intensity rain, and so
it is at the outset that new The
used
reviewed to see which, if any, are s fol:" the purpose"
bearing in mind the particular requirements of the present
1)
2)
• 1bese are:
from earlier exne:r1.1 a wide appears to be a
feature of size size distribution, a large number samples
rains of and probably
more seasons .. method be such the
processing and cbmputation of results from a large of
• the other hand, one,feature the
( common to of the so-called
'underdeveloped' countries) of
quantitative measurements not a
or estimation.. .A nrl~elS
involving a is thus not
unsui table on those grOUnds with some
countries where the
of such labour
high cast or non·availability
well make such a
or the distinguishing of tropical is
the very high intensities which occur .. p~ apparatus used must
be for use intens up to per hour.
The very heavy storms occur
they are largely unpredictable; end liable to occur at any hour
of the day or a team of observers to
the hours of for
and so when in 19,u (39 ) higher, using modern high a
very careful search was for errors.
none , and were later confirmed by
studies of Kinzer and Gunn(40). The method of
Gunn (49) was water crops were and
.l.WlfJu.J"i::)"" when the drop fell through were fed
through amplifiers to an so that the time to the
measured between the two '-'1..1,'''''''' be
Gunn's
Laws 3%, and been for ____ ,._ of
in the
present experiments •
•
As climates, or low intensity rain, and so
it is at the outset that new The
used
reviewed to see which, if any, are s fol:" the purpose"
bearing in mind the particular requirements of the present
1)
2)
• 1bese are:
from earlier exne:r1.1 a wide appears to be a
feature of size size distribution, a large number samples
rains of and probably
more seasons .. method be such the
processing and cbmputation of results from a large of
• the other hand, one,feature the
( common to of the so-called
'underdeveloped' countries) of
quantitative measurements not a
or estimation.. .A nrl~elS
involving a is thus not
unsui table on those grOUnds with some
countries where the
of such labour
high cast or non·availability
well make such a
or the distinguishing of tropical is
the very high intensities which occur .. p~ apparatus used must
be for use intens up to per hour.
The very heavy storms occur
they are largely unpredictable; end liable to occur at any hour
of the day or a team of observers to
the hours of for
Univers
ity of
Cap
e Tow
n
months, some system of
... ..L.1.u.\., .... '" recording
of catching everyone of the few
automa.tion must be very
The wide varie ty of
drop siZes,
been compared
of
J or
The importance
storms means that such
which have been to measure
liquid drops, has therefore
of
sizes must be
influences fall velocity, the Qistribution
distribution among certain groups of
total mess of all drops is of no
collection of individual drops for measuring
as required. The preservation of individual
them in a heavier fluid been used successfully
in Gunn and Kinzer (4D) applied this to determine
the size of water whose fall ve 10 ci ty was being measured, using
a to measure the diameter. and
oil in
use was made of
record rx, HLLru
the a , used a similar
of very small drops caught in
(51) that some
Hageman in Germany to
is not for natural
of the probability of drops in the catching
and Harmon(52) the method of catching water drops
in a tank of super cooled liquid ,and obtaining drop sizes from the
rate of fall of the frozen drop through the s uper-cooled liquid.
Longwell (53) used a freezing technique to study drop size of
atomised fuel oils. Maintaining the liquids at very low temperatures
under field conditions difficulties, and
even if these vU~l..!.u.. be overcome the
of or
for use on a
4.4.2. Radar.
number
The amount of
size of
recent years
is not for
.. None
of
of waves is a function of the
been in
equipment both on the ground
mounted in • Estimates of intensity and amount
are by (54), Marshall (55), Smith (56), Twomey (57), Wexler (58)" and Jones (59). A general estimate of
was by Marshall and Pa.lmer (60), and
months, some system of
... ..L.1.u.\., .... '" recording
of catching everyone of the few
automa.tion must be very
The wide varie ty of
drop siZes,
been compared
of
J or
The importance
storms means that such
which have been to measure
liquid drops, has therefore
of
sizes must be
influences fall velocity, the Qistribution
distribution among certain groups of
total mess of all drops is of no
collection of individual drops for measuring
as required. The preservation of individual
them in a heavier fluid been used successfully
in Gunn and Kinzer (4D) applied this to determine
the size of water whose fall ve 10 ci ty was being measured, using
a to measure the diameter. and
oil in
use was made of
record rx, HLLru
the a , used a similar
of very small drops caught in
(51) that some
Hageman in Germany to
is not for natural
of the probability of drops in the catching
and Harmon(52) the method of catching water drops
in a tank of super cooled liquid ,and obtaining drop sizes from the
rate of fall of the frozen drop through the s uper-cooled liquid.
Longwell (53) used a freezing technique to study drop size of
atomised fuel oils. Maintaining the liquids at very low temperatures
under field conditions difficulties, and
even if these vU~l..!.u.. be overcome the
of or
for use on a
4.4.2. Radar.
number
The amount of
size of
recent years
is not for
.. None
of
of waves is a function of the
been in
equipment both on the ground
mounted in • Estimates of intensity and amount
are by (54), Marshall (55), Smith (56), Twomey (57), Wexler (58)" and Jones (59). A general estimate of
was by Marshall and Pa.lmer (60), and
Univers
ity of
Cap
e Tow
n
"' ... , ... JJ<:;..... further by Sp~us (61) , . but a COIW~lrJ.
cu. •.• "' ...... by , showed
radar methods in the
pJ.(me~er work of
of
of
experiments.
1904, photographic
used to s falling drops, and the use of
apparatus, strobosoopes J
is now a developed
very spee d cameras
), but
cameras been successfully used te measure the velocity
drops and rain by Laws (39), of
Green ( ), but in Case were
sufficiently In Germany recently Schladerbusoh
( size and of from
rohieved successf'ul results the
Ekern (65), photographically recorded the of' f'alling
and Blanchard (66 67), sses of'
but
numbers of'
formation
C;;d..J"'-Ul'" of' d
nrllro" .en T"V me thods not
The only known case of' nnnT.nfY'I"'".·r.n"l being
used to record the s
(68 and 69)
ments f'or the
high
size
the cost of the
Median
n a are
were calculated
were
Diameter
been by SpUhaus (42), Neuman (
suffer the same deductions
at of' 0 - 3
up to of or 12
is work of'
f'or
J f'or example: 1
f'all of' rain,
there
values of'
intensities is not
have
but
,er
should not
hour experimental
"' ... , ... JJ<:;..... further by Sp~us (61) , . but a COIW~lrJ.
cu. •.• "' ...... by , showed
radar methods in the
pJ.(me~er work of
of
of
experiments.
1904, photographic
used to s falling drops, and the use of
apparatus, strobosoopes J
is now a developed
very spee d cameras
), but
cameras been successfully used te measure the velocity
drops and rain by Laws (39), of
Green ( ), but in Case were
sufficiently In Germany recently Schladerbusoh
( size and of from
rohieved successf'ul results the
Ekern (65), photographically recorded the of' f'alling
and Blanchard (66 67), sses of'
but
numbers of'
formation
C;;d..J"'-Ul'" of' d
nrllro" .en T"V me thods not
The only known case of' nnnT.nfY'I"'".·r.n"l being
used to record the s
(68 and 69)
ments f'or the
high
size
the cost of the
Median
n a are
were calculated
were
Diameter
been by SpUhaus (42), Neuman (
suffer the same deductions
at of' 0 - 3
up to of or 12
is work of'
f'or
J f'or example: 1
f'all of' rain,
there
values of'
intensities is not
have
but
,er
should not
hour experimental
Univers
ity of
Cap
e Tow
n
A number of widely
venienoe under the del~berately
methods may be grouped for con-
loose term 'eleotronio S I •
et .. (73) mounted
on a
tube I
sorted by
fifteen size
the
whioh
a souroe and a '
oell?) from whioh the impulses were amplified,
seleotors., and oounted. Very in
IJV''''''JI.U..L.V, but
was
be oaught on but
diffioul ties arise when are involved. A ·number of
similar scanning devices have been produced (Oourshee and , 74;
study from
of a camera are
now commercially available, but are all dependent upon produoing a
Mason used a 'photo-eleotric
passes
a light light is fooussed on the of a
sizes
and counted. The
in England but fails to fulfill the of a of
over A very
of oausing to a beam fooussed on a
photo-eleotrio cell linked to an osoillosoope, was used in an
"'''''''''m,:>TI'' by Cunningham (77). This recorded
in
not be ... "',..._ •• _
suooession as the flew through rain, and could
of a large number of drops at
a point.
Smith ( and 79) oauseddrops to between the
a oondenser,
deduced both
raindrops.
from of the induced
natural
The primary , of , but a praotioal for
of the natural
WaS the study
of appear have been developed. All
devices must be ruled as to fulfill the
during of
from a diaphragm I suoh as a miorophone, when
a drop have been mounted on
the impulse
reoei ves the
(77) and
of
of
I.. (80) and • Katz (81), and on (82)
and (83) .. These _1"," ..... 11 drops, or
A number of widely
venienoe under the del~berately
methods may be grouped for con-
loose term 'eleotronio S I •
et .. (73) mounted
on a
tube I
sorted by
fifteen size
the
whioh
a souroe and a '
oell?) from whioh the impulses were amplified,
seleotors., and oounted. Very in
IJV''''''JI.U..L.V, but
was
be oaught on but
diffioul ties arise when are involved. A ·number of
similar scanning devices have been produced (Oourshee and , 74;
study from
of a camera are
now commercially available, but are all dependent upon produoing a
Mason used a 'photo-eleotric
passes
a light light is fooussed on the of a
sizes
and counted. The
in England but fails to fulfill the of a of
over A very
of oausing to a beam fooussed on a
photo-eleotrio cell linked to an osoillosoope, was used in an
"'''''''''m,:>TI'' by Cunningham (77). This recorded
in
not be ... "',..._ •• _
suooession as the flew through rain, and could
of a large number of drops at
a point.
Smith ( and 79) oauseddrops to between the
a oondenser,
deduced both
raindrops.
from of the induced
natural
The primary , of , but a praotioal for
of the natural
WaS the study
of appear have been developed. All
devices must be ruled as to fulfill the
during of
from a diaphragm I suoh as a miorophone, when
a drop have been mounted on
the impulse
reoei ves the
(77) and
of
of
I.. (80) and • Katz (81), and on (82)
and (83) .. These _1"," ..... 11 drops, or
Univers
ity of
Cap
e Tow
n
I drops J entering a
volume of rain to
suitable
Anj~"'1:"""'p, and upon passage through a
samPte J and so are un
on the ground. Maulard (84) used the
as the receiver of a ~.~n~~ device
to count number of raindrops, but .neither this nor an
version of the same principle by (85) gave an accurate
assessment of use of a
effect of a
or collector to
measure the combined J,;illJpa<.;\; ..... "'~'v ..... in
Chapter 7, but the problem is to measure the individual drop
within a To this f rom recorded droP. impacts ,. would an impractically large number (of the order ·of 100) of
elements since the minimum area required for sampling
distribution is estimated to be ab,ou.t one square foot.
communication from the Director, Federal Meteorological
:J):}partment) •
both
size and size distribution is to catch the drops on some Vv~.""'CI"'" which
a or ,::,,::,~"UtJ. which can be , and whose size can
be related to the of 'be was
first by Lowe (86) who in' caught on of
slate, into squares for ease of measuring the of the
Very small droplets may be caugl1t on glass slides coated with
a substance s uah as ""!:2.i!'!r''''' oxide in which a minute crater is formed
by • The size of
measured under a
size sorting into groups was
of orifices to regulate the approach speed.
and hence size, is
. and ,88).
May (89) by means
Such methods are not
suitable for large drop sizes because of the splashing effects when
the drop stirkes a solid surface.
problem of splash was overcome by Blanchard (90) and
(91), who used fine mesh screens (actually ladies I nylon
stock1hgs) coated with a powder of contrasting colour such as lamp
black or white icing sugar. Raindrops pass through 'the screens
without splashing and each removes the powder from an area whose
diameter corresponds to that of the The screens were photographed
for permanent record and ease of , but the measure-
ment could be done directly on the screens. a very iJ.I. ... :a,;.I.i:iCl
and but not for
of the screens
or con
time coo-
A.IJ'Ji:>~u."", or
previously T'I'I"\~T'I •• 'Y"A,n screens must be stored in constant ................ ..... .chambers.
I drops J entering a
volume of rain to
suitable
Anj~"'1:"""'p, and upon passage through a
samPte J and so are un
on the ground. Maulard (84) used the
as the receiver of a ~.~n~~ device
to count number of raindrops, but .neither this nor an
version of the same principle by (85) gave an accurate
assessment of use of a
effect of a
or collector to
measure the combined J,;illJpa<.;\; ..... "'~'v ..... in
Chapter 7, but the problem is to measure the individual drop
within a To this f rom recorded droP. impacts ,. would an impractically large number (of the order ·of 100) of
elements since the minimum area required for sampling
distribution is estimated to be ab,ou.t one square foot.
communication from the Director, Federal Meteorological
:J):}partment) •
both
size and size distribution is to catch the drops on some Vv~.""'CI"'" which
a or ,::,,::,~"UtJ. which can be , and whose size can
be related to the of 'be was
first by Lowe (86) who in' caught on of
slate, into squares for ease of measuring the of the
Very small droplets may be caugl1t on glass slides coated with
a substance s uah as ""!:2.i!'!r''''' oxide in which a minute crater is formed
by • The size of
measured under a
size sorting into groups was
of orifices to regulate the approach speed.
and hence size, is
. and ,88).
May (89) by means
Such methods are not
suitable for large drop sizes because of the splashing effects when
the drop stirkes a solid surface.
problem of splash was overcome by Blanchard (90) and
(91), who used fine mesh screens (actually ladies I nylon
stock1hgs) coated with a powder of contrasting colour such as lamp
black or white icing sugar. Raindrops pass through 'the screens
without splashing and each removes the powder from an area whose
diameter corresponds to that of the The screens were photographed
for permanent record and ease of , but the measure-
ment could be done directly on the screens. a very iJ.I. ... :a,;.I.i:iCl
and but not for
of the screens
or con
time coo-
A.IJ'Ji:>~u."", or
previously T'I'I"\~T'I •• 'Y"A,n screens must be stored in constant ................ ..... .chambers.
Univers
ity of
Cap
e Tow
n
The 1nird variation of or measurements,
been used in the of studies both of natural rain
of the drops on sheets of
absorbent material suoh as blotting paper,
has
\mder
size of the
)
each
) or
to the water makes the
The same effect is rohieved in
a
(Hanson, 93; permanent
easior to measure.
measurements by
amount of a "001iVfu:!l:'e
onto the
such as A ..... '-""'"Cl ....... or Methylene
(Blanchard - , 95, 96). This
German meteorologists who
method was extensively used
out many experiments on
natural
Of
.. (Weisner, 97; fS work is of interest in the
, 98; , 99). because it
is other known whioh includes thunderstorms,
both Weisner and Defant considered probabJ e sources of error.. This
early work was by (100) who
into no than 97 size groups.
American meteorologists considered using technique,
a of errors (51). He
the' sheets of
paper or filter paper may cause errors, even when using a
paper, and that the of the drop the stain it
a funotion of both the humidity of the and the
velocity which the drop the
In of these defects the
to widely studied, but "''''''''Nc>,,..1 S arguments are
between the equations suggested by
the
where D is
is D ::::
drop
the same
to relate the of the crop ahd
(i.e. diameter of a ~nl,pr"
as the drop which
S is the average stain
a is a depending on the thickness
of the paper
b a constant for which "' ..... , .. "'-" . ., <:i. ..... ~L<JU. are 0 by
and from
'"'v ............ ..., have
Gunn (49) in
Riley (107),
irrigation
D' (102), 0 .. 84 depending on
Jarman (103)
(Magarvey, 1(4).
of to n~rr-loot;eoro.lo~~1C~a~
Lee (105) in a stuny of
studies of velocity, by
.... 'au .... '" & & 109), t and Davis & Elliot (no) far
atom1sation,
(106), of
The 1nird variation of or measurements,
been used in the of studies both of natural rain
of the drops on sheets of
absorbent material suoh as blotting paper,
has
\mder
size of the
)
each
) or
to the water makes the
The same effect is rohieved in
a
(Hanson, 93; permanent
easior to measure.
measurements by
amount of a "001iVfu:!l:'e
onto the
such as A ..... '-""'"Cl ....... or Methylene
(Blanchard - , 95, 96). This
German meteorologists who
method was extensively used
out many experiments on
natural
Of
.. (Weisner, 97; fS work is of interest in the
, 98; , 99). because it
is other known whioh includes thunderstorms,
both Weisner and Defant considered probabJ e sources of error.. This
early work was by (100) who
into no than 97 size groups.
