the influence of climatic, hydrologic, and soil
TRANSCRIPT
THE INFLUENCE OF CLIMATIC, HYDROLOGIC, AND SOIL FACTORS ON
EVAPOTRANSPIRATION RATES OF TANARISK (Tamarix pentandra Pall.)
by
Arnett C. Mace, Jr.
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF WATERSHED MANAGEMENT
In Partial Fulfillment of the RequirementsFor the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
1968
ON EVAPOTRANSPIPATION PATES OF TANARISK ( Tamarix pentandrPall. )
be accepted as fulfilling the dissertation requirement of the
degree of Doctor of Philosophy
eertation Director
After inspection of the dissertation, the following members
of the Final Examination Committee concur in its approval and
reconitnend its acceptance:*
THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
I hereby recoamend that this dissertation prepared under my
direction by Arnett C. Mace, Jr.
entitled THE INFLUENCE OF CLIMATIC, HYDROLOGIC, AND SOIL FACTORS
2-'2- :'z
c2/,;z /i? /Z3 /67q/...2.3 /6:7
*This approval and acceptance is contingent on the candidatetsadequate performance and defense of this dissertation at thefinal oral examination. The inclusion of this sheet bound intothe library copy of the dissertation is evidence of satisfactoryperformance at the final examination.
STATENENT BY AUTHOR
This dissertation has been submitted in partial fulfillmentof requirements for an advanced degree at The University of Arizonaand is deposited in the University Library to be made available toborrowers under rules of the Library.
Brief quotations from this dissertation are allowable with-out special permission, provided that accurate acknowledgement ofsource is made. Requests for permission for extended quotation fromor reproduction of this manuscript in whole or in part may be grantedby the head of the major department or the Dean of the GraduateCollege when in his judgment the proposed use of the material is inthe interests of scholarship. In all other instances, however,permission must be obtained from the author.
SIGNED: C. >'a e. 1.
ACKNOWLEDGENENTS
The author wishes to acknowledge the guidance of the late
Professor P. B. Rowe in planning the initial phase of the research.
He wishes to express his appreciation to Professors A. R. Croft and
P. R. Ogden for suggestions in collection and analysis of data.
The helpful suggestions of Professors J. H. Ehrenreich, D. B.
Thorud, R. F. Wagle, J. L. Thames, D. D. Evans and L. G. Wilson dur-
ing the research, preparation, and writing of this dissertation are
gratefully acknowledged.
To Drs. R. 0. Kuehi and A. B. Humphrey for statistical suggest-
ions and Mrs. Janet Beauchamp and the personnel of the Numerical
Analysis Laboratory for their help in computer analysis, the author
wishes to express his appreciation.
Research for this dissertation was made possible by a contract
with the Bureau of Reclamation. The author wishes to acknowledge the
helpful suggestions and support of Mr. Curtis Bowser and the personnel
of the Bureau of Reclamation. The recommendations and material assist-
ance provided by the Bureau of Indian Affairs and the San Carlos Apache
Tribe, the U. S. Forest Service, and the U. S. Geological Survey are
gratefully acknowledged.
To his wife, Judy, for her inspiration, encouragement, and help
throughout the graduate program, the author wishes to express his
gratefulness.
111
TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS vi
LIST OF TABLES ix
ABSTRACT x
iv
1. INTRODUCTION 1
2. REVIEW OF LITERATuRE 3
2.1 Origin of Tamarisk 3
2.2 Anatomical Structure of Taniarisk 4
2.3 Measurement of Evapotranspiration 5
2.31 Climatological Methods 5
2.32 Aerodynamic Method 7
2.33 Water Budget Methods 7
2.4 Measurement of Evapotranspiration Usingthe Evapotranspiration Tent 8
2.41 Enclosure Effect 8
2.5 Effect of Salinity on Transpiration Rates 10
3. THE RESEARCH AREA 13
3.1 Location of Field Study Sites 13
3.2 Climate 13
3.3 Vegetation 16
3.4 Soil 16
4. METHODS AND PROCEDURES 19
4.1 Evapotranspiration Tent Technique 19
4.2 Theory 19
4.3 Evapotranspiration Tent 21
4.4 Ventilation Assembly 23
4.5 Humidity Sampling Equipment 23
4.6 Evapotranspiration Measurement 23
4.7 Evaluation of Tent Enclosure Effect 264.71 Air Temperature of the Tent Enclosure 264.72 Net and Total Radiation of the Tent Enclosure . 27
4.73 Sapflow Velocity of Enclosed Tamarisk Plants. . 27
4.8 Measurement of Climatic and Hydrologic Data 274.81 Soil Moisture 284.82 Depth to Water Table 28
TABLE OF CONTENTS--Continued
Page
4.83 Wind Speed and Direction 294.84 Pan Evaporation 294.85 Air Temperature and Relative Humidity 294.86 Precipitation 294.87 Net Radiation 30
4.9 Laboratory Study of Effect of Salinity onTranspiration Rates 30
4.91 Nutrient Solution and Environmental Control . 304.92 Salinity Treatments 314.93 Transpiration Measurements 324.94 Root Permeability Studies 34
RESULTS AND DISCUSSION 36
5.1 Evaluation of Enclosure Effect 365.11 Effect on Radiation Exchange 375.12 Effect of Ventilation Rate 445.13 Ventilation Rates and Air Temperatures
of the Decker Evapotranspiration Tent . . . 47
5.15 Heat Pulse Velocities 47
5.2 Tent Modifications 49
5.21 Enclosure Effect of the Triple-Inlet Tent . . 535.22 Measurement Errors 65
5.3 Evapotranspiration Rates of Tamarisk 67
5.4 Laboratory Study of the Effect of Salinityon Transpiration Rates 71
5.41 Effects of Salinity on Transpiration Rates. . 715.42 Atmospheric Vapor Pressure Deficit
Versus Transpiration 79
5.43 Salinity Effects on Root Permeability 88
5.44 Plant Adjustment to Increased Salinity andInteractions 93
CONCLUSIONS 96
SELECTED REFERENCES 99
V
LIST OF ILLUSTRATIONS
Figure Page
3.100 Location of Gila River and Salt Creek StudySites 14
3.200 Precipitation distribution at Globe, Arizonafrom 1894 to 1957, and at the Salt Creeksite for 1965 15
3.210 Mean monthly air temperature at Globe from1894 to 1957 and at the Salt Creek andGila River sites for 1965 17
4.300 The Decker "evapotranspiration tent". Air isboth introduced and removed from one sidewhich causes poor air circulation patternsand heat trapping at the upper levels of thetent 22
4.600 Diagrametric sketch of Gila River site deline-ating the three study areas 24
4.930 Diagram of plywood cover used to seal containerand support plant and equipment for measuringtranspiration 33
4.940 Root permeability chamber. (A) Top view of topplate. (B) Sectioned side view of chamber . . . . 35
5.110 All-wave incoming and net radiation of an en-closed and unenclosed tamarisk plot. The meanreduction in incoming and net radiation was 10.6and 3.1 per cent respectively 39
5.111 All-wave incoming and net radiation of anenclosed and unenclosed bare soil plot.Mean incoming and net radiation was reduced19.5 and 17.8 per cent respectively 40
5.140 Air circulation pattern of "Decker evapotrans-piration tent" showing heat trapping andstill air pockets 48
vi
LIST OF ILLUSTRATIONS--Continued
Figure Page
5.150 Heat pulse velocities of an enclosed andunenclosed tamarisk plant growing on anarea in which the water table depth was30-feet 50
5.151 Heat pulse velocities of an enclosed andunenclosed tamarisk plant growing on anarea in which the water table depth wasthree feet 51
5.200 Air circulation of the first University ofArizona evapotranspiration tent. Note thestill air pockets 52
5.201 Average temperature at three heights in thefirst University of Arizona evapotranspirationtent and an adjacent area 54
5.202 Diagram of second University of Arizona triple-inlet evapotranspiration tent 55
5.203 Air circulation pattern of the second Universityof Arizona triple-inlet evapotranspirationtent. The still air pockets were eliminated. . . . 56
5.210 Air temperatures in a tamarisk canopy at aheight of 18-inches inside and outside thetent when the tree occupies only one-halfthe volume of the tent. The tent was re-moved at 3:10 p.m. 59
5.211 Air temperatures in a tamarisk canopy at aheight of 57-inches inside and outsidethe tent when the tree occupies only one-half the volume of the tent. The tent wasremoved at 3:10 p.m. 60
5.212 Air temperatures in a tamarisk canopy at aheight of 75-inches inside and outside thetent when the tree occupies only one-halfthe volume of the tent. The tent was re-moved at 3:10 p.m. 61
5.213 Air temperatures in a tamarisk canopy at aheight of 20-inside and outside the tentwhen the tree occupies the entire volume of thetent. The tent was removed at 2:00 p.m. 62
vii
5.411
LIST OF ILLUSTRATIONS--Continued
Figure
5.214 Air temperatures in a tamarisk canopy at aheight of 52-inches inside and outside thetent when the tree occupies the entirevolume of the tent. The tent was removedat 2:00 p m
5.215 Air temperatures in a tamarisk canopy at a
height of 72-inches inside and outside thetent when the tree occupies the entire vol-ume of tent. The tent was removed at 2:00 p.m.
5.300 Evapotranspiration rates in inches per monthfor 1965 from the Gila River study site 68
5.410 Effect of salinity on transpiration rates atdifferent vapor pressure deficits. Eachpoint represents 66 measurements 72
Effect of salinity ongram of growth (F.W.pressure deficits.of six replicationsment s
transpiration rates per
) for different vaporEach point is the sumand represent 66 measure-
viii
Page
63
64
75
5.420 Estimation of mesophyll saturation deficit oftamarisk plants by regression analysis oftranspiration and vapor pressure deficit.Extrapolation of the regression lines to theX-axis is a measure of the saturation deficit.Each point represents 66 measurements 82
5.421 Transpiration rates affected by vapor pressuredeficits at four salinity levels. Water lossis linearily related to vapor pressure deficitat low salinity levels (0.3 and 4.0 Atm.). Athigh vapor pressure deficits high salinity(8.0 and 12.0 Atm) becomes a limiting factor 89
5.431 Effect of salinity on root permeability atdifferent vapor pressure deficits. Eachpoint represents 66 measurements 90
5.440 Analysis of the effect of time since treatmenton transpiration rates. Plotted pointsrepresent the means of 44 measurements 94
LIST OF TABLES
Table Page
4.600 Variations of depth to water table, crowndensity, and height of vegetation onthe upper, middle, and lower study areasat the Gila River site 25
4.610 Sampling procedure for evapotranspirationrates on the Gila River site 25
5.220 Accuracy of transpiration rates in relationto errors in temperature measurements 66
5.300 Comparison of evapotranspiration ratesdetermined by the evapotranspirationtent and Penman's method 69
5.410 Summary of analysis of covariance ofsalinity effects on transpiration ratesat different vapor pressure deficits 73
5.411 Summary of analysis of covariance forsalinity effects on transpiration ratesper unit growth (fresh weight) atdifferent vapor pressure deficits 76
5.430 Summary of analysis of covariance of salinityeffects on root permeability at differentvapor pressure deficits 91
5.440 Summary of analysis of covariance of timeafter salinity treatments on transpirationrates at different vapor pressure deficits . . . . 93
ix
ABS TRACT
In the arid southwestern United States, where water is a
limiting factor in agricultural and industrial development, a sizeable
portion of the annual precipitation may be lost through evapotranspir-
ation. In Arizona such losses account for approximately 95 per cent
of the annual precipitation.
Tamarisk (Tamarix pentandra Pall.) is estimated to occupy over
one million acres of the flood plains and streambanks in the southwest.
Although reported to use a large quantity of water, accurate estimates
of evapotranspiration are unknown. Evapotranspiration processes are
complex and depend on many interrelationships of the soil-plant-
atmosphere system. Although, water use by tamarisk has been intensively
studied, evapotranspiration measurements under different climatic and
hydrologic conditions are not available.
The evapotranspiration tent was selected to measure evapotrans-
piration rates of tamarisk under varying climatic and hydrologic
conditions. Intensive investigations of the enclosure effect of the
tent were performed. Modifications of the tent reduced serious enclosure
effects of the original tent.
Evapotranspiration rates measured by the tent agreed favorably
with rates computed by Penman's equation. Evapotranspiration rates for
an area where the water table depth was approximately 20-feet was greater
than an area where the Water table depth was 14-feet. This deviation,
x
xi
which may be attributed to salinity, led to a laboratory investiga-
tion of the effects of salinity on transpiration rates of tainarisk.
An intensive laboratory study was conducted to determine the
effect of salinity on transpiration rates of tamarisk at different
vapor pressure deficits. Results indicated that the effect of
salinity is dependent on vapor pressure deficit. Transpiration
rates were linearily related to vapor pressure deficits at low
salinity levels, but a curvilinear relationship was obtained at high
salinity levels.
An estimate of saturation deficit of the mesophyll cells was
determined by extrapolation of transpiration and vapor pressure defi-
cit relationships. These data indicate minimial increases in salt
concentrations in the stomatal cavities as indicated by small increases
in the mesophyll saturation deficits as the salinity of the root sub-
strate was increased.
Root permeability tests were conducted on plants subjects to
varying salinity and vapor pressure deficit levels. Results indicated
a significant reduction only at the highest salinity and vapor pressure
deficit levels.
1. INTRODUCTION
The term phreatophyte is derived from two Greek words meaning
"well plant". This group of plants includes many families which de-
pend on ground water or moisture in the capillary fringe for water.
The principal species of importance in Arizona is tainarisk (Tamarix
pentandra Pall). Phreatophyte communities occupy an estimated 17
million acres along flood plains and streainbanks in the southwest.
