Weather and Evapotranspiration Studies 

in a Saltcedar Thicket, Arizona 

G E 0 L 0 G I CAL SURVEY P R 0 FE S S I 0 N A L PAPER 4 9 1-F 

Weather and Evapotranspiration Studies 

in a Saltcedar Thicket, Arizona 

By T. E. A. VAN HYLCKAMA 

STUDIES OF EVAPOTRANSPIRATION 

G E 0 L 0 G I CAL SURVEY P R 0 FE S S I 0 N A L PAPER 4 9 1-F 

U N I T E D STATES G 0 V ERN M EN T P R I N T I N G 0 F F I C E, WAS H I N G T 0 N 1 9 8 0 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

Van Hylckama, T. E. A. 

Weather and evapotranspiration studies in a saltcedar thicket, Arizona. 

(Studies of evapotranspiration) (Geological Survey professional 

paper; 491-F) 

Bibliography: p. F48-F51. 

1. Tamarix Chinensis. 2. Evapotranspiration-Arizona-Buckeye 

region. 3. Vegetation and climate-Arizona-Buckeye region. 

4. Microclimatology-Arizona-Buckeye region. 5. PhreatophytesArizona-Buckeye region. I. Title. II. Series. Ill. Series: United 

States. Geological Survey. Professional paper; 491-F. 

QE75.P9 no. 491-F [QK495.T35] 557.3s 80-11316 [583'.158] 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 
CONTENTS 

Page 

Abstract __ _ __ _ _ __ __ _ _ __ __ __ __ __ _ _ _ _ _ _ __ __ __ _ _ _ _ __ __ ____ _ _ _ F1 

Introduction________________________________________________ 1 

Saltcedar and other phreatophytes ______________________ 1 

Literature______________________________________________ 2 

Acknowledgments ______________ ________________________ 3 

Methods of studying water use ------------------------------ 3 

A bit of history _ __ _ _ _ _ __ __ __ __ __ _ _ ______ __ _ _ _ _ __ _ ______ 3 

Hydrologic methods _ __ ___ ___ __ _ _ _ _ _ _ __ __ __ _ _ _ _____ _ _ ___ 4 

Micrometeorologic methods______________________________ 4 

Empirical methods______________________________________ 5 

The Buckeye test site -------------------------------------- 5 

Location, soil, and water ------------------------------ 5 

Climate________________________________________________ 6 

Vegetation _ _ _ __ _ _ _ _ _ ___ __ __ _ _ __ _ _ __ __ _ _ _ _ _ _ __ __ ____ __ _ 8 

Evapotranspiration and its calculation ______________________ 10 

Evaporation____________________________________________ 10 

Transpiration ------------------------------------------ 11 

Flow resistance ___ _ _ _ _ __ __ ____ __ _ _ __ __ __ __ __ _ _ _ ___ __ _ _ _ 12 

Vapor-flow methods ------------------------------------ 12 

The atmospheric water balance ______________________ 12 

The eddy-correlation method ________________________ 13 

Bulk-aerodynamic methods______________________________ 13 

The Dalton approach________________________________ 14 

The aerodynamic or profile approach ________________ 14 

Energy-budget method _ ____ __ __ _ _ _ _ _ _ __ __ __ _ _ ____ __ __ ___ 15 

Combination methods _ __ _ _ _ _ __ _ _ _ _ _ _ __ __ __ __ __ _ _ __ __ _ _ _ 16 

Instrumentation _ _ _ _ _ __ __ _ _ _ _ _ _ __ _____ _ _ _ _ _ __ ____ _ _ _ _ __ __ _ __ 17 

The atmosphere: Wind and temperature __________________ 17 

Wind---------------------------------------------- 17 

Air temperature _ _ _ _ _ ___ _ __ _ _ _ _ _ __ _ __ _ _ _ _ ____ __ __ ___ 18 

Precipitation and water vapor __________________________ 18 

Rainfall ____________________________________________ 18 

Water vapor ________________________________________ 18 

Instrumentation-Continued Page 

Radiation ____________________________________ --- _______ F19 

Soil temperature, heat flow, and soil moisture ____________ 20 

Soil temperature ____________________________________ 20 

Heat-flow plates------------------------------------ 20 

Soil moisture ------~------------------------------- 20 

Water-use measurements ________________________________ 21 

The microclimate of a saltcedar thicket ---------------------- 22 

Why the microclimate?---------------------------------- 22 

The special observation periods-------------------------- 22 

Daily cycles of temperature and humidity ---------------- 23 

Winds ------------------------------------------------ 27 

Radiation ---------------------------------------------- 30 

Soil heat flux __ _ _ _ ____ __ _ _ __ _ _ _ _ _ _ __ ___ _ __ _ _ ______ _ ____ 30 

Soil temperatures -------------------------------------- 34 

Heat flux and water use -------------------------------- 34 

Turbulent transport ___ ___ _ __ _ _ _ _ __ ____ _ ___ _ _ _ _ __ ____ _ __ 35 

Carbon dioxide _ _ _ __ __ ______ ___ _ _ _ _ ________ _ _ _ __ __ _ _ _ __ 36 

Predicting water use________________________________________ 40 

Use of long-term means -------------------------------- 40 

Use of daily averages ---------------------------------- 42 

Short-term predictions _ __ _ _ _ _ __ _ _ ___ _ __ _ _ __ _ _ __ _____ _ ___ 43 

Discussion ------------------------------------------------ 47 

References cited _ _ _ _ _ __ ____ _ _ _ _ _ _ _ ___ _ _ _ _ __ __ __ __ ____ _______ 48 

Appendices ------------------------------------------------ 53 

A. Tables 

A-1 Hourly averages of windspeeds and wind direction 

A-2 Hourly averages of air temperature and vapor pressure 

A-3 Radiation data 

A-4 Windspeed, temperature, and water vapor data 

B. A nomogram for data reduction 

C. Symbols and dimensions 

ILLUSTRATIONS 

FIGURE 

Page 

1. A reproduction of plate 1 in Hales' (1727) "Vegetable Staticks" ------------------------------------------------ F4 

2. Map showing location of the project site---------------------------------------------------------------------- 5 

3. Sketch of the project site, southeast of Buckeye, Ariz., showing position of main features ________________________ 6 

4. Aerial photograph of the project site _ _ _ _ _ _ _ __ _ _ _ _ __ _ _ _ _ _ _ __ ____ _ _ _ _ __ __ _ _ _ _ __ _ _ __ _ _ _ _ __ __ _ _ _ _ __ __ _ _ _ _ __ _ _ __ _ 6 

5. Photograph of the project site from a 4-m-high platform ------------------------------------------------------ 6 

6. Graph showing mean monthly temperature __ ___ __________________________ ______ _____________ _________ ____ ___ 7 

7. Circular graph showing wind roses ___________________ ______________ ____________ _________ ____ _____ __ ____ _ _ ___ 8 

8. Circular graph showing wind directions ___ ____ ________ _________________ _________ _________ ____ __ _ ______ ____ ___ 8 

9-12. Graphs showing: 

13. 

14. 

15. 