American meteorologists considered using technique,
a of errors (51). He
the' sheets of
paper or filter paper may cause errors, even when using a
paper, and that the of the drop the stain it
a funotion of both the humidity of the and the
velocity which the drop the
In of these defects the
to widely studied, but "''''''''Nc>,,..1 S arguments are
between the equations suggested by
the
where D is
is D ::::
drop
the same
to relate the of the crop ahd
(i.e. diameter of a ~nl,pr"
as the drop which
S is the average stain
a is a depending on the thickness
of the paper
b a constant for which "' ..... , .. "'-" . ., <:i. ..... ~L<JU. are 0 by
and from
'"'v ............ ..., have
Gunn (49) in
Riley (107),
irrigation
D' (102), 0 .. 84 depending on
Jarman (103)
(Magarvey, 1(4).
of to n~rr-loot;eoro.lo~~1C~a~
Lee (105) in a stuny of
studies of velocity, by
.... 'au .... '" & & 109), t and Davis & Elliot (no) far
atom1sation,
(106), of
Univers
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sprays.
Successful size size distribution of
a number of on same basic method have
been by Atlas & Plank (Ill), Anderson (112), (113)
and Marshall and Palmer & 60), but of
rain so de not
solution of the immediate problem. elimin~ted errors to
in the absorbtion of the paper by using a "" .... ,:l."'t):1..I. , and
that this is "an ,. A an unrolling of
paper was for use in aircraft by Eowen (114) J but
Was intended for measurements of comparative of total water
content cloud levels, the U."·lHlt:: of drops
could .. be measured
In more for ground use Eomn &
( ) allowed rain to fall through f.:, into a
wind tunnel where an air stream the falling drops onto a
of sensitised paper. The deflection is
to the mass and velocity of the drop and so the
on the paper as from a datum line is a. measure of
of A is obtained the size of
at various distances of deflection from the base • The
same Was modified hy Turner (116), who WI >Hlt"::LL to establish
a size and ""u .......... · ....
to catch groups of by a number of
delfected stream. The total volume of
rain collected in the of 1" aindrops
falling within a size group.
Several recording instruments been used by
(94, 95, 96), & manchard (117) during the operation
of Project Shower in It -m:s measure drop and
distribution at different mountainous
country a light portable recorder was which ""'"1 Itl measure
a for a few minutes each time,
fell thll"'r'it'IQ a small sampling
narrow strip of on
the battery ..riven motors used to
..
intervals
onto a
A time
sections of '""'0' .. "".... for intervals.. La tel"
models a shutter close off the aperture when not rp',ClOJr'd:'i.nQ,
and a heating to before it
spool. The were very
purpose, could be adapted to the
to give an adequate (only SllU:I...I....I...
occur in , 2) a much faster tape speed
onto the
using
of drop
sprays.
Successful size size distribution of
a number of on same basic method have
been by Atlas & Plank (Ill), Anderson (112), (113)
and Marshall and Palmer & 60), but of
rain so de not
solution of the immediate problem. elimin~ted errors to
in the absorbtion of the paper by using a "" .... ,:l."'t):1..I. , and
that this is "an ,. A an unrolling of
paper was for use in aircraft by Eowen (114) J but
Was intended for measurements of comparative of total water
content cloud levels, the U."·lHlt:: of drops
could .. be measured
In more for ground use Eomn &
( ) allowed rain to fall through f.:, into a
wind tunnel where an air stream the falling drops onto a
of sensitised paper. The deflection is
to the mass and velocity of the drop and so the
on the paper as from a datum line is a. measure of
of A is obtained the size of
at various distances of deflection from the base • The
same Was modified hy Turner (116), who WI >Hlt"::LL to establish
a size and ""u .......... · ....
to catch groups of by a number of
delfected stream. The total volume of
rain collected in the of 1" aindrops
falling within a size group.
Several recording instruments been used by
(94, 95, 96), & manchard (117) during the operation
of Project Shower in It -m:s measure drop and
distribution at different mountainous
country a light portable recorder was which ""'"1 Itl measure
a for a few minutes each time,
fell thll"'r'it'IQ a small sampling
narrow strip of on
the battery ..riven motors used to
..
intervals
onto a
A time
sections of '""'0' .. "".... for intervals.. La tel"
models a shutter close off the aperture when not rp',ClOJr'd:'i.nQ,
and a heating to before it
spool. The were very
purpose, could be adapted to the
to give an adequate (only SllU:I...I....I...
occur in , 2) a much faster tape speed
onto the
using
of drop
Univers
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of time-operated triggering mechanism.
However, in of many succe ssful
was not considered the most
two main reasons:
the
the
1) The standards of acouracy are
2)
of the nature for
is to be
such as
will not
squared TV\u"" ....
be
drop stains are
or where the
measured, or non-concurrent, characteristics
• In such cases the
the
affect the result, but when,
group
present
by splash
and hence
Neuberger (51) of
, the error in
of mass by this of measurement may be as as
of a instrument
certainly overcome but the use of a
scanner ..is impossible in the pre case, 'by
of a number of drop would a very
All the possible methods so .LLLC'.Luu.. have thus been
for one or more reasons it to
discuss 1 and was used in the
ndronR in oil or on paper
may be in a 'Oowaer
are cement, househol~ flour) and plaster of
paris.. Provided an exact can be the
(or mass) of a (or mass) of a form
a distribution of raindrops can be
calculated from by a .. Of the
nor
as the of an individual drop
very
into the
a discrete pellet which oven drying
and
The flour is dish or tray, exposed to the
for an with catching an adequate number of
without there some on top of the
formed a drop. Sorting of ovell dried
..
of time-operated triggering mechanism.
However, in of many succe ssful
was not considered the most
two main reasons:
the
the
1) The standards of acouracy are
2)
of the nature for
is to be
such as
will not
squared TV\u"" ....
be
drop stains are
or where the
measured, or non-concurrent, characteristics
• In such cases the
the
affect the result, but when,
group
present
by splash
and hence
Neuberger (51) of
, the error in
of mass by this of measurement may be as as
of a instrument
certainly overcome but the use of a
scanner ..is impossible in the pre case, 'by
of a number of drop would a very
All the possible methods so .LLLC'.Luu.. have thus been
for one or more reasons it to
discuss 1 and was used in the
ndronR in oil or on paper
may be in a 'Oowaer
are cement, househol~ flour) and plaster of
paris.. Provided an exact can be the
(or mass) of a (or mass) of a form
a distribution of raindrops can be
calculated from by a .. Of the
nor
as the of an individual drop
very
into the
a discrete pellet which oven drying
and
The flour is dish or tray, exposed to the
for an with catching an adequate number of
without there some on top of the
formed a drop. Sorting of ovell dried
..
Univers
ity of
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-36-
can be a of
smaller , and the number of groups
number of sieves used.. From the weight of
it is possible to calculate the total weight
falling within the size
is limited the
in anyone
or number of ,.. .... , .... .,....,.
The method was fir st used by ( in 190!.- measure
sizes in a number of research workers
have on his used it for various Laws
(39) and (66) of was
the and Laws later (119) extended
to Schleusener
(120) studied the effects of on spray
(121) tested rain-
fall _ and Chapman (122) compared the erosive effects of
rainfall tmder a tree canopy and in the open. of the size
distribution of natural rain have also been
and Ker (22) ..
by ID.anchard (123)
The main disadvantage associated in the
been the of in the between the
maSS of a and the mass of the which forms a
this mass. This is to detailed study in the following
of
chapter, and the probable sources of error in the previous studie s are
identified and eliminated.. The of a machine to allow automatic
----"------'0 of this me thod, the procedure
the are ae:SC1",-u::)ea in
KINETIC ENERGY OR MOMENTUM.
An association between erosion and kinetic energy can be
deduced from the fact that erosion is a work process involving the
expendi ture of energy in can::ving out the several processes, viz. the
or breaking down of soil particles or aggregates, the
soil the and the transportation of soil
the u. ........ .I.1J,~ rain, and both
over the surface.
than that of
has an average
the run-off is only a
sources of energy are kinetic energy from
and kinetic energy of water flowing
The energy of is usually far
flow. The
of about 20 - 30 feet
of the total rain
the soil
second, whereas
velocity. There are exceptions to this when the run-off from a -'-i':LL lI<.C, ...
area is concentrated onto a smaller area, or on long where
energy is to kinetic energy during run-off,
in Cases such as the ellergy of the rain is several orders
than the of run-off. It is probable, then,
that Kinetic energy is an of power.
I -36-
can be a of
smaller , and the number of groups
number of sieves used.. From the weight of
it is possible to calculate the total weight
falling within the size
is limited the
in anyone
or number of ,.. .... , .... .,....,.
The method was fir st used by ( in 190!.- measure
sizes in a number of research workers
have on his used it for various Laws
(39) and (66) of was
the and Laws later (119) extended
to Schleusener
(120) studied the effects of on spray
(121) tested rain-
fall _ and Chapman (122) compared the erosive effects of
rainfall tmder a tree canopy and in the open. of the size
distribution of natural rain have also been
and Ker (22) ..
by ID.anchard (123)
The main disadvantage associated in the
been the of in the between the
maSS of a and the mass of the which forms a
this mass. This is to detailed study in the following
of
chapter, and the probable sources of error in the previous studie s are
identified and eliminated.. The of a machine to allow automatic
----"------'0 of this me thod, the procedure
the are ae:SC1",-u::)ea in
KINETIC ENERGY OR MOMENTUM.
An association between erosion and kinetic energy can be
deduced from the fact that erosion is a work process involving the
expendi ture of energy in can::ving out the several processes, viz. the
or breaking down of soil particles or aggregates, the
soil the and the transportation of soil
the u. ........ .I.1J,~ rain, and both
over the surface.
than that of
has an average
the run-off is only a
sources of energy are kinetic energy from
and kinetic energy of water flowing
The energy of is usually far
flow. The
of about 20 - 30 feet
of the total rain
the soil
second, whereas
velocity. There are exceptions to this when the run-off from a -'-i':LL lI<.C, ...
area is concentrated onto a smaller area, or on long where
energy is to kinetic energy during run-off,
in Cases such as the ellergy of the rain is several orders
than the of run-off. It is probable, then,
that Kinetic energy is an of power.
I
Univers
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of
measure of erosi va power
momentum or servES as a
later. As both are
functions of a of mass and velocity J either can be computed
in
measurements of these iwo
chapter. Direct
be discussed in Chapter 7.
WU~vU were discussed
of both and mo:mellt1.:Ull
of
measure of erosive power
funct:Lons of a
from of the se iwo
in Direct
be in Chapter 7.
or energy serva;; as a better
As are
either ca:n be
which were
of both and momentum
Univers
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I
5
the flour
arrived in
method
, it in detaiL any
of rain upon the relationship between the
size (or mass) of the drop and the size (or mass) of the flour pellet,
and this must first be by a
• A of have been
but accuracy of of is doubtful as the necessary control
were not applied" Before analysing the present calibrations,
and presenting a new more accurate one, it is to
the effects of these • are to ensure
, others to ensure that it
may be applied without error to observations. For example the
of upon how much are Provided
the same in the and in the errors are
not necessarily introduced, although compiete drying
is the most method.. On the other hand if the laboratory
calibration uses the flour at a than that
of natural raindrops,
influenced by this speed,
when the
ifj as will be shown the mass
t hen an error will inevitably be
is app~~Y~ to field samples.
consists of allowing " ...... ~T\'" of ........ un... size
flour and determining the size of the
must be repeated at of rain
which is from mgm. to 50 mgm. Many drop sizes to be
measurements are LLLL.CU at each size I so uniform drops of constant
size are .. A suitable method to water to
from nozzles whose of can be very controlled
arrived in
in detail.
of rain upon
size (or mass) of the and the size (or of the flour
and this mus t first be
A of have been
but acouracy of of is doubtful as the necessary control
oondl.tions were not Before
and a new mo:re accurate one" it is ",nn'l"lnn''''' to
consider the effects of these
that the
may error to
weight of the upon how much
to ensure
ensure that it
the
the same and in the errors are
not nece
is the most
calibrat::i .. on uses
method .. On the other
01~
when the
::lnto flour and
This !1l.u.st be at
d::t:'op sizes to be
measurements are
size are;
from whose of
the flour at
be shown
error
size of the
which is from
of
of
masS
size
rain
.. at each size, so uniform 11 ...... ''\'''\., of """,.. ,,.",",
method water to
can be very controlled
Univers
ity of
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I suitable valves, or clamps on The
may be turned from brass drawn out
to surface tension
, drops will not detach themselves
by alone, but these may be blown off by an ai;r' .. Gunn
and Kinzer (40) verti it to
to micrograms using a
varying the rate of water the
was also used
ofTa mass • "'..I.j,j~.'" hypodermic
of
(124)
by
The
of
small and size in connexion with fire-fighting research.
of '\eloci ty of
the air the into
ve used for catching the drops, and in the present this
was: adopted, for a different reason
tte also a
stream for very
drops vmose mean size can be determined with accuracy,
was approximatelY
(126) to
even
so tha. of
the me an • This
more slowly, but the standard of mass was 27'0 for
0.6 mgm. mgm .. drops.. Alternative of
off from a
, and the use of a blade to
cut off droplets from a column of and
(130) used the ingenious method of
VVH"LVU on bursting the water
project very small the air, and was means
to make down to 2 microns ter.
mayoocur is insignificant.
lower limit at vmich the , and
the resultant o mgm ..
workers to produce of size
away when the use of an
The
5.1. drops of sizes
that to avoid "' .. r,"',..",.' .... '"
the air should be
water .. It is
to air
is shovm in
combinations of nozzle and stream
the range ..
the formation of
it through several flasks of
test this as to
in the air
from saturated
the
change in any of the conditiams of re
by
suitable , or
may be turned from brass
sizes, or
, drops
to
alone, but these may be blown off by an
The
dra\"ll'l out to
tension
themselves
Gu."ln
and Kinzer (40)
verti to ofra mass
to a by
the rate of water of The
method was also used (124) to of
and size in connexion with research.
of
the air
ve used for the arops, this
reaSon
Lane (125) ,..YO· ....... \1'y""'tte also a
stream for very
'Whose mean size can be accuracy, but eVen
so the. waS approximately of
the mean. This (126) to
but the staDdard mass was for
0.6 mgm. methods of
, and the use of a blade to
cut off droplets from a column of
(130) used the
on
method of
the water
and
very small the air, and was means
to make ~n to 2 microns of
may occur is The
lovrer
it to isolate the is about 0 mgm ..
met to produce of size
away when the use of an
The is shovm in
5 .. 1 .. combinations of stream
of sizes the range. (121)
the
water ..
to
that to avoid
should be
the
It is to test this as to
in the air
in a..'1y of the of
formation of the
several flasks of
from saturated
the
1'1.9
il
Univers
ity of
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-4-0-
Plate 5.1. ~op Producing Apparatus.
The flow of water to the hypodermic need-Ie is
controlled by the stop cock and screw clamp on the
left. The air stream from the glass tube is
controlled by the air valve at the top centre.
-40-
Plate 5.1. ~op Producing Apparatus.
The flow of water to the hypodermic nee ale is
controlled by the stop cock and screw clamp on the
left . The air stream from the glass tube is
controlled by the air valve at the top centre.
Univers
ity of
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drop mass~ not in a change of
drops of a size, but to drops as 1.ll'lif'ormly
as of • Using a non-coaxial the
humidity of will only influence
) of a second
it passes through ihe into fall
through the general mass of unblown air the laborator.v. The
of evaporation during the period of fall is
and (131) showed very or turned with
are required to minimi.se drop variation, Morgan )
emphasised the importance of all control conditions, particularly
temperature, from both Vibration contamination ...