Evapotranspiration loss from these areas is approximately 25 million
acre feet annually, and is a major factor in the hydrologic cycles of
the arid southwest where 95 percent of the annual precipitation may
be lost through evapotranspiration and where water is in short supply
(Robinson, 1957). The substantial loss by evapotranspiration from
phreatophyte communities is of great interest to agriculturists, hydro-
logists, watershed managers, and metropolitan water planners involved
in water management.
Evapotranspiration processes are complex and depend on many
factors including vapor pressure gradient, net radiation, wind speed,
depth to water table, soil moisture, soil and water salinity, and
other soil and plant characteristics. Water use by tamarisk and
other phreatophytes has been extensively studied under specific cli-
matic and hydrologic conditions. However, measurements of evapotrans-
piration by phreatophytes, especially tamarisk, under different
hydrologic and climatic conditions have been difficult to obtain be-
cause the significant factors that control evapotranspiration are
1
2
difficult to integrate.
The extent to which water now utilized by tamarisk, which
has no known economic value, can be salvaged is not accurately known.
Although theoretical and empirical methods have been developed for
predicting water loss by tamarisk, none are generally applicable or
without significant sources of error when measurements are made under
varying climatic and hydrologic conditions.
Salinity has been shown to reduce transpiration rates of num-
erous plants. However, tamarisk exists under a wide range of saline
conditions and thrives under saline conditions that prevent growth of
other species. The ability of this plant to prosper under these
conditions may be due to an adaptation for exuding salt through salt
glands. The effect of this mechanism on transpiration rates under
high saline conditions is unknown and may be a significant factor
regulating water use.
The objectives of this study were: (1) to evaluate the
evapotranspiration tent as a method of measuring evapotranspiration
by tamarisk, (2) to determineevapotranspiration losses of tamarisk
in relation to different climatic and hydrologic conditions, and
(3) to determine the effect of salinity of the root medium on
transpiration rates of tamarisk.
2. REVIEW OF LITERATURE
2.1 Origin of Tamarisk
Water shortages in the arid southwest became apparent about
the time that tamarisk, commonly called saltcedar, spread throughout
the flood plains of the southwest. Robinson (1966) estimated that
this species presently occupies more than one million acres on reser-
voir flood plains and deltas, and continues to spread rapidly. The
rapid spread is due to prolific reproduction by seed and sprouting
under moist conditions and high temperatures.
Tamarisk was introduced as an ornamental and as a windbreak
early in the last century. It was sold in nurseries in New York as
early as 1823, and was spread as an ornamental through the entire
United States (Horton, 1964). Thornber (1916) advocated the use of
tainarisk for windbreaks and shade around dry-land homesteads in An-
zona. He also noted that tamanisk was beginning to grow along the
rivers of the state - perhaps the initiation of its spread in Arizona.
Tamarisk was originally thought to be French tamarisk (Tamanix
gallica), but recent studies by Horton (1964) indicated that French
tamarisk has been naturalized only in Texas and the Gulf of Mexico
area. The western tamarisk (Tmanix pentandra) appears to be similar
to a species growing in Asia from China to Mongolia and Turkestan.
However, Baum (1966) recently questioned the name of western tamarisk,
and the origin of tamarisk still concerns ecologists.
3
4
2.2 Anatomical Structure of Tamarjx
Compared with a typical woody dicot, the only "apparent" ana-
tomical variations in taniarisk are salt glands and sub-epidermal
cuticularization (Wilkinson, 1966). Salt glands are initiated in
the deciduous cladophylls (cylindrical leaf like branches) of the shoot
apex and in leaf primordia. They commonly occur on the abaxial epi-
dermis of scale-like leaves and very young stems. Mature glands
consist of eight cells derived from division of a single protoderm
cell surrounded by thick walled epidermal cells compared to relatively
thin walled epidermal cells surrounding the guard cells of the stomata
(Campbell and Strong, 1964).
Campbell and Strong observed an opening or pore space between
the cap cells of the salt gland. It was located in the center of the
gland between adjacent walls of the paired four-celled structures and
was traced down to the third cell below the cuticle layer. Decker
(1961) previously inferred from his observations of "salt whiskers,"
that additions to the whiskers were probably made at several places on
the external surfaces of the salt gland.
These glands secrete excess salts and provide a mechanism for
tamarisk to thrive under highly saline conditions. Apparently active
salt glands are not associated with vascular bundles but are primarily
desalting organs capable of reducing the salt content of leaf mesophyll
cells. However, the mechanism has not been isolated.
Scalelike leaves are approximately three mm in length. Cauline
leaves are eight to nine mm in length and are characterized by large
irregular epidermal cells and a cuticle about seven microns thick. The
5
vascular system Consists of a large median vascular bundle with two
smaller lateral bundles occasionally present (Wilkinson, 1966). The
guard cells are usually sunken, and the bases of the guard cells are
even with the base of the epidermis.
The cladophyll anatomy is similar to woody dicots except for
a well oriented palisade layer (Wilkinson, 1966). As the growing
season progresses, cladophylls thicken secondarily, due to a progress-
ive sub-epidermal disposition enveloping four or five layers of cells
which reach a maximum thickness of about 50 microns by late June.
Morphologically, tamarisk is similar to a mesophyte adapted to
a xeric ecosystem through (1) a deep root system, (2) small leaf area,
(3) sunken stomates and (4) the development of salt glands. However,
with access to ground water via a deep root system, evapotranspiration
from a stand of tamarisk may be very high compared to xerophytes.
2.3 Measurement of Evapotranspiration
The first quantitative measurements of transpiration were made
with potted plants by Stephen Hale prior to 1927 (Kramer and Kozlowski,
1960). Since then numerous theoretical and empirical methods have
been devised to quantify evapotranspiration, a term which combines
transpiration and evaporation from soil and plants. The main methods
can be divided into three general classes: climatological; aerodynamic;
and water budget methods.
2.31 Climatological Methods
Clitnatological methods are usually dependent on the assumption
that when water is nonlimiting the amount of evapotranspiration is
6
more dependent on the energy supply than on the type of vegetation.
The most widely accepted methods may be grouped under two categories:
(1) those based on air temperature, and (2) those based more directly
on components of the energy budget equation.
Thornthwaite (1948), as a result of studies of irrigation
projects in the western United States developed an empirical relation-
ship between potential evapotranspiration and mean monthly temperature
in degrees centigrade. Nixon et al. (1963) found poor correlation
between Thornthwaite's equation and the measured evapotranspiration
from irrigated alfalfa fields. They attributed extraneous variation
in Thornthwaite's equation to dependence on temperature rather than
on solar radiation. Conversely, Shakur (1964) found close agreement
in evapotranspiration rates for tamarisk from Thornthwaite's equation
and from large lysiineter tanks.
Blaney-Criddle (1950) developed an equation to estimate evapo-
transpiration which utilizes mean temperature and percentage of annual
daylight hours for the period of interest. An empirical coefficient
for a particular crop, season and site has been added to the equation
to take into account crop differences. Recent studies by Blaney etal.
(1961) have added refinements to the empirical coefficient. This
method has been used extensively due to its simplicity.
Penman (1948) proposed an approximate energy balance method
using simplifying assumptions and combined vapor flow and energy bal-
ance methods to estimate potential evapotranspiration. The energy
balance approach is potentially more accurate than mean temperature
methods, but the data are expensive to collect and the requirements
7
more stringent. Tanner and Pelton (1960) found estimates based on
the Penman method to be highly correlated with those obtained from
detailed energy balance measurements, although the absolute values of
the Penman estimates were much too small.
The energy balance approach is a method of accounting for
incoming and outgoing thermal energy, partitioning it into component
parts, and determining the amount of energy available for evapotrans-
piration. Tanner (1960) presented a detailed description of this
method. Advected heat from adjacent areas and extensive and expen-
sive equipment are disadvantages of this method.
2.32 Aerodynamic Method
In the aerodynamic method the turbulent transfer of atmospheric
moisture in the air layer above the vegetation or soil is estimated.
It involves theories of turbulent diffusion of water vapor and approp-
riate coefficients which are not agreed upon. The methods require
intensive measurements of the micro-climate above the evaporating
surface. An evaluation of these data by van der Bijl (1958) pointed out
the possible discrepencies of the method as well as some of the advan-
tages.
2.33 Water Budget Methods
Numerous methods have been developed for estimating evapotrans-
piration rates by measuring water losses directly. Measurement of
evapotranspiration rates of tamarisk stands using this method was
reviewed by Robinson (1966).
8
Convenient suuhivaries of methods available for measuring evapo-
transpiration have been given by Thornthwaite and Hare (1965) and the
American Society of Civil Engineers (1966). Other review articles
related to the preceding methods are available in Penman (1956) and
Van Wijk and de Vries (1954).
2.4 Measurement of Evapotranspiration Using the Evapotranspiration Tent
Recent adaptations of the 18th century bell-jar technique for
measuring evapotranspiration using ventilated systems are numerous.
Glover (1941) and Anderson et al. (1954) used small containers to
enclose a leaf, while Thomas and Hill's (1937) enclosure was a small
ventilated greenhouse.
Decker et al. (1962) were the first to develop a large inflat-
able plastic "tent" to measure evapotranspiration of large shrubs and
small trees. The tent was originally designed to determine comparative
rates of evapotranspiration between different cover types. The "Decker
tent" has been adopted and modified by others (Shachori et al., 1962;
1966; Lewis and Burgy, 1963; and Bowman, 1963), for research studies
of evapotranspiration phenomena.
Ventilated enclosures such as the evapotranspiration tent pro-
vide one of the most attractive approaches for evapotranspiration
measurement since the plant remains in its "natural" state. However,
the tent method has been criticized due to its interference with the
natural environment.
2.41 Enclosure Effect
Decker et al. (1962) surmised that enclosed plots would reduce
evapotranspiration rates, but would not yield exaggerated estimates of
9
comparative water loss by tamarisk. Assuming this, the reduction of
actual rates would minify rather than magnify differences between
cover types. Decker et al. concluded that "Although enclosure effect
could not create serious difficulty in the primary use for which the
technique was intended, analysis and evaluation of it would enable
one to compute rates for unenclosed plots and would thus extend the
usefulness of the technique." In several exploratory studies, Decker
found no completely satisfactory method of accurately measuring the
enclosure effect. Using potted tamarisk plants, they measured a 22
per cent reduction in transpiration rates for the enclosure as a first
approximation.
Lee (1966) severely criticized the tent as designed by Decker
etal. (1962) on the basis of enclosure effect. Lee's measurements
of enclosure effects using potted plants indicated a relative increase
in transpiration rates of two to 70 per cent in the tent with definite
strata and niches present. These results are opposite to those ob-
tained by Decker. However, Lee (1966) indicated that soil moisture
was not measured, but kept near field capacity. Small differences in
soil moisture under high atmospheric vapor pressure deficits and root
distribution could account for his large variations (Gardner, 1960).
He also ascertained that leaf stomata were closed on all plants sampled
during midday periods when the enclosure effect was measured. This
indicates perhaps that water loss was occurring only as cuticular
transpiration since the soil surface was sealed.
Lee (1966) concluded that:
(1) Absolute vaporization rates in the tent-enclosed space mayvary considerably from those in the open.
Expected differential enclosure effects rule out the useof the tent, with present design, to determine even therelative water consumption among cover types.
With proper design modifications, and simultaneous moni-toring of environmental parameters within and outside ofthe enclosure, a tent system might provide reasonablyaccurate estimates of water loss rates; but other tech-niques are usually more satisfactory and less expensive.
The tent technique's potential lies in the study of pro-cesses where it is desired to have a controlled or knownenvironment, but should not be used to obtain estimatesof actual or relative evapotranspiration from wildlands.
Shachori et al. (1962) described adaptation of the "Decker
tent" to measure evapotranspiration rates of maqui-shrub cover types
in Israel. Evaluation of enclosure effects indicated temperatures
were 1.00 to 1.5° C higher in the enclosure than outside and the
variation within the tent was less than 1.0° C. They also reported
a 2.5 per cent insignificant increase in transpiration rates inside
the tent by weighing potted plants.
Further evaluation of the enclosure effect by Shachori et al.
(1966) indicates a total possible positive error of 11.5 ± 9.5 per
cent. Seven ± 4.0 per cent was attributed to enclosure effect and
4.5 ± 4.0 per cent to humidity and air flow measurements.
2.5 Effect of Salinity on Transpiration Rates
Numerous investigators, including Arisz etal. (1951), Bern-
stein (1961) and Nieman (1965) have observed reduced growth and water
loss from crop plants with increased salinity in the root medium.
However, growth reductions have not been observed in tamarisk plants
growing in a very saline medium. Van Hylckama (1963) found decreased
growth and development of taniarisk plants to parallel a diminishing
10
11
use of water, even though water seemed freely available. He specu-
lated that decreases in the growth rate and development could be
due to: (1) a increase in plant density which may not be the optimum
density for plant growth and water use; (2) a decrease in the CO2 con-
tent of the air for growth and development due to the increase in
density; (3) and the effects of increased salinity of the ground
water in the tanks. He stated that the third possibility is admittedly
remote, since tamarisk is a highly salt tolerant plant. Further stud-
ies revealed that even tamarisk, which is a very salt tolerant plant,
grows, develops and transpires more water under non-saline conditions
(van Hyickama, 1966).
Lagerwerff and Eagle (1962), surmised that it is not logical
that the rate of water uptake, after it has been corrected for growth
reductions, should diminish with increased salinity. Their data indi-
cated that transpiration based on total leaf surface area steadily
decreased with decreased growth. However, when transpiration was
based on unshaded leaf area, the transpiration rate appeared to be
fairly independent of the growth stage of the plant and was mainly
influenced by the osmotic pressure of the root media. Eaton (1941)
and Lunin and Gallatin (1965) found transpiration rates to be inde-
pendent of growth rate. It appears that the salinity of the root
medium should be taken into account when estimates of transpiration
rates are made.
