Agricultural Meteorology, 21(1980) 213--226 213 @) Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

CROP EVAPORATION, SURFACE RESISTANCE AND SOIL WATER STATUS

G. RUSSELL*

Department of Physiology and Environmental Studies, University of Nottingham, School of Agriculture, Sutton Bonington, Loughborough LE12 5RD (Great Britain)

(Received July 14, 1978; accepted October 20, 1978)

ABSTRACT

Russell, G., 1980. Crop evaporation, surface resistance and soil water status. Agric. Meteorol., 21: 213--226.

Rates of evaporation from barley and from pasture were calculated from rainfall and from changes in soil water content measured with a neutron probe. Corrections were made for drainage. The potential evaporation was calculated for ten-day periods using the Penman--Monteith equation and mean surface resistances of the crops were derived. Relationships were established between the supply of water (as measured by the soil water deficit or soil water potential) and the response to a shortage of water (as measured by the surface resistance or the ratio of actual to potential evaporation). For pasture, these relationships held for the entire growing season and for barley for most of the period from mid-May to the end of July. They did not hold for barley earlier in the year when the leaf area index was small or towards harvest when the crop was ripening. The differences in response of the two crops were apparently due to differences in the water release curves of the soils rather than to species differences.

INTRODUCTION

The publication of Penman's combinat ion equation in 1948 was a land- mark in studies of evaporation from field crops. Penman calculated the potential rate of evaporation from standard meteorological data bu t he recognised that actual evaporation would be less than the potential amount if there was a shortage of soil water. Many later workers explored the relation between soil water status and the ratio of actual to potential evaporation rates, e.g. Penman (1949), Veihmeyer and Hendrickson (1955) and Thornthwaite and Mather (1954). Monteith (1965) generalised the Penman equation by specifying two resistances to vapour transfer: from evaporating surfaces to the air within a canopy (rs) and through the air (ra) to a reference height. This equation is applicable to all crops but its use requires that rs be measured or related to simple attributes of the soil and crop in the same way that r a can be

* Present address: Edinburgh School of Agriculture, West Mains Road, Edinburgh EH9 3JG (Great Britain)

214

estimated from crop height and windspeed. Only a few workers have tried to relate rs to some measure of soft water status (Szeicz and Long, 1969; Trivett, 1972). The aim of the present paper is to examine the rates of water loss from barley and permanent pasture growing on a sandy loam soil in the Midlands of England and to relate the seasonal trend of r.~ to crop development and soil water status.

METHODS

The rate of evaporation was calculated from measurements of rainfall and changes of soil water content measured with a neutron probe. A graphical me thod developed by Williams (1971) was used to separate drainage losses from losses of water due to uptake by the plant roots (McGowan, 1973). This method gave results which were in good agreement

NOTATION

Cp

E

E0 Ep e

es(T)

G

Rn

r s

ra

S u

Z

z 0

7 A

P ~s

specific hea t o f air at cons t an t pressure J kg -1K -1

evapora t ion rate g m -2 s - l

po ten t ia l evapora t ion rate (r s = 0) g m -2 s- 1

po ten t i a l evapora t ion rate (r s = p) g m~2s -1

water vapour pressure at screen height Pa

sa tura ted vapour pressure at screen t empera tu re T Pa

soil hea t f lux dens i ty W m -2

ne t radia t ion flux dens i ty W rn -2 -1 crop resis tance to water vapour t ransfer s m

ae rodynamic resistance to water vapour t ransfer s m -1

shor t wave irradiance W m -2

windspeed at he igh t z m s -1

height m

roughness length m p s y c h r o m e t e r cons t an t Pa K -]

rate of change of sa tu ra ted vapour pressure with temperature Pa K -1

latent heat o f vaporisation of water j g-i

densi ty of air kg m -3

soil water potent ia l MPa

with those from another method in which tensiometers were used to define a specific depth below which all movement of water was down- wards and above which all movement was upwards or into the plants (Russel l 1976) . Evaporation was calculated for ten-day periods. The soil water deficit was defined as the amount of water needed to return the soil to the water content which prevailed when the roots began to take up water. The soil water potential at 0 .2 m depth was calculated from the soil

215

water content using water release curves obtained in the laboratory. The resultant figures agreed well with measurements made in the field with tensiometers and thermocouple psychrometers.