In the studies the were formed from turned brass
and the of vnl~ru~rnnC uC'~=O whose
ends were carefully lJ"ound to a flat s1.li'face at right
length of the needle, the ins ide burr removed.
the
Distilled water
was used, s~plied from a tank whose surface area is enough that
be a of the .. Once
a test has rate of water supply J and the
supply, must absolutely constant be shown later,
of f
not touched during tests on a
size. It was found that even under these conditions there was
a tendency for the rate of to decrease
is that minute impurities in the
a progressive choking effect at the clamps controlling the
water so once a size is
being produced the test as as ,
constant checking by stop-watch of the rate of drop production.
of , which
of the wa tar,
of partition is not solely
affect the viscosity or surface
be The moment
of
......... ,. ............ force to surface tension forces, but is
by
later), so
Tibration. Particularly
drop (as will.be
must be free from
, a high
pump is used to produce the air must
be a different room.
In formation which
there some mass of individual drops, and it
mass.
size, but to
of a. seoond
it passes
of eV'A,t:)iCI'A
ends were
was
a
a
a .......... '''' ... ......,.'''
water
oonstant
by
be
5
there
studies the
and the
test has
to
the
)
were formed from turned brass
of
at
burr removed.
area is
the
tests on a
whose
the
... ... "' ....... "'iU. water
be shown
even under the se there was
for the rate to decrease
the
so once a size is
drop (as will be
be free from
, a
must
a room ..
In which
some mass of it
Univers
ity of
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not only to measure the
which has been
as as ..,"' .... .., .... t:J, but
covered in previous
investigations, to estimate the magnitude of this variation.
difficulties in the of discussed the
the
accuracy are
of ) and the measurement under a of the
diameter of drops immersed in oil. Ta te and Marshall (50) studying
the size distribution of very small pressure
nozzles,
bottom s
in brass cells a 1'S ..... 1cl."''''
was chosen
as it has the of not absorbing any of droplets,
a specific gravity so near to that of we.ter that the drops rest
lightly on bottom with no from a truly
* bottom, the
of the Gunn
and Kinzer (40) used a variation of this in
u shallow dish of vacuum pump oil, measuring the
.. this method accurate better 2 for
droplets of mass 10 micrograms and less than ° per cent for
of 10,000 1\ but not show how these figures were
To of , a
immersed in the was used to meaSure the in
diameter.
of haS the advantage
should be more since
upon the cube of the measured diameter. Gunn and Kinzer in
the same used for the larger drops but found
impractioable to the .. fresumably this was
of the minute size, as of mass 0 must be
the naked eye. In the
direct .J..e;. . .LJ..L.ue;. of down to 0.08 mgm. was found to be both
and accurate, but
weighing errors would become of
cau;:1.t in "",L,a."""'-"'''
is the at which
Groups of were
0.5 cm .. depth of S.A • 20
motor WC.J..~l=U on a sensitive balance to
.. 05 Ill.L .......... LI'S ... For studied (0.08 mgm.),
groups of JI the weighing error is
about l1z%, but· this decreases as the drop maSs increases It These
were
mass/pellet maSs
to confirm the trend of the drop
the range in which it
not only to measure the
which has been
as as ;::';::'.I.U..I.c, but
and the two
"' .... i".r''' ... H.;<, of
diameter of
,to
iynmersed oil.
covered in
of this
, discussed
the
accuracy are
of
the size distribution of very small pressure
nozzles,
bottom
as of
so near to that
in brass cells
of Wt!l.ter
was chosen
rest
the
Gmm
in and Kinzer (40) used a
u shallow dish of vacuU!ll pump oil,
ter
that this method for
of
of 10,000
accurate
less ° per cent for
not show how these
To of , a
immersed in the was used to meaSure the
diameter.
The of
of should be more
upon the cube of the ..... .I.1:UIl"'ter.
the same
the
of the minute size, as of mass ° the naked eye. In the
in
haS the
Gunn and Kinzer in
but found
this was
be
direct of down to ° . was found to be both
and accurate, but
WC.l.~l4LH~ errors would become of
cau.' :1.t in
mot.or
''2ishes
on a sensitive
For
groups of ,
about l~, but· this decreases as the
at which
were
of S.A
mgm.), error is
mass increases It These
were to confirm the trend of the
the range in which it
Univers
ity of
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-43-
actually be The lowest mass at which the relationship.
fC1.md to be 0.17 mgm., and at this level the
maximum weighing error is 0,,75%.
from the of
loss can be reduced • which are closed immediately
after collection, but collection directly into oil is b~th
and more accurate. also the the
can be in oil to ensure that the correct
number have in fact been caught. air
a high pressure lettve the
The drops formed with
so fast
of in
eye.
in the
If a. missed the dish this would be tected
the
only
out in D.
some of
care is
on clean
and
but is always the possibility that a drop might
the a of be in
check of number of
VU':l..LJ.I',wS in weight of the vessel are
about three hours, and as each test was
of minutes no error was introduced. Absorption of
in to the oil also take s slowly, and it is
since it does not extreme
them the
them to dust.
In the number of drops of each
a
The largest
for accurate
as
between two
number of
of drop size
to 1ll..LJ"'.J."".J."
variation with time of the
but the test
the risk of
co-efficient of variation of to decrease as
mass ases • 5 .) and so for compar8~le
maSS of r for the smaller
drops. In the tes ts, groups of t en ten flour pellets)
were
mass so a
as the Case
groups of for the
physically impossible to use the same
both for a measure of mas s and the
procedure was Once an
a group of ten (or
in oil, the next ten
the flour pans, the next ten ro on for four groups of
drops and four A few are lost during
from the drops oil them as
in
-43-
actually be The lowest mass at which the relationship.
fC1.md to be 0.17 mgm., and at this level the
maximum weighing error is 0,,75%.
from the of
loss can be reduced • which are closed immediately
after collection, but collection directly into oil is b~th
and more accurate. also the the
can be in oil to ensure that the correct
number have in fact been caught. air
a high pressure lettve the
The drops formed with
so fast
of in
eye.
in the
If a. missed the dish this would be tected
the
only
out in D.
some of
care is
on clean
and
but is always the possibility that a drop might
the a of be in
check of number of
VU':l..LJ.I',wS in weight of the vessel are
about three hours, and as each test was
of minutes no error was introduced. Absorption of
in to the oil also take s slowly, and it is
since it does not extreme
them the
them to dust.
In the number of drops of each
a
The largest
for accurate
as
between two
number of
of drop size
to 1ll..LJ"'.J."".J."
variation with time of the
but the test
the risk of
co-efficient of variation of to decrease as
mass ases • 5 .) and so for compar8~le
maSS of r for the smaller
drops. In the tes ts, groups of t en ten flour pellets)
were
mass so a
as the Case
groups of for the
physically impossible to use the same
both for a measure of mas s and the
procedure was Once an
a group of ten (or
in oil, the next ten
the flour pans, the next ten ro on for four groups of
drops and four A few are lost during
from the drops oil them as
in
University of Cape Town
· ,
.. c
.Ij en
" Q
. ::::E
0 "-
« V
) 0
~
« 0
::::E N
Q
.
i .....
0 0
0 en
d 0.. :l
0 a: C)
UJ
N
ii Q
.
~ o. LLI C
) 0
<
-« LLI
~ u.. 0 Z
0 -l-.e( -« ~ ....;, , . 1ft
0 ;0
In
....
M
0 C»
.Y\-, N
(o/o) -/
NO
lJ.Vn:lV
A
:f0
J.N31:>
I:f.:lO:l
&L
I r
'I
6
'7' ;" - s z 0
~ " a:: « ;>
3 ILL 0
~ z: 2 !!! u i:i.: l.IJ 0 U
0 0.1
"
~.O 10.0 DROP MASS
OF AVERAGE DROP SIZE IN 2
.. ~.<",
,r
.. ~
i,
Univers
ity of
Cap
e Tow
n
-45-
pellets, by using previously prepared flour pans and collecting
dishes for the d raps, was reduced to a minimum and whole
completed in a few minutes. A test was used to
de tect any in the te a
with time was
in section 5.1 reasons for
It is unfortunately impracticable to catch botn the drops .. and pellets the same height the dropping point. be
shown that the the flour
, of fall of far
drops.
constant,
The point of impact is by no means
;::"uc:t...L.,l.. drops, as edd,y currents and drop
vary. A of flour can be
used to catch , but for of the size of
the vessel used for catching the drops is limited, so the
at the beginning of the fall, a few inches below the
.. reason for not drops at the
bottom of fall is the of
TT,""""""'''' to catch the drops in conical
wool, , etc., "'1L1'W'-:U. that errors due to
" .. ;" ................ " could not be
in
splash, either of the~op or the
avoided~ It is interesting that a , dry,
surface such as does not
was
(134 dOes not offer a solution as the evaporation
wouJ.d be
considered
The effect of
in section 5.1.6. will be
Since in the the of drop
at an orifice or nozzle have been ov.~~~
by and physicists interested in of
the surface of , by bacteriologists or
pathologists interested in producing d of known size.
In the surface tension was determined the
weight of the drop I as measured by weighing after to the
tension effects the periphery of the
UW;-,.u:lL,Il, drop formation. differed considerably
from those by other methods led to the
Guthrie (136) in 1963 that a or trsatelli tell
the of separation, he that the
-45-
pellets, by using previously prepared flour pans and collecting
dishes for the d raps, was reduced to a minimum and whole
completed in a few minutes. A test was used to
de tect any in the te a
with time was
in section 5.1 reasons for
It is unfortunately impracticable to catch botn the drops .. and pellets the same height the dropping point. be
shown that the the flour
, of fall of far
drops.
constant,
The point of impact is by no means
;::"uc:t...L.,l.. drops, as edd,y currents and drop
vary. A of flour can be
used to catch , but for of the size of
the vessel used for catching the drops is limited, so the
at the beginning of the fall, a few inches below the
.. reason for not drops at the
bottom of fall is the of
TT,""""""'''' to catch the drops in conical
wool, , etc., "'1L1'W'-:U. that errors due to
" .. ;" ................ " could not be
in
splash, either of the~op or the
avoided~ It is interesting that a , dry,
surface such as does not
was
(134 dOes not offer a solution as the evaporation
wouJ.d be
considered
The effect of
in section 5.1.6. will be
Since in the the of drop
at an orifice or nozzle have been ov.~~~
by and physicists interested in of
the surface of , by bacteriologists or
pathologists interested in producing d of known size.
In the surface tension was determined the
weight of the drop I as measured by weighing after to the
tension effects the periphery of the
UW;-,.u:lL,Il, drop formation. differed considerably
from those by other methods led to the
Guthrie (136) in 1963 that a or trsatelli tell
the of separation, he that the
Univers
ity of
Cap
e Tow
n
-46-
drop was ejected upwards from
oscillations immediately after separation.
pursued , and observed that the
extended neck which just before
(138) in 1903 the
workers observed
resolved when me<:iel'''n T:,"',",''1Yn
drop by violent
Worthington (137) formed from
..
of the
(62 and 139). Hauser and (140)
- "The actual "'y' .... "", ..... .J."'''' of fall
The drop grows the end of the
instability sets in, and a part of the breaks off it
1881
and
then
a narrow stem to the remainder of the drop.. At moment
of the drop is spherical. The narrow neck then
breaks off from the main body constricts to form one or more
• Using liquids like
may form from
However, it
heck, but water into a drop.
interesting that Guthrie's observation on the upward
movement of a satellite drop was very close to the truth, which is
that the secondar,y oscillates so violently that it sometimes
a
the dropping
the formation of the main and
This of the
of the
drop. This tertiary drop is
the water remaining on
s book it shows
arose from the
to the
eye to detach itself the main drop after a fall dis tance of a
foot or ro.. In this Calibration exercise it may lead to error
if the collection vessel is held a few inches below the
dropping point the secondar,y drop will also be caught, but the
secondary pellet will not be noticed in the flour.. It is not
from Hauser's work Vilhether this secondary drop is produced during the
formation of of all sizes, or only when drops are formea.
Laws (39) implies that it only occurs with drops of 6 mm .. diameter,
but this may be dUG to the fact by the writer the t it is
such drop is visible to the
U<:l.Z"'''''''''' eye. .As to the of the secondar,y drop, Laws
from "" ..... ,... ... , ......... ..,
6 rom. diameter ).
was found to be 3.12% but does not
when this was determined. Kelkar
as than one
Hauser (l4O ) states that it
the size of the main drop
) refers to the drop
as Plateau's spherule, and used it as a souroe of "' .. 'e ........ ur,uu-,.",
but the ratio of the sizes of the main and se.~lae~
-46-
drop was ejected upwards from
oscillations immediately after separation.
pursued , and observed that the
extended neck which just before
(138) in 1903 the
workers observed
resolved when me<:iel'''n T:,"',",''1Yn
drop by violent
Worthington (137) formed from
..
of the
(62 and 139). Hauser and (140)
- "The actual "'y' .... "", ..... .J."'''' of fall
The drop grows the end of the
instability sets in, and a part of the breaks off it
1881
and
then
a narrow stem to the remainder of the drop.. At moment
of the drop is spherical. The narrow neck then
breaks off from the main body constricts to form one or more
• Using liquids like
may form from
However, it
heck, but water into a drop.
interesting that Guthrie's observation on the upward
movement of a satellite drop was very close to the truth, which is
that the secondar,y oscillates so violently that it sometimes
a
the dropping
the formation of the main and
This of the
of the
drop. This tertiary drop is
the water remaining on
s book it shows
arose from the
to the
eye to detach itself the main drop after a fall dis tance of a
foot or ro.. In this Calibration exercise it may lead to error
if the collection vessel is held a few inches below the
dropping point the secondar,y drop will also be caught, but the
secondary pellet will not be noticed in the flour.. It is not
from Hauser's work Vilhether this secondary drop is produced during the
formation of of all sizes, or only when drops are formea.
Laws (39) implies that it only occurs with drops of 6 mm .. diameter,
but this may be dUG to the fact by the writer the t it is
such drop is visible to the
U<:l.Z"'''''''''' eye. .As to the of the secondar,y drop, Laws
from "" ..... ,... ... , ......... ..,
6 rom. diameter ).
was found to be 3.12% but does not
when this was determined. Kelkar
as than one
Hauser (l4O ) states that it
the size of the main drop
) refers to the drop
as Plateau's spherule, and used it as a souroe of "' .. 'e ........ ur,uu-,.",
but the ratio of the sizes of the main and se.~lae~
Univers
ity of
Cap
e Tow
n
-47-
FORMATIO ! OF A DROP, 11 A small droi' follows the larger and during its oscillations takes the unexpected shape revealed in 5, Picture!\ ShO"'5 ;\ dn)p that has fallen six or eight feet. During this fall the oscillations have ceased, and the fOl'ces of \\'indagc and surface tcn~ion gl'adually reach a balance, resulting in the stahle shape shown , The drop in <) has fallen 14 fe e t and flattened slighrly hut h" s the same contour as the un)p ill S
fl a te 5.2. The Process of DTop Formation.
(Reproduced from "Fla sh" by Edgerton end Killian)
-47-
FORMATION OF A DROP. 11 A small dro" follows thc larger and during its oscillations takes the unexpected shape revealcd in 5 . Picture R sho,vs a drop thar has fallcn six or cight feet. During this fall the oscillations haye ceased, and the fot·ccs of windage and surface tension grrldually rcach a b,lIance , resulting in the stahle shape shown. The drop in <) has fallen 14 fe c t and flattencd slightly hut h;l s the same contour as the d.·op in R
Pla te 5.2. The Process of Drop Formation.
(Reproduced from "Fla sh" by Edgerton and Killian)
Univers
ity of
Cap
e Tow
n
II undergo es In estimates
USin~i1 ter paper technique that the drop had
a maSs of 1 or 2"fo of the main drop.
The actual magnitude is not It is an A"""f"'IT"
as such must be eliminated Can be
easily directing a gentle air ands
sideways so that the small secondary drop is deflected more
than the main drop.. The collecting dish is then held a~ a point
about 6 inches below the dropping point in such a that the main
drop is caught) but not the secondary drop.. To check that the secondary
drop missed the collecting dish it is caught on dye-treated filter
paper held the dish.. Not knowing whether the
seciondary form with medium and small drops, the technique
was used for all For the drops where the air stream is
not to the air stream >Av."' ....... ".., was
mounted 2 inches below and to one side of the
for the the faster air stream to de tach the drops
was a to deflect any secondary drops,
(Plate 5.1 .. ).