It is generally agreed that the osmotic pressure of the plant
increases as the osmotic pressure of the root medium increases (Janes,
1966; Bernstein, 1961, 1963; and Slatyer, 1961). However, authors
12
disagree on the mechanism of adjustment within the plant and the in-
hibition of plant growth and transpiration rates.
Bernstein (1961) proposed that osmotic subcellular units,
plastids or mitochondria, may not adjust to high osmotic pressures
despite the apparent capacity of the vacuole to do so. Or that
osmotic adjustment is performed at the expense of reduced growth. But,
he indicated that unpublished data by R. H. Nieman indicated an in-
crease in respiration (principal process by which mitochondria act-
ivity is evaluated) for some species.
Nieman (1965) showed that increased salinity suppressed the
rate of RNA and protein synthesis and cell enlargement. But all
three processes were prolonged so that the total amount of RA, pro-
tein, and cell enlargement was comparable to the levels that the
control plants reached at an earlier date. He concluded that his
results were consistant with the older ideas that salinity affects
growth by imposing a water stress, possibly resulting from decreased
permeability of the roots to water. Arisz et al. (1951), Eaton (1941)
and Hayward and Spurr (1943) also implied that decreased root perme-
ability is the mechanism by which salinity affects growth and water
loss.
3. THE RESEARCH AREA
3.1 Location of Field Study Sites
The study was conducted on two sites located above the San
Carlos Reservoir on the San Carlos Indian Reservation (Figure 3.100).
The Gila River site was located five miles east of the confluence
of the Gila and San Carlos Rivers, approximately 1000 feet from
the Gila River stream channel. The Salt Creek site was located
approximately four miles east of the Gila River site on Salt Creek,
1/8 mile north of the confluence of Salt Creek and the Gila River.
3.2 Climate
The research area is located in a semi-arid region (Thornth-
waite, 1948). Sellers (1960) indicated that the annual precipitation
varies from 8.0 to 24.0 inches at Globe, a town 35 miles west of the
study site ,and occurs mainly during summer and winter months (Figure
3.200). The average annual precipitation for the period 1894 to 1957
was 15.75 inches at Globe (Ibid, 1960) and the annual precipitation
for 1965 at the study site was 10.03 inches. The plotted annual
distribution of precipitation shows two maximum periods, the dominant
one in July and August, the other during the winter months. Summer
precipitation occurs when warm moist air from the Gulf of Mexico
flows over the mountains from the southeast. Winter precipitation
results from Pacific Ocean storms that move into Arizona from south-
em California.
13
.\S
AN
CA
RLO
S,
EX
IST
ING
HIG
HW
AY
S
EX
IST
ING
AC
CE
SS
RO
AD
SP
RO
PO
SE
D A
CC
ES
S R
OA
DS
BO
UN
DA
RY
OF
GIL
A R
IVE
R F
LOO
D P
LAIN
0 P
LOT
ST
UD
Y A
RE
AS
U.S
70
I-'
----
-TO
CO
OLI
DG
E D
AM
/
/.-
.-1,
-,j
c
N
0.
Figure 3.100 - Location of Cila River and Salt Creek study sites.
I Ii' P HOENI dSA
FF
OR
DT
UC
SO
N
I0.
5 0
I2
MIL
ES
II
I
1140
112°
110°
36°
34°
3.0
2.5
I .5
1.0
0.5
0.0
Globe (1894-1957)
Salt Creek (1965)
MONTHS
Figure 3.200 - Precipitation distribution at Globe, Arizona from
1894 to 1957 and at the Salt Creek site for 1965.
15
JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT OCT NOV. DEC.
16
The mean annual temperature for Globe is 62.4° F (I1id, 1960).
Mean monthly temperatures at Globe for the period of 1894 to 1957 and
at the Salt Creek and Gila River Study sites are shown in Figure (3.210).
3.3 Vegetation
The dominant vegetation cover on the two study sites is
tamarisk (Tamarix pentandra Pall.). Tamarisk densities on the Salt
Creek and Gila River sites are approximately 40 and 70 per cent,
respectively. Major understory vegetation consists of spangle-top
(Leptochloa sp.), brome (Bromus sp.), needle grama (Bouteloua
aristidoides H.B.K.), and pursiane (Portulaca sp.). Dominant species
growing on the slopes above the tamarisk stands includes mesquite
(Prosopis -juliflora var. velutina Woot.) and creosotebush (Larrea
tridentata (D.C.) Coy.).
Most plants on the area begin growth in late April or May
and lose their leaves after the first frost. The boundary between
the tamarisk stands and the mesquite and creosotebush is distinct
and is delineated by the high water mark. Most tamarisk plants
growing at the upper edges of the stands survive only on the annual
precipitation, and are stunted and widely scattered.
3.4 Soil
The depth of alluvium on the Salt Creek site varies from 1 to
5 feet, is a silt loam to a sandy loam, and is underlain by sand and
gravel (Baldwin, 1965). The p1-I at the 0 to 48-inch depth is slightly
basic varying from 7.0 to 8.5 and the soils are generally non-saline.
Alluvium deposits on the Gila River site are deeper ranging
100
90
U-0
80
70
60 '0;"I'.I.,
z501ii
40
17
X----X Salt Creek (1965)Gila River (1965)
g \ 0 0 Globe (1894-1957)
S
IIII x
ci :I
. '
0JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT OCT NOV. DEC.
MONTHSFigure 3.210 - Mean monthly air temperature at Globe from 1894 to
1957 and at the Salt Creek and Gila River sites for
1965.
18
from 8 to 30 feet. The silt loam top soils are also underlain by
sand and gravel. The pH at the 0 to 48-inch depth is slightly basic
and the soluble salts of the 0 to 12-inch depth range from approxi-
mately 1000 ppm at the upper edge of the tamarisk stand to 33,000
ppm near the stream channel.
4. METHODS AND PROCEDURES
4.1 Evapotranspiration Tent Technique
The determination of evapotranspiration rates for tamarisk
plants under different climatic and hydrologic conditions required
a method which was portable and accurate. The evapotranspiration
tent technique developed by Decker et al. (1962) which was an
adaptation of the method described by Thomas and Hill (1937) was
selected for this purpose.
4.2 Theory
The evapotranspiration tent technique requires that a plant
be enclosed with a ventilated, bottomless transparent tent. The
absolute humidity difference between the inlet and outlet, as
determined by wet and dry bulb hygrometry, is multiplied by the
ventilation rate and cross-sectional area of the outlet is an
estimate of the evapotranspiration rate of the enclosed plots. The
following equation was used to calculate the evapotranspiration rate:
H 217 e/T [4. 20]
where:
3= absolute humidity (gm/rn );
e = vapor pressure (mb);
and T = temperature (°A)
19
20
3 3 33Ha was converted from gm/rn to gm/ft by multiplying by 0.02832 tn /ft
and 1.33322 mb/mm to give:
H 8.193 e/t [4.21]
where:
and 8.193
3Ha = absolute humidity (gm/ft );
e = vapor pressure (mmllg);
T = temperature (°A);
= constant (gm-°A/ft3-mnillg).
The Vapor pressure in equation [4.21] is computed from the
psychrometic formula:
e = e' - 0.00066 B (t-t') (1.0 + 0.00115 t')
[4.22]
where:
e = vapor pressure (imi11g);
et = saturation vapor pressure at temperature (t1)
(mrnHg)
B = barometric pressure (mrnHg)
t = dry bulb temperature (°C)
t'= wet bulb temperature (°C)
Combining equations [4.21] and [4.22], the difference in absolute
humidity (Ha) between incoming and outgoing air is obtained by
the following equation:
21
Ha = 8.193 e - 0.00066 B (t - t ') (1.0t +273.15 ° 0 0
0
+ 0.00115 t ') -0
8.193 et - 0.00066B (t - t') (1.0t + 273.15
1
+ 0.00115 t ')1 [4.23]
Subscripts i and o in equation [4.23] refer to inlet and outlet
respectively. Values for e.' and e' may be obtained from standard
tables relating saturated vapor pressure to temperature, t', or
programmed into a computer program. The rate of evapotranspiration in
gm/mm is determined by:
ET = AVH [4.24]a
where:
A = cross-sectional area of outlet (ft2);
V = velocity of air at outlet (ft./min.)
Ha = absolute humidity (gm/ft.3)
4.3 Evapotranspiration Tent
The evapotranspiration tent was a frameless and bottomless
vertical cylinder 11 feet in diameter and 12 feet in height construct-
ed from four-mil transparent polyvinyl plastic with an eight-inch
inlet and outlet as shown in Figure (4300). The component parts of the
tent were chemically welded together with TBC technical grade cyclohex-
anone.
8 OUTLET
8" INLET
Figure 4.300 - The cker "evapotranspiratiOn tentT1. Air is bothintroduced and removed from one side which causes
poor air circulation patterns and heat trapping at
the upper levels of the tent.
22
4.4 Ventilation Assembly
The ventilating blower was an eight-inch squirrel cage fan
driven by a four cycle gasoline engine. Air introduced at speeds
of 1400 to 1900 feet per minute through inlet resulted in an aver-
age air velocity of 6.2 to 8.4-feet per minute inside the tent.
Air speeds were measured at the center of the eight-inch outlet
with a Florite indicating anemometer.
4.5 Humidity Sampling Equipment
Absolute humidity was determined by the wet and dry bulb
hygrometry method at the inlet and outlet. Chemical mercury-in-
glass thermometers with a range of 0 to 120° F graduated by 0.2° F
intervals were inserted into a rubber stopper and the sensing ele-
ment was placed at the center of the eight-inch metal seive which
provided support and shielded the thermometers from direct radiation.
Thermometers were placed parallel in a vertical plane to avoid
errors in measurement of moisture contents of the air due to the
presence of the wet bulb. The type of placement also reduced turbur-
lence effect of one bulb on the other.
4.6 Evapotranspiration Measurement
Evapotranspiration measurements were made at upper, middle,
and lower locations on the Gila River site (Figure 4.600). The
three areas are different in depth to water table, crown density,
and height of vegetation as shown in Table 4.600.
23
1'200'
200'
200'
1
Figure 4.600 - Diagramatic sketch of Gila River site delineating
the three study areas.
24
k 800'
UPPER AREA
MIDDLE AREA
LOWER AREA
Table 4.600. Variations of depth to water table, crown density, andheight of vegetation on the upper, middle, and lowerstudy areas at the Gila River site.
CrownStudy Area Depth to Water Table (ft.) Density Vegetation (Ft.)
Evapotranspiration measurements were made on seven randomly
selected trees in each area. Sampling was conducted during eight 4-
hour periods each week during June, July and August and each month
during the remainder of 1965. Two of the three areas could be
sampled simultaneously, and the middle area was chosen as the control
as shown in Table 4.610.
* X denotes areas sampled
25
Table 4.610 - Sampling procedure for evapotranspiration rates on theGila River site.
Week Upper
Area*LowerMiddle
1 x x
2 X x
3 x x
4 x x
5 x x
6 x x
7 x x
8 X x
Upper 20 40 8
Middle 14 70 10
Lower 3 90 12
26
Four wet and four dry bulb readings were taken at both the inlet
and outlet at 15-minute intervals during the four-hour sampling period.
Wind velocity was measured at the beginning and end of the temperature
measurement period. Temperature and velocity measurements were averag-
ed for each 15 minute interval and evapotranspiration rates computed
from these average readings. Evapotranspiration rates, computed by
equation [4.24]in grams per minute were converted to inches per month
assiirrring a 10 hour day and 30 days per month for comparison with results
of methods previously described.
4.7 Evaluation of Tent Enclosure Effect
Use of the evapotranspiration tent to sample evapotranspiration
rates has been criticized because of the interference with the natural
environment, a phenomena commonly referred to as the enclosure effect.
Decker et al. (1962) assumed enclosure effects would not create serious
difficulty if only relative water loss rates among various cover types
were to be determined. However, this is based on a dubious assumption
that enclosure effects are proportional for each species and plant
size. An investigation of the microclimate of the tent was performed
to determine its exciosure effects on evapotranspiration rates.
4.71 Air Temperature of the Tent Exclosure
Air temperatures of an enclosed and unenclosed plant were
simultaneously determined at three levels in the crown canopy using
shielded 24-guage copper-constantan thermocouples. The thermocouple
emf output was measured with a Honeywell pointerlite potentiometer at
15 minute intervals, the same interval used for transpiration readings.
27
Measurements were conducted on different size plants to determine the
relationship between the constant tent volume and enclosed plant
volume.
4.72 Net and Total Radiation of the Tent Enclosures
Simultaneous measurements of net and total radiation of an
unenclosed and enclosed plant were determined above the crown canopy
using two all-wave radiometers (Suomi al., 1954) as modified by
Goodell (1962). The tent was alternately placed over each radiometer
using the unenclosed radiometer as the control. These readings were
also taken at fifteen minute intervals.
4.73 Sapflow Velocity of Enclosed Tamarisk Plants
Concurrent measurements of sapflow velocities of an enclosed
and unenclosed plant were made by a heat pulse (sap velocity) indica-
or (Swanson, 1962) to determine comparative rates of sap movements as
affected by the evapotranspiration tent. Measurements were made at
thirty minute intervals on typically "wet" and "dry" tamarisk sites.
Wet sites were located near the perennial Gila River a few feet above
the water table. Dry sites were located along the ephemeral Salt
Creek where the water table depth was approximately 30 feet.
4.8 Measurement of Climatic and Hydrologic Data
In order to: (1) characterize the study area, (2) interpret
evapotranspiration data and (3) develop a prediction equation for
evapotranspiration of tamarisk, the following supplementary measure-
ments were made: soil moisture, depth to water table, wind speed and
direction, pan evaporation, air temperature and relative humidity,
precipitation, and net radiation.