9. Mean monthly wind speeds__________________________________________________________________________ 8 

10. Monthly rainfall____________________________________________________________________________________ 9 

11. Solar radiation ------------------------------------------------------------------------------------ 9 

12. Dew-pointtemperatures ---------------------------------------------------------------------------- 9 

Schematic drawings of leaf stomata__________________________________________________________________________ 11 

Example of a 16-point strip chart _ _ _ __ _ _ __ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ __ __ _ _ _ _ __ _ _ _ _ __ _ _ _ _ _ _ __ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 17 

Example of a windspeed record _ __ _ _ __ __ __ __ _ _ _ _ _ _ __ _ _ _ _ _ _ __ __ _ _ _ _ _ _ _ _ _ _ _ _ __ __ _ _ _ _ _ _ _ _ __ _ _ _ _ __ _ _ _ _ _ _ _ _ __ _ _ _ _ _ 18 

III 
IV CONTENTS 

Page 

FIGURE 16. Sketch of an air-temperature sensing system ______________________________________________ --------- _________ F19 

TABLE 

17. Diagram of the ventilation system -------------------------------------------------------------------------- 19 

18. Nomogram to compute vapor pressure________________________________________________________________________ 20 

19-22. Photographs showing: 

19. Radiation mast showing instrument installed -------------------------------------------------------- 21 

20. Adjustable nomogram for computing heat flow-------------------------------------------------------- 21 

21. Tensiometers installed in one tank ______ _ _________________ __ ___ ____ __ __ ________ ______ ____________ __ _ 21 

22. Reading of tensiometers at night _________ ________________ _ _________ _ ___ __ ____ ____ ___ _ ____ ______ __ 22 

23. Sample of an event recording chart -------------------------------------------------------------------------- 22 

24. Graph showing microclimate characteristics of the Buckeye project site ---------------------------------------- 23 

25. Graph showing the distribution ofwindspeeds within and above a stand of saltcedar ---------------------------- 27 

26. Example of a border-punched card used for analyzing wind profiles -------------------------------------------- 28 

27-45. Graphs showing: 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

27. How to determine roughness length, friction velocity, and zero displacement---------------------------- 29 

28. Relation between modified roughness length and windspeed ratios ------------------------------------ 30 

29. Modified roughness length versus stability ratio ------------------------------------------------------ 30 

30. March of radiation and heat flow for seven 3-day periods ---------------------------------------------- 31 

31. Methods of computing heat flow in the ~oil ---------------------------------------------------------- 32 

32. Heat flow into and out of the ground in an evapotranspirometer near Buckeye, Ariz. ____________________ 33 

33. Heat flow into and out of the ground in a lysimeter near Tempe, Ariz.---------------------------------- 33 

34. Sinusoidal 24-hour heat flow into and out of soils ---------------------------------------------------- 33 

35. Relation between amplitudes of daily temperature waves, damping depths, and depths of 0.05 x amplitudes 35 

36. Relation between soil heat flux and water use -------------------------------------------------------- 35 

37. Relation between windspeeds, temperatures, and vapor pressures -------------------------------------- 37 

38. Fluctuations of C02 content of the air and windspeeds over a saltcedar thicket __________________________ 40 

39. C02 fluctuations over a saltcedar thicket and its harmonic analysis ------------------------------------ 40 

40. Relation between C02 and water vapor fluxes -------------------------------------------------------- 40 

41. Average water use as computed by various formulas and compared with measured use __________________ 41 

42. Water use as predicted by the Konstantinov model versus measured use ________________________________ 45 

43. Water use as predicted by the Rider model versus measured use -------------------------------------- 45 

44. Hourly rates of evapotranspiration for two 3-day periods near Buckeye, Ariz. __________________________ 46 

45. Hourly "rates of evapotranspiration as measured and as computed using equation 26 ____________________ 47 

TABLES 

Page 

Analyses of soils at the Buckeye project______________________________________________________________________ F7 

Analyses of irrigation water at the Buckeye project ---------------------------------------------------------- 7 

Plants (other than saltcedar) observed growing at the Buckeye project site, 1961-66 ---------------------------- 10 

Values of'Y(il.T!Il.e) for a 24-hour period, May 5 and 6, 1966 -------------------------------------------------- 16 

Heat-flow plate calibration factors ___ _______ _________ _ ___ __ _ _ _____ _ ____________ __________ ______ __ __ ________ _ 21 

Some results of harmonic analyses of soil-temperature fluctuations inside and outside tank 6 for two 3-day periods 35 

Variables used in some models for predicting potential evapotranspiration ______________________________________ 41 

Comparison of different methods of estimating evapotranspiration for 1963 using the harmonic analysis data ____ 42 

Water use in tanks 2 and 4 during special observation periods with some results of harmonic analyses of the 

hourly data -------------------------------------------------------------------------------------------- 43 

Some climatic data pertaining to seven special observation periods of 3 days each ------------------------------ 43 

Water use during special observation periods of 72 hours each as measured in tanks 2 and 4 and estimated 

according to various equations __ _____ __ _ ______ _ ________________ _ ___________ _ __ ______ _____ _ ___________ _ _ _ _ 44 

Water use by saltcedar for 3 days in 1966 -------------------------------------------------------------------- 47 
Multiply SI Unit 

Area 

square centimeters (cm2) 

square meters (m2) 

square kilometers (km2) 

Electricity and Magnetism 

millisiemens (mS) 

Energy 

joules (J) 

Energy/Area Time 

watt/meter2 (W/m2 ) 

Heat 

watt/meter2·kelvin 

(Wm- 2 K- 1) 

joules/kilogram· kelvin 

(J kg-t K-tl 

Length 

nanometers (nm) 

micrometers (J.Lm) 

millimeters (mm l 

centimeters (em) 

meters (m) 

kilometers (km) 

Mass 

kilograms (kg) 

Mass IV olume (Density) 

kilogram/meter3 

(kg/m3 l 

Power 

watt (W) 

Pressure 

kilopascal (kPa) 

Temperature 

degrees Celsius (°Cl 

kelvin (K) 

Velocity 

meter/second (m/s l 

Volume 

cubic meters (m3 ) 

CONTENTS 

CONVERSION TABLE 

By 

0.155 

10.76 

247.1044 

1.0 

9.4 787 X 10-~ 

0.2388 

5.2895 X 10-3 

1.434 X 10-3 

0.1761 

2.388 X 10-~ 

0.3937 X 10-~ 

0.03937 

0.03937 

0.3937 

3.281 

0.621 

2.205 

6.24 X 10-2 

0.001 

3.412 

0.2388 

0.2953 

10 

1.8°C +32 

(K-273.151 1.8 +32 

2.237 

0.81 X 10-3 

To obtain inch-pound 

or cgs units 

square inches (in. 2) 

square feet (ft. 2

) 

acres 

millimhos (mmho) 

British thermal units 

(mean) (Btu) 

calories (call 

Btu/foot2·minute 

cal!centimeter2·minute 

(cal cm- 2 min- 1) 

Btu/poundmass ·oF 

mils 

inches (in.) 

foot (ftl 

miles 

pound mass (lbm) 

poundmass/foot3 

Btu/hour 

calories/second (calls) 

inches of mercury (in. Hg) 

millibar (mb) 

degrees Fahrenheit (°F) 

miles/hour (mph) 

acre-feet (a/fl 

v 

STUDIES OF EVAPOTRANSPIRATION 

WEATHER AND EVAPOTRANSPIRATION STUDIES 

IN A SALTCEDAR THICKET, ARIZONA 

By T. E. A. VAN HYLCKAMA 

ABSTRACT 

Water use by saltcedar, Tamarix chinensis, was studied from 1961 

through 1967 near Buckeye, Ariz. The test site was located on the 

rim of the Gila River flood plain and was bordered on the north, east, 

and west sides by fetches of dense saltcedar thickets 1 or more 

kilometers wide. On the south side, however, the fetch was less than 

100 meters. 