EarlY in the present studies it was observed that on
the average size produced from a particular
from test to test under apparently identical
.. Eventually it was found that this variation was in fact
due to
A
been
of formation of the drops, that
drop size and rate of drop formation ..
v<:l..J.c· ... that this waS no new phenomenon, but had
T'le earliest studies of Guthrie and Worthington refer to
effect.
drops were presumably
was not used in these
and it was found that the mass
as the to form incre ase d. Hauser (140)
same effect" but not any data. (142) and
and (143) in connexion the
of for of their
are shown in • 5 • and 5 is the only known
en the behaviour of small drops and his restll.t
confirms that of the writer that for small drops the effect
reversed - that is small drops inorease as the time to form increases.
Fildes and Smart criticise Donald's inference that for small the
mass becomes almost constant at rates than one per
II undergo es In estimates
USin~i1 ter paper technique that the drop had
a maSs of 1 or 2"fo of the main drop.
The actual magnitude is not It is an A"""f"'IT"
as such must be eliminated Can be
easily directing a gentle air ands
sideways so that the small secondary drop is deflected more
than the main drop.. The collecting dish is then held a~ a point
about 6 inches below the dropping point in such a that the main
drop is caught) but not the secondary drop.. To check that the secondary
drop missed the collecting dish it is caught on dye-treated filter
paper held the dish.. Not knowing whether the
seciondary form with medium and small drops, the technique
was used for all For the drops where the air stream is
not to the air stream >Av."' ....... ".., was
mounted 2 inches below and to one side of the
for the the faster air stream to de tach the drops
was a to deflect any secondary drops,
(Plate 5.1 .. ).
EarlY in the present studies it was observed that on
the average size produced from a particular
from test to test under apparently identical
.. Eventually it was found that this variation was in fact
due to
A
been
of formation of the drops, that
drop size and rate of drop formation ..
v<:l..J.c· ... that this waS no new phenomenon, but had
T'le earliest studies of Guthrie and Worthington refer to
effect.
drops were presumably
was not used in these
and it was found that the mass
as the to form incre ase d. Hauser (140)
same effect" but not any data. (142) and
and (143) in connexion the
of for of their
are shown in • 5 • and 5 is the only known
en the behaviour of small drops and his restll.t
confirms that of the writer that for small drops the effect
reversed - that is small drops inorease as the time to form increases.
Fildes and Smart criticise Donald's inference that for small the
mass becomes almost constant at rates than one per
Univers
ity of
Cap
e Tow
n
• 5 • ) •
combines
-49-
is unfO'lmded since
for very large
5.2. and 5.3. to show that
own
Fig .. 5.4. minimum
of formation for constant size does as .. , and as said it is about one
for of 5 mgm. , there no justification for
It however, most
unlikely that the
this to very small
would be reversed , and
the in that minimum
should be one seoond for all up to 5 mgm.,
and for larger the minimum time should be not less than the
of
value from Fig. 5 .. 4. of
a further cheok the total time
was measured and when
were
The reasons for this of drop mass
of the effect is in nTIlnn."
with time
directions
for small , were pursued in Case such might
suggest ways of ,u<:u"-L,I.,"5 predictable drops. Worthington
(137) be due to an influx of through the
narrow neck of about to
of this water. The writer the possibility of
of of the with the time of
of No
of the
are known for
IJU"'''''I''.H this can be clearly
seen in the large drops, but values are available for
Rayleigh (144) derived a formula, and .j...l..Litu"' .. Heu. ......
..
Both values for of
(the largest with which we are concerned) of about 30 to 40
second. There is no oertainty that the pendent drop has the same
but is from ob that
vibration is very it is to see how a period
of vibration CQU inter~ct with th& period ot formation,
one probably orders of ..
Worthington suggested (137) that effects of the
through the elongated neok
the moment of .. is conceivable that some suoh effect
combines with gravitational in such a way as to a net
re which is in and for , but
this is pu.re and does not
towards improvement of the of .. What is
..
I • 5 • ) •
combines
-49-
is unfO'lmded since
for very large
5.2. and 5.3. to show that
own
Fig .. 5.4. minimum
of formation for constant size does as .. , and as said it is about one
for of 5 mgm. , there no justification for
It however, most
unlikely that the
this to very small
would be reversed , and
the in that minimum
should be one seoond for all up to 5 mgm.,
and for larger the minimum time should be not less than the
of
value from Fig. 5 .. 4. of
a further cheok the total time
was measured and when
were
The reasons for this of drop mass
of the effect is in nTIlnn."
with time
directions
for small , were pursued in Case such might
suggest ways of ,u<:u"-L,I.,"5 predictable drops. Worthington
(137) be due to an influx of through the
narrow neck of about to
of this water. The writer the possibility of
of of the with the time of
of No
of the
are known for
IJU"'''''I''.H this can be clearly
seen in the large drops, but values are available for
Rayleigh (144) derived a formula, and .j...l..Litu"' .. Heu. ......
..
Both values for of
(the largest with which we are concerned) of about 30 to 40
second. There is no oertainty that the pendent drop has the same
but is from ob that
vibration is very it is to see how a period
of vibration CQU inter~ct with th& period ot formation,
one probably orders of ..
Worthington suggested (137) that effects of the
through the elongated neok
the moment of .. is conceivable that some suoh effect
combines with gravitational in such a way as to a net
re which is in and for , but
this is pu.re and does not
towards improvement of the of .. What is
..
I
Univers
ity of
Cap
e Tow
n
50 \
til +
49 .
:l
'" If. ::e I DONALD
'" '" 1,7
'" ::e Q. 4 0 u: ~ + 0
.-i 4$'
0 " 8 10 II TIME TO FORM EACH DROP - SECONDS.
"
~ ...
~ SS '~O-------------------4------------------~--------'-O---------'.-
Q.
o g 134
1.31
+
It;u~ ________ ~ ________________ ~~ __________________________ _
o '2. 4- " IS 10 II
FIG. .2. VARIATION OF DROP MASS WITH CHANGE
IN TIME OF FORMATION.
50 \
til +
49 .
:l
'" If. ::e I DONALD
'" '" 1,7
'" ::e Q. 4 0 u: ~ + 0
.-i 4$'
0 " 8 10 II TIME TO FORM EACH DROP - SECONDS.
"
~ ...
~ SS '~O-------------------4------------------~--------'-O---------'.-
Q.
o g 134
1.31
+
It;u~ ________ ~ ________________ ~~ __________________________ _
o '2. 4- " IS 10 II
FIG. .2. VARIATION OF DROP MASS WITH CHANGE
IN TIME OF FORMATION.
Univers
ity of
Cap
e Tow
n
Q.
o IX o
5.8 ,---~-----------+
5.7
S.3
5."2 ...... ----------------------------o 2 5.,
FIG.S.3. VARIATION OF DROP MASS WITH CHANGE
IN TIME OF FORMATIO'N.
Q.
o IX o
5.8 ,---~-----------+
5.7
S.3
5."2 ...... ----------------------------o 2 5.,
FIG.S.3. VARIATION OF DROP MASS WITH CHANGE
IN TIME OF FORMATIO'N.
Univers
ity of
Cap
e Tow
n
f
FIG. 5.4. LEAST TIME OF FORMATION REQUIP.ED FOR PRODUCTION OF DROPS OF UNIFORM SIZE,
.- \ \
f
FIG. 5.4. LEAST TIME OF FORMATION REQUIP.ED FOR PRODUCTION OF DROPS OF UNIFORM SIZE,
.- \ \
Univers
ity of
Cap
e Tow
n
relevant is
some of the
•
the to pay
may have
calibra tions •
In order to
the curve to flour
, it is important that the c
whole of the expected
to the of
to errors
of extrapolation, when
i::ili\)U.W!U. cover the
is the maximum The upper
in values, estimated values,
vary • (66) found that drops supported in an
upward current of air were
50 mgm.) and definitely
with a zone
of
up to sizes of 4.6 rom. (
above 5.4. rom. (aPllr'o.Xl.llla
where the
stream. LenbI'd (46)
85 mgm.),
drops
of 5.2 rom. i1'3. natural rain, (97) a normal maximum
of 5.3. rom. up to 6.7 nun .. , but
can occur due to coalescence short distances
ground
between • In fact it will be shown that the relationship
mass becomes constant sizes
above about 4 ( mgm.). An limit of
restricts the c
to slightly larger with
confidence.
With "'wc, .......... 1"1 ..... ,'"..."', however, the departure from a linear
v.L'.Ju~)a.l..1-' between mass increases as
• Chosen mass are to have even
distribution on a logarithmic in order to
ship, and the smallest drop should correspond to
of the groups into which it is intended to sort
It is able to sieve, sort I we igh , the
from rain size groups with a mean maSs less than about 0.2 mgm.. and SO that lower in the is the
corresponding drop mass of about 0.16 mgm.
In the sent
down to Ll.rL)lJ'" of a mass 0
apparatus already
mgm.,the
in an
the calibration was
obtainable with the
to better the
of the ~~A~.,~~ between drop mass and mass.
relevant is
some of the
•
the to pay
may have
calibra tions •
In order to
the curve to flour
, it is important that the c
whole of the expected
to the of
to errors
of extrapolation, when
i::ili\)U.W!U. cover the
is the maximum The upper
in values, estimated values,
vary • (66) found that drops supported in an
upward current of air were
50 mgm.) and definitely
with a zone
of
up to sizes of 4.6 rom. (
above 5.4. rom. (aPllr'o.Xl.llla
where the
stream. LenbI'd (46)
85 mgm.),
drops
of 5.2 rom. i1'3. natural rain, (97) a normal maximum
of 5.3. rom. up to 6.7 nun .. , but
can occur due to coalescence short distances
ground
between • In fact it will be shown that the relationship
mass becomes constant sizes
above about 4 ( mgm.). An limit of
restricts the c
to slightly larger with
confidence.
With "'wc, .......... 1"1 ..... ,'"..."', however, the departure from a linear
v.L'.Ju~)a.l..1-' between mass increases as
• Chosen mass are to have even
distribution on a logarithmic in order to
ship, and the smallest drop should correspond to
of the groups into which it is intended to sort
It is able to sieve, sort I we igh , the
from rain size groups with a mean maSs less than about 0.2 mgm.. and SO that lower in the is the
corresponding drop mass of about 0.16 mgm.
In the sent
down to Ll.rL)lJ'" of a mass 0
apparatus already
mgm.,the
in an
the calibration was
obtainable with the
to better the
of the ~~A~.,~~ between drop mass and mass.
Univers
ity of
Cap
e Tow
n
-54-
The must be considered of
oauses a in the mass of the between the point
of point of It_llet formation. The losses are certainly
and are not detectable direct weighing , as explained
earlier, it is not to with sufficient acc~acy
been
a fall of several metres.
elsewhere, and a
Namekawa ( )
The has ~ however,
of the is
an
... a..I.uvu. from "''''',,'''' ..... velocities up to 2
second only. There is justification for ex·tr~~n()ia to
terminal velocities of the order of 9 metres/second, but
to obtain a first estimate of the order of the loss, we
so
that the
loss of weight of a drop of mass .51 mgm. mgm. a
through 8 metres, and this is not •
KinZ'er and Gunn (146) used much more methods to
measure evaporation losses over a wide range of conditions. Their
resul ts are in the form of two evaporation factors" one
size air the second dependent
on the and relative of the ambient air. The
of the two factors the loss in per unit time.
The for the sizes, of fall, and air
used in the present investiga tiona are in Table 5.1. The
time of fall has been calculated from the data of Laws (39) who
measured various
was as 10v: as
when air conditions were
drop size increases, ranging from
• The
out the calibration on
error decreases as
for this effeot 1 but in hot
no correction was
air the loss could be three
times and this would correction.
5.1.7.
Variations in the of the flour
measurement other weighing, as with the
preclude
it is
necessary not only to measure the mean pellet mass but also to
the varie.tion about this mean. This was done in the same
way as for the drops, that is by weighing 4 groups of 10 large pellets
and 20 pellets. It might be expected that the variation
would be than the drop variation, beCause the pellet variation
includes the in size and due to flour
effects .. the
to the variation (Fig. 5 .)
shown in
that
5.5. is similar
dependent on
-54-
The must be considered of
oauses a in the mass of the between the point
of point of It_llet formation. The losses are certainly
and are not detectable direct weighing , as explained
earlier, it is not to with sufficient acc~acy
been
a fall of several metres.
elsewhere, and a
Namekawa ( )
The has ~ however,
of the is
an
... a..I.uvu. from "''''',,'''' ..... velocities up to 2
second only. There is justification for ex·tr~~n()ia to
terminal velocities of the order of 9 metres/second, but
to obtain a first estimate of the order of the loss, we
so
that the
loss of weight of a drop of mass .51 mgm. mgm. a
through 8 metres, and this is not •
KinZ'er and Gunn (146) used much more methods to
measure evaporation losses over a wide range of conditions. Their
resul ts are in the form of two evaporation factors" one
size air the second dependent
on the and relative of the ambient air. The
of the two factors the loss in per unit time.
The for the sizes, of fall, and air
used in the present investiga tiona are in Table 5.1. The
time of fall has been calculated from the data of Laws (39) who
measured various
was as 10v: as
when air conditions were
drop size increases, ranging from
• The
out the calibration on
error decreases as
for this effeot 1 but in hot
no correction was
air the loss could be three
times and this would correction.
5.1.7.
Variations in the of the flour
measurement other weighing, as with the
preclude
it is
necessary not only to measure the mean pellet mass but also to
the varie.tion about this mean. This was done in the same
way as for the drops, that is by weighing 4 groups of 10 large pellets
and 20 pellets. It might be expected that the variation
would be than the drop variation, beCause the pellet variation
includes the in size and due to flour
effects .. the
to the variation (Fig. 5 .)
shown in
that
5.5. is similar
dependent on
Univers
ity of
Cap
e Tow
n
Loss
I I Relative tion tion tion Time of
Drop . A B as %
No. I .. • __ ) ( (%) (from and (sees.) of
Gunn 146) Wt.
1 2 0 21.7 1 0
2 0 18 1 1 o. 0 0.47
3 3 0 2 1 1 1
4- 3 1 21 2 2 1 2 I VI
.5 3 1 21.1 3. 1.02 2.61 VI
0 o. 2 0 , 6 3 1 1 4 3 0.97 3.
7 3 ~ 1 .7 5 0
8 6 5 2 9 0 5 1 7
9 7 9 2 0 11 1 16 0
10 7 3 .. 2 .80 0 1 0
11 • 0 O • 1
12 0 1. 0.10 ~-.,.. .. -",
tion tion Time of Loss
B as %
No. ) ( (sees.) of
Wt.
1 2 0 21.7 1 0
2 0 1 1 0 0
3 3 0 2 1 1 1
4- 3 1 21 2 2 1 2
5 3 0 1 21.1 3. 0 .. 2 1.02 2. 0
6 3 1 1 4 3 3.
7 3 ~ 1 .7 5 0
8 6 5 2 9 0 5 1 7
9 7 9 2 0 11 1 0
10 7 3 .. 2 0 1 0
11 • 0 o • 1
12 0 1. 0
Univers
ity of
Cap
e Tow
n
size. The ----r----,." method. for the drops is therefore
for the
5 .1.8.
Laws (39) coined the for the factor
mass mass, and a useful way of
the both in
to field of .the control
of the
the
(147) and
• The statistical method
confidence limits of the maSs
shown in Appendix 1.
for
is from
The mass of the pellet depends upon the extent to which it
is forma tion • The of flour
the
n of their
in , as he used
based upon a
is not an
the of polletsvaries, almost
an
method
for small but dough-nut shaped with an almost hollow centre
for ts. There is considerable variation in the
of formed by of the same size. Laws first
dried the
comments (119)
in air, then in an oven at 1100 for one hour but
"two hours would he.ve been a better drying time
as after one hour the were still
Ker (22) who later followed Laws I
a short of 1000 0 before
for one hour at
Provided that
calibration exercise
method will not
same
for the
the res ul t •
rapidly".