4.81 Soil Moisture
Soil water content above the water table was measured once
each week during June, July, and August and once each month during
the remainder of the year with a neutron scattering device manufac-
tured by Troxler Laboratories, Inc. One-minute readings were made
at six inch increments and counts per minute were expressed as in-
ches of water per foot of soil using a factory calibration curve.
The 3 mc source was Ra-Be and was located at the end of the probe.
Shelby seamless, 1.75 in. i.d., 2.00 in. o.d.,aluminum
access tubes were installed to 24 feet on the Salt Creek site and
12 feet on the Gila River site. Drilling was done inside the tube
to avoid large cavities around the tube. The bottoms were sealed
with a rubber stopper and roofing compound, and the tops closed
with rubber stopper and an empty can to prevent the entrance of
moisture.
The probe and scaler used in soil moisture determinations
were calibrated at Troxler Laboratories, Inc. Field calibration
was not attempted due to large variation of the soils in the area.
4.82 Depth to Water Table
Water table depths were determined in wells (at the Salt
Creek site) with Steven Type F water level recorders. Weekly
measurements of the water table depth were made at six wells on
the Gila River site with an electrical resistance meter.
28
4.83 Wind Speed and Direction
At the Salt Creek site wind speed was measured with 3-cup
anemometers placed at 2, 10, and 18 feet from the soil surface and
continuously recorded on event channels of an Esterline Angus
Analog-Event Recorder, Model No. A609. Wind direction was measured
by a wind direction transmitter with eight electrical contacts for
recording on the event recorder.
4.84 Pan Evaporation
Evaporation was measured weekly from a Class A Weather
Bureau evaporation pan, stilling well and hook gage at the Salt
Creek site. Evaporation was summarized in inches per week and
month.
4.85 Air Temperature and Relative Humidity
Air temperature and relative humidity were continuously
recorded on U.S. Weather Bureau hygrotherinographs. The instru-
ments were checked for accuracy at weekly intervals with a maximum-
minimum thermometer and a Friez psychrometer and adjusted when
necessary.
4.86 Precipitation
A recording rain gage and six standard 8-inch gages were
located on the Salt Creek site. Precipitation at the Gila River
site was measured by a recording rain gage only.
29
30
4.87 Net Radiation
Net radiation was measured with a Thornthwaite miniature net
radiometer, Model No. 605, with a typical output of 2,986 my per
cal. per sq. cm. per mm. The net radiometer output was amplified
with a Burr-Brown Model 1503 operational direct current amplifier
with a gain of 20,000 and recorded on the 0-i ma range analog
channel, of the Esterline Angus recorder.
4.9 Laboratory Study of Effect of Salinity on Transpiration Rates
In the laboratory experiments the effect of salinity of the
root medium on transpiration rates of tamarisk cuttings from the
field study area was evaluated. Four tests were conducted with
vapor pressures deficits of 37, 42, 87, and 112 mui Hg. The cuttings
were approximately 3/8 inches in diameter, 3 inches in length, and
were rooted in vermiculite flats. The flats were placed in a
Sherer Controlled Environment Lab, Model CEL 37-14 on a 12 hour
light and dark cycle (90° F in the light, 72° F in the dark). The
growth chamber beds were adjusted until the cuttings received
approximately 2600 foot candles of light from the flourescent and
incandescent lights.
4.91 Nutrient Solution and Environmental Control
After the plants were approximately 15 inches in height,
24 uniform plants were removed from the vermiculite, washed, and
transferred to twenty four 6-liter plastic containers. Each con-
tainer had been filled with 5-liters of a 1-molar aerated nutrient
solution of the following composition: 15 ml - Ca (NO3)2
31
10 ml - KNO3; 10 ml - 1(112 PO4; and 10 ml - MgSO. Five milliliters
of micro-nutrient solution composed of 2.5g of H3BO; 1.5 g of
NnC1 .41120; 0.22 g of ZnSO4 .7H0; 0.05 g of CuC1 .2H0; and
0.05 g of MoO3 per liter of water was added to the above solution.
The plants were transferred to a second controlled environmental
chamber with humidity controls and allowed to adjust to the desired
vapor pressure for a period of 7 days.
Vapor pressure was maintained at the prescribed level by
programming a wet and dry bulb ARCS thermostatic control system on
the environment chamber. A Dryomatic Dry Conditioner dehumidifier
was connected to the chamber to maintain the chamber at low relative
humidites at prevailing high temperatures and to increase the accur-
acy of the vapor pressure control system.
4.92 Salinity Treatments
Six replications of four treatments corresponding to 0.30
(control with nutrient solution), 4.0, 8.0 and 12.0 atmospheres
osmotic pressure or 351, 5612, 11,224 and 16,836 ppm of NaCl, res-
pectively, were used in each test. On the 8th day, 96 meg. of NaCl
(4 atmospheres) per liter was added to the plants of the 12.0
atmosphere treatment. On the 9th day, 96 meg. of Naci per liter
was added to the plants of both the 12.0 and 8.0 atmosphere treat-
ments. On the 10th day, salinity treatments were completed by an
addition of 96 meg. of Maci per liter to all treatment except the
control. Two milliliter samples were analyzed on an Advanced
Osmometer to determine if all treatments were at their appropriate
level.
32
4.93 Transpiration Measurements
All containers were covered with 1/4-inch sheet of plywood
and sealed with masking tape. The plywood sheet was drilled with
holes for the plant support, aeration hose, resistance meter and
the addition of water (Figure 4.930). Plants were inserted into
the top, wrapped with strips of paper toweling around the stem and
sealed to the plywood with silicon rubber to prevent evaporation.
Plants were supported with dowel pins and aeration lines were
connected with plastic tees and rubber tubing. Holes for the
electrical resistance meter and addition of water were plugged with
rubber stoppers except during periods of measurement. The electrical
resistance meter,which was used to determine a reference level at the
beginning of transpiration measurements ,consisted of two 45-volt
batteries, a 68K ohm resistor connected to a flash light bulb, and two
metal tips. Distilled water was added using a pipette with 0.2 ml
graduations until the reference level was reached. The amount of water
added was considered to be equivalent to transpiration, assuming water
used for photosynthesis was negligible. Measurements were initiated 24-
hours after salt additions had been completed for all treatments and
were continued at 24-hour intervals for 11 days.
Plants were continuously aerated at a constant rate by two
small aquarium pumps attached to an aeration line. Small glass tubing
was drawn to a specified diameter and connected with plastic tubing to
a tee inserted in the plywood top. The small diameter glass tubing
was used to obtain a constant rate of aeration for all plants.
Aeration hose openingWater addition openingPlant openingPlant support openingResistance meter opening
Figure 4.930 - Diagram of plywood cover used toseal container and support plantand equipment for measuringtranspiration.
33
34
4.94 Root Permeability Studies
To determine the effect of salinity treatments on root permea-
bility to water, the plants were removed from the growth chamber at the
end of the transpiration measurements, roots and stems excised from the
tops and placed in a root permeability chamber.
This chamber is a modified pressure plate apparatus which was
constructed to accommodate plant roots as shown in Figure(4.940). Plant
stems were inserted in rubber tubing and clamped with a hose clamp to
prevent air leakage. The plant roots were submerged in distilled water
and pressure was applied to the contents of the container. Ten milli-
liter pipettes were inverted and inserted into the rubber tubing to
measure exudation. Permeability rates expressed as milliliters per
hour were determined for three hour periods at a pressure of two
atmospheres or 29.4pounds per square inch.
A
7/16 Tee bolts/
Pressuregauge
eli
Openings for_'plant stems
Tee bolts for clampingtop to chamber
if
6/16 Opening for plant in l rubber tubinginserted in I opening
7/16Steeltop
5/16Steel flange
7/16 Steelcylinder
3/4Plexi-glass
5/16 Steel flange
Pie xi-glass
Figure 4.940 - Root permeability chamber. (A) Top view oftop plate. (B) Sectioned side view ofchamber.
35
5. RESULTS AND DISCUSSION
It became apparent at the beginning of this study that an
artificial microclimate was created by the evapotranspiration tent
designed by Decker et al. (1962). Mesquite and tamarisk trees were
defoliated after being enclosed for an eight-hour period on May 18
1964, and June 16, 1964, respectively. This indicated a serious
enclosure effect, and the internal microclimate in the tent was there-
fore evaluated. Design modifications of the tent based on the
evaluation were made before tamarisk evapotranspiration rates were
measured under different hydrologic and climatic conditions.
5.1 Evaluation of Enclosure Effect
Evidence for a diversity of plant transpiration responses to
environmental variation is numerous (Daubenniire, 1959; Kozlowski,
1960). Contrasting root, anatomical, and physiological systems of
major plant groups such as xerophytes, mesophytes, and halophytes
contribute to the variations in responses to environmental fluctua-
tions. Variation within a group or species is not accurately known
due to phenological variation. For these reasons, a physical rather
than biological evaluation of the tent enclosure effect was made.
36
5.11 Effect on Radiation Exchange
The microclimate surrounding a plant, and the sources of
energy for biological processes, surface heating, and vaporization
are dependent on the magnitude of the various components of the
energy balance equation presented in simplified form as follows
(Rose, 1966):
R5 (1-a) =RL+G+H+LE[5.1101
where:
R5 = flux density of total short-wave radiation;
a = albedo or reflection coefficient of plant or groundsurface;
RL = net flux density long-wave radiation emitted by thesurface, the difference between that emitted and absorb-ed;
G = heat flux density into the soil;
H = sensible heat flux density into the atmosphere;
L = latent heat of vaporization of water; and
E = evaporation rate.
The difference between the incoming and outgoing components
of equation [5.1101 is the net radiation flux which is:
RN = R (1-a) - RL
Combining equations [5.1101 and [5.1111, RN is equivalent to:
37
RN = G + H + LE [5.1121
38
A measure of the enclosure effect on transpiration rates may
be demonstrated by measurement of RN of equation [5.112]. Total and
net radiation measurements were conducted over enclosed and unenclosed
tamarisk plants using two all-wave radiometers (Soumi et al., 1954),
as modified by Goodell (1962), which have an accuracy of ± 4 per cent.
Total and net radiation over a tamarisk canopy were found to be re-
duced 10.7 and 2.1 per cent, respectively, in the tent (Figure 5.110).
Measurements over bare soil indicated a reduction in incoming and net
radiation of 19.5 and 17.8 per cent, respectively, inside the tent
(Figure 5.111). The differential reduction can be attributed to the
difference in albedo of tamarisk and bare soil and the distribution
of energy in the tent.
According to Businger (1963), the "greenhouse effect" (increase
in temperature by trapping of long-wave radiation) is of minor impor-
tance compared to the increase in temperature resulting from a lack
of ventilation. He showed that 22 per cent of the temperature increase
in a glasshouse could be attributed to the "greenhouse effect", while
78 per cent was ascribed to the lack of ventilation.
Net radiation was theoretically compared under completely
transparent and 4-mu polyvinyl plastic, used for the tent, having
transmjssjtjvjtjes of 0.90 and 0.25 for short and longwave radiation,
respectively, by the following equation:
RN = R (1-a) + Ra (1-B) - Rb [5.113]
NETRADIATION
0171 1 I I I I I
10 II $2 $3 14 $5 $6 $7
TIME (Hours after Midnight)
Figure 5.110 - All-wave incoming and net radiation of anenclosed and unenclosed tamarisk plot.The mean reduction in incoming and netradiation was 10.6 and 3.1 per centrespectively.
39
3.0 TENTOPEN
/ '.. INCOMING/ RADIATION/25
-.-.. NN./ \ \/z
2.0 -.---
2.5-
2.0
0.5
TENTOPEN
INCOMING RADIATION
NET RADIATION
TIME (Hours after Midnight)
Figure 5.111 - All-wave incoming and net radiation of anenclosed and unenclosed bare soil plot.Mean incoming and net radiation was reduc-ed 19.5 and 17.8 per cent respectively.
.7
I I I I I I I
0 830 840 850 900 910 920 930 940 950
40
where:
RN = net radiation (ly/min);
R5 scattered and direct beam solar radiation (ly/min);
Ra = long-wave radiation from the atmosphere (ly/min);
Rb = long-wave back radiation (ly/min);
a albedo for solar radiation assumed 0.15 for plantcover;
B = 1 -a
a = absorptivity (s); and
= .05;
Ra = aT4( a + b %I);
4= EaT ; and
a
= 8.14 x 10-11 ly/min - OK4
It was assumed that:
= 1.20 ly/min;
= Ta = 3l0°K; and
e = 9 mb.
where:
T5 = temperature of ambient air (°K);
Ta = temperature of crown canopy (°K); and
e = vapor pressure (mb).
The assumed values are similar to those measured in the field.
A climatological estimate of Ra was made using Brunt's equation
(Brunt, 1932) with constants of a = 0.605 and b = 0.048 as follows:
41
Ra = (8.14 x l011) X (310)4 (0.605 + 0.048 9)
Ra = 0.563 ly/min.
Lee (1966) computed Ra using Swinbank's (1963) equation which
gave a value of 0.653 ly/min. Sellers (1965) pointed out that Swin-
bank's equation does not yield favorable results in the southwestern
United States. He indicated that the failure is due to a singular
dependence on temperature.
Net radiation for a completely transparent tent from equation
For a plastic cover having transmissitivities of 0.90 and
0.25 for short and long-wave radiation, respectively, two sources of
incoming long-wave radiation are present, the atmosphere and the
plastic, and Ra becomes:
Ra = 0.25 (oT54 (a + b ) + [5.114]
where:
Ev = emmissivity of plastic layer and;
T = temperature of plastic layer.
42
[5.113] for R5 = 1.20 ly/min is:
RN = 1.20 (1.00 - 0.15) + 0.563 (1.0 - 0.05) -
(0.95) (8.14 x 10I1) (310)4
RN = 1.020 + 0.534 - 0.714
RN = 0.840 ly/min.