The climate of the area, typical of the Sonoran Desert, is characterized by low humidities, strong winds, and temperature extremes 

of <-lOoC in winter and >50°C in summer. Potential evaporation 

values are among the highest observed in the United States. 

Rates and quantities of evapotranspiration were observed in six 

plastic-lined evapotranspirometers (tanks) whose 81 square-meter 

surfaces were planted to saltcedar having heights and density equal 

to those of the surroundings. 

The test site was further equipped with instrumentation to measure the following data: solar short-wave radiation; long- and shortwave net radiation; albedo; humidity, temperature, and wind profiles 

in and over the vegetation; soil-temperature gradients and soil-heat 

flux; moisture content of the soil; and carbon dioxide content of the 

air. Detailed studies were made of the microclimate in and over a 

typical saltcedar thicket. Analyses showed that, above the vegetation, the wind profiles in more than 80 percent of the observations 

were logarithmic. Within the thickets considerable turbulence and 

irregular wind inversions (tunneling) occurred during daylight 

hours. 

It was concluded that transport constants for momentum, heat, 

and vapor are the same more than 80 percent of the time because 

plots of windspeed versus temperature at different heights and of 

windspeeds versus vapor pressure at those heights fall on straight 

lines. Fluxes of carbon dioxide and vapor are closely related. Vapor 

fluxes diminish, as do rates of photosynthesis, during hot afternoons 

when temperatures exceed 40°C, suggesting a variable stomatal resistance factor. 

Estimates of potential evapotranspiration rates using various 

models were plotted against measured values. For rough estimates of 

total yearly quantities of evapotranspiration, three independent 

methods gave values closest to those measured by evapotranspirometers with the shallowest (1.5 m) depth to water and with low 

salinity of soil moisture. (The effects of depth-to-water and of salinity 

have been discussed extensively in "Water use by saltcedar as measured by the water-budget method" (van Hylckama, 1974, U.S. 

Geological Survey Professional Paper 491-E). 

For short-term estimates (of the order of 1 hour) the 1966 combination method of C. H. M. van Bavel gave results that were too high 

during daytime hours. When appropriate corrections were made by 

taking stomatal and aerodynamic resistances into account, the calculated values fitted the measured ones very well. This shows that 

saltcedar reacts to extremely high windspeeds and temperatures by 

stomatal closure, thus diminishing evapotranspiration even though 

water is freely available (as it is in evapotranspirometers with favorable water and soil conditions). That riparian vegetation always uses 

water at a potential rate cannot be taken for granted, and quantitative estimates of salvageable water based upon that assumption may 

at times be far too large. 

INTRODUCTION 

SALTCEDAR AND OTHER PHREATOPHYTES 

Saltcedar, Tamarix chinensis, 1 is one of more than 70 

species listed by Robinson (1958) as phreatophytes, a 

word derived by Meinzer (1923; 1927) from the two 

Greek words cppeaTocr (phreatos = well) and cpvTov 

(phuton plant). These plants are called 

phreatophytes because they have root systems with access to ground water or to water in the capillary fringe 

above it. It has sometimes been assumed that they can 

use water at a rate equal to, or greater than, the rate of 

potential evapotranspiration, even during long periods 

of drought. Whether this is always true is debatable, as 

will be discussed later in this report. A fact is that 

many phreatophytes, especially saltcedar, grow densely along rivers ·and reservoirs, and the expression riparian vegetation (from the Latin ripa=border) seems to 

be more appropriate and descriptive. 

Phreatophytes were estimated by Robinson (1958) to 

cover 16 million acres (65,000 km2 in the 17 Western 

States and to consume nearly 25 million acre-feet (31 x 

109 m3) of water per year; they were expected to spread 

and to consume even more water in the years after 

1958. As for saltcedar alone, Robinson (1965) estimated that in 1961 there were 900,000 acres (3600 

km2

) covered by saltcedar using 3.5 million acre-feet 

(4.3 x 109 m3) of water per year. 

However there are reasons to believe that these 

numbers may be greatly exaggerated. For instance, 

'In earlier studies the names T. gallica L. and T. pentandra Pall. are used. The name T. 

chinensis Lour. is preferred now by taxonomists (Horton and Campbell, 1974). 

F1 
F2 STUDIES OF EVAPOTRANSPIRATION 

Robinson reported that of the 900,000 acres (3600 km2 ) 

just mentioned, about half were in Texas and that 

nearly half of those, or about 218,000 acres (880 km2 ), 

were located along the Pecos River and its tributaries. 

But that is far larger an area than the entire flood 

plain of that river and its feeders, including all agricultural land (J. S. Horton, written commun., 1977). 

Nonetheless, Robinson's data served to create active, 

even alarmist2 interest in the phreatophyte problem 

and have encouraged the development of many action 

programs. The data also created the desire to know 

how much water these plants actually use and whether 

there are means of making this amount available for 

more beneficial use. 

In this report procedures of studying water use are 

discussed, methods, of measuring evapotranspiration 

are presented, the microclimate of vegetation on a typical flood plain is described, and methods of indirectly 

estimating water use by saltcedar are evaluated. 

LITERATURE 

0. E. Meinzer (former Geologist-in-charge, Division 

of Ground Water, U.S. Geological Survey) who did 

much to call attention to the economic significance of 

phreatophytes in arid and semiarid regions, might 

have been surprised if he could have foreseen the 

amount of study that would be devoted to evapotranspiration and related topics. A single bibliography (Robinsort and Johnson, 1961) emphasizing papers on evaporation and transpiration from the United States from 

the early 1800's until 1958 contains more than 600 

titles. Humphreys (1962) assembled 804 references 

(mostly American) dated between 1802 and 1960. Horton (1973) collected over 700 references to evapotranspiration as related to phreatophyte management. 

There is very little overlap in these compilations, and, 

although there is a lot of speculation and empiricism, 

there is also very little in all this that specifically deals 

with the physical relationships between water use by 

phreatophytes and climatologic and meteorologic 

events. 