'-' ..... , .... "'"..I...y, used
.LLlJVVC;U. by further
for the
if a short
period is , errors are more to arise from variations in
time, and so a standard of drying to constant weight
is to be Fig. 5.6. shows the effeot upon the mass ratio
of for various lengths of time using two different
temperatures, and led to the adoption ill the
of a time of 12 hours at
After
, both for the
been oven
up moisture to the air. To reduce this
to the
cool in a de asica tor and only removed ..L.IUIUv,"L..1.ct
should be
before
to
size. The ----r----,." method. for the drops is therefore
for the
5 .1.8.
Laws (39) coined the for the factor
mass mass, and a useful way of
the both in
to field of .the control
of the
the
(147) and
• The statistical method
confidence limits of the maSs
shown in Appendix 1.
for
is from
The mass of the pellet depends upon the extent to which it
is forma tion • The of flour
the
n of their
in , as he used
based upon a
is not an
the of polletsvaries, almost
an
method
for small but dough-nut shaped with an almost hollow centre
for ts. There is considerable variation in the
of formed by of the same size. Laws first
dried the
comments (119)
in air, then in an oven at 1100 for one hour but
"two hours would he.ve been a better drying time
as after one hour the were still
Ker (22) who later followed Laws I
a short of 1000 0 before
for one hour at
Provided that
calibration exercise
method will not
same
for the
the res ul t •
rapidly".
'-' ..... , .... "'"..I...y, used
.LLlJVVC;U. by further
for the
if a short
period is , errors are more to arise from variations in
time, and so a standard of drying to constant weight
is to be Fig. 5.6. shows the effeot upon the mass ratio
of for various lengths of time using two different
temperatures, and led to the adoption ill the
of a time of 12 hours at
After
, both for the
been oven
up moisture to the air. To reduce this
to the
cool in a de asica tor and only removed ..L.IUIUv,"L..1.ct
should be
before
to
Univers
ity of
Cap
e Tow
n
COEflCIENT Of VARIATION (0/0 ) "TI 0 N .. Gl P • US . UJ .
< + >-XJ .... ~ .. -0 Z
0 + "TI
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COEflCiEIllT Of VARIATION (0/0) o ..
us .
+
.. -o
r
+
is o
8 o
Univers
ity of
Cap
e Tow
n
( importance of pellets being
to core before placing in the oven cannot be over-
If' still moist they tend to bake brown bre ad
dough, causing in size and
affect
and
gave a result.
No evidence is given to
to this
Several groups of pellets were made,
and allowed to air dry for varying before drying to
constant weight. The shown in 5 .. 7., show that when the
pellets are dried to constant the of air before
oven drying is Measurements were also size
and "'H~'.Ur:-; of the of
"' .... "'''',..'''.''''''', but the size and were as as
the .. It was necessary to establish this point because
studies have involved the direct ,",U_L...L.v of
rain present
between the rain and removing the
be to
used an
of the day or
was reached. The time
whenever a
thus varied, and al t~ough
for a fixed
much more convenient to the 12 hour
time, it was
at fixed time s ~
the so the drying was completed either
After oven drying, the
sieved without any l..I.l:l.J.-';';<:;J. of A
and may be
loose flour
S to of is dislodged during sieving,
and the r----- are fUrther
is described in
5.1.10.
The mass ratio is
before The
8.2.6.
to be a of
1he flour. In most cases all that is required
is same conditions obtain for both the calibration and the
collection of
by some of the
$ but the accuracy may
Even all other v s have been
remain in the calibration results of
It is ,are due to .. this .
as an for which are more
errors in technique I it is likely that there
to Cause .. The is to
be influenced
workers.
been
has been
by
is some
the risk
( importance of pellets being
to core before placing in the oven cannot be over-
If' still moist they tend to bake brown bre ad
dough, causing in size and
affect
and
gave a result.
No evidence is given to
to this
Several groups of pellets were made,
and allowed to air dry for varying before drying to
constant weight. The shown in 5 .. 7., show that when the
pellets are dried to constant the of air before
oven drying is Measurements were also size
and "'H~'.Ur:-; of the of
"' .... "'''',..'''.''''''', but the size and were as as
the .. It was necessary to establish this point because
studies have involved the direct ,",U_L...L.v of
rain present
between the rain and removing the
be to
used an
of the day or
was reached. The time
whenever a
thus varied, and al t~ough
for a fixed
much more convenient to the 12 hour
time, it was
at fixed time s ~
the so the drying was completed either
After oven drying, the
sieved without any l..I.l:l.J.-';';<:;J. of A
and may be
loose flour
S to of is dislodged during sieving,
and the r----- are fUrther
is described in
5.1.10.
The mass ratio is
before The
8.2.6.
to be a of
1he flour. In most cases all that is required
is same conditions obtain for both the calibration and the
collection of
by some of the
$ but the accuracy may
Even all other v s have been
remain in the calibration results of
It is ,are due to .. this .
as an for which are more
errors in technique I it is likely that there
to Cause .. The is to
be influenced
workers.
been
has been
by
is some
the risk
Univers
ity of
Cap
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Univers
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1.4
1.3
."L2
i THE UPPER a L.IMITS 95 0", CONFIDENCE
1.1
o 3 , 9 12. 15 18 21 a4 STANDING TIME - nUUKllIo
FIG. 0)( RATIO OF 'THE nMJ: BEfORE FOR 12 HOURS.
1.4
T Cc -, 1.3 1 1
1 0-
w l .2-d
~ ::I! UPPER a LIMITS THE 95 CONFIDENCE
DRYING I
Univers
ity of
Cap
e Tow
n
-61-
of the same batcb and brand by buying and
as can be s of experiments.
b)
Once the
it
diameter the mass
using flour used in a
c)
oven dried its are
drops of about 4 mm.
changed from 1 fresh flour to 1.29
A uniform method of filling the sample pans with flour must
be adopted, and a convenient simple way is to sprinkle it evenly
into the pan a full of a few inches the
pan. La1lYS and () felt the of may vary
sieved or had been for some time,
and used pans filled less than two hours before use. This point
also to be investigated because the automati~ sampler would involve
pans having to for various of time. The results of
this • 5 • show was
which may reduce the accuracy of tbe
have been out by Blanchard (123). the depth of flour
is sufficient to cushion the , some
may occur, in
than 2 cm. is required to
found that of the factors likely to Cause this drop spatter, the most
is flour surface. and Ker both struck off the
excess flour with a Ie a smooth
this causes drops more than 4 rom. to
out on~ to three droplets, and
this could be the flour so that
the irregular surface with its and causes a less
violent were carried out wi th rough and smooth
flour 5.2. When large fall
on smooth and to
possible source of errors flour was used
both for the and
made another positive improvement in the flour
by introducing into flour very small
of
o blue dye.. He found the
per oont of which had been
mesh sieve to remove any
is hardly the dry flour, but the
blue and seen in the flour. the
to be
..
-61-
of the same batcb and brand by buying and
as can be s of experiments.
b)
Once the
it
diameter the mass
using flour used in a
c)
oven dried its are
drops of about 4 mm.
changed from 1 fresh flour to 1.29
A uniform method of filling the sample pans with flour must
be adopted, and a convenient simple way is to sprinkle it evenly
into the pan a full of a few inches the
pan. La1lYS and () felt the of may vary
sieved or had been for some time,
and used pans filled less than two hours before use. This point
also to be investigated because the automati~ sampler would involve
pans having to for various of time. The results of
this • 5 • show was
which may reduce the accuracy of tbe
have been out by Blanchard (123). the depth of flour
is sufficient to cushion the , some
may occur, in
than 2 cm. is required to
found that of the factors likely to Cause this drop spatter, the most
is flour surface. and Ker both struck off the
excess flour with a Ie a smooth
this causes drops more than 4 rom. to
out on~ to three droplets, and
this could be the flour so that
the irregular surface with its and causes a less
violent were carried out wi th rough and smooth
flour 5.2. When large fall
on smooth and to
possible source of errors flour was used
both for the and
made another positive improvement in the flour
by introducing into flour very small
of
o blue dye.. He found the
per oont of which had been
mesh sieve to remove any
is hardly the dry flour, but the
blue and seen in the flour. the
to be
..
University of Cape Town
1.4
u
u
6 120 US 21 STANDING TIME - '
FIG. ON MASS RAT I 0 ALLOWING
PAN TO STAND BEFORE EXPOSURE PREPARED
TO RAIN.
0-N
1.4
L3
1.1
I.~--------~--------~--------~~--------~--------~--------~--------~---------d o 3 6 9 12. 24 TIME - ...... ' ...... ,"'"
University of Cape Town
-63-
dye was the s so that" the
could be seen the correct number
of each
was not used in the
pass the whole flour mass
rain as it waS
sieve than to
to
by;hand the number of
immediately
a
on top ensures
this cannot be
(121) oovered the with a of flour
each , on the tha t as the
into the flour than smaller ones they would
of contact with the flour
surrounded
sn.mpler it was not in the
, but the effect was
might have affected Meyer's
to see if it
5.3. shows that the
size is the
will have been
but are no
calibration
s
accurate as the were covered both in the
in •
5.1 •
of varying the impact of the water drops
on the flour waS tested by to into
the flour from show in • 5.9. that the mass ratio is The deorease mass
when cannot be accounted for
by redmtion of in 5.1.6. the
effects were shown to be HQ~.I-.l-~.l-U.l-t:;
conditions, so the must be due to
Presumably the fast-moving
and make a the
the of sizes the same
expressed as % of terminal velocity, will be
, and the calibration curve
mass ..
into the flour
ex:r:;eriments
I their
for
be in error.
) showed that the of a
is seldom outside the ran~e 90 - 100% of the still
Thee
conditions. • 5
size, with 95% as the most usual value. ....",{""'.,...'" be
• shows Laws f
under the se same
of
for a drop to reach velocity, and
in the present calibration. The were the hel~hts of
difficul ty of the of ft.
which is required for large crops has workers ignore
this
was the s v"" ......... "'''' sc the cculd be seen the correct
of each
was nct used in the
pass flour mass
the
covered the
rain as it waS to
~".~~A~ of flour
that as the
than smaller ones
the
Vlould
flour
on surrounded
cannot be it was not
, but the effect was to sce if it
• shows the
the
will have been
but are no were both in the
5
of the
on .~~ .. ~ the flour waS tested into
is
Thec
were
which is
this
the same
The deorease mass
be accounted for
5.1.6. the
mass ..
into the
as be
the calibration curve be error.
) showed that the
• shows Laws'
to reach
the
for has
of a
of the still
as the most value.
under the se same
The
of ft.
workers
Univers
ity of
Cap
e Tow
n
Tes
:Fe11ets
of variance
j ::= ( ::= 1 N .S.
..)
homogeneity of variance
F3•3 1.42 N.S.
Small 3 ::= 1
of mean mass ratio
Large drops t4
:;::: 3
N .S.
t4 4.31 Significant P
@ 1%) Tes
:Fe11ets
of variance
j ::= ( ::= 1 N .S.
..)
homogeneity of variance
F3•3 1.42 N.S.
Small 3 ::= 1
of mean mass ratio
Large drops t4
:;::: 3
N .S.
t4 4.31 Significant P
@ 1%)
University of Cape Town
."
." -c :£
1.2S +--+---IIIIIIIIIIIiIII==~
'.26
DIA. 2.44 MM.
1.24 DROP DIA.4.75 MM.
1.22
L20·L __________ ~ ________ ~ ________ ~~ ________ ~ ________ ~~ ________ ~ ________ ~ __________ d_ ________ -d ________ ~
o 10 20 30 40 50 60 70 80 90 100 VELOCITV OF DROP EXPRESSeD AS PERCENTAGE OF TERMINAL VELOCITV.
FIG. 5.9 EFFECT OF DROP VELOCITY ON MASS RATIO.
'.26
L2 ..
1.22
o 10 20
+---
30 <40 VELOCITY OF
50 EXPRESSeD AS
DIA. 2.44 MM.
90 VELOCITY.
100
Univers
ity of
Cap
e Tow
n
5.2.
The pre s was carried out under the
5 • is a
large number of and
groups of ten for
from the
, four of twenty for small drops),
errors an estimate was of the 95%
for the drop mass, .. "1"(~1"1'" were formed a rate of formation.
.An air was used to blow away the , ,and the
evaporation. The pellets were
formed at of terminal velocity into uncompacted
unsmoothed flour, and were oven dried before
weighing. The results are in Table 5.4.
5.2.2.
the mass is pellet mass (Fig. 5 it is that there is a very close and apparently direct
relationship, and the least squares regression equation is
Drop mass (mgm) ::: 1.2624 Pellet Mass (mgm) - 0 (1)
correlation coefficient is at r ::: 0 .. 999.
This is, The
form of the curve must pass through the origin as a drop of zero mass
cannot a , and so there must be a departure
at very small values. To this
the in 5 • The
relationship is almost , but the is in
fact The confidence limits are not shown in Figs. 5
and 5 .. because as be seen from would be
of
is
mass. In
in the
from the
is masked
on these scales. The best
using Laws l ra tio ", the of to
the mass of a raindrop will be by
the measured mass
of 10% in the mass ratio
this maSS ratio, and so a
result in a lq%variation
drop mass. • 5 ., the
of the mass ratio of a line
to any particular point, and in fact this
O"'Cl.J..".L. values, although the variation
the fact that the of the line
(i.e .. tho maSS
passes through the origin.
is ne very nearly
5.2.
The pre s was carried out under the
5 • is a
large number of and
groups of ten for
from the
, four of twenty for small drops),
errors an estimate was of the 95%
for the drop mass, .. "1"(~1"1'" were formed a rate of formation.
.An air was used to blow away the , ,and the
evaporation. The pellets were
formed at of terminal velocity into uncompacted
unsmoothed flour, and were oven dried before
weighing. The results are in Table 5.4.
5.2.2.
the mass is pellet mass (Fig. 5 it is that there is a very close and apparently direct
relationship, and the least squares regression equation is
Drop mass (mgm) ::: 1.2624 Pellet Mass (mgm) - 0 (1)
correlation coefficient is at r ::: 0 .. 999.
This is, The
form of the curve must pass through the origin as a drop of zero mass
cannot a , and so there must be a departure
at very small values. To this
the in 5 • The
relationship is almost , but the is in
fact The confidence limits are not shown in Figs. 5
and 5 .. because as be seen from would be
of
is
mass. In
in the
from the
is masked
on these scales. The best
using Laws l ra tio ", the of to
the mass of a raindrop will be by
the measured mass
of 10% in the mass ratio
this maSS ratio, and so a
result in a lq%variation
drop mass. • 5 ., the
of the mass ratio of a line
to any particular point, and in fact this
O"'Cl.J..".L. values, although the variation
the fact that the of the line
(i.e .. tho maSS
passes through the origin.
is ne very nearly
Univers
ity of
Cap
e Tow
n
lII. o
67
I
~t~
./ .~. 7
6
5
l
FLOWER
O~------~------~------~~------~------~------~ o 2 3 4 S DROP DIAMETER - M.M.
FIG. 5.10. RELATION BETWEEN DROP SIZE AND HEIGHT OF
FALL REQUIRED TO REACH 95 % OF
TERMINAL VELOCITY.
I&. o
I
7
6
s
3
61
FLOWER
O~------~------~--------~------~----------------~ o 2. 3 4 S 6 DROP DIAMETER - M,M.
F 5. BETWEEN HEIGHT OF
REQUIRED TO REACH 95 % OF
TERMINAL VELOCITY.
Univers
ity of
Cap
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n--~,v, S.E •. a,v'la,v'l
)1 IS.EfV 'I W:~er (20 in (20) cps) groups ( .. of maE'S 1ia tio
(C.v.) or 20 in groups
of' or 20)
"
1 0.0800 .004l 5.08 5.08 1.96
2 20 O. 2 2 2
3 0.3117 8 8 .044-
4- 20 0 2 2 2 .78C
5 20 0 3 3 6 20 1 2.74- 2 1. 1
7 2 3
8 5 1 1.172
9 9 .055 1 1.2ll
.063 2 1.169
1 1 .. 226
155 I .24- 0 1.00 0.71 1.40 1 ,.-
-- ----- "~"-"
Standard S.E. ' C.V. C.V. T'r""''Oer in (20) LO\'\i'er L~.mits
) groups ( of lrla~ s l1a tio (C.V.) or 20
in groups of
(10 or
1 0.0800 .004l 5 5 1 .. 96 1.96
2 20 0.2038 .0048 2 2 .0]-5 2.82
3 0.3117 8.02 8 .080 .044-
4 20 0.5787 2. 2 .041 2 .78<'
5 20 0.8587 3 3 .086 .009
6 20 1 .. 567 2,,74 2 .137 1. 1.006
7 20 2 .. 382 3 .. 51
8 5 .20 1 1
9 10 ,,055 0,,60 1 1
10 10 ,,063 0,,27 0,,19 .. 20 2 1.169
.. 04 0.08 .12 1 1 1 .' .