RN then becomes:
RN = 0.90 R (1-a) + Ra + 0.75aT4 - 0.95cT4
RN 0.90 R5 (1-a) + Ra - 02OGTv4a
RN + 0.90 (1.020) + 0.25 (0.563) - (0.20) (0.714)
RN = 0.916 ly/min.
The "greenhouse effect" is the difference between 0.916 and 0.840
ly/min or 0.076 ly/min. Net radiation is theoretically shown to
increase inside the tent having transmissitivities of 0.90 and 0.25
for short and long-wave radiation. Figure (5.110) indicates that
net radiation inside the tent is greater than in the opening in the
late afternoon. This increase can be attributed to the increase in
incoming long-wave radiation from the tent due to an increase in tem-
perature as shown in equation [5.114]. Lee (1966) indicated that net
radiation in the tent varied by ± 15 per cent from measurements in the
open during midday periods. The magnitude of variation between net
radiation in the tent and open will depend on temperature and the
short-wave radiation reduction. As temperature increases, the contri-
bution of long-wave radiation to incoming radiation increases. The
reduction of net short-wave radiation is dependent on the age and
cleanliness of the polyvinyl tent material. The increase in tempera-
ture associated with the long-wave blocking effect is shown in the
following computations:
Volume of air in tent = 2.55 x i0 cm3
Air replaced in tent 1.72 times/mm
43
Effective volume/minute 2.55 x cm3 x l.72/mjn
4.386 x cm3/min
Specific heat of air = 0.24 cal/gm/°c or 2.64 x
cal/cm3/°c
The energy available is 0.076 ly/minute received on a surface
of 88,288 cm2 or 6710 calories per minute. Assuming this energy is
used for sensible heat, the calories needed to raise and maintain the
temperature of the tent 10 C above inlet temperature is:
4.386 x lO cm3/min x 2.64 l0 cal/cm3/°C =
11.58 x l0 cal/°C-min;
and the temperature increase in the tent is:
6.710 x l0 cal/mm 1.158 x l0 cal/°C - mm =
0.58°C = 1.04° F
At a temperature of 45° C (113° F) the blocking effect is
equivalent to 0.186 ly/min which would increase the temperature of
the enclosure 1.42° C (3.55° F).
5.12 Effect of Ventilation Rate
If 0.076 ly/min is assumed to be the sensible heat flux due
to ventilation, the increase in temperature due to lack of ventilation
of the enclosure can be shown by Busingerts equation (Businger, 1963).
44
45
ga-H = C . V S (t - t0) [5.121]yen aiw
where:
g-}i = sensible heat flux retained in tent due to in-yen adequate ventilation (ly/min);
Cai = volumetric heat capacity (cal/cm3 - °C);
V = volume of tent (cm3);
A = wall surface (cm2);
S = number of times fresh air volume replaced perminute (min-);
= air temperature inside;
to = air temperature outside; and
(t1-t0) = mean temperature difference between inside andoutside air (°C)
Then:
.076 ly/min = (2.64 x lO cal/cm3-°C) 2.55 x 107cm3 (1.72 min-)(t-t0)4.09 x 105cm2
.076 ly/niin = .028 cal/cin2-min - °C (t-t0);
therefore:
(t1-t0) = 2.71 °C or 4.87°F.
If the sensible heat flux due to a lack of adequate ventilation is
0.186 ly/min, the temperature increase is 6.64°C or 11.75°F at 45°C
(113.0°F). These calculations indicate that the reduced ventilation
46
rate of the enclosure (4.87°F) causes a more significant internal
energy increase than the "greenhouse effect" (1.04°F).
The calculations are based on isothermal conditions which
usually do not occur. The long-wave "greenhouse effectt' under non-
isothermal conditions may be obtained by the expression:
R = [R (1-B) - RL}. - [Ra (1 - RL]o [5.122]g a
where:
Rg = long-wave "greenhouse effect"'
= inside enclosure; and
0 = outside enclosure.
Then:
Rg = [0.25 T54 (a + b e) + CvGTv4 _ETa4] i
- [T4 (a + b e) -EJT4]
R = -0.75 T 4 (a + b e) + EvTv4 + T -EaTg a0 ai
[5.123]
Equation [5.123] indicates that the magnitude of Rg under non-
isothermal conditions is dependent on the temperature of the tent (Tv)
and the crown canopy (T ), which are opposite in sign. On a cleara1
day, canopy temperature should exceed the tent plastic temperature
(T), and the value of Rg computed for isothermal conditions is the
upper limit of the blocking effect. On a cloudy day or during low
47
radiation intensities, T will be reduced inside the enclosure, anda1
R will increase as a function of the difference: e T -scyTg vv a1
5.13 Ventilation Rates and Air Temperatures of the Decker Evapo-transpiration Tent.
Air introduced by means of portable gasoline blowers at speeds
of 1400 to 1900 ft/mm through an eight-inch inlet resulted in an
average air velocity of six to eight ft/mm through the tent. Air
movement inside the enclosure was traced with a jeweled anemometer
(activation velocity of 1.76 ft/mm). The anemometer was activated
only near the inlet, outlet, and in the area of the flow pattern as
shown in Figure (5.140). Measurements of air temperatures inside the
enclosure indicated definite areas of increased temperatures which
were associated with low air velocities. The maximum increase was
25.0°F above ambient conditions.
5.15 Heat Pulse Velocities
Decker and Skau (1964) reported a good correlation for coni-
ferous species between heat pulse velocity (sap flow velocity) and
transpiration rates determined with the tent technique. Skau and
Swanson (1963) showed that heat pulse velocity is closely correlated
with water forced through stem sections. Field experiments conducted
by these authors indicated a close response between heat pulse velo-
city and environmental variables such as shading, leaf wetting,
irrigation, and natural soil drying.
8" OUTLET
8" INLET
STILL AIR POCKET
WARM AIR RISING
'r r I t I ii
STILL AIR POCKET
Figure 5.140 - Air circulation pattern of "DeckerEvapotranspiration Tent" showingheating trapping and still air pockets.
49
(1962) instrument was used to detect the nature of
the enclosure effect on the taniarisk plants. According to Swanson,
this technique should be valid for obtaining an index of sap flow
rates in tamarisk.
Measurements made on tamarisk plants on Salt Creek where the
water table depth was 30 feet indicated a significant increase in
sap flow velocities inside the enclosure compared to plants outside
(Figure 5.150). Similar measurements on the Gila River site where
the water table depth was three feet also indicated a significant
increase in sap flow velocities inside the tent (Figure 5.151), but
the enclosure effect was less pronounced on the moist site.
5.2 Tent Modifications
These data indicated that the evapotranspiration tent as
designed by Decker et al. (1962) was inadequate for measuring evapo-
transpiration rates of taniarisk plants because of enclosure effects.
Changes in design were undertaken to reduce the temperature buildup
inside the tent. No attempt was made to reduce the radiation "green-
house effect" since it is relatively small compared to the effect of
reduced ventilation rates. Also, the difficulty of keeping the tent
material clean and dry in the field prevents attainment of a natural
radiation exchange. Design changes to eliminate heat trapping in
the upper portion of the tent (Figure 5.200). Air temperature was
measured and flow patterns were determined with smoke bombs. Figure
5
4
I'
i
'I ' / it /I ,' I.1
50
I
2NUMBERNUMBER
'7TENT OVER TENT OVER
l6 PLANT 'PLANTI NO. I I NO. I
5
'4
E° 2
II I J I I
0 5 7 9 II 13 5 7 19 21 23
TIME (Hours after Midnight)
Figure 5.150 - Heat pulse velocities of an enclosed andunenclosed tamarisk plant growing on anarea in which the water table depth was30-feet.
18
7
I6
5
-c
EC-)
>13H0o 12-JuJ
>11uJ(I)-Jj103-
Hw
19-
8
NUMBER 2NUMBER 37
6
5
' - -
TENT OVERI PLANT
NO.2
' I$ I
'I'I
TENT OVERI PLANTI NO.3
I /\ /N.±/ \
/ I\ \\_
'V
51
I I I I I 1 I
8 9 0 II 12 13 14 5 6 7 18
TIME (Hours after Midnight)
Figure 5.151 - Heat pulse velocities of an enclosed and unenclosedtamarisk plant growing on an area in which the watertable depth was three feet.
8" INLETi
STILL AIR POCKET4'
8" OUTLET
Figure 5.200 Air circulation of the first University of
Arizona evapotranspiration tent. Note the
still air pockets.
52
53
(5.201) indicates that the temperature buildup was still present with
this design. Tests conducted to determine air flow patterns demon-
strated a pattern similar to that in Figure (5.200).
In a second design more desirable air flow patterns and venti-
lation rates were achieved to reduce the long-wave blocking effect
(Figure 5.202). Figure (5.203) shows the theoretical air flow pat-
terns of the triple inlet tent. Smoke bombs placed at the inlet
demonstrated air flow patterns similar to those shown in Figure (5.203).
Air was introduced by means of a squirrel cage blower through
three ten-inch inlets located at heights of two, six, and nine feet
above the ground surface. A square metal extension (1.2 ft2 in area)
conducted air from the blower to the upper inlets. The blower was
run by a three-quarter horsepower electric motor. This ventilating
assembly had a capacity of 2900 ft3/min compared to the blower of
Decker et al. (1962) which had a capacity of 547 ft3/min. Polyvinyl
perforated sleeves extending the height of the tent were attached at
the inlet and outlet to obtain a more uniform air flow pattern at all
heights.
5.21 Enclosure Effect of the Triple-Inlet Tent
Air was replaced in the tent 3.22 times per minute, which
theoretically reduces the temperature increase due to inadequate
ventilation from 4.87 to 2.06°F at an ambient temperature of 98.6°F.
At an ambient temperature of 113°F, the reduction in increased air
temperature due to the increased ventilation rates was from 11.75°F
130
70 4 5 6 8 tO 4 18 2022
TIME (Hours after Midnight)
Figure 5.201 - Average temperature at three heights in thefirst University of Arizona evapotranspira-tion tent and an adjacent area.
TENTOPEN
' \ Ii\ v
54
90
I'I' ,.I I
I
Is' /''-I-I
80 I/I
/I/
70 /
60
50
120
ito
tOO
I.0
INC
H D
IAM
ET
ER
INLE
T
PO
LYV
INA
L IN
LET
EX
TE
NS
ION
SLE
EV
ES
ME
TA
L E
XT
EN
SIO
N
SQ
UIR
RE
L C
AG
E B
LOW
ER
INLE
T H
UM
IDIT
YT
HE
RM
OM
ET
ER
S
PE
RF
OR
AT
ED
PO
LYV
INA
LC
UR
TA
INS
PO
LYV
INA
L O
UT
LET
EX
TE
NS
ION
SLE
EV
E
OU
TLE
T M
ET
AL
CY
LIN
DE
RO
UT
LET
HU
MID
ITY
TH
ER
MO
ME
TE
RS
18 IN
CH
DIA
ME
TE
RO
UT
LET
4.
Figure 5.202 - Diagram of second University of Arizona triple-inlet evapotranspiration tent.
10" INLET
PER FO RAT EDINLET SLEEVE
loll INLET
0" INLET
i-I
Figure 5.203 - Air circulation pattern of the secondUniversity of Arizona triple-inletevapotranspiration tent. The stillair pockets were eliminated.
THEORETICAL AIR FLOW PATTERN
PERFORATEDOUTLET SLEEVE
8" OUTLET
56
57
to 6.40°F. An increase in air temperature of 6.40°F inside the
enclosure at an ambient temperature and relative humidity of 113°F
and 29 per cent, respectively, would cause an increase in the vapor
pressure deficit of 6.66 mm Hg, assuming leaf and air temperatures
were equal. Sebenik (1967) found an average increase in air and
leaf temperature at heights of 24-, 54-, and 72-inches of 5.5 and
3.8°F respectively at ambient temperatures of 98 to 99°F. Assuming
a relative humidity of 29 per cent, the increase in vapor pressure
deficit inside the tent would be less than 4.0 mm Hg. These results
agree with the vapor pressure deficit of 6.66 mm Hg theoretically cal-
culated for a relative humidity of 29 per cent and an ambient air
temperature of 113°F.
The effect of an increase in vapor pressure on the transpira-
tion rates may be obtained by:
V = [5.20]R
where:
V = net gas exchange rate (g/cm2-sec);
AP vapor concentration difference between leaf and atmos-phere (g/cm); and
R = total resistance to gaseous diffusion (sec/cm).
Each of these independent variables in equation [5.201 are
affected by the enclosure effect of the tent environment. The ex-
change rate (V), computed for an enclosed and unenclosed environment
58
using the computed temperature increase of 6.40°F, indicated an
increase in the net gas exchange rate of 11 per cent in the enclo-
sure. Shachori et al. (1966) reported a positive error of 7 ± 4 per
cent in the transpiration of the potted plants inside the enclosure.
Air temperatures were measured at three heights in the cano-
pies of tamarisk plants in the tent. Figures (5.210) through (5.212)
represent the air temperature at these heights inside the canopy,
when the plant does not occupy the entire volume of the tent. Abrupt
changes in temperature may be attributed to the presence or absence
of cloud cover. Figures (5.213) through (5.215) indicate that tem-
perature is not significantly increased if the plant occupies the
entire volume of the tent. These data imply that if the plant does
not occupy the entire volume of the tent, the plant acts as a barrier
diverting air around it. However, if the plant occupies the total
volume of the tent, air is forced to move through the canopy similar
to a natural environment. Another possible explanation is that
temperature does not increase if the plant occupies the entire vol-
ume because the increased foliage uses the energy for transpiration.
However, previous theoretical calculations at an ambient temperature
of 113°F indicated that the maximum possible increase is 6.40°F.