Evaporation rates were also calculated from the Penman--Monteith equation (Monteith, 1965):

E = A(Rn- G) +pCp [e s ( T ) - e ] / r a (1)

A + "y (1 + rs/ra)

Initially, a potential rate of evaporation (E o) was calculated by setting rs = 0. Calculations were made for daylight hours on meteorological measurements averaged over a period of ten days. All the meteorological measurements were made on the meteorological site at Sutton Bonington 7 km from the principal experimental sites and no a t tempt was made to modify the data to account for possible differences between the two places. Net radiation was calculated by regression against the short wave radiation. For Sutton Bonington Rn = 0.46 S (W. Kyle, personal communication, 1977). There was no seasonal trend between May and September. Soft heat flux was expressed as a fraction of net radiation which varied with the crop and the time of year. From measurements made at Sut ton Bonington (P. V. Biscoe, personal communication, 1977) the appropriate figures were 0.12--0.35 for barley and 0.15--0.20 for pasture. The mean temperature during the hours of daylight was assumed to be the average of the daily maximum and minimum and the 09h00 measurement of vapour pressure was considered to represent the daily mean (Trivett, 1972). The aerodynamic resistance was calculated using the method of Thom and Oliver (1977) assuming that the windspeed at z = 2 m at Sut ton Bonington represented the windspeed at a similar height at the field sites, and that the roughness length (z 0) was 0.13 times the height of the crop.

4.72 In ( z / zo) 2 ra = (2)

1 + 0.54 u

Eq.1 was combined with the equation for E 0 (i.e., eq.1 with r s = 0) and rearranged to provide the following expression from which rs could be calculated:

r~ = r a [1 + (A/T)] [ (Eo/E) - 1] (3)

Since E, the actual rate of evaporation, includes evaporation of intercepted water, rs will be depressed whenever the crop is wet (for wet leaves r s ~ 0 ) .

When a minimum crop resistance is inserted in the Penman--Monteith equation another potential rate (Ep) can be calculated. For a minimum resistance of p s m -1 , E/Ep can be found from the equation: E r a [1 + (A/3')] + p

Ep r a [1 + (A/T)] + r s (4)

216

S I T E S

Measurements were made on two commercial farms 20 km south east of Nottingham. The two sites, one of permanent pasture, predominantly perennial ryegrass (Lolium perenne L), Yorkshire fog (Holcus lanatus L.) and white clover (Trifolium repens L.); and the other of barley (cv Zephyr) were typical of the gently undulating countryside of the eastern Midlands of England. The softs have developed in a sandy loam drift which gives way to clay well below the rooting zone and have been assigned to the Arrow (pasture) and Newport (barley) soil series (Thomasson, 1971). The "available" water in both soils, defined as the soil water deficit when the potential falls to -1 .5 MPa, is 100 mm m -1 . Measurements were made at both sites during the growing seasons of 1970 to 1972. In the summer of 1973 there were several very heavy rain storms: the soil water deficit was close to zero for most of the summer and the evaporation from pasture could be computed only for parts of June, August and September. There are no measurements for barley in 1973 because the field had been sown to oil- seed rape. In addition r s was calculated for two more sites in 1972. The first was a field of barley near Sut ton Bonington growing on the Newport soil series. The second represented pasture growing on clay soils (principally Ragdale series). The estimates from these sites are less reliable than from those of the others because they were further from the meteorological station at Sut ton Bonington and because r s was calculated only for month ly periods. The grass was assumed to be a constant 0.2 m high for the calculation of r, .

RESULTS

Evaporation rates

The seasonal trends in measured evaporation are depicted in Figs. 1 and 2. From about the 21 May to 31 July, both crops lost water at about the same

i

Fig. 1. The seasonal trend o f evaporation from a field of barley. Measurements were made in 1970 (e) , 1971 (o) and 1972 (m). H is harvest time,

rate (2.1 mm d "I compared with the long term potential rate of 2,9 mm d -I ) although the highest ten.day mean was: for barley (3.2 mm d -l ). The rate of evaporation from pasture exceeded that from the barley field in April and

217

3 E E

2

E o

o> 0 Ld

o

Apri l I May I June I July I August ISeptember lOctober I

Fig.2. The seasonal trend of evaporation from a field of pasture. Measurements were made in 1970 ( , ) , 1971 (o), 1972 (m) and 1973 (D).

early May when the barley had a small leaf area index and in August and September when the grass was still growing but the barley had been harvested.