Research into the ecology of phreatophytes in general, and of saltcedar in particular, has been going on 

now for more than 25 years. Gatewood and others 

(1950) were probably the first to draw attention to the 

potentially large amount of water that saltcedar could 

transpire under favorable conditions. Since that time 

literature and study projects have proliferated enormously. The Phreatophyte Subcommittee of the Pacific 

'In the literature, saltcedars have been described as "water hogs'' !Douglas. 19541, "aggressive" !Robinson, 196.51, "greedy"IDouglas, 19651, "insidious" ISebenik and Thames, 

19681, "thieves" !Robinson, 19521, and "water-stealing culprits" I U.S. Information Service, 

19651. These expressions indicate the emotional and propagandistic attitude that is sometimes taken towards the phreatophyte problem. 

Southwest Interagency Committee (1965) compiled 

more than 500 references on phreatophytes and related 

subjects, nearly all more recent than 1950, while 

Robinson (1964) listed 48 projects pertaining to 

phreatophyte research. By 1965 a large number of 

these projects had been in progress for 2 or more years. 

In spite of the dedicated efforts of many agencies, including the U.S. Department of Agriculture's Agricultural Research Service, the U.S. Forest Service, the 

U.S. Geological Survey, and several University Experiment Stations, the complexity of the problem is such 

that we are still unable to just go into the field, set up 

some, preferably simple, instrumentation and determine in the course of 2-6 months how much water is 

used by phreatophytes in that area and to estimate 

with reasonable certainty if and how much water can 

be diverted to economical use by eradication or conversion of the vegetation. 

The present study clarifies some of the problems and 

suggests possible solutions, but also describes in detail 

the difficulties one can expect while trying to solve 

such problems. Many of them have been discussed in 

the following papers, reporting on one or more aspects 

of the Buckeye Project (in chronological order): 

van Hylckama, T. E. A., 1960, Measuring water use by saltcedar: 

Proc. Fourth Ann. Ariz. Watershed Symposium, Watershed 

Management Div., State Land Dept., State of Ariz., Phoenix, p. 

22-26. 

--1961, Natural recharge of ground water, in Fletcher, J. E., and 

Bender, G. L., eds, Ecology of ground water in the southwestern 

United States: 37th Ann. Meeting of the Southwest and Rocky 

Mountain Div. of Am. Assoc. for the Adv. of Sci., Arizona State 

Univ., Tempe, Ariz., p. 21-41. 

--1963, Growth, development, and water use by saltcedar 

(Tamarix pentandra) under different conditions of weather and 

access to water: Publ. 62, Intern at. Assoc. Sci. Hydrology, 

Comm. for Evaporation, UNESCO, Paris, p. 75-86. 

Shakur, Abdul, 1964, Evapotranspiration from a stand of saltcedar: 

Masters Thesis, Dept. of Hydrology, Tucson, Univ. of Ariz., 73 p. 

van Hylckama, T. E. A., 1966a, Evaporation from vegetated and 

fallow soils: Water Resources Research, v. 2, p. 99-103. 

--1966b, Effects of soil salinity on the loss of water from vegetated and fallow soil: Publ. 83, Internat. Assoc. Sci. Hydrology, 

UNESCO, Paris, p. 636-644. 

--1968a, Water level fluctuations in evapotranspirometers: 

Water Resources Research, v. 4, p. 7 1-768. 

--1969a, Photosynthesis and water use by saltcedar: Bull. 

Internat. Assoc. Sci. Hydrology, v. 14, p. 71-83. 

--1970a, Water use by saltcedar: Water Resources Research, v. 

6, p. 728--735. 

--1970b, Winds over saltcedar: Agr. Meteorology, v. 7, p. 217-

233. 

Ripple, C. D., Rubin, Jacob, and van Hylckama, T. E. A., 1972, Estimating steady-state evaporation rates from bare soil under 

conditions of high water tables: U.S. Geol. Survey Water-Supply 

Paper 2019-A, 39 p. 

van Hylckama, T. E. A., 1974, Water use by saltcedar as measured 

by the water budget method: U.S. Geol. Survey Prof. Paper 

491-E, 30 p. 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F3 

--1975, Estimating evapotranspiration by homociimates: Geog. 

Rev. , v. 65, p. 37-48. 

The following papers were presented but only 

printed in abstracts: 

van Hylckama, T. E. A., 1962, Use of water by saitcedar as measured 

by the water budget method: Jour. Geophys. Research, v. 67, p. 

3535. 

--1964, Evaporation from bare soil: Am. Meteorol. Soc. Bull. , v. 

45, p. 540. 

--1968b, Wind profiles over saltcedar: Am. Meteorol. Soc. Bull., 

V. 49, p. 769. 

--1968c, Carbon dioxide and saltcedar growth: Southwestern 

and Rocky Mountain Div. of the Am. Assoc. Adv. Sci., Proc. 44th 

Ann. Meeting, El Paso, Texas, p. 5. 

--1969b, Plant growth and water use as affected by salinity of 

soil moisture and density of stand: Arid Lands in a Changing 

World, Abstracts of contributed papers, Am. Assoc. Adv. Sci., 

Comm. on Arid Lands, Tucson, Ariz., 2 p. 

--1972, Leaf resistance in saltcedar, in Adams, W. P. and Helleiner, F. M., eds., International Geography 1972, Univ. Toronto 

Press, p. 204. 

ACKNOWLEDGMENTS 

Under a memorandum of understanding dated May 

4, 1959, "concerning studies of consumption of water by 

phreatophytes and related problems," the U .S. Geological Survey "will take the leadership in the preparation 

of reports and the publication of results of the studies 

conducted under the agreement" while the U.S. 

Bureau of Reclamation would be mainly responsible 

for the construction of the tanks and the flood protection of the area. In particular, the help and assistance 

of Curtis W. Bowser, Robert Mason, and Leslie C. 

Pampel, all members of the Bureau, is gratefully acknowledged. 

The late Thomas W. Robinson together with Curtis 

W. Bowser conceived the idea of using 20-mil (about 

0.5 mm) black vinyl plastic as a cheap and easy-toinstall lining for the evapotranspirometers and supervised the installation of the first two. G. Earl Harbeck, 

Jr., gave general supervision to the work of the Survey. 

Oliver E. Leppanen designed and constructed the 

wind-recording equipment. Jack L. Sidders supervised 

the construction of the tanks and aided in the installation of the complex plumbing and the electrical circuitry required for the study. He also performed nearly 

all the maintenance during the 6%-year duration of the 

project. 

Many others, among them a number of graduate and 

undergraduate students, helped in taking around-theclock readings for periods of 3 or 5 days and assisted in 

the very time-consuming transcription of the striprecording data into numerical and calibrated values. 

I am grateful for very helpful criticisms of the first 

draft of this report made by J . S. Horton, U.S. Forest 

service, (retired), and D. C. Davenport of the University of California at Davis (Dept. of Land, Air, and 

Water Resources). 