As shown in Figure (5.211), temperature increases of approximately
24°F occurred in the tent when the plant occupies only one-half the
tent volume, which would indicate reduced ventilation rates inside
the canopy. Although air velocity measurements were not made during
/ t.\
Il0
4-/
-\V
/ /\i
102-
/ //
/--
.H
EIG
HT
1811
98-
/--
- T
EN
T96
-/
OPEN
94 9 2
II
II
II
II
II
I
O0O00Ot0t0
GQ_ 00= =c_CJc T
IME
Figure 5.210 - Air temperature in a tamarisk canopy at
a height of 18-inches inside
and outside the tent when the tree occupies only one-half the volume
of the tent.
The tet was removed at 3:10 pm.
I
120
AIR
TE
MP
ER
AT
UR
ET
EN
T V
S O
PE
N11
6-G
ILA
RIV
ER
114-
UP
PE
R A
RE
AIL .- 1
128/
23/6
5I'
'/\
I08-
/\
\/ /
106
-/\
\8-
/
\I/
I04-
.._
\/\/
\<
102-
\10
0-/
HE
IGH
T 5
7"98
-.-
----
- T
EN
T96
-/N
.N/
OPEN
94 92I
j_L
II
II
I
c3
CO
O=
C'J
_ çj
OJ-
C'.J
C'J
C'r(
)TIME
Figure 5.211 - Air temperatures in
a tamarisk canopy at a height of 57-inches inside
and outside the tent when the tree
occupies only one-half the volume
of the tent.
The tent was removed at 3:10
p.m.
uJ
26 120-
F.
/'S
iO/
AIR
TE
MP
ER
AT
UR
E12
2-T
EN
T v
s O
PE
NG
ILA
RIV
ER
UP
PE
R A
RE
A'
/11
8-8/
23/6
51
tIl6-
/11
4-/
I/
112
/1/
/ItO
-7
'S...
.../
-....
--.
/.-
.--S
.--
/\I
/Ii
I
/I
/'I
//
/
/
too
- ci-0
rOci
-0 =
'ci- 0
rOci
0 =
rOci
0r(
)0
rOci
-
TIM
E
Figure 5.212 - Air temperatures in a tamarisk canopy at a height of 75-inches inside
and outside the tent when the tree occupies only one-half the volume
of the tent.
The tent was removed at 3:10 p.m.
//
,/--.
.--/ /./ H
EIG
HT
-75'
TE
NT
OP
EN
I12
2 20-
118-
116-
Z11
4-0
112-
LJ a:
110--
AIR
TE
MP
ER
AT
UR
ET
EN
T V
S O
PE
NG
ILA
RIV
ER
UP
PE
R A
RE
A8/
23/6
5
/
/ / //\\ /
/\!
/
98-
96 -
/N/
92I
II
/08
-a:
/ / /I0
6-/
I04-
----
./
I-/
a: 1
02
lOt - 8/9/65100 /,/-99
98
97- /196- I,,'
0 I,95- /!94- HEIGHT- 20"
93 /,i\// OPEN
92 / \\TENT
9 I -\._./ /
90-". /11
/r.r \ /'I88 I I I I I
Q - _\JC'JJ(\Jr0TIME
Figure 5.213 - Air temperatures in a tamarisk canopy at aheight of 20-inches inside and outside thetent when the tree occupies the entirevolume of the tent.. The tent was removedat 2:00 p.m.
62
105
104- AIR TEMPERATURE
03- TENT VS OPENGILA RIVER
102 LOWER AREA
112
III110
109
108
07
106
105
104LL
103Ui
102
lOtcrLiia-
u99cr98d97
96
95
9493
929'90
0 LI) 0 ii)
AIR TEMPERATURETENT VS OPEN
GILA RIVERLOWER AREA
8/9/65
QU)0-TIME
HEIGHT 52'OPEN
TENT
u)0N)
,.s'/ ..'
U) 0c'i
Figure 5.214 Air temperatures in a tamarisk canopy at a
height of 52inches inside and outside the
tent when the tree occupies the entire
volume of the tent. The tent was removed
at 2:00 p.m.
63
LI)
oJ00
00c'J
U) 0F1)c'J
C%J
105
104 AIR TEMPERATURE
I03 TENT VS OPENGILA RIVER
102LOWER AREA
(01 8/9/ 65/Ui
//
I'98 'I97 I'- I I
Iuj96-i I
I II
94-' / / I
93-\ \ I,, /\\/
92 \ \ /V /9I
HEIGHT- 72"
OPEN
TENT
9c I I I I I i I I I I I I I
89
88 TIME
Figure 5.215 - Air temperatures in a tamarisk canopy at aheight of 72-inches inside and outside thetent when the tree occupies the entirevolume of the tent. The tent was removedat 2:00 p.m.
64
65
air temperature measurements, Lee (1966) found lower air velocities
inside the canopy than around the periphery of the plant. These
results suggest that the tent size should be scaled to the size of
the plants to be measured to reduce increased temperatures in the
enclosure.
Tent instrumentation was calibrated by sealing the bottom
with polyvinyl plastic and an error in the evapotranspiration rate
of 0.00029 ± 0.00029 inches per hour was determined. Variation in
wick lengths of the wet bulb thermometers was found to be very criti-
cal and laboratory calibration of wet bulb thermometers was performed
before tent runs.
5.22 Measurement Errors
Two possible sources of error associated with use of the tent
method are inaccurate temperature and air velocity measurements. An
error of ± 0.1°F in the dry bulb reading at the inlet, at a dry bulb
reading of 98.4°F, would cause a calculated change in the evapotrans-
piration rate of ± 0.0006 inches per hour (Table 5.220).
No significant variation in temperature was measured due to
variation in measurement positions within the inlet and outlet. How-
ever, variations in temperature within a five-minute period were as
great as 3.6°F. Such variation may suggest that temperatures should
be recorded continuously, but this could reduce the portability of
the method.
66
Table 5.220--Accuracy of transpiration rates in relation to errors intemperature measurements.
Inlet Temperatures (°F) Outlet Temperatures (°F)Error in Transpiratioit.
Rate (In/Hr)
dry bulb wet bulb dry bulb wet bulb
±0.1
±0.2
---
---
±0.1
±0.2
---
---
---
---
±0.1
±0.2
--
---
---
---
---
±0.1
±0.2
±0.0006
±0.0012
±0.0020
±0.0050
±0.0006
±0.0012
±0.0025
±0.0050
67
Air velocity measurements at the center of the outlet
varied between 1550 and 1650-feet per minute. An error of ± 50
feet per minute in air velocity would lead to a possible error of
± 0.0002-inches per hour in evapotranspiration rate determinations.
Measurements made at the center of the outlet do not represent the
average velocity of the air moving through the outlet. Velocity
measurements made at 41 points in the 18-inch outlet indicated that
the average velocity was 0.834 times that measured at the center of
the outlet. Sebenik (1967) indicated that the measured velocity at
the center of the outlet should be multiplied by a factor of 0.83521.
All evapotranspiration values were reduced by the factor of 0.834.
Atmospheric pressure is included in calculations of absolute
humidity at the inlet and outlet. Atmospheric pressure was two
mm Hg higher inside the tent. An error of ± 100 mm Hg at 93°F affects
the evapotranspiration rate by only ± 0.001-inch per hour; therefore,
errors in measurement of atmospheric pressure were considered insigni-
ficant.
5.3 Evapotranspiration Rates of Tamarisk
Evapotranspiration rates for the Gila River study areas are
shown in Figure (5.300). These data indicate that evapotranspiration
rates from taniarisk stands are low during February, March, and April
(periods of defoliation). Measurements during July and August indi-
cate the greatest loss occurred on the lower area of the research site
where the water table was three feet below the ground surface.
0
10z
16 14 12 8 6 4 2 0
uppe
r ar
ea
mid
dle
area
low
er a
rea
2/27
327
3/28
4/21
7/12
7(13
/17/
207/
267/
273/
33/
43/
93/
233/
74
DA
TE
Figure 5.300 - Evapotransiration rates in inches per month for ]965 from th
Gila River study sites.
69
Evapotranspiration rates on the upper area where the water table is
approximately 20-feet were greater than the middle area where the
water table is approximately 14-feet. Theoretically, evapotranspira-
tion rates should be higher on the middle area than on the upper area
due to larger plants and a greater access to the ground water table.
This deviation from theory may be accounted for by increased salinity
levels of the soil and ground water on the middle area. Salinity levels
of the surface 12-inches on the upper area was 1425 ppm compared to
21,090 ppm on the middle area. Salinity of the ground water samples
on the middle area was 17,900 to 20,400 ppm. Unfortunately, before an
intensive investigation of salinity effects on the three areas could
be initiated, the areas were inundated by San Carlos Reservoir. A
laboratory study was subsequently initiated to determine the effects of
salinity on transpiration rates of tamarisk plants.
Evapotranspiration rates were computed by Penmants equation to
compare with rates measured by the triple-inlet evapotranspiration
tent. Table 5.300 compares evapotranspiration rates for these two
methods for the months of July and August, 1965.
Table 5.300--Comparison of evapotranspiration rates determined bythe evapotranspiration tent and Penman's method.
ItemArea
Upper Middle Lower
Sample size 208 441 173
Tent ET (in/mo.) 9.58 7.15 10.13
Penman ET (in/mo.) 7.24 7.12 11.73
Penman/tent ratio 0.755 0.996 1.158
R2 0.798 0.714 0.928
70
These data indicate a close relationship between the tent
and Penman's method. The greatest deviation occurs on the upper
area where the tent would be the least accurate due to a greater en-
closure effect because of the smaller plants. Sebenick's (1967) data
for July and September, 1966 indicated evapotranspiration determined
by the tent method exceeded pan evaporation from three nearby weather
stations by three, twenty-five, and ten per cent. Van Bavel (1966)
found actual evapotranspiration rates of alfalfa to be greater than
calculated potential evapotranspiration by at least ten per cent on
numerous occasions.
Estimates computed by the Blaney-Criddle method indicated an
average evapotranspiration rate for July and August, 1965 of 7.93
in/mo. Estimates by the tent exceeded values obtained by the Blaney-
Criddle method by 17 and 22 per cent for the upper and lower areas,
respectively. The tent method was 11 per cent less than the Blaney-
Criddle method for the middle area and 11 per cent greater than the
average of the three areas.
These results indicate that the tent method overestimates
evapotranspiration losses computed for the average of all areas by
the Blaney-Criddle and Penman method by eleven and one per cent,
respectively. This variation is greater on the upper area and on
the lower area compared to the Blaney-Criddle method. These varia-
tions may be expected because of the factors considered in the
derivation of the two prediction equations. The deviation of the
71
tent method could be due to variations in the water supply, inherent
plant characteristics, soil, hydrologic, and climatic factors, and
the enclosure effect of the tent.
5.4 Laboratory Study of the Effect of Salinity on Transpiration Rates
Transpiration rates were determined for tamarisk cuttings
with roots subjected to nutrient medium with 0.3, 4.0, 8.0, and
12.0 atmospheres osmotic pressure by the addition of NaC1 to the
nutrient medium. Measurements of transpiration were made at 24-hour
intervals for an 11-day period. Four consecutive experiments were
conducted at vapor pressure deficits (V. P. D.) of 37, 42, 87, and
112 mm Hg, respectively, under environmental conditions specified
in section 4.9.
5.41 Effects of Salinity on Transpiration Rates
The effects of increasing salinity of the root substrate
on transpiration rates at different vapor pressure deficits are shown
in Figure.4l0). These data were analyzed by an analysis of co-
variance using initial plant weight minus cutting weight as the
independent variable. Coefficients of variation, degrees of freedom,
and corresponding F-values are shown in Table 5.410.
10.0-
9.0
8.Ow
Ca
7.00
C
. 6.0U)
0I--c
5.0E
z0I-
400(I)z
3.0
2.0-
- \
\
\\
NN
N
mm Hgmm Hgmm Hgmm Hg
N
NN
NN
NN
NN
NN
72
vpd 37vpd 42vpd 87vpd 112
0 4.0 8.0 2.0
SALINITY (ATMOSPHERES)
Figure 5.410 - Effect of salinity on transpiration rates atdifferent vapor pressure deficits. Each
point represents 66 measurements.
* Significant at 5 per cent level**Significant at 1 per cent level
These results indicate a highly significant difference (1 per
cent level) in transpiration rates per unit initial plant weight for
different salinity levels at vapor pressure deficits of 37 and 112
mm Hg; a significant difference (5 per cent level) at 87 mm Hg; and
no difference at 42 mm Hg. High coefficients of variation may be a
contributing factor to non-significance at 42 mm Hg vapor pressure
deficit. Transpiration rates of two of the six control plants (0.3
atmospheres) at 42 mm Hg were unusually low compared to the other
salinity levels and vapor pressure deficits. Possible explanations
for these variations are location of cuttings taken from the plant,
mesophyll development or anatomical differences between cuttings.
73
Table 5.410--Summary of analysis of covariance of salinity effectson transpiration rates at different vapor pressuredeficits.
Vapor Pressure Deficit(mm Hg)
Degrees ofFreedom
Coefficient ofVariation (per cent)
F-
Value
37 3 and 14 58 to 102 9.52**
42 II 86 to 156 3.09
87 70 to 100 535*
112 'I 94 to 231 6.99**
74
Analysis of the data by standard error of the difference
between two adjusted treatment means indicated no significant dif-
ferences in transpiration rates between 0.3, 4.0, 8.0 atmospheres
osmotic pressure of the substrate at all vapor pressure deficits.
Significant differences existed between 12.0 atmospheres and 0.3,
4.0,and 8.0 atmosphere treatments at all vapor pressure deficits
except 42 mm Hg.
These results indicate that transpiration rates per unit
initial plant weight were reduced when the salinity level of the
root medium was between 8.0 and 12.0 atmospheres osmotic pressure
or 11,224 and 16,836 ppm of NaC1, respectively. However, effects
of salinity on plant growth have not been taken into account by this
analysis. Bernstein (1961) reported that total transpiration per
plant decreases markedly with increasing salinity, but this is the
result of sharply inhibited growth and large decreases in leaf area.