Seasonal change in surface resistance

Seasonal changes in r~ from about 15 to 200 s m -1 are shown in Figs.3 and 4. For barley, r~ tended to be high in April falling to a minimum in May and rising again towards harvest. Pasture had a minimum r~ in early June

2001 ~" g 120

t_

5

0 April' I May I June I July I August }September I

Fig.3. The seasonal change in the surface resistance of barley. Symbols as in Fig.1.

followed by a gradual increase. The dry year of 1970 produced much higher values of r S for both crops. The changing green leaf area of the annual crop presumably accounts for the greater seasonal change with barley. Early in the year the soil and not the plant canopy is the effective exchange surface so that rs will be small as long as the soil is very wet. Kristensen (1974) has shown that the evaporation rate from a growing crop does no t reach its potential rate till the leaf area index exceeds 3. As the barley reaches matur i ty the leaves senesce and die and the green leaf area falls, resulting in an increase in r s. The leaf area index of the pasture will generally be greater

2 1 8

than 3 but will be reduced after grazing and during severe drought when many leaves can die. In addition r~ depends on the degree of stomatat open- ing which is a function of the leaf water status and consequently on the soil water status.

2OC

i E

16C

~2q

i

80,

~c

o I I A p r i l Moy

I I ! u t ' S e p t e m b e r ' June uly ug S

Fig.4. The seasonal change in the surface resistance of pasture. Symbols as in Fig,2.

The effect of soil water deficit (swd) on r~

The most convenient measure of soil water status is the soil water deficit (swd). The calculated values of r~ axe plotted against swd in Figs.5 and 6. In these figures and throughout the paper, averages have been calculated for swd ranges of 10 or 20 mm. In this and in succeeding graphs of the barley results, only measurements made between the 21 May and a date three

2°° I 1 6 0

v

12C

~ • Q X X

• ® X

I I i i i I I i i •

0 20 4 0 60 8 0 100

S o i l w a t e r c l e f i c i t ( r am)

Fig.5. The r e t a t i ~ i p I~tween :thesttrface res ist~ce of barley and the soil water deficit. Measurements were made in May (o), ~ (o) and July (is). F_,stimates from the second site (X) have been marked. Circliad poln4~ represent periods with no rain ( 0 = June, O'ffi July). In..this and succeeding Figs., o represents the class means and the lines are drawn by e y e u~l~ss o t h e F w i s e Stated.

219

2 0 C

1 6 C

12C

8 C • ®

bq • X • • D

® ®

I • I I I l | I I o 4'0

$o~1 w a t e r d e f i c i t ( m m )

Fig.6. The relationship between the surface resistance and soil water deficit for pasture. The estimates from the clay site (X) have been marked. Circled points (®) represent periods without rain.

weeks before harvest were inc luded to avoid the e f fec t of a changing green leaf area. Per iods w i t h o u t rain have been marked . F o r the per iods w h en there was rain the measu red evapora t ion includes evapora t ion o f i n t e rcep ted water. However , a l though rs approaches zero when the leaves are wet , this source of var ia t ion seems to be insignif icant a l though there is an indicat ion tha t rs fo r bar ley t e n d e d to be slightly higher when the plants were d ry ( the e f f ec t should be more no t i ceab le wi th bar ley as r a is smaller and changes in r~ should have a larger inf luence on evapora t ion rate) .

A pa t t e rn is clear fo r bo th crops. F o r barley, r~ remains cons t an t at a b o u t 30 s m - l till the def ic i t exceeds 30 m m af te r which i t increases steadily to 210 s m -1 at 100 mm. The pas ture results were m o re variable perhaps because ra, which was d i f f icul t to es t imate accura te ly , assumes m o re impor t ance with the shor te r crop. Unti l the def ic i t r eached 40 m m , rs r ema ined at a b o u t 40 s m -1 bu t as the soil dr ied still fu r ther , rs increased rapidly to 180 s m -1 at an swd o f 100 mm. Fig.5 also conta ins points f rom the second bar ley site which suggest a curve m o re akin to tha t of the pasture site than to the first bar ley site. In Fig.6, es t imates of r~ f rom pas ture clay sites fall on the same curve as those f rom the pas ture sandy loam site.