METHODS OF STUDYING WATER USE 

A BIT OF HISTORY 

All plants need water for growth and development 

and most of them must transpire considerable quantities in order to grow and develop. That plants need 

water has been known undoubtedly since the time our 

earliest ancestors began to practice agriculture. One of 

the first to measure this water use was van Helmont 

(1655), who found that a 5-pound willow sapling in 5 

years grew up to a 169-pound tree and concluded that 

the 164 pound increase came exclusively from the water.3 

That plants transpire however, was first described by 

Stephen Hales (1727). He presented data on the quantities of water transpired by different plants under different conditions. Figure 1 shows just two of the dozens 

of experiments described by Hales and gives an idea of 

his painstaking work. Hales' studies were not improved upon until nearly a century later when many 

physiologists and physicists began to study the plantsoil-water relationships more intensively. I~itially the 

transpiration phenomenon was only of scientific or 

academic interest; later, when economic considerations 

became urgent, the amount of water transpired without apparent benefit to man became the center of attention and the concept that transpired water is wasted 

became prevalent. 

To find out how "wasteful" saltcedar is, one must 

know how much water the plant uses under various 

circumstances. The basic method is essentially the 

same as that used by van Helmont and Hales: measure 

the water applied and after a definite time reweigh 

container plus plant and compute from these data the 

water lost by transpiration and evaporation from the 

soil. The crux is that in this manner only one or at most 

a few plants are studied, and one cannot safely extrapolate that use to hectares or acres much less to 

thousands of them planted to such crops or covered by 

such vegetation. 

Therefore, since vegetation conversions, of which 

eradication is the extreme, cost money, one must make 

an estimate of the quantities of water that are used by 

the total vegetation to be converted or eradicated for 

the purpose of computing cost-benefit ratios. 

:l"Omnia***vegetabilia***ex solo aquae elemento prodire hac mechanica didici "- freely 

translated: "That all vegetable matter comes solely from elements of water, l proved by this 

experiment." Then fOllows the descr!ption of how van Helmont planted, weighed, watered, 

and reweighed his willow tree. The, actual weight of van He I mont's pounds is not certain. 
F4 STCOIES OF EVAPOTRAI'\SPIRAT!Ol'\ 

-----------------------~~~~= ~:~ 

FIGURE 1.-A reproduction of plate 1 in Hales' 

(1 727) "Vegetable Staticks." The drawings were 

used to illustrate experiments I and VI. Plant on 

the left is a spearmint, Mentha spicata; the one 

on the right is a sunflower, Helianthus annus. 

HYDROLOGIC METHODS 

The most straightforward way of determining water 

use by saltcedar by means of containers (the lysimeter 

method, mentioned in any textbuok on hydrology) was 

discussed in another chapter of the Professional Paper 

491 series (van Hylckama, 1974). However, finding a 

suitable location for evapotranspirometers or lysimeters is often difficult; the construction is expensive; and 

the maintenance, until reliable results are obtained, 

very time consuming, especially if one is, as in this 

case, dealing with tall vegetation. (Saltcedar can reach 

a height of 5-7 m by the time it is 10 or more years old.) 

There are two other ways to measure water use by 

hydrologic (sometimes called water-balance) methods. 

The first is the natural-catchment or watershedhydrology method of which Culler'(1 970) described a 

good example. Troubles with this method are discussed 

in the reference and will not be dealt with her e. The 

second, called the "change-in-soil-moisture" method, 

consists of monitoring the changes in soil moisture, 

usually over the depth of the root zone. However, instrumentation difficulties do not make this method 

very reliable over periods of less than 7- 10 days, and 

large errors may occur if drainage is not measured or 

goes unnoticed (van Bavel and others, 1968) or if fluxes 

from the water table are neglected (E. P. Weeks, written commun., 1978). 

MICROMETEOROLOGIC METHODS 

Answers could be obtained much faster, and certainly with less destruction, if one could evaluate the 

climatic and meteorological inputs that determine the 

reactions of the plants and the amount of water used or 

transpired by them. Such methods of measuring water 

use or evapotranspiration are available, do not require 

lysimeters, and do not depend on empiricism or educated guesses. They can be classified as vapor-flow, 

bulk-aerodynamic, energy-balance, and combination 

methods. 

There are two types of vapor-flow methods. The 

atmospheric-water-balance method is analogous to the 

hydrologic balance in that it considers the quantities of 

moisture entering and leaving a column of air (rather 

than that entering or leaving a mass of soil or a 

watershed ). The other is known as the eddycorrelation, eddy-flux, or eddy-transfer method. In this 

method the vertical turbulent fluxes of vertical wind 

and vapor in the atmosphere are measured over very 

short intervals, a few seconds at most. The method requires extremely sensitive wind and water-vapor sensors and needs elaborate recording systems to assimilate the enormous amount of data accumulating during 

a day or even an hour. Since neither the sensors nor the 

integrating circuits or computers were available at the 

Buckeye Project, this method was not used but it will 

be mentioned together with the theory of the other 

methods and their instrumentation. 

The bulk-aerodynamic and the energy-balance 

methods go back, in principle at least, more than 2000 

years. When Aristotle noticed that water, put on a 

plate, slowly disappeared, he said, "the wind does it." 

Others, during his lifetime, accused the sun. While the 

wind causes the aerodynamic or turbulence process, 

the sun causes the energy- or heat-balance process. 

These two systems still operate today; they are no 

longer theory as they were in Aristotle's days because 

now we have the instruments to measure them and 

their effects. 

The fourth method is actually a fusing of the effects 

of wind and sun and a number of equations have been 

proposed to estimate water losses by this combination 

method. 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F5 

EMPIRICAL METHODS 

Faster methods (apart from more or less educated 

guesses) involve one of the many empirical equations 

for computing potential evapotranspiration from readily available climatological parameters. Because such 

equations are empirically derived, claims as to their 

accuracy or inaccuracy are often without experimental 

foundation. In order to test the claims, many field experiments have been set up, mostly in the southwestern parts of the United States, where vegetation conversion is considered to be most effective. However, too 

often only scant attention is paid to the effects of the 

ambient weather upon water losses, and efficient verification of accuracy and applicability of an empirical 

relationship remain debatable if not impossible. For 

instance, only a few of the projects listed by Robinson 

(1964) and only a few of the articles compiled by the 

Phreatophyte Subcommittee (1965) are concerned with 

the input forces and ambient conditions that determine 

the rate of water use by phreatophytes, and even fewer 

articles if one considers saltcedar alone. Of these few 

the papers by Gatewood and others (1950), Blaney 

(1957), Dylla and Muckel (1964), McDonald and 

Hughes (1968), Robinson (1970), Culler (1970), Culler 

and others (1970), Hughes (1972), and McQueen and 

Miller (1972) are probably the most pertinent. 

THE BUCKEYE TEST SITE 

LOCATION, SOIL, AND WATER 

The study site and its characteristics have been described in detail elsewhere (van Hylckama, 1974), and 

only a brief statement is required here. Figures 2 and 3 

show the location and the layout of the project. Figure 

4, an aerial photograph of the area, and figure 5, a 

photograph of the test site as seen from a 4-meter-high 

platform, show the density of vegetation on the flood 

plain at the experimental site. 