12 10 .24 ° 0 .. 30 • 76 1.00 1 .. 40 1 .. 281 1 t.\: ,.-
Univers
ity of
Cap
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'Mn· 151M S~. ______ ~~~ ______ ft?-____ ~F-______ s~t ______ ~oer-______ Srl _______ O~I _______ S~ ____ ~o
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Univers
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Univers
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I."
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!'\ILl. LIlliE 1$ UIIE OF IIi!ST FIT
T • 95 '110 COHFIDINCE L,W'TII
1
,~ I EOII. a. MASS RATIO .. l.aU4 ,;;::I!It'i~=~",
I I I j
I I
~50~.~I----------------------~I~~~--------------------~'~O~O~PU4---ET--~-----~----------~roo~·
FIG. 5.13. RELATION BETWEEN MASS RATIO • PELLET MASS.
I."
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I I I j
I I
~50~.~I----------------------~I~~~--------------------~'~O~O~PU4---ET--~-----~----------~roo~·
FIG. 5.13. RELATION BETWEEN MASS RATIO • PELLET MASS.
Univers
ity of
Cap
e Tow
n
.. 5 • the mass is on semi-logarithmic In
against confidence limits for mass ratio 's me thad (Appendix I). The
of the of .. 5.12., and of the
Fig .. 5.11. and equation (1), is now s.hown by the
mass ratio for small pellets.
5.11. and 5.12. transform to
of
from ..
Mass Ratio == 1.2624 - - (2)
(:Fellet Mass)
and Mass ::: - (3) 1.0745
the
Equation ( fits the data well for
and it is clear that
OVer the whole range ..
but not for
pellets and is the Case for
to a
a equation have not
trend line shown
true
can be
A
that of best fit by eye.
which
the
The fact
(2). .J.Ul.L", •. L.J.jJ in order
, and the
the nature of the
(2)
line
cuts the with a implies a
of zero mass requires a negative drop mass. This intrinsically
is nonsense, but what it may suggest that a certain proportion
of the does not go the, but is dissipated
the flour. If one assumes that this
is area of the , then it
will be a function of the of the diameter, and since the
maSs is a function of the cube of the dia.meter, the would
be that as the increase the mass more
than the moisture. In other words the
be most marked with
of this
, and the
mass ratio will be lowered more and more as drop size decreases.
in
constant intercept,
or
.. of 5 .. ,
that this dissipated moisture
value. So when the equation is
its
.. 5.13. it gives values
too low for the
too large) and it becomes asymptotic to the
effect of the dissipated moisture becomes ._,~_~.~
as the
This thus offers a rational explanation for
the masS r Wha t is less easy to for the
..
.. 5 • the mass is on semi-logarithmic In
against confidence limits for mass ratio 's me thad (Appendix I). The
of the of .. 5.12., and of the
Fig .. 5.11. and equation (1), is now s.hown by the
mass ratio for small pellets.
5.11. and 5.12. transform to
of
from ..
Mass Ratio == 1.2624 - - (2)
(:Fellet Mass)
and Mass ::: - (3) 1.0745
the
Equation ( fits the data well for
and it is clear that
OVer the whole range ..
but not for
pellets and is the Case for
to a
a equation have not
trend line shown
true
can be
A
that of best fit by eye.
which
the
The fact
(2). .J.Ul.L", •. L.J.jJ in order
, and the
the nature of the
(2)
line
cuts the with a implies a
of zero mass requires a negative drop mass. This intrinsically
is nonsense, but what it may suggest that a certain proportion
of the does not go the, but is dissipated
the flour. If one assumes that this
is proportional to the surface area of the , then it
will be a function of the of the diameter, and since the
maSs is a function of the cube of the dia.meter, the would
be that as the increase the mass more
than the moisture. In other words the
be most marked with
of this
, and the
mass ratio will be lowered more and more as drop size decreases.
in
constant intercept,
or
.. of 5 .. ,
that this dissipated moisture
value. So when the equation is
its
.. 5.13. it gives values
too low for the
too large) and it becomes asymptotic to the
effect of the dissipated moisture becomes ._,~_~.~
as the
This thus offers a rational explanation for
the mass r Wha t is less easy to for the
..
Univers
ity of
Cap
e Tow
n
-73-
fact that the masS ratio apparently reaches a maximum at a
pellet mass of 18 mgm. and then decreases slightly. The shape
and form of the pellets does vary, and while small pellets are circular
the large ones have a characteristic shape best likened to that of a
ring type doughnut with a thick membrane acrose the hole. This may
account for the largest pellets having a relatively higher mass and
lower mass ratio. However, while the variation from a uniform mass
ratio involves a lCJO% difference at the lower range, this reduction at
the high range is less than 2% and not worth pursuing. In the
practical application of these results it has been assumed that this
slight decrease in mass ratio is real, and the trend line of Fig. 5.13. has been used to determine the values of mass ratio actually used in
the calculation of the resul~s.
5.3. DISCUSSION OF E~IER CALIBRATION~.
The relationship between the mass of the drop and the mass
of the flour pellet has been determined experimentally by a number of
other workers,and their results will now be discussed briefly in
relation to the control conditions established in Section 5.1., and the
results presented in Section 5.2. It was shown in 5.2. that plotting
drop mass against pellet mass is not sufficiently sensitive to show
the relationship-adequately, and so in Fig. 5.14. the more sensitive
mass ratio plot is used to compare the several results.
The earliest calibration was undertaken by Bentley (118)
who in 1904 first used the flour pellet method. He did not establish
a very pre~ise relation, merely saying that the pellets were found
"to correspond very closely in size with the raindrops that made them".
This may mean that he assumed the average diameters of drop and
pellet to be equal, or that a simple ratio was established between
the two diameters. Since no more detail is given by Bentley we
are not able to discuss how his result may-have been influenced by
this over-simplification.
Laws (39) next reports on the method in 1941 when he used
it to measure the size of water drops whose fall velocity was being
studied, and again (Laws and Parsons, 119) to study natural rainfall.
The same calibration was used in both studies. Laws appreciated
the need for precision and plotted mass ratio against log. pellet
mass as the most accurate and convenient way of presenting the data.
His original data is not available and has been carefully extracted
from his graphs. The fact that the whole trend is for much lower
mass ratios i~ due to his having only partially dried the pellets,
and need not necessarily have caused an error when applied to his
resul ts. The effect of drying to constant weight would be to
-73-
fact that the masS ratio apparently reaches a maximum at a
pellet mass of 18 mgm. and then decreases slightly. The shape
and form of the pellets does vary, and while small pellets are circular
the large ones have a characteristic shape best likened to that of a
ring type doughnut with a thick membrane acrose the hole. This may
account for the largest pellets having a relatively higher mass and
lower mass ratio. However, while the variation from a uniform mass
ratio involves a lCJO% difference at the lower range, this reduction at
the high range is less than 2% and not worth pursuing. In the
practical application of these results it has been assumed that this
slight decrease in mass ratio is real, and the trend line of Fig. 5.13. has been used to determine the values of mass ratio actually used in
the calculation of the resul~s.
5.3. DISCUSSION OF E~IER CALIBRATION~.
The relationship between the mass of the drop and the mass
of the flour pellet has been determined experimentally by a number of
other workers,and their results will now be discussed briefly in
relation to the control conditions established in Section 5.1., and the
results presented in Section 5.2. It was shown in 5.2. that plotting
drop mass against pellet mass is not sufficiently sensitive to show
the relationship-adequately, and so in Fig. 5.14. the more sensitive
mass ratio plot is used to compare the several results.
The earliest calibration was undertaken by Bentley (118)
who in 1904 first used the flour pellet method. He did not establish
a very pre~ise relation, merely saying that the pellets were found
"to correspond very closely in size with the raindrops that made them".
This may mean that he assumed the average diameters of drop and
pellet to be equal, or that a simple ratio was established between
the two diameters. Since no more detail is given by Bentley we
are not able to discuss how his result may-have been influenced by
this over-simplification.
Laws (39) next reports on the method in 1941 when he used
it to measure the size of water drops whose fall velocity was being
studied, and again (Laws and Parsons, 119) to study natural rainfall.
The same calibration was used in both studies. Laws appreciated
the need for precision and plotted mass ratio against log. pellet
mass as the most accurate and convenient way of presenting the data.
His original data is not available and has been carefully extracted
from his graphs. The fact that the whole trend is for much lower
mass ratios i~ due to his having only partially dried the pellets,
and need not necessarily have caused an error when applied to his
resul ts. The effect of drying to constant weight would be to
Univers
ity of
Cap
e Tow
n
) "74 "
l4
HIJOSOH I.iI
1(1111 ... --., .. - .... -!,.AM .. PMS()NS " ...... _-------MiVEII ~ ---IllEAN .. WEI.J.I< --~-- .. -BI.ANCHAIID I
~1~~~I------------------------~l~O~-----------------------:~~O~----------------------~~O. "E!.I.U ... ~ _ ......
FIG. 5.14. COMPARISON OF SEVERAL MASS RATIO CALIBRATIONS.
; , .
~ m 11111 .;1\ --" .. -,.~-I..AW$ .. PMS()M e ...... _---- ...... -MlVE'I <t ---IIIIW\! il WEU.I< --~-- .. -IIIl.iU'lCHAIiD ~
100.
FIG. 5.14. OF SEVERAL MASS RATIO CALIBRATIONS.
Univers
ity of
Cap
e Tow
n
increase the mass ratios, with a for the
, Le .. bringing it more into line 's
results.. Probably the most feature of Laws' results is
ths t he records values of mass ratio at both ends of his
range, but to t this. The mass ratio value of 0.845
at mass of 1.035 mgm. was to be error
on the theory that the mass could not be
mass i.e. mass ratio could not be less there
are no at for this , below unity
can and do occur. Laws was unfortunately working before
the development of the use of co-axial air to produc~ very
small drops and was unable to such small drops to
his .. He no comment on the fact that his mass ratio
also falls for the st Other
\A"" •• v"" ..... Laws I results are
of the (
but considered ), and flour pans with surface.
The of is not mentioned, nOT the variation among the
measured groups of drops or The trend line shown on Fig ..
5.14. is that shown J but it will be seen that an equally good
or better fit could be
to Ker's
(
distribution in natural
TIn~~ stain method as
a curve concave downwards.
used the method to size
J although turned to the
to requirements.
"'6~'."U the
better s
are not , but are graphically
as the relation between drop diameter and cube of
(i.e. a function of average pellet diameter).
which, assuming a
Drop maSS (mgm)
or Mass Ratio
When the are
::::
Mass (mgm)) O.
of 1 for
l.l~ x Pellet mass (mgm)
1
s and
5 .. it is seen that the maSs ratio varies between 0.87 1.21 so his is an over-simplification.
to
arises from assuming a straight line relationship passing through
the origin, and the ever present danger of extrapolation beyond
experimental values is demonstrated. says !tAl-though
no drops Vlere produced below 1.62 mm. in it is reasonable to
assume the of the down to
have been more to ·say that no were
produced below size, there are no for assuming that
increase the mass ratios, with a for the
, Le .. bringing it more into line 's
results.. Probably the most feature of Laws' results is
ths t he records values of mass ratio at both ends of his
range, but to t this. The mass ratio value of 0.845
at mass of 1.035 mgm. was to be error
on the theory that the mass could not be
mass i.e. mass ratio could not be less there
are no at for this , below unity
can and do occur. Laws was unfortunately working before
the development of the use of co-axial air to produc~ very
small drops and was unable to such small drops to
his .. He no comment on the fact that his mass ratio
also falls for the st Other
\A"" •• v"" ..... Laws I results are
of the (
but considered ), and flour pans with surface.
The of is not mentioned, nOT the variation among the
measured groups of drops or The trend line shown on Fig ..
5.14. is that shown J but it will be seen that an equally good
or better fit could be
to Ker's
(
distribution in natural
TIn~~ stain method as
a curve concave downwards.
used the method to size
J although turned to the
to requirements.
"'6~'."U the
better s
are not , but are graphically
as the relation between drop diameter and cube of
(i.e. a function of average pellet diameter).
which, assuming a
Drop maSS (mgm)
or Mass Ratio
When the are
::::
Mass (mgm)) O.
of 1 for
l.l~ x Pellet mass (mgm)
1
s and
5 .. it is seen that the maSs ratio varies between 0.87 1.21 so his is an over-simplification.
to
arises from assuming a straight line relationship passing through
the origin, and the ever present danger of extrapolation beyond
experimental values is demonstrated. says !tAl-though
no drops Vlere produced below 1.62 mm. in it is reasonable to
assume the of the down to
have been more to ·say that no were
produced below size, there are no for assuming that
Univers
ity of
Cap
e Tow
n
-76-
the rela.tionship holds beyond this point". Blanc~d's experimental
oonditions are not given in sufficient detail to warrant attempts to
explain the large and random scatter of his results, and the work is
noteworthy mainly for the introduction of methylene blue dye to the
flour, and for es tablishing the f aO t that an unsmoothed flour surf ace
is desirable ..
K.er (22) followed Laws and Parson I s work very closely, the
only modification being that the pellets were dried a little longer,
al though still not to constant weight. A height of fall· of only
8 feet was used, and although pellets larger than 30 mgm. were
discarded because their shape was observed to differ from those of . natural rain, the error would in fact have applied to the whple
calibration since even the smallest drop would only have reached
about 75% of 'terminal velocity after such a small height of fall.
Other possible sources of errors nre variation of drop size,
evaporation during collection, collection of satellite drops, and a
smooth flour surface. The variation of pellet weight had a maximum
range of 11.4%, corresponding to a coefficient of variation of
approximately 3%, a reasonable value, but no measurement was made of
drop variation so no confidence limits can be established for the
variation in maSs ratio.
Schleusener (120) used the pellet method to stuQy the size
and kim tic energy of drops from irrigation sprinklers. His
calibration was done with plaster of paris, and the drops of various
sizes projected from the periphery of a spinning disc. After
establishing a correction factor for the difference between plaster
of paris and flour, this correction was applied to Laws' calibration.
Apart from possible sources of error in Laws' original work. this
method of drop production is suspect because the smaller drops are
projected at the greatest velocity, and the largest drops more
slowly, which is the reverse of the natural conditions of both
rainfall and drops from an irrigation spray ..
Beans and Wells (92) gave a calibration curve shown as
log .. pellet mass (mgm) against drop diameter (rom).. Transposed to
the mass ratio form the curve is shown in Fig~ 5.14., but the authors
are unable to recall the original source of the information, and
in the absence of the experimental data and conditions it would be
unfair to comment further than that it appears to follow the present
writer's results fairly closely.
Meyer (121) undertook the most carefully controlled
calibration previously reported. Some unnecessary controls were in
-76-
the rela.tionship holds beyond this point". Blanc~d's experimental
oonditions are not given in sufficient detail to warrant attempts to
explain the large and random scatter of his results, and the work is
noteworthy mainly for the introduction of methylene blue dye to the
flour, and for es tablishing the f aO t that an unsmoothed flour surf ace
is desirable ..
K.er (22) followed Laws and Parson I s work very closely, the
only modification being that the pellets were dried a little longer,
al though still not to constant weight. A height of fall· of only
8 feet was used, and although pellets larger than 30 mgm. were
discarded because their shape was observed to differ from those of . natural rain, the error would in fact have applied to the whple
calibration since even the smallest drop would only have reached
about 75% of 'terminal velocity after such a small height of fall.
Other possible sources of errors nre variation of drop size,
evaporation during collection, collection of satellite drops, and a
smooth flour surface. The variation of pellet weight had a maximum
range of 11.4%, corresponding to a coefficient of variation of
approximately 3%, a reasonable value, but no measurement was made of
drop variation so no confidence limits can be established for the
variation in maSs ratio.