To evaluate the influence of salinity on growth and transpira-
tion rates, transpiration rates adjusted for differences in initial
plant weight minus cutting weight were based on unit fresh weight
growth for the study period (Figure 5.411). F-values, degrees of
freedom and coefficients of variation determined by analysis of co-
variance using initial plant weight minus cutting weight as the
independent variable and transpiration rates per unit fresh weight
of growth are shown in Table 5.411.
5.0
4.5
4.0
0
3.5tj
E
-c
E
z0I-czZ 2.5
vpd 37 mm Hgvpd 42 mm Hgvpd 87 mm Hgvpd 112 mm Hg
SALINITY (ATMOSPHERES)
1igure 5.411 - Effect of salinity on transpiration rates pergram of growth (F.W.) for different vaporpressure deficits. Each point is the sum, ofsix replications and represent 66 measurements.
75
a-(I)z
2.0
1,5
I.0,
\\
\\
\\\
\ - -
0 4.0 8.0 12.0
* Significant at 5 per cent level** Significant at 1 per cent level
These results indicate significant differences in transpira-
tion rates at high vapor pressure deficits, and that differences at
37 mm Hg vapor pressure deficit (Table 5.410) can be attributed to the
differences in growth.
Discussion of the effects of salinity on transpiration rates
will be restricted to passive absorption of water by plants which ap-
pears to be the predominant process for water absorption (Slatyer,
1962). Movement by the process occurs across a potential gradient
according to the equation by Cowan (1965):
q = [5.40)
where:
q flux of water (transpiration) per unitarea of the crop;
76
Table 5.4ll--Summary of analysis of covariance for salinity effectson transpiration rates per unit growth (fresh weight)at different vapor pressure deficits.
Vapor Pressure Deficit Degrees of Coefficient of F-(mm Hg) Freedom Variation (per cent) Value
37 3 and 14 22 to 27 1.11
42 25 to 41 3.17
87 6 to 10 5.46*
112 47 to 97 15.97**
77
AlP total potential difference between the waterpotential at the external root surface and thewater potential of the atmosphere and;
= internal impedance of a unit area of plant tothe transport of water.
Philip (1966) indicated that impedance in the original elec-
trical context meant the ratio, in a harmonically alternating circuit,
between the root mean square current and the root mean square voltage.
Since a non-oscillating system is encountered in plants, the term
resistance should be used instead of impedance and equation [5.40]
becomes:
q = AlP [5.41]R
The total potential gradient (AlP) can be written as:
- [5.42]
where:
water potential at the external root surfaceand;
water potential of the atmosphere surroundingthe leaf.
Transpiration occurs if 'P2 is greater than 'Pi, and the rate
of flux is dependent on the magnitude of the difference and the
internal resistance.
The addition of NaC1 to the aqueous medium surrounding the
roots increases 'Y1; therefore, M decreases and according to equation
78
[5.42] transpiration decreases assuming resistance remains the same.
Conversely, as the vapor pressure deficit increases with an increase
in temperature,2' iV , and transpiration increase. At some criti-
cal range in vapor pressure deficit, the internal resistance of the
plant for water transport will become the limiting factor, and trans-
piration will become dependent on the capacity of the plant to trans-
port water from the substrate to the external surface of the
mesophyll cells.
Therefore, at a low vapor pressure deficit, the flux of water
due to a low P probably does not exceed the transport capacity of
the plant at any salinity level. Although AP at 12.0 atmospheres
osmotic pressure would be less than M' at 0.3 atmospheres osmotic
pressure, a significant difference was not detected. At a high V. P. D.
the flux of water possibly would exceed the plant's capacity to trans-
port water from the root substrate with a high osmotic pressure.
Therefore, the plant would transpire at a rate corresponding to the
maximum value of the transport function, not a rate equivalent to its
potential gradient. This could explain statistically significant
differences in transpiration rates between salinity treatments at
high vapor pressure deficits and non-significant differences at low
vapor pressure deficits.
Lunin and Gallatin (1965) and Eaton (1941) reported that
transpiration rates were reduced by increased salinity levels of the
root substrate and were independent of growth rates. Both authors
79
used non-salt tolerant corn and tomato plants. Their data indicated
salinity was effective in reducing transpiration rates at approximately
2.0 atmospheres osmotic pressure in the root substrate. Lagerwerff
and Eagle (1962) reported a similar relationship with kidney beans,
a non-tolerant plant. Boyer (1965) found no reductions in transpira-
tion rates of cotton at osmotic pressures as high as 8.5 atmospheres.
However, cotton is a relatively salt tolerant plant and Boyer's ex-
periments were conducted at low vapor pressure deficits of 17.33
mm Hg.
Variation in these studies may be due partly to differences
in salt tolerance of the species, physiological conditions of plants,
and to a large extent the environmental condition under which the
experiments were performed. Data from the present study indicate
that environmental conditions that increase vapor pressure deficit
contribute significantly to the results obtained from transpiration-
salinity studies and should be a major factor in the interpretation
of previous studies.
5.42 Atmospheric Vapor 1ressure Deficit Versus Transpiration
Whiteman and Killer (1964) reported that under constant tur-
bulent conditions and stomatal resistance, transpiration is a linear
function of the vapor pressure gradient - between the evaporating
surface and the atmosphere. Decker et al. (1964) inferred that this
concept has been widely accepted in principle, but experimental con-
firmation is lacking. Decker et al. (1964) obtained a linear
80
relationship between transpiration of oleander and allepo pine and
vapor pressure deficit by assuming leaf temperatures equal to air
temperature; however, their data indicated a transpiration rate of
approximately 15 gm/hr at zero vapor pressure deficit. This would
imply that leaf temperatures were higher than air temperatures or
that transpiration may be an active process rather than the generally
accepted passive process (Slatyer, 1962).
Regression equations relating transpiration at four salinity
levels to vapor pressure gradient indicate a linear relationship at
salinity levels of 0.3 and 4.0 atmospheres osmotic pressure and
curvilinear relationships at 8.0 and 12.0 atmospheres osmotic pres-
sure for the data. Results at low salinity levels agree with those
obtained by the previous authors, although the magnitude and inter-
cepts are different. Differences in species and environmental
conditions would contribute to the variation between studies reported
in the literature.
Whiteman and Koller (1964) proposed a method for estimating
the saturation deficit (less than 100 per cent) of the evaporating
surfaces of the mesophyll cells in plants. This method resulted in
an estimate of the equilibrium vapor pressure of the external atmos-
pheric at which the net flux of water vapor between the plant and the
surrounding air was zero. Extrapolation of the linear relationship
between transpiration and water potential to the abscissa (transpira-
tion equals zero) was assumed to be the saturation deficit of the
81
mesophyll cells. This is based on the assumption that at zero
transpiration the vapor pressure of the evaporating surface of the
mesophyll cells was in equilibrium with that of the atmosphere, and
that stomatal resistance was constant. They indicated that the slope
of the regression line is an indirect measure of the stomatal resis-
tance.
Similar results were obtained in the present study by extra-
polation of the regression lines to the abscissa as shown in Figure
(5.420). Unfortunately, data were not available at low transpiration
rates which would have increased the validity of these results.
Discussion here will be based on the assumption that the linear and
curvilinear relationships apply to transpiration rates in the ex-
trapolated region.
Whiteman and Koller (1964) assumed that if transpiration was
zero (vapor pressure equilibrium); if the evaporating surface was
water saturated; and if the leaf and air temperatures were equal,
then the plotted curve of transpiration against vapor pressure gradi-
ent would pass through the origin. If the curve did not pass through
the origin and the leaf and air temperatures were equal, the magni-
tude of the deviation from zero would be equal to the saturation
deficit of the evaporating surface of the mesophyll cells.
Highest transpiration rates in the present study were associ-
ated with the lowest mesophyll saturation deficit and salinity levels
of the root substrate. This may indicate that salt accurnmulation on
32
28
24
C0
20
C
w
16
00I.--c
E 12
z0I-cr
8(I)z
I-
4
o 0.3 ATMOSPHERES OSMOTIC PRESSURE. 4.0 ATMOSPHERES OSMOTIC PRESSUREo 8.0 ATMOSPHERES OSMOTIC PRESSURE
12.0 ATMOSPHERES OSMOTIC PRESSURE
z-8.53+O.3550X
//
0-
VAPOR PRESSURE DEFICIT (mm Hg)
/
Figure 5.420 - Estimation of mesophyll saturation deficit oftamarisk plants by regression of transpirationand vapor pressure deficit. Extropolation of
the regression lines to the X-axis is a measureof the saturation deficit. Each point represents
66 measurements.
//
- 948 + 03505 X
82
A 2Y-I6.2I+O.65O8X -0.003IX
- 10.09 + O4083X 0.00I8X2
0 I I
0 20 40 60 80 100 120
83
the evaporating surface of the mesophyll cell walls reduces the
saturated vapor pressure in the stomatal cavity for high salinity
levels. As far as is known, salt does not enter the mesophyll cells
due to the presence of salt glands. However, neither the salt trans-
port mechanism nor its efficiency is known. These data indicate that
only a minimal concentration of salt may enter the mesophyll cells
as shown by the possible difference between the mesophyll saturation
deficits at 0.3 and 12.0 atmospheres osmotic pressure of the root
substrate assuming this extrapolation procedure is valid (Figure
5.420).
Resistance will be used to describe conductance in the follow-
ing discussion since it is the reciprocal of conductance. The slopes
of the regression equations (dY/dX) are equal to the total conductance
and are a measure of the total resistance to transpiration, not just
the stomatal resistance as indicated by Whiteinan and Koller (1964).
Equation [5.41] indicates that the resistance to transpiration is
the total resistance associated with the potential gradient from the
substrate of the roots to the atmosphere. Rose (1966) illustrated
the various soil-plant-atmospheric resistances encountered by the
transpiration stream as shown by:
R=r5+r +r +r +r +r +rr x s c bi e
where:
Rt = total resistance
r5 = water supply resistance
re = external air resistance
He further stated that while no resistance is fixed, stomata,
boundary layer, and external air resistances are particularly variable.
Regression equations for transpiration (Y) versus vapor pres-
sure deficit (X) are:
Y = -9.48 + 0.3505X [5.421]
for 0.3 atmosphere osmotic pressure of the root substrate, and
y -8.53 + 0.3550X [5.422]
for 4.0 atmosphere osmotic pressure of the root substrate. These
equations indicate that transpiration is proportional to the vapor
pressure deficit for isothermal conditions.
The derivative for equation [5.421] is
dY/dX = 0.3505 [5.423]
and for equation [5.422] is
dY/dX = 0.3550 [5.424]
84
rr = root resistance
= xylem resistance
= stomatal resistance
rc = cuticle resistance
rb 1 = boundary layer resistance
85
Equations [5.423] and [5.424] indicate that total conductance
(dY/dX), a measure of resistance, to transpiration at low salinity
levels is a constant in the range of environmental conditions measured.
Low salinity levels should not affect water supply or root resistance
since the change in potential is not of sufficient magnitude to be
effective for a salt tolerant plant. Stomata resistance has been
shown to be largely dependent on light intensity (constant in present
study) with an unlimited water supply (Slatyer and Bierhuizen, 1964).
Xylem resistance is relatively constant under all conditions since
the conducting tissue is dead (Ray, 1965). At a constant air velo-
city such as in an environmental growth chamber, the external air
resistance is probably a constant. Therefore, the total resistance
to transpiration will be dependent on the resistance to molecular
diffusion across the laminar boundary layer if the previous statements
are assumed to be true. Slatyer and Bierhuizen (1964) indicated
that the relationship between evaporation (Y) from a wet surface
and the vapor pressure deficit (X) at a given air velocity (V)
and where (a) is a constant exponent is:
Y = o.l28XVa [5.425]
The derivative of equation [5.425] at a constant air velocity
(Va = K) is
dY/dX - 0.128K [5.426]
which is a measure of the boundary layer resistance at a constant
86
air velocity. Thus, the total resistance to transpiration at low
salinity levels, constant light intensity, and wind movement is
shown to be dependent on the boundary layer resistance which is a
constant as shown in equation [5.426].
Regression equations for transpiration versus vapor pressure
deficit are:
Y = -16.21 + 0.6508X - 0.0031X2 [5.427]
for 8.0 atmospheres osmotic pressure of the root substrate and
Y = -10.09 + 0.4083X - O.00l8X2 [5.428]
for 12.0 atmospheres. These curvilinear equations indicate that
transpiration is non-linear to the vapor pressure deficit as the
salinity level is increased above a critical level. Analogous to
this non-linearity Thames (1966) and Swartzendruber (1963) observed
a similar effect for flow equations in unsaturated soils.
The derivative for equation [5.427] is:
dY/dX = 0.6508 - 0.0062X [5.429]
and for equation [5.428] is:
dY/dX = 0.4083 - 0.0036X [5.430]
Equations [5.429] and [5.430] indicate that total conductance
(dY/dX), a measure of resistance, is a linear function of vapor
87
pressure deficit at high salinity levels. The constant in equations
[5.429] and [5.430] implies that vapor pressure deficit indirectly
affects resistances within the Soil-plant-atmospheric system under
highly saline conditions. The magnitude of the direct effect of
the vapor pressure deficit under the conditions measured is small
compared to the constant. External air resistances have been shown
to be constant at a given air velocity and can be eliminated as a
resistance indirectly affected by vapor pressure deficit (equation
[5.4261). The magnitude of cuticular transpiration is negligible
(6 per cent) compared to stomata transpiration and will be considered
constant in the following discussion (O'Leary, 1966). Water conduct-
ing tissue of the xylem is considered dead and substrate salinity
or vapor pressure deficit would not have an effect on its resistance.