The effect o f soil water deficit on the ratio o f actual to potential evaporation rate

The re la t ionships be tween swd and the ra t io o f actual to po ten t ia l evapor- a t ion rate (E/Eo) are shown in Figs.7 and 8. This ra t io should reach un i ty on ly if the foliage r ema ined we t t h r o u g h o u t the ent i re t en-day per iod because a d ry c anopy has a f ini te value of r s. F o r bar ley the ra t io r emained near 0.6 till the def ic i t e x c e e d e d 30 m m and then decl ined l inear ly to 0.17

220

1.©

G L~

O ~

O.6 e l o

0 . 2

0 . 0 ! 0

o

! i i i | l i i i

2 0 4 0 6 0 8 0 1 0 0

Soil wQter " d e f i c i t ( r a m )

Fig.7. The relationship between E/Eo and soil water deficit for barley. The lin e was cal, culated by least squares linear rqressic~ for swd values between 30and 100 mm~

g uJ

1.0

0.8

0.6

0.4

0.2

g

C).Cm | I ! I I | t i: ! l o 20 40 6o eo 1oo

Soil w o t e r d e f i c i t ( r a m )

Fig.8. The relationship between E/Eo and soft water deficit for pasture.

at a swd of 100 mm. For pasture the ratio declined gently from about 0.77, when there was no deficit, to 0 .74 at 50 mm and then more sharply to 0.45 at 100 ram. Monteith (1965 ) quotes a range o f 0 .62- -0 .76 for field crops adequately supplied with water.

In the two graphs o f Figs.9 and 10 the analysis has been improved by incorporating a minimum ~ l ~ n c e ( found by inspection) of 80 s m -1 for barley and 4 0 s m -I for pasture. The ratio should be unity when there~is no deficit. In fact it declines linearly from 0 .98 to 0 .29 at a swd of 100 mm for barley while the pasture provided a ratio of 0 .95 at 50 mm and 0 .60 at a

221

deficit of 100 mm. The figures for the other pasture sites are in close agree- ment while the second barley site shows a pattern intermediate between that for the pasture sites and the first barley site.

o

~J

Ld

1.2

1.0,

0,8

0.6 i ~

O.zl

0.2

I I i I I I l I I I 0'00 20 40 coo 80 100

Soil wa te r deficit (rnrn)

Fig.9. The relationship between E/E30 and soil water deficit for harley. The solid line was fitted to all the points from the first site by linear regression, the pecked line was fitted by eye to the figures from the second site (X).

~J

1.0

0.8

0.6

0.4

0.2

J¢ O • O•

x•

0 0 B

• x

o o .

• • • x

I I I I I I I 8 1 0 I I O.% 20 40 60 100

SOil water' deficit (m m)

Fig.10. The relationship between E/E40 and the soil water deficit for pasture. The figures from the clay site have been marked (X).

2 2 2

The relationship between rs and soil water potential

The difference in response of the sites may be a consequence of differences in soils, in species or in both. Since water moves into the plant in response to a gradient of water potential between the soil and the roots the rate of uptake of water, and consequently the leaf water potential and rs, should depend on ~ . Where soils differ in their physical properties, especially in texture, ~s is not uniquely related to swd and consequently the relationship between r~ and swd will vary from soil to soil. If the differences

2 0 0

120

8 0 o x

4O

0 I I I I , ' i • - o . o l - G o 2 - o . o 5 - o . 1 0 - 0 . 2 o - o , 5 o - l x30 -1 .5o

Soil w • t e r potent iGI ( M PCl)

Fig . l l . The relationship between surface resistance and soil water potential (at 0.2 m depth) for barley.

2OO

m

~ 12o

c 8C

•

ffl 4 0 ~ ' ~ ' u -

l

i

I ] • I I | I I

- 0 . 0 1 - 0 . 0 2 - 0 0 5 - 0 . 1 0 ~0 .20 - o , 5 o - 1 ~ 3 0 4 . 5 0

Soil w a t e r po ten t i a l ( M P~)

Fig.12. The relationship between surface resistance and soil water potential (at 0.2 m) for pasture.