In choosing a location for evapotranspirometers an 

important consideration is the extent of the vegetation 

surrounding the proposed area. As a rule of thumb, it is 

assumed that homogeneous vegetation should extend 

over a radius 100 times the height of the vegetation 

under study (see Monteith, 1973, for a detailed discussion). As figure 4 shows, the vegetation does not extend 

that far to the south, and the map of figure 3 indicates 

such a desirable fetch only in a southwesterly direction. However, under natural conditions saltcedar predominates in this region as flood-plain vegetation 

along rivers and washes or as strips of riparian vegetation along lakes and reservoirs. Especially when winds 

blow normal to rather than parallel to the river, the 

fetch will quite likely be too short and oasis effects are 

T1S 

R3W 112°31' 

LOWER RIVER ROAD 

0 

I 

0 

l 

/ 

I 

1 KILOMETER 

I 

3000 FEET 

COLORADO 

RIVER 

FIGURE 2.- Location of the Buckeye test site. 

to be expected. The large rates of evaporation one finds 

in the literature are most likely due to these facts of 

nature. 

Whereas then the location of the evapotranspirometers at the Buckeye Project was as good as one can get 

in an area of flood-plain vegetation, and whereas the 

instrumentation was quite adequate to provide reliable 

results, the construction had one big drawback: no provisions were made for draining the tanks. The consequences of the resulting increase in salinity of the 

ground water in the tanks have been discussed 

elsewhere (van Hylckama, 1974). However, when salinity was abated by proper flushing, reliable waterbudget data could be derived for short periods of the 

order of days and hours as will be shown later on. 

Even though flushing made possible short-period determination of evapotranspiration, one can never be 

sure how representative such evapotranspirometers 

are. The Buckeye tanks probably gave results that 

would be valid for the flood-plain vegetation along the 

Salt River between Tempe, Ariz., and the confluence 

with the Gila River, from that confluence a few tens of 
F6 STUDIES OF EVAPOTRANSPIRATION 

Dense saltcedar 

,..---__ - ( ---- ~--------~-; 

J 0 Bare ground / 

Low fence 

0 0 0 N 

,oo~ 

1 120m [I] 0 p 

~ 

4 o!l ~ ""C 

Q)Q> "' w ""C "' (.) (.) Q) Q) 

.... c >a~ (.) (.) 

-Q) c .... 

1ll-+- Q) ~~ Q)::: [Q]o ~ ffi] ::: Q) "'0 

~....I 0 ~ 

....1 "' 0 M 0 c. Platform 0 (/) 

oc 0 Office, pump, 

and storage 

rv.~ 0) 

0 Low fence 

Cotton field 

0 50 METERS ""C 

"' 0 

0 !50 FE ET II: 

FIGURE 3.-Sketch of the project site, southeast of Buckeye, Ariz., showing position of main features. P, R, Ware 

instrument masts for wind and temperature profiles, 

radiation, and wind direction respectively; M and 0, additional masts. Numbers 1 through 6 indicate evapotranspirometers forming part of the saltcedar thickets, 

constructed in 1959. Tanks 7 through 11 (not indicated 

in the figure) were constructed in 1962 in the area 

marked "bare ground." Open circles indicate location of 

access tubes to determine soil moisture outside the 

tanks. 

kilometers upstream along the Gila and downstream to 

Gila Bend, Ariz., provided depth to ground water and 

soil-moisture salinity are taken into account. 

The soil at the project site is a sandy to fine sandy 

loam belonging to the Entisol soil order. It consists 

mainly of alluvial sediments deposited by floods of the 

Waterman Wash. Table 1 presents some physical and 

chemical characteristics. The physical ones are averages of 12 samples taken from evapotranspirometers 

number 1 through 6 (see fig. 3). The chemical ones are 

presented as the range of samples taken from the 6 

tanks in 1963 and as the range of samples taken from 

all 11 tanks in 1966. The large range and the changes 

between the 1963 and 1966 data are mostly due to the 

treatments of the tanks, such as flushing, change in 

quality of water, and so on. 

Water for irrigating tanks and surroundings came 

FIGURE 4.-View from an airplane of the project site shortly after 

planting the first five tanks (1959). Camera looks to the north. 

FIGURE 5.- View of the project site in 1963 as seen from a 4-m-high 

platform. The main instrument mast is visible right of center. 

White Tank Mountains north of Buckeye in the left background. 

Camera looks to the northeast. Arrow points to the mast carrying 

radiation instruments. See fig. 19. 

from a well drilled to a depth of 25 m in 1959 and 

deepened to 43 min 1960. The change in quality due to 

this procedure is clearly shown in the data of table 2. 

CLIMATE 

The climate at the project site is arid. Mean annual 

temperature at Buckeye is 20.6°C and mean annual 

rainfall is 190 mm. 

The monthly mean temperatures at the project site 

do not vary much from year to year. The means of the 

61Jz years of operation are presented in figure 6, which 

also shows the long-time means for the town of Buck-
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F7 

TABLE 1.-Analyses of soils at the Buckeye project TABLE 2-Analyses of irrigation water at theBuckeye project 

[USWCI:: United States Water Conservation Laboratory, (USDA I, Tempe, Arizona, U of A: [All analyses made by the U.S. Geological Survey. All values in milligrams per liter except 

The Umversity of Anzona, Agricultural Experiment Station, Tucson, Arizona; USBR: where otherwise indicated] Uni~ States Bureau of Reclamation, Phoenix Development Office, Soils Laboratory, 

Phoemx, Anzona. Physical characteristics are expressed as mean percentage of 12 samples and standard deviations. Moistu~e characteristics are expressed as percentage H20 by volume. One milhbar = 100 N/m = 100 Pa; a- = standard deviation] 

Mechanical analysis of sandy loam, USWCL 

Classification Particle diameter 

clay 

silt 

silt 

less than 2 JLm 

2-20 JLm 

21-50 JLm 

sand over 50 JLm 

Chemical analysis 

Chemical sample Depth 

(cml 

Ca + Mg (Meq/100 gl __________ 25 

75 

125 

Na + K (Meq/100 gl __________ 25 

75 

125 

C1 (mg/U ____________________ 25 

Total dissolved 

75 

125 

solids (g/L) __________________ 25 

75 

125 

pfl ____________________________ 25 

75 

125 

U of A 1963 

1.8-4.7 

1.8-4.8 

1.6-3.5 

1060-1850 

530-2530 

200- 830 

7.9---8.1 

7.9---8.1 

8.0-8.1 

Moisture characteristics 

Millibars Percent 

20 __________________________ 40.0 

40 __________________________ 33.5 

80 __________________________ 29.2 

120 __________________________ 26.9 

160 __________________________ 24.7 

200 __________________________ 22.2 

300 __________________________ 17.3 

500 __________________________ 13.2 

1000 __________________________ 10.6 

Percent 

7.0 ± 0.8 

9.2 ± 3.0 

24.5 ± 3.0 

59.2 ± 7.4 

USBR 

1966 

0.3-1.9 

0.3-1.3 

0.2-2.0 

0.6-4.7 

0.8-4.4 

1.0-5.3 

620-5950 

1110-7650 

1280-9650 

2.6 

2.2 

1.9 

2.4 

3.8 

4.7 

4.1 

3.2 

2.2 

1.7-14.0 

2.6-13.6 

3.0-10.7 

7.5-8.1 

7.5-7.9 

7.5-7.8 

eye according to Weather Bureau data (U.S. Dept. of 

Commerce, 1955). 