Therefore, the constant in equations [5.427] and [5.428] does not
include resistances of the boundary layer, external air, cuticle,
and xylem.
Vapor pressure deficit could indirectly affect other resist-
ances by increasing transpiration rates beyond the transport capacity
of the plant as previously discussed. If the plant is unable to
supply enough water to maintain guard cell turgidity surrounding
the stomates, these cells will shrink and cause stomate resistances
to increase as the vapor pressure deficit increases. A second factor
influencing the water transport capacity is the effect of high salinity
on the permeability of the roots to water. As discussed below, root
permeability was significantly reduced at high V. 2. D. at 12.0
88
atmospheres osmotic pressure in the root medium. Thus, as root
permeability is reduced as salinity is increased, reductions in
stomata size can be expected to be reduced due to transpiration
exceeding the transport capacity of the plant at high vapor pres-
sure deficits. In this manner, the vapor pressure deficit could be
indirectly related to total resistances as shown in equations
[5.429] and [5.430].
Although quadratic equations were fitted to the data at os-
motic pressures of 8.0 and 12.0 atmospheres, Figure (5.421) indicates
that two populations may exist and two linear relationships might
better describe the trends. Unfortunately, the data were not of
sufficient range to test this hypothesis. If such a relationship
exists, it would appear that at some vapor pressure deficit between
42 and 87 mm Hg, the limiting factor for transpiration becomes sali-
nity rather than vapor pressure deficit.
5.43 Salinity Effects on Root Permeability
Results of increased salinity of the root substrate on root
permeability of plants subjected to atmospheric vapor pressure
deficits of 37, 42, 87, and 112 mm Hg are shown in Figure (5.431).
Differences between salinity levels at each deficit were determined
by an analysis of covariance using initial plant weight minus cutting
weight as the independent variable. Coefficients of variation, de-
grees of freedom, and corresponding F-values are shown in Table 5.430.
C
34
30
22
2
a I i I I I I i
0 20 40 60 80 100 120
VAPOR PRESSURE DEFICIT (mm Hg)
Figure 5.421 - Transpiration rates affected by vapor pressure deficitsat four salinity levels. Water loss is linearily re-lated to vapor pressure deficit at low salinity levels(0.3 and 4.0 atm.). At high vapor pressure deficitshigh salinity (8.0 and 12.0 atm.) becomes limiting.
0.3 ATMOSPHERES OSMOTIC PRESSURE4.0 ATMOSPHERES OSMOTIC PRESSURE8.0 ATMOSPHERES OSMOTIC PRESSURE
- 12.0 ATMOSPHERES OSMOTIC PRESSURE
//II/I/1"
/ ,1
/,,.
89
(.4
'; (.2
CQ
I.0
C
0.8a
0.6>-I--J
4w
w0
I-o 0.20
00
-- \-
mm Hgmm Hgmm Hgmm Hg
\\
N \\\/\\
N
NN
90
NN
NN
NN
N
37vpd
vpd 42vpd 87vpd 1(2
4.0 8.0 12.0
SALINITY (ATMOSPHERES)Figure 5.431 - Effect of salinity on root permeability at different
vapor pressure deficits. Each point represents 66
measurements.
** Significant at the 1 per cent level
These results show a highly significant difference in root
permeability at different salinity levels for a vapor pressure deficit
of 112 mm Hg, but non-significant differences at lower vapor pressure
deficits. Hayward and Spurr (1943) reported a significant reduction
in root permeability of non-salt tolerant corn plants at 4.8 atmos-
pheres osmotic pressure of the root substrate. Plants preconditioned
to the substrate for 5 to 7 days showed a higher root permeability
than nonconditioned plants, but were still significantly lower than
the control plants. This indicates that corn plants have the capa-
city to adjust to increased salinity in the root medium. Bernstein
(1961, 1963) and Slatyer (1961) reported an osmotic adjustment in
all plant parts to changes in the osmotic pressure of the root sub-
strate. The magnitude of the adjustment was not equivalent to the
91
Table 5.430--Summary of analysis of covariance of salinity effectson root permeability at different vapor pressure deficits.
Vapor Pressure Deficit(mm Hg)
Degrees ofFreedom
Coefficient ofVariation (per cent)
F-Values
37 3 and 14 57 to 99 0.58
42 I!56 to 73 0.25
87
112 ti
51 to 111
32 to 140
1.73
6. 71**
92
increase in salinity, but the potential gradient was maintained.
If such an adjustment takes place, the decrease in transpiration
cannot be attributed to water stress.
Experiments by Bernstein (1961, 1963) and Slatyer (1961)
showed that less adjustment occurs in the roots and that the poten-
tial gradient is not maintained to the same extent in roots as in
other parts of the plant. Oerti (1966) hypothesized a model to
describe water and salt transport using kinetic equations. He con-
cluded that it is impossible for osmotic adjustment to occur in the
root xylem when plants are grown in saline solutions. He showed that
for various laws governing the rate of salt and water transport,
osmotic adjustment within the xylem is not possible under saline and
high transpiration conditions. Measurements of the osmotic pressure
of the root tissue and other plant parts were not attempted in this
study due to the lack of a satisfactory method.
Eaton (1941) attributed the reduction in root permeability
to reduced growth and inhibited meristematic activity. Hayward
and Spurr (1943) reported reduced growth with increased salinity
and a shift in the zone of maximum water entry toward the apical
region of the root. This shift was primarily due to increasing the
rate of lignification and suberization of the endodermis and Casparian
strip as salinity was increased. Therefore, the zone of water entry
was reduced in surface area as salinity was increased.
This study indicates that the root permeability of tamarisk
was reduced by increased salinity only at the highest vapor pressure
** Significant at 1 per cent level
93
deficit (112 mm Hg). Thus, resistance to water entry through the
roots is not only a function of water potential (osmotic pressure of
the root substrate), but also the function varies depending on how
the potential arises. As indicated previously by the curvilinear
equations, variation in resistances is dependent on many complex
interrelationships in the soil-plant-atmosphere system.
5.44 Plant Adjustment to Increased Salinity and Interactions
Bernstein (1961, 1963) and Slatyer (1961) have reported
adjustment within the plant system to increased osmotic pressure of
the root medium. Figure (5.440) indicates that transpiration rates
increase with time after salinity treatment at all vapor pressure
deficits. Coefficients of variation, degrees of freedom, and cor-
responding F-values are shown in Table (5.440).
Table 5.440--Summary of analysis of covariance of time after salinitytreatment on transpiration rates at different vaporpressure deficits.
Vapor Pressure Deficit Degrees of Coefficient of F-(mm Hg) Freedom Variation (per cent) Value
3 7 3 and 14 10 to 18 51. 07 **
42 13 to 19 22.26**
87 16 to 39 24.35**
112 7 to 25 206. 67**
10.0
9.0
8.0
C
0.7.0
0
.! 6.0U,
'00
-c
5.0E
z0F-
4.0a-(I)2
3.0
2.0
vpd 37 mm Hg \vpd42 mm Hgvpd 87 mm Hgvpd 112 mm Hg
/ '
/
94
I I I I I I I
0 I 2 3 4 5 6 7 8 9 10
DAYS AFTER SALINITY TREATMENT
Figure 5 440 - Analysis of the effect of time since treatmenton transpiration rates. Plotted pointsrepresent the means of 44 measurements.
95
These results show a highly significant difference in
transpiration rates between days after salinity treatments at all
vapor pressure deficits. This increase in transpiration rates
can be attributed to either an adjustment within the plant to
salinity or an increase in growth. Since changes in plant weight
were impossible to measure during the study period, growth effects
cannot be reported. However, it was previously shown (Figures
5.410 and 5.431) that transpiration rates expressed as a function
of growth and root permeability were significantly reduced at high
vapor pressure deficits. This may indicate that both adjustment to
salinity and increases in growth cause increased transpiration rates
with time after salinity treatments.
Unusual increases in transpiration rates occurred at vapor
pressure deficits of 37 and 112 mm Hg on the sixth day after treat-
ment. No satisfactory explanation for this increase can be offered,
but it is interesting to note that they occurred on the same day.
Analyses were conducted to determine if interactions existed
between salinity levels, days after treatment, and replications
(measure of variation within growth chamber for covariance analysis).
No significant differences existed for any combination of factors,
which indicates that the analyses were unaffected by interactions.
6. CONCLUSIONS
The evapotranspiration tent developed by Decker et al. (1962)
created serious enclosure effects. Measurements of the enclosure
effect indicated increased air temperatures and sapflow velocities.
Incoming and net radiation and ventilation rates were lower than
unenclosed plots. Distinct still air pockets were detected at the
bottom and top of the tent.
Reduced ventilation rates In the tent enclosure caused a more
significant increase in internal energy than the "greenhouse effect".
Reduced ventilation rates were shown to theoretically increase air
temperature 4.87° F. compared to an increase of 1.04° F. due to the
"greenhouse effect" above an ambient temperature of 98.6° F. At an
ambient temperature of 113° F, the increase due to reduced ventila-
tion rates was 11.75° F compared to an increase of 3.55° F due to the
"greenhouse effect". These calculations show that the reduced venti-
lation rate is the significant factor contributing to increased
temperatures inside the tent.
Modification of the "Decker tent" were made to increase
ventilation rates and improve the air flow pattern. Ventilation
rates were increased four-fold which reduced the possible tempera-
ture increase inside the tent approximately 50 per cent. Smoke bombs
used to determine air flow patterns showed that the still air pockets
were eliminated.
96
97
Air temperature measurements were made at three heights in
the crown canopy of tamarisk plants. These measurements indicated
an increase in air temperature inside the enclosure if the tent was
not fully occupied by plant. If the plant occupied the entire
volume of the tent, air temperatures in the crown canopy were not
significantly increased above unenclosed temperatures. These results
suggest that the evapotranspiration tent should be scaled to the
size of the plant to be measured to reduce increased temperatures
in the enclosure.
Evapotranspiration rates measured by the tent exceeded values
calculated by Penman's equation by twenty-five and one per cent on
the upper and middle areas respectively, but was fifteen per cent
less on the lower area. Values computed by the Blaney-Criddle method
were seventeen and twenty-two per cent less than the values obtained
by the tent method on the upper and lower areas respectively. However,
rates determined by the tent method were eleven per cent less than
those computed by the Blaney-Criddle method on the middle area. The
tent method overestimated evapotranspiration losses computed for the
average of all areas by Blaney-Criddle and Penman method by eleven
and one per cent respectively.
Evapotranspiration rates were consistently higher on the upper
area than on the middle area, although the depth to the water table
is greater on the upper area. The only possible explanation was
increased soil salinity on the middle area which could reduce evapo-
transpiration rates.
A laboratory study was conducted in a plant growth chamber to
98
determine the effect of salinity on transpiration rates of tamarisks.
Transpiration rates per unit weight of growth (F.W.) were signifi-
cantly reduced by increased osmotic pressure of the root medium at
high vapor pressure deficits.
An estimate of mesophyll saturation deficit was determined
by extrapolation of regression equations for transpiration versus
vapor pressure deficit. These data indicated a minimal increase in
mesophyll saturation deficit with increased osmotic pressure of the
root substrate.
These regression equations showed a linear relationship
between vapor pressure deficit and transpiration rates at low osmotic
pressures (0.3 and 8.0 atm.); whereas, a curvilinear relationship
was present at high osmotic pressures (8.0 and 12.0 atm.). These
results imply that increased osmotic pressure of the root medium
only reduces transpiration rates at high vapor pressure deficits.
Root permeability rates were measured to determine if increas-
ed osmotic pressure of the root substrate reduced the permeability of
the roots to water. Results showed that the root permeability was
significantly reduced only at the highest vapor pressure deficit.
Transpiration rates significantly increased with time after
the additions of NaC1 to the root medium at all vapor pressure
deficits. These results imply that tamarisk adjust to some extent
to increased osmotic pressure of the root medium.
SELECTED REFERENCES
American Society of Civil Engineers. 1966. Methods for estimatingevapotranspiration. Irrigation and Drainage Specialty Con-ference. Las Vegas, Nev. 236 pp.
Anderson, N. E., C. H. Hertz, and H. Rufelt. 1954. A new fast re-cording hygrometer for plant transpiration measurements.Physiologia Plantarum. 7:753-767.
Arisz, W. H., R. J. Helder and R. van Nie. 1951. Analysis of theexudation process in tomato plants. Jour. Exptl. Botany,2:257-97.
Baldwin, T. 1964. Soil inventory of Salt Creek. Soil Survey, SanCarlos Indian Reservation. 13 pp. (mimeo).
Baum, B. 1966. Monographic revision of the genus TANARIX. FinalRes. Rep. for the U. S. Dept. Agr. Proj. No. AlO-FS--9. Dept.Bot., Hebrew Univ., Jerusalem. 193 pp., illus.
Bernstein, L. 1961. Osmotic adjustment of plants to saline media.Steady state. Amer. Jour. Bot. 48:909-18.
1963. Osmotic adjustment of plants to saline media.Dynamic phase. Amer. Jour. Bot. 50(4):36O-370.
Blaney, H. F. and W. D. Criddle. 1950. Determining water require-ments in irrigated areas from climatological and irrigationdata. S. C. S. Technical Paper 96.
H. R. Haise and M. E. Jensen. 1961. Monthly consump-tive use by irrigated crops in western united states. Pro-visional Supplement. S. C. S. Technical Paper 96.
Bowman, C. 1963. Net interception and its effects on precipitationdisposal: Transpiration phase. Progress Report, MontanaAgricultural Expt. Sta., Western Regional Research ProjectW-73. 29 pp. (mimeo).
Boyer, J. S. 1965. Effects of osmotic water stress on metabolisrates of cotton plants with open stomata. Plant Physiol.40:229-234.
Brunt, D. 1932. Notes on radiation in the atmosphere. QuarterlyJour. Roy. Meteor. Soc. 58:389-420.
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