Schleusener (120) used the pellet method to stuQy the size
and kim tic energy of drops from irrigation sprinklers. His
calibration was done with plaster of paris, and the drops of various
sizes projected from the periphery of a spinning disc. After
establishing a correction factor for the difference between plaster
of paris and flour, this correction was applied to Laws' calibration.
Apart from possible sources of error in Laws' original work. this
method of drop production is suspect because the smaller drops are
projected at the greatest velocity, and the largest drops more
slowly, which is the reverse of the natural conditions of both
rainfall and drops from an irrigation spray ..
Beans and Wells (92) gave a calibration curve shown as
log .. pellet mass (mgm) against drop diameter (rom).. Transposed to
the mass ratio form the curve is shown in Fig~ 5.14., but the authors
are unable to recall the original source of the information, and
in the absence of the experimental data and conditions it would be
unfair to comment further than that it appears to follow the present
writer's results fairly closely.
Meyer (121) undertook the most carefully controlled
calibration previously reported. Some unnecessary controls were in
Univers
ity of
Cap
e Tow
n
5
2 3 ::f
1'\ : " j'
o--------~~------~------~------------------o
~8 __________________________ ~ __________________ ~ ______ ~
I 10 50 100 PELLET MASS - MGM.
FIG. 5.15. BLANCHARD'S CALIBRATION DATA.
5
2 3 ::f
1'\ : " j'
o--------~~------~------~------------------o
~8 __________________________ ~ __________________ ~ ______ ~
I 10 50 100 PELLET MASS - MGM.
FIG. 5.15. BLANCHARD'S CALIBRATION DATA.
Univers
ity of
Cap
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-78-
fact applied,such as saturating the air used to blow the small drops
off the dropping point, and air drying the pellets before oven
drying. Other necessary controls were not applied and possible
sources o~ error are due to variation in drop size, evaporation during
collection, catching s8.telli te drops, the use of levelled flour and
the use of a height of fall of 12 feet only. The regression is
expre Esed by Meyer as
Drop mass (gm) = 1.617 (Pellet weight (gm)) 1.061
on the assumption that the data can be rectified to a straight
line ona plot of log Drop mass against log. Pellet mass. In the
masS ratio form, this regression line (Fig. 5.14.) shows a curvature
conCave up''V'ards, which suggests that this equation is not in the
best form, as all the other calibrations are straight lines pr curves
conCave downwards, but the scatter is too wide to permit of a trend
line being fitted by eye with any greater accuracy.
In considering all these calibrations there is one variable
vvhich cannot bo quantitatively evaluated and which may account for
some of the differences, and that is the flour used. Variations may
be caused by milling methods or additives, and although a number of
possible errors in teChniques hove been eliminated, and standard
control conditions have been established, use of the flour pellet
method should always be acoompanied by a careful calibration.
-78-
fact applied,such as saturating the air used to blow the small drops
off the dropping point, and air drying the pellets before oven
drying. Other necessary controls were not applied and possible
sources o~ error are due to variation in drop size, evaporation during
collection, catching s8.telli te drops, the use of levelled flour and
the use of a height of fall of 12 feet only. The regression is
expre Esed by Meyer as
Drop mass (gm) = 1.617 (Pellet weight (gm)) 1.061
on the assumption that the data can be rectified to a straight
line ona plot of log Drop mass against log. Pellet mass. In the
masS ratio form, this regression line (Fig. 5.14.) shows a curvature
conCave up''V'ards, which suggests that this equation is not in the
best form, as all the other calibrations are straight lines pr curves
conCave downwards, but the scatter is too wide to permit of a trend
line being fitted by eye with any greater accuracy.
In considering all these calibrations there is one variable
vvhich cannot bo quantitatively evaluated and which may account for
some of the differences, and that is the flour used. Variations may
be caused by milling methods or additives, and although a number of
possible errors in teChniques hove been eliminated, and standard
control conditions have been established, use of the flour pellet
method should always be acoompanied by a careful calibration.
Univers
ity of
Cap
e Tow
n
6 EF.F.EOT OF WIND ON MEASUREMENTS OF QUANTITY
If w.ind blows horizontally over the ground surfaoe, a
horizontal oomponent of velooity to the
and an ................... .u<;>\.L
a ,..................... than would obtain il:" air.
Turbulence may add ~ULl:I...I..L vertioal components to the wind velooi ty,
but slloh variations are usually local or temporary, and consideration
of wind as a horizontal veotor only is not an over-eimplif'ioatian.
A gauge set vertically presents to inclined rain an effeotive
oollecting'area smaller than the area presented to vertical rain.
When the ground surfaoe is flat no problem arises as the rain colleo-
ted is a true of that reaching the gro1.md, but since quantities
(or intensities) of rainfall oonventionally relate to depths (or
rates) per unit area of the (map area) of
ground , elTers arise when is recorded by a
of to IT...,,, IT. n surface is i::j .... ~'ftU Fig. 6 .. 1 ..
case from
Hamil ton (148) it ~ er.ror of a ve,c-~' .. UML gauge is proportional
to secant "" , where 0( is the angle of the ground surface with
and mountainous country where 0( is of the order
of 450 or lllOre error can be has to the
of setting
ground slope. !£be present studies are to
where 0( seldom exceeds -,0 (approximately 12% slope) and the
elTor to this slope (1 .. 0075) can be ignored. measurements
and have therefore been made with horizontal.
collecting areas.
6.2 .. The angle of inclination of
and the resultant velocity of
values of wind speed and
the (eo),
d.e,Det1lO. on relative
of' rain. The terminal
6 EF.F.EOT OF WIND ON MEASUREMENTS OF QUANTITY
If w.ind blows horizontally over the ground surfaoe, a
horizontal oomponent of velooity to the
and an ................... .u<;>\.L
a ,..................... than would obtain il:" air.
Turbulence may add ~ULl:I...I..L vertioal components to the wind velooi ty,
but slloh variations are usually local or temporary, and consideration
of wind as a horizontal veotor only is not an over-eimplif'ioatian.
A gauge set vertically presents to inclined rain an effeotive
oollecting'area smaller than the area presented to vertical rain.
When the ground surfaoe is flat no problem arises as the rain colleo-
ted is a true of that reaching the gro1.md, but since quantities
(or intensities) of rainfall oonventionally relate to depths (or
rates) per unit area of the (map area) of
ground , elTers arise when is recorded by a
of rain normal to fFOund surface is soo'Wn in Fig.. 6.1. taken from
Hamil ton (148) it ~ er.ror of a gauge is proportional
to secant "" , where 0( is the angle of the ground surface with
and mountainous country where 0( is of the order
of 450 or lllOre error can be has to the
of setting
ground slope. !£be present studies are to
where 0( seldom exceeds -,0 (approximately 12% slope) and the
elTor to this slope (1.0075) can be ignored. measurements
and have therefore been made with horizontal.
collecting areas.
6.2 .. The angle of inclination of
and the resultant velocity of
values of wind speed and
the (eo),
d.e'Det1ld on relative
of' rain. The terminal
University of Cape Town
1 inch of rain falls through opening of 50 sq. in.
Slope area catch J of 50 cu. in. reaches depth of linch on projectional area at 50 sq. in.
I-----l VERTICAL RAIN GAGE SLOPEAREA CATCH IS 50c.u,in.
I I I I , I
§~
I inch of ro in falls throug h openin\! of 35 sq. in.
T Slope area cetch I of 3 5 cu. in. ree ches I" depth of 1 inch on
projectional orea of 35 sq. in.
HORIZONTAL PROJE~1JQ.N.._OF ttlllSIDE (~~~~EAl _ ..
Ho"- rai n, fa lling vert ically on a slope, i~ ;':/Ullple<.l I"qually W E'n by a rain gage placed either vertically or norlllal to the slope.
FIG. 6.1. THE EFFECT O F
INCLINED & VERTICAL RAIN GAUGES.
(FROM HAMILTON, 148)
VERTICAL RAI~AGE SLOPE AREA CATCH IS 35 cu. tn.
Slope orea catch of 3 5 cu. in. reaches depth of 0.7 inch on -projectional area of 50 SQ. in. O.
I I I I • I
~
Sample of same rain falls through opening of 35 sq. in.
I
TILTED RAIN GAGE SLOPE ·AREA CATCH IS 50 Cu. in.
r 5'op, " .. , "'" 1.4" of 50 cu. in. reaches
depth of 1.4 inches 01\
projectional area of 35 '<I. in.
HORIZONTAL PROJECTION OF HILLSIDE (MAP AREA)
How Inclined rain falling on a slope is incorrectly sampled by a yerUeal rain gage aud correctly sampled by a tilted gage.
00 o
I inch of rain falls through open ing of 50 sq. in.
Slope oreo cotch J of 50 cu . in. reaches depth of linch on I" projectional area of 50 sq. in.
RA IN FALLING VERTICALLY
1111111 j ill
I ioch of rain falls through openino of 35 SQ. in.
VERTICAL I-----i RAI N GAGE
I I I I , I
SLOPEAREA CATCH IS 50c.u,in.
§~ T Slope oreo colch I of 3 5 cu. in. reoches
," depth of ' inch on projec ti onal area of 35 sq. in.
HORIZONTAL PROJECliQ.f"'!. .. OF ttlLLSIDE (~~~~~.Al .•..
Ho"- rai n. fa lling l'ert ically on a slope. is ;<nmIJle<.l equally wen by a rain gage placed eIther vertically or normal to the slope.
FIG. 6.1. THE EFFECT O F
INCLINED & VERTICAL RAIN GAUGES.
(F~M HAMlLTON,14S)
VERTICAL RAI~AGE SLOPE AREA CATCH IS 35 cu. in.
I I I I • I I
~ Slope area catch I of 3 5 cu. in. reaches depth of 0.7 inch 00
projectional area of 50 sq. in. 0.
Sample of some rain falls through opening of 35 sq. in.
TILTED RAIN GAGE SLO~AREA CATCH IS 50 Cu. in.
Slope area catch of 50 cu. in. rea ches depth of 1.4 inches on pro jechonO' area of 3 5 '<I. in.
HORIZONTAL PROJECTION OF HILLSIDE (MAP AREAl
How Inclined rain falling on a slOlle is incorrectly sampled by a yertLcal raw gage I1nd correctly sampled I>y a tilted gage.
0)
o
Univers
ity of
Cap
e Tow
n
-81-
velocity of water drops falling in still air varies fI~m 12 feet
per second for very small drops 1 mm. in alilineter, to about 30 feet
per second for the largest drO?S (al:out 5 mm. diameter). Values
of 8 of 450 will tIlu:::; be enconl:.terecl \~.e:tl wind speeds are of the
same order, i.e. from 8 - 22 m.p.h., and for any given wind speed
e will be greater for the small drops tllo'l.n the larg0r. It would be
imprac"bicable, though not impossible, to measure separately -the in
clination of the various sizes of drops, e..'1d instead the average
inclination of the whole rain i~ rne8.Sm'ed.
If, as in t:i.le present studies, the object is to seek
correlA.tions b'3tween quantity (or 5.ntensi ty) of rain and functions
of velocity f.luoh as momentum or energy, then the effects .)f varying
angles of inclination mud be pl:im::1na-l;ed (i.t' they are significant)
by relating both to the common standard of what the m3asure~ value
wou..1.d be without wind - i .. e 0 "lith vertical rain. The quantity
of inclined rain is correctly mea;:mred lJY a hoT.':i.zontal gauge but
the velocity of inclL'1eo. rain is :i.ncre ased by seca..'1 t 9. 'lbe
correlation must therefore be tested between
Q (measured) and(V (measure,l) ~ secant e)
i.e. Q = f(V • cos 0) m m
La ter (6.4.) the significance of th1.s correction for wind
will 'be considered in relation to measured values of 9. It will
of course be more important 1.."'1 the C9.se of kinetic energy ex v 2 )
than of momentum (t.?(' v) •
6.3. MSASgREMF:NT OF D.f....RE...Q!lQti ... !¥l!LJNCLIi'lt\TICN OF RA.TIi.
EA.rlier meas~ments of direction and inclination show SOQe
strik:L':lg wind effects, but this is not surprising as these studies
were in areas where v:ind. was obviou:3ly inlporiant. Wicht (149)
measuring rainfall in the mountainous experimental catcl1Ill.ent of
.Tonkershoek (S. AflC'ica) found that "of a total of fifteen rains measured
at one locality, eleven fell at an incljnation greater than 45 degrees
with the vertical". The same data further analysed by van Heerden
(20) sl~ow that twelve of the same fifteen rains came from directions
lying within a 60 degree sector in azimuth. Free (150) fou.'1d that
in ten out of thirty nine storms recorded in New York State, U.S.A.
9 was greater t.'1an 45°, and this inclination was so combined with
prevailing direction that slop:i.:.g erosion test plots facing directly
into the prevailing ~dnd lost three times as much soil as plots
facing the opposite direction. In the light of these and similar
results it was obvious that the effect shouJd be measured in the
present studies, but at the same time it was antiCipated that thG
-81-
velocity of water drops falling in still air varies fI~m 12 feet
per second for very small drops 1 mm. in alilineter, to about 30 feet
per second for the largest drO?S (al:out 5 mm. diameter). Values
of 8 of 450 will tIlu:::; be enconl:.terecl \~.e:tl wind speeds are of the
same order, i.e. from 8 - 22 m.p.h., and for any given wind speed
e will be greater for the small drops tllo'l.n the larg0r. It would be
imprac"bicable, though not impossible, to measure separately -the in
clination of the various sizes of drops, e..'1d instead the average
inclination of the whole rain i~ rne8.Sm'ed.
If, as in t:i.le present studies, the object is to seek
correlA.tions b'3tween quantity (or 5.ntensi ty) of rain and functions
of velocity f.luoh as momentum or energy, then the effects .)f varying
angles of inclination mud be pl:im::1na-l;ed (i.t' they are significant)
by relating both to the common standard of what the m3asure~ value
wou..1.d be without wind - i .. e 0 "lith vertical rain. The quantity
of inclined rain is correctly mea;:mred lJY a hoT.':i.zontal gauge but
the velocity of inclL'1eo. rain is :i.ncre ased by seca..'1 t 9. 'lbe
correlation must therefore be tested between
Q (measured) and(V (measure,l) ~ secant e)
i.e. Q = f(V • cos 0) m m
La ter (6.4.) the significance of th1.s correction for wind
will 'be considered in relation to measured values of 9. It will
of course be more important 1.."'1 the C9.se of kinetic energy ex v 2 )
than of momentum (t.?(' v) •
6.3. MSASgREMF:NT OF D.f....RE...Q!lQti ... !¥l!LJNCLIi'lt\TICN OF RA.TIi.
EA.rlier meas~ments of direction and inclination show SOQe
strik:L':lg wind effects, but this is not surprising as these studies
were in areas where v:ind. was obviou:3ly inlporiant. Wicht (149)
measuring rainfall in the mountainous experimental catcl1Ill.ent of
.Tonkershoek (S. AflC'ica) found that "of a total of fifteen rains measured
at one locality, eleven fell at an incljnation greater than 45 degrees
with the vertical". The same data further analysed by van Heerden
(20) sl~ow that twelve of the same fifteen rains came from directions
lying within a 60 degree sector in azimuth. Free (150) fou.'1d that
in ten out of thirty nine storms recorded in New York State, U.S.A.
9 was greater t.'1an 45°, and this inclination was so combined with
prevailing direction that slop:i.:.g erosion test plots facing directly
into the prevailing ~dnd lost three times as much soil as plots
facing the opposite direction. In the light of these and similar
results it was obvious that the effect shouJd be measured in the
present studies, but at the same time it was antiCipated that thG
Univers
ity of
Cap
e Tow
n
be less severe in convective thunder-
showers.
stud:ies (Rose, and
has in f'ac't be':ln cor..:f'irmec1 both by ","'1"1",,..
the
A number of instruments have been measure
the of inclination and of Pers (151) in
1932 a lLa,.u<.c;; f'or which coined the name
orifices f'acing the
compass .. The amounts of rain caught by
are both in to each othel'" to
the caugh t by a f'~Lfth orif'ice,
the of' both
A varietion on design by
) was usee, by Wicht the
previously mentioneCl.