223

between sites are due only to differences in the water-holding properties of the soils one relationship be tween r~ and ~ should suffice. Although ~ varies considerably down the profile, a good index might be the potential in the upper 0.30 m of the soil where most of the roots are. In F igs . l l and 12, r s is plot ted against ~s at 0.2 m (r~ has been used because E/Eo includes the effect of a difference in r a ). The differences between sites seen in Figs.5 and 6 have been reduced. For all three sites on the sandy loam soil r s remained relatively constant till ~s was about -0 .15 MPa and then rose to about 150 s m -1 at - 1 . 0 0 MPa. Potentials were not measured in the clay soils.

DISCUSSION AND CONCLUSIONS

The results in this paper show that simple measurements of evaporation combined with Penman--Montei th equations using climatological data averaged over ten-day periods can give values of r s which appear to show realistic variation with changes in soil water deficit. A useful relationship between crop resistance and soil water deficit was obtained for pasture from April to September bu t because of changes in the green leaf area index there was a good relationship for barley only during the period from 21 May to a date 20 days before the crop was harvested. Interception of rainfall by the vegetation did no t appear to affect the results significantly.

Both the soil water deficit and the soil water potential (at a depth of 0.2 m) can be used to predict r~ although both can give misleading results. The swd is an inappropriate index when rain falls on a dry soil; and ~ is in- appropriate when a significant propor t ion of the water is contr ibuted by the subsoil (at the height of a dry spell in 1970, 75% of the evaporated water came from below 0.25 m). Although the present work suggests that one curve could be used to relate rs to ~ for all three sandy loam soils, the response differs from the suggested universal relationship of Szeicz and Long (1969). Penman (1968) modelled the uptake of water by roots and showed that by changing the values of rooting density and soil water diffusivity within real- istic limits almost any relationship between the actual evaporation and the swd (and consequently ~ ) could be obtained. In any case, r~ depends on the soil water status only indirectly. The controlling factor is the leaf water status which depends on the balance between the evaporative demand of the atmosphere and the ability of the plant to take up water from the soil. The correlation between r~ and swd would be spurious if the main control on leaf water status was the evaporative demand of the atmosphere (i.e., E0 ) and periods of high swd coincided with periods for which E0 was large. To ex- amine this possibility, r, was plot ted against E0 (Figs.13 and 14). There was no good relationship between the pasture r s and E 0 and consequently it is unlikely that r~ was affected by atmospheric conditions independently of the swd although Denmead and Shaw (1962) reported that the evaporation rate from maize was reduced when the potential evaporation was high. Their maxi- mum rates of potential evaporation were, however, twice those encountered

2 2 4

in the present work. The apparent correlation for barley (Fig.13) is not necessarily causal since, during the restricted part of the year when the mea- surements were made, periods of high evaporative demand often coincided with periods when the swd was high.

The uptake of water was probably limited either by small hydraulic conductivity of the soft or by an insufficiency of functional roots. The latter possibility could occur when a large proportion of the water was coming from parts of the soil with few roots. Reicosky and Ritchie (1976) have shown that the soil resistance for moderate root densities becomes important only when the hydraulic conductivity of the soil falls below 10 -s to 10 -6 mm d -~ . Hydraulic conductivity was calculated for a depth of 1.55 m for the two main sites although it proved impossible to perform the calcu-

lations accurately for the rooting z o n e . The figures suggested that 10 -s m m d -~ would occur at a soil water potential of 0.1 MPa, i.e., near the break in slope in F igs . l l and 12.

The scatter in the diagrams was not increased significantly when E/Ep was calculated rather than r s although another source of variation (ra) was included. Figs.9 and 10 show that the evaporation rate was certainly related to the dryness of the soil. Although the barley seemed to be more affected

1 6 C

m 12C

8 c

~ ec

a 4C ~C

© 0 Potential evaporation rate

( W i n - 2 )

I I I i I I

lOO 200 3OO

16C

21~c

c cJ

m_ 8c

~ 4c ~c

0 ©

@ @ @

~ e

• e o

i . ' ~ I i i J

Potential evaporation rate (W m -2)

F i g . 1 3 . The relationship between surface resistance and the potential evaporation rate for barley.