Maximum temperature at Buckeye may reach 49°C, 

usually in July and August, but at the test site, temperatures as high as 52oC have several times been recorded and, whereas the lowest temperature at the 

Buckeye weather station is given as -l0°C, temperatures as low as -l2°C at the test site were not uncommon. In winter, the above-ground plumbing had to be 

protected with electrical heating coils. 

Wind directions are highly variable. Figure 7 shows 

wind roses for the calendar year and for the growing 

seasons at the project, compared with long-term averages observed at Sky Harbor Airport east of Phoenix, 

June 9, May 11, Feb. 20, Apr. 26, May 6, 

1959 1960 1961 1962 1963 

Si02 ---------------------- 27 31 46 35 31 

Ca --------- -------------- 290 475 505 475 396 

Mg ------------------------ 39 91 126 125 122 

Na + K ____________________ 512 960 1200 1250 1150 

HC03 ------- -------------- 207 366 416 403 405 

so, ________________________ 265 662 836 878 836 

Cl --------------- ___ 1140 1880 2250 2250 1980 

Total dissolved solids ____________ 4280 4860 5210 4720 

Hardness as CaC03 -------- 880 1560 1780 1700 1490 

pH-- ----------- 7.4 7.4 7.5 7.3 6.8 

Specific 

conductivity 

1-'mho/cm 

at 25°C _______ ----------- ___ 4.30 6.97 8.21 8.32 7.57 

40 

(/) 

:::> 

(/) 

....J 

UJ 

u 30 (/) f UJ 7 UJ 

a: / <.:) 

UJ / 0 20 / z /( 

UJ- /"-..BUCKEYE, ARIZONA 

a: '/ :::> / 1-

<( / a: 10 / UJ 

a.. 

~ 

UJ 

1-

J F M A M J J A S 0 N D 

FIGURE 6.-Mean monthly temperatures (1961-67) at the 

project site and 30-year means atBuckeye, Ariz. 

less than 80 km to the east of the test site. It is clear 

that the main directions at the airport are E and SE, 

while those at the project are NE and SW or W., This 

difference is undoubtedly due to the effect of the 

mountains to the north and east of the project site. The 

wind directions also vary considerably by time of day 

as shown in figure 8. The significance of these wind 

directions will be shown in the discussion of the advective term in the energy- and mass-transfer equations. 

Windspeeds also are quite variable. The monthly 

means for the 2 years for which complete data were 

available are given in figure 9; the data are compared 

with long-time averages observed at the Sky Harbor 

Airport. That the speeds at the airport are generally 

higher is not surprising because they were measured at 

the control tower level and those at the project at only 

4m. 
F8 STUDIES OF EVAPOTRANSPIRATION 

Whole year April-September 

Project 1962 

Project 1965 

Sky Harbor 28-year mean 

FIGURE 7.-Wind roses for the Buckeye project site (1962, top; 

1965, middle) and for Sky Harbor Airport, Phoenix, Ariz. 

(1930-58 mean values, bottom). Means for the calendar year 

(left); for the 6 warmest months of the year (right). Numbers in 

the centers are mean windspeeds, in meters per second. 

Rainfall, as shown in figure 10, is quite erratic and 

scanty although 1965 and 1966 showed annual totals of 

more than 200 mm each and are higher than the longterm averages given by the broken line in that figure. 

Solar radiation is intense, as can be expected. Figure 

11 shows the observed radiation as a percent of the 

maximum possible radiation at the project-site 

latitude; the atmospherical transmission coefficient is 

assumed to be 0.9. Note the dip in radiation intensity 

due to thunderstorm activities in the late summer. 

Finally, the fluctuations in the dew-point temperature, given in figure 12, were computed from observed 

air temperatures and relative humidities. 

VEGETATION 

As can be seen from figure 4, a considerable amount 

of clearing of the saltcedar vegetation was necessary 

~'" ·ff} ··~/T -~ ·-: .• :.-" ~ ' ·-: .• :.-" 

~ "'" ~ ~- "M /~ 

~r ~~- '"'m 

~" """ ~ ~~ :.~,::~ 13 - 18 13 - 18 19 - 24 

July August 

FIGURE 8.-Mean wind directions for 6-hour and daily periods for the 

months of May, June, July, and August, 1965. Numbers in the 

center are windspeeds for the periods, in meters per second. 

0 

z 

0 

u 

w ___ ........ '', 3.0 C/) \,/28-year mean / 

0: //1\ ', values w 

c.. // \ .... ..... 

// ..... C/) ' ' 0: r;r-- ' w 

1- )/ ' w ' ~ ' ' 

z /I ' ' / l ' ..... ..... 

6 

w 

w 

c.. 2.0 C/) 

0 

z 

~ 

>-

...J 

I 

1-

z 

0 

~ 

z 

<( 

w 1.0 ~ J F M A M J J A s 0 N D 

FIGURE 9.-Mean monthly windspeeds at the Buckeye project 

site in 1962 and 1965, and at Sky Harbor Airport, Phoenix, 

Ariz. (1930-58 mean values). 

before the evapotranspirometers could be installed. 

But as figure 5 shows, by 1963 vegetation on the 

cleared area was nearly equal in height and density to 

the original one. In open places, inside as well as outside the test area, a number of grasses and forbs grew 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SAL TCEDAR THICKET, ARIZONA F9 

I 

1-

z 

0 

100 

50 

1962 1963 

~ oL-~~L------i~~~--~--~--~~=-~~ 

a: 

w 

~ 

C/) 100 a: 

w 

rw 

~ 

j 50 

~ 

z 

_j 

...J 

<( 

LL. 

z 

<( 

a: 

1967 

/

BUCKEYE, 

ARIZONA I 

/--" I ~ 

S N J M M J S N J M M J 

FIGURE 10.-Monthly rainfall at the Buckeye project site and 

monthly mean (1930-58) at Buckeye, Ariz. 

up from seeds washed in with floods of the Waterman 

Wash. Some of these are roadside weeds, others are 

members of the mesquite associations occurring outside the riparian areas along the lower Gila River and 

its tributaries. Table 3 gives a list of these plants together with some characteristics. According to Kearney and Peebles (1960), quite a few of the species normally grow at higher altitudes. They are washed down 

in the frequent floodings of the Waterman Wash. It is 

also noteworthy that none of these plants is known to 

be an indicator or edificator4 (Chikishev, 1965) of soilor soil-moisture characteristics, except possibly pigweed, Chenopodium rubrum, and the quail plant, 

Heliotropium curassavicum, which seem to prefer alkali or saline soil. 

Soon after the replanting of the cleared areas (fig. 4 

is an example) most of this herbaceous vegetation disappeared, leaving saltcedar as a nearly 100-percent 

dominant species. There is undoubtedly a shading effect, but an important cause of this phenomenon is the 

capacity of saltcedar to exude salt (Campbell and 

Strong, 1964; Hem, 1967) through openings in the 

• An edificator is a plant species that is an indicator of more than one soil or soil·moisture 

characteristic. 