A of instrumer:. t was used
154) f'o:>:' the San .v-LLUCLO, .. This
a f'unnel mounted on a rotating
he ad 'iJhich was the wind by a vane. P..a.in
by the
tank, one
into of a
of' the compass for
Reerden (20) used a S .\-w."' .... e,. ... 'into-wind' , but
with seven f'unnels dlfferent
to the to
funnel had a
..
was .llounted on a freely rotating platform kept into the wine: by a va.'1e ..
Such are to
between free rotation _~ ... ",~ .. ,... of
winds. Hamilton f'ound San to
build use ll and turned to a model the Pers
Van Heeril.en (155) subsequently that, from
def'iciencies, an v-.. ~,~~ instrument is inherently unsound on
as is the case, of direction
the R torm.. Such a storm may
be described by a ntean ,
direction, but cmch a vector must be the summation of similar vectors
for each of' the Nei ther the nor vertical
of the vectors may be as occurs with
gauges. The Dim&s t)~e
( but averages the of'
whereas van s machine
(wi thin the seven classes) but averages the
The of th~se t have been o~Jercome
be less severe in convective thunder-
showers.
stud:ies (Rose, and
has in f'ac't be':ln cor..:f'irmec1 both by ","'1"1",,..
the
A number of instruments have been measure
the of inclination and of Pers (151) in
1932 a lLa,.u<.c;; f'or which coined the name
orifices f'acing the
compass .. The amounts of rain caught by
are both in to each othel'" to
the caugh t by a f'~Lfth orif'ice,
the of' both
A varietion on design by
) was usee, by Wicht the
previously mentioneCl.
A of instrumer:. t was used
154) f'o:>:' the San .v-LLUCLO, .. This
a f'unnel mounted on a rotating
he ad 'iJhich was the wind by a vane. P..a.in
by the
tank, one
into of a
of' the compass for
Reerden (20) used a S .\-w."' .... e,. ... 'into-wind' , but
with seven f'unnels dlfferent
to the to
funnel had a
..
was .llounted on a freely rotating platform kept into the wine: by a va.'1e ..
Such are to
between free rotation _~ ... ",~ .. ,... of
winds. Hamilton f'ound San to
build use ll and turned to a model the Pers
Van Heeril.en (155) subsequently that, from
def'iciencies, an v-.. ~,~~ instrument is inherently unsound on
as is the case, of direction
the R torm.. Such a storm may
be described by a ntean ,
direction, but cmch a vector must be the summation of similar vectors
for each of' the Nei ther the nor vertical
of the vectors may be as occurs with
gauges. The Dim&s t)~e
( but averages the of'
whereas van s machine
(wi thin the seven classes) but averages the
The of th~se t have been o~Jercome
Univers
ity of
Cap
e Tow
n
trument used in the " Farbrother (156)
i'or of cottcn
and it
shQ'lJlred that it
of rain. It
vLllA..!..',"" also be used to the
set
on a with each vV'~.J..VV
vertical,
funnel at to t..."':le
1200
leads "vo a
catch in
a calcula. tion of both
three
~o.-,~ ......... '..... analysis
, e ach with each
direction of the storm"
the '-'~J'WIJu. P~T.~V area of each fumlel for all
are
reduced to a single
direction may be
catch to catch f, and t
and
catch to second
method, which was used in the "nT',P .... ,c'Tl
sources or error.
are two
a) If conventional gauges
are used possibility of scme rain
funnel. but
the ~ Snow"den t gauge
where Q its less is
witll o 8.-'1 indicated average f) cf say 40 may include
of o at say 50 not been
intrcducing .Jl1 error. erro:'::' decreases
average below
studies onlY one storm was not
onc may be in error (9
• (9
40 0 and
out of
by
the
, a storm
some portions
so
), so this
does not apply to studies of •
Tw-o
low
the
errors were 1) The tilted gauges have a
the lower side of funnel which is not
tUbe. This was overcome by
this space with putty. 2) of the '-' ... ' ........... '-''-'
was by running the water through hl.bing in to
closed with an U-tube vent.
There one defect to any
measures total effect over the duration of the whole sterm.
That of each other
out and an ... vhich
To an extreme case, a sterm
rain from the North with at say from the ,
trument used in the " Farbrother (156)
i'or of cottcn
and it
shQ'lJlred that it
of rain. It
vLllA..!..',"" also be used to the
set
on a with each vV'~.J..VV
vertical,
funnel at to t..."':le
1200
leads "vo a
catch in
a calcula. tion of both
three
~o.-,~ ......... '..... analysis
, e ach with each
direction of the storm"
the '-'~J'WIJu. P~T.~V area of each fumlel for all
are
reduced to a single
direction may be
catch to catch f, and t
and
catch to second
method, which was used in the "nT',P .... ,c'Tl
sources or error.
are two
a) If conventional gauges
are used possibility of scme rain
funnel. but
the ~ Snow"den t gauge
where Q its less is
witll o 8.-'1 indicated average f) cf say 40 may include
of o at say 50 not been
intrcducing .Jl1 error. erro:'::' decreases
average below
studies onlY one storm was not
in error (9
• (9
40 0 and
out of
by
the
, a storm
some portions
so
), so this
does not apply to studies of •
Tw-o
low
the
errors were 1) The tilted gauges have a
the lower side of funnel which is not
tUbe. This was overcome by
this space with putty. 2) of the '-' ... ' ........... '-''-'
was by running the water through hl.bing in to
closed with an U-tube vent.
There one defect to any
measures total effect over the duration of the whole sterm.
That of each other
out and an ... vhich
To an extreme case, a sterm
rain from the North with at say from the ,
Univers
ity of
Cap
e Tow
n
Pla te 601- 111e 3-gauge instr~ent f or det0~inL~g angle
of L'1.cl ination a:':ld directiun of r ainfall.
Plate 601. 111e 3-gauge instr~ent for detQ~ining angle
of' :L.-.,.clination a:':ld directiun of rainfa1.1.
Univers
ity of
Cap
e Tow
n
then swing suddenly as the storm centre
the s a.rne , but amount of rain 1
vector for the WijU~.~ waldd. be
S:n:i.th.
to an equal
total
m::lght be construed
as
cf
tht'l.t the rain
.,U.J.'v ..... ,,J.J.J.'vu. and
A comparison of th3 ea teh
wiD. give some measure of the
to whioh
a full
continuous
study.
has occurred the
of the storm would.
recarding of rates of
incl:ined gauges or
of
the scope of the
was :in operation at
dUI":ing the whole of the 1959/60 and 1960/61
dil"eotion were for
those of 103s inches which a:r:-e of no
to .I'UI'l-off and erosion. The results of two
seasons only tentative conclusions about rainfall
as a vlhole, snd any infel"el1Ce which may have to
or shou.ld be to further
The inferences <t;hese results
are purpose which waS to record effeots
of the other "' ....... ........
6 .. six
seasons it was that the angle of inclination v,rould
substantial~ less than in the Case of the non-oonvective rain
6 • and • 6.2. show this is
:in one rain C'u t of was the
vector at more the vertical, and only one other storm
fell
results are
that the
is
valid for angles up to [.,.50• Small storns of inches
show little in the dis bribution among groups of specified
of 9, as in 6.1. and • 6 ~
storms, i.e. those
between ~~tity and
are ,,71th more
than 0.25
of inclination 9,
more o inches this nr",.....""
is the
For all of
at the 4%
then swing suddenly as the storm centre
the s a.rne , but amount of rain 1
vector for the WijU~.~ waldd. be
S:n:i.th.
to an equal
total
m::lght be construed
as
cf
tht'l.t the rain
.,U.J.'v ..... ,,J.J.J.'vu. and
A comparison of th3 ea teh
wiD. give some measure of the
to whioh
a full
continuous
study.
has occurred the
of the storm would.
recarding of rates of
incl:ined gauges or
of
the scope of the
was :in operation at
dUI":ing the whole of the 1959/60 and 1960/61
dil"eotion were for
those of 103s inches which a:r:-e of no
to .I'UI'l-off and erosion. The results of two
seasons only tentative conclusions about rainfall
as a vlhole, snd any infel"el1Ce which may have to
or shou.ld be to further
The inferences <t;hese results
are purpose which waS to record effeots
of the other "' ....... ........
6 .. six
seasons it was that the angle of inclination v,rould
substantial~ less than in the Case of the non-oonvective rain
6 • and • 6.2. show this is
:in one rain C'u t of was the
vector at more the vertical, and only one other storm
fell
results are
that the
is
valid for angles up to [.,.50• Small storns of inches
show little in the dis bribution among groups of specified
of 9, as in 6.1. and • 6 ~
storms, i.e. those
between ~~tity and
are ,,71th more
than 0.25
of inclination 9,
more o inches this nr",.....""
is the
For all of
at the 4%
University of Cape Town
. l'!!fL~ §..:1. AN(Ef,¥ OJ<' mC!dNA~::ON OF F'ALL1!:tG RAIN.
Angle of Falls of Less }lialls greater Falls greater Totals for Percentage of Inclination than 0 .. 25 ins. than 0.25 ins. than 0 .. 50 ins. 1959/60 & 1960/61 total in each with Vertical
(degrees) Number .Amount Number .AL10unt l~ll.lJlber A-:rrount Number AmotL."'lt group.
o - 5 9 1018 11 10 .. 02 6 8.05 20 11.20 18.1 5 - 10 7 1 .. 43 21 22.74 14 20~15 28 24-.1.7 39.1
10 - 15 4 .35 11 8.54- 4 5 .. 45 15 8.89 L+.L:~ 1.5 - 20 6 .. 73 13 6,,30 3 2~66 19 7.03 11.4 20 - 25 6 .80 5 2~63 3 1 .. 95 u 3.1]-3 5.6 I
~ 25 - 30 1 ~19 3 2.01 2 1.74 4 2.2.0 3~6 i.
30 - 35 4 .,33 3 1~62 2 L21 7 le95 3,,1 35 - 40 5 .40 2 1,,61 2 ' 7," ...L. ... , ~j'...J 7 2.,01 .:\.3 40-45 1 .. 86 1 .86 1 .86 1.4
Falls of Less Falls gre for of
vii th Vertical than 0 .. 25 ins. 0 ins. 1959/60 & 1960/61
( ,
Number Number Al.'lount !~ll..rnber .knount Number )
o - 5 9 1.,18 11 6 8.05 20 5 - 10 7 1 21 14
10 - 4- 11 8.54 4- 8,.89 - 20 6 .73 6,,30 3 7.03
6 .80 5 2~63 3 1 .. 9.3 u 3 • .li-3 25 - 30 1 3 2.01 2 1 4- 2.2.0
4- 3 1 2 1.,21 7 1 3 , 5 2 1,,61 2 .,
7 2 .~~ ,,3 .....
1+D - 1 .86 1 1 .86 1
Univers
ity of
Cap
e Tow
n
87
STORMS LESS THAN 0.25 INCHES
O~ ______________________ ~ __ __
o 5 10 IS 20 25 30 35 40 45
z o ~ z -..J U Z
ffi 10 2:
ANGLE OF INCLINATION - DEGREES fROM VERTICAL..
STORMS MORE THAN 0.25 INCHES
5 10 15 20 25 30 35 40 45
STORMS MORE THAN 0.50 INCHES
~ o~--------------------------~ o 5 fO 15 20 25 30 35 40 4S
AL.L STORMS - HENDERSON
Ii.
o 0 0 5 10 15 20 2S 30 35 40 4S II..!
~ Z ILl u ffi3 Q. UGANDA - FROM ROSE.
5 10 15 20 2S 30 35 40 45
FIG. 6. ANGLE OF INCLINATION
FALLING RAIN
87
STORMS LESS THAN 0.25 INCHES
O~ ______________________ ~ __ __
o 5 10 IS 20 25 30 35 40 45
z o ~ z -..J U Z
ffi 10 2:
ANGLE OF INCLINATION - DEGREES fROM VERTICAL..
STORMS MORE THAN 0.25 INCHES
5 10 15 20 25 30 35 40 45
STORMS MORE THAN 0.50 INCHES
~ o~--------------------------~ o 5 fO 15 20 25 30 35 40 4S
AL.L STORMS - HENDERSON
Ii.
o 0 0 5 10 15 20 2S 30 35 40 4S II..!
~ Z ILl u ffi3 Q. UGANDA - FROM ROSE.
5 10 15 20 2S 30 35 40 45
FIG. 6. ANGLE OF INCLINATION
FALLING RAIN
Univers
ity of
Cap
e Tow
n
level,
the
The
so some
when
•
of more than 0
is
inn1.'ease with t:le
mind these of the
are
level ..
out by changes
the
the
that the covers only seasons, the indi(l~tions are that
1) 9 ov .... u. ..... !U exceeds 1;50•
2) to O. inches) of Q are
distributed thr01::gh thc ra...'1ge 0 _. with
3) For lilGI..L..LUlll (0,,25 to 0.50 ) the :beavier storms
values of 9Q are a d with the
4) For heavy rains (more than 0 in::)hes) this associa"tion
is
shovvn in
uut
.. may be of Rose, also
• 6 .. 2", which 'Were in
of 3,800 feet in Uganda. There it was
also found very few with 9 more than , but
90 pe!"eent 01' the total rain o
more tha...1. 10 •
per cent o
Q more than 10 •
6 02.
'I'he quantity of from
are shown in
in thP arc between 0450 and 'Illis does
not this is the dire ction from
which rain clouds come, the local winds ground
surface below a thunderstorm are influenced by and the
Since the
unlikely
to va-.::y
of
of the observer relative the centre of the
of , this concentration of oirection will cause erosion
waS the case New
tha t of the storms had e than 1% in the experiments.
tl1.e size distribution
the at any time ..
and small drops f'ormed at the sam:::: time and
pa. ths as the smaller are more
level,
the
The
so some
when
•
of more than 0
is
inn1.'ease with t:le
mind these of the
are
level ..
out by changes
the
the
that the covers only seasons, the indi(l~tions are that
1) 9 ov .... u. ..... !U exceeds 1;50•
2) to O. inches) of Q are
distributed thr01::gh thc ra...'1ge 0 _. with
3) For lilGI..L..LUlll (0,,25 to 0.50 ) the :beavier storms
values of 9Q are a d with the
4) For heavy rains (more than 0 in::)hes) this associa"tion
is
shovvn in
uut
.. may be of Rose, also
• 6 .. 2", which 'Were in
of 3,800 feet in Uganda. There it was
also found very few with 9 more than , but
90 pe!"eent 01' the total rain o
more tha...1. 10 •
per cent o
Q more than 10 •
6 02.
'I'he quantity of from
are shown in
in thP arc between 0450 and 'Illis does
not this is the dire ction from
which rain clouds come, the local winds ground
surface below a thunderstorm are influenced by and the
Since the
unlikely
to va-.::y
of
of the observer relative the centre of the
of , this concentration of oirection will cause erosion
waS the case New
tha t of the storms had e than 1% in the experiments.
tl1.e size distribution
the at any time ..
and small drops f'ormed at the sam:::: time and
pa. ths as the smaller are more
Univers
ity of
Cap
e Tow
n
89
N
195 0 165 0
FIG.6.3. DIRECTION OF INCLINATION OF RAIN ..
89
N
195 0 165 0
FIG.6.3. DIRECTION OF INCLINATION OF RAIN ..
Univers
ity of
Cap
e Tow
n
f'rom vertical, and so land farther doW!l.vd..nd , 157) .. the milch
00 may either
thundershowers), or
observed at the onset of
the same
and "1"""""'''''1',,,,'''' , Hitschfeld 1 ) .. Studies of the of
were by D.Li9nr;na..r: Plank
to deduce facts t.'lJ.e and time of
formation~ However, as it has been shovm that
in as measured
of are J 'i:;he effects of wind will not be
when consider-lug drop size o.is·cributj ons.
f'rom vertical, and so land farther doW!l.vd..nd , 157) .. the milch
00 may either
thundershowers), or
observed at the onset of
the same
and "1"""""'''''1',,,,'''' , Hitschfeld 1 ) .. Studies of the of
were by D.Li9nr;na..r: Plank
to deduce facts t.'lJ.e and time of
formation~ However, as it has been shovm that
in as measured
of are J 'i:;he effects of wind will not be
when consider-lug drop size o.is·cributj ons.