F i g . 1 4 . The relationship between surface resistance and the potential evaporation rate for p a s t u r e .

than the pasture by thesoit water status, the evidence of the second barley site and of Figs . l l and 12 suggests that this effect was due more to physical properties of the soil, perhaps because of the way in which they influence rooting, than to the species of plant. If this is so, then a compar- ison of the rates of evaporation from the two main sites can be misleading. Over the whole year it was found that the rates o f evaporation of the two

225

crops were similar. The effect of removing the influence of the difference in soil can be seen by calculating what the expected rates of evaporation would have been had the soil water deficit remained less than 40 mm. Taking as an example the period between the 21 May to 31 July 1972, the actual mean rates of evaporation for pasture and barley were 1.94 and 2.09 mm d -1 . Had the swd remained low, the figures would have been 2.05 and 2.60, respectively.

The use of an empirical correlation between rs (or E/Ep ) and soil water deficit to calculate the actual evaporation seems promising especially as the soil water deficit can be estimated from a water balance. However, some means must be found to generalise the relationship to other soil types. Soil texture might be a good measure of the soil properties since both soil water potential and hydraulic conductivity can be related to it. It may be possible to produce a family of curves of rs against soil water deficit for different soil textures. Alternatively it may be worth investigating whether other soils give the same relationship between r~ and the soil water potential.

Modifications to a model will be necessary to take account of periods when the leaf area index is small and periods when the surface soil is wet while the swd is large. Grant (1975) has shown that the latter correction is necessary and has suggested a way in which the swd figure could be modified to account for evaporation from the soft.

ACKNOWLEDGEMENTS

I would like to thank Professor J. L. Monteith and Dr. M. McGowan for their advice and encouragement, Dr. M. McGowan for providing data from the subsidiary sites and Dr. J. B. Williams who measured the soil water in 1970 and 1971.

REFERENCES

Denmead, O. T. and Shaw, R. H., 1962. Availability of soil water to plants as affected by soil moisture content and meteorological conditions. Agron. J., 54: 385--390.

Grant, D. R., 1975. Comparison of evaporation from barley with Penman estimates. Agric. Meteorol., 15: 49--60.

Kristensen, K. J., 1974. Actual evapotranspiration in relation to leaf area. Nordic Hydrol., 5: 173--182.

McGowan, M., 1973. Depth of water extraction by roots. In: Proc. Syrup. Isotope and Radiation Techniques in Soil Physics and Irrigation Studies. IAEA, Vienna.

Monteith, J. L., 1965. Evaporation and environment. In: The State and Movement of Water in Living Organisms. Syrup. Soc. Exp. Biol., 19th, pp. 205--234.

Penman, H. L., 1948. Natural evaporation from open water, bare soil and grass. Proc. R. Soc. Lond., Ser. A, 193: 120--146.

Penman, H. L., 1949. The dependence of transpiration on weather and soil conditions. J. Soil Sci., 1: 74--89.

Reicosky, D. C. and Ritchie, J. T., 1976. Relative importance of soil resistance and plant resistance in root water absorbtion. Soil Sci. Soc. Am. Proc., 40: 293--297.

Russell, G., 1976. Crop Evaporation and Soil Water Status. Ph.D. Thesis, Nottingham.

226

Szeicz, G. and Long, I. F , 1969. Surface resistance of crop canopies. Water Resour. Res. 5: 622--633.

Thorn, A. S. and Oliver, H. R., 1977. On Penman's equation for estimating regional evaporation. Q.J.R. Meteorol. Soc., 103: 345--357.

Thomasson, A. J., 1971. The Soils of the Melton Mowbray District: Soll Survey of England and Wales, Harpenden.

Thornthwaite, C. W. and Mather, J. R., 1954. The computat ion of soil moisture. Publ. Climatol., Centerton, N.J., 7: 397--402.

Trivett, N. B. A., 1972. The Estimation of Evaporation from Climatological Measure- ments. Ph.D. Thesis, Nottingham.

Veihmeyer, F. J. and Hendrickson, A. H., 1955. Does transpiration decrease as the soil moisture decreases? Trans. Am. Geophys. Union, 36: 421--468.

Williams, J. B., 1971. The Water Balance of an Agricultural Catchment. Ph.D. Thesis, Nottingham.