C/) 

...J 

w 

() 

C/) 

ttl 10 a: 

(.9 

w 

Cl 

~ 

1-. 

z 

0 

~ 

$: 

w -10L__L __ L__L __ ~~--~~--~--~~--~~. 

Cl J F M A M J J A S 0 N J 

FIGURE 12.-Dew-point temperatures at the Buckeye project; 

plot based on mean values for 1961-67. 

scalelike leaves that are constructed very much like 

the water-guttating hydathodes one finds in hydrophytes and other plants growing under condition~ of 

frequent high relative humidities. Since saltcedar 1s a 

halophyte, such glands could well be named halohodes 

(van Hylckama, 1966). The exuded crystals are hygroscopic and on cool mornings tiny drops are formed on 

the leaflets. This salty ndew ," from which the saltcedar 

gets the first part of its common name, falls on the 

ground and kills any emerging seedling. One rar~ly 

finds an intruder in a saltcedar thicket. The conductivity of the soil-moisture extracts taken from th~ surfaces of the soils at the project site was often as h1gh as 

45 mmho/cm at 25°C (millimho per centimeter), or 45 

mS/cm (millisiemen per centimeter) at 298°K. 

Another point of interest is the genotypical variability of saltcedar. The plant can have the appearance of a 

small tree, as along the lower Colorado River near 

Yuma, Ariz., or it can take on a more shrublike appearance as at the Buckeye Project site, or the plants 

can be tall but spindly, as along the Rio Grande near 

Bernardo, New Mex. The color of the flower can also 

vary from red through shades of pink to near white; the 

time and character ofbloom can also vary greatly. Variations are mostly due to genetic characteristics and 

not to environmental factors (J. S. Horton, written 

commun., 1977). 
FlO STUDIES OF EVAPOTRANSPIRATION 

TABLE 3.-Plants (other than saltcedar) observed growing at the Buckeye project site, 1961-66 

[Abbreviations used in columns are the following: For value of plant: W ~ymbolizes weeda1 ~d F

1 forage pladn~. For abuhndaFce: a,_ a_bunlpi~~~~~ cEc~~pem~~-1°fn¥::;lc~db~r~~ ;i~c~l:::~~; saline soils, dr, dry areas; gs, gravelly s01l; rna, mOist areas; rs, road sides; ss, sandy so1 s, w , waste an s, ws, was es. or ongin o · , ' ' than European; and (-I, no specific information available. Nomenclature follows Kearny and Peebles (1960)] 

Plant Value 

Annuals and Biennials 

Grasses: 

Eragrostis cilianensis (love grass)--------------------------------W 

Schismus barbatus (six week grass) ______________________________ F 

Polypogon monspeliensis (rabbit foot grass) ________________________ F 

Echinochloa colonum (cock spur) ___ ---------------------------- _F 

Forbs: 

Eriogonum deflexum (skeleton weed) ---------------------------- -

Chenopodium incisum (goose foot) ------------------------------ - Chenopodium rubrum (pig weed) --------------------------------W 

Chenopodium leptophyllum (goose foot) -------------------------- - Salsola kali (Russian thistle)------------------------------------W 

Acanthochiton wrightii (green stripe) ____________________________ F 

Abronia villosa (sand verbena) ---------------------------------- - Argemone intermedia (prickle poppy) ---------------------------- - Tribulus terrestris (caltrop) _________ ---- ____ ---------------------W 

Mentzelia albicaulis (blazing star)_ _______________________________ - !Vama demissum ________________________________________________ -

Amsinckia tessellata (fiddle neck) - __ -----------------------------

Plantago purshii (plantain)-------------------------------------- - Verbesina encelioides (crown beard) _---- ____ --------------------- Palafoxia linearis _________________ --- -------------------

Pectis papposa (fetid marigold) --- __ -----------------------------

Sonchus oleraceus (sow thistle)----------------------------------W 

Perennials 

Grasses: 

Cynodon dactylon (Bermuda grass) ------------------------------W 

Sorghum halepense (Johnson grass)------------------------------W 

Forbs: 

Sphaeralcea ambigua (globe mallow) ---------------------------- Petalonyx thurberi (sandpaper plant) _______ ---- ____ ------------- Ment?elia puberula (stick leaf) _____ ---- ____ -- ____ --------------- Heliotropium curassavicum (quail plant) _________________________ _ 

Solanum elaeganifolium (horse nettle)----------------------------

!Vicotiana trigonophylla (tobacco) _ ------------------------------- Shrubs: 

Prosopis juliflora (mesquite) _____ ------------------------------- - Cercidium floridum (palo verde) --------------------------------

Lycium fremontii (desert thorn)----------------------------------

Encelia farinosa (brittle bush) ___ -------- __ ---------------------

Quantity 

a 

a 

c 

c 

a 

a 

a 

a 

a 

a 

c 

Habitat 

wl 

ss 

rna 

rna 

ss 

dr 

as 

dr 

wl 

dr 

ss 

ss 

rs 

dr 

ss 

ss 

dr 

wl 

ss 

gs 

wl 

wl 

dr 

ss 

gs 

as 

ss 

ws 

ws 

ws 

ss 

gs 

Origin 

E 

E 

I 

E 

E 

E 

E 

E 

EVAPOTRANSPIRATION AND 

ITS CALCULATION 

Although evaporation, transpiration, and evapotranspiration all involve the same process, namely the 

change of water into vapor, a distinction should be 

made between· evaporation and transpiration. Such a 

distinction has not always been made. For instance, 

Thornthwaite (1948), Penman (1956), and many other 

workers, at times considered vegetation to be a mere 

inert transporter of water or vapor, but later investigators found that at least some plants exercise a pronounced control over the rate of their transpiration. 

See also Monteith (1963), Tanner (1963), and (for lively 

discussions) Lee (1967; 1968a; b), Idso (1968), and van 

Bavel (1968). 

EVAPORATION 

methods consider the transformation of water into 

vapor as a surface phenomenon, similar to what happened to the water on Aristotle's plate when ex~osed to 

sun and wind. If the plate is covered and remains at a 

constant temperature, the number of molecules leaving the water surface eventually become equal to the 

number entering the surface, at which time the air 

above the water is said to be saturated with water 

vapor. Any increase in water molecules in this space 

would cause condensation and the moisture would fall 

back to the water surface. If the cover is removed and 

wind blows over the plate, vapor is taken away. The 

original balance is upset, an excess of molecules leaves 

the water surface and the supply is depleted at a rate 

which (among other things) depends upon the speed of 

the air moving over the surface. This in brief is the 

principle of the aerodynamic method. 

Originally two methods to compute evaporation indirectly were· tested over open water surfaces. Both 

On the other hand, assuming that there is no wind 

but that there is a change in temperature, (which 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET. ARIZONA F11 

under natural conditions would be caused by the sun's 

energy), the water is heated, the molecules move faster 

and more leave the surface. The air in turn also becomes heated and capable of absorbing more water 

molecules since the solar heat represents an incoming 

supply of energy which is transposed into the energy 

represented by warmer water and warmer air. Computations based on this principle are called energybudget methods. 

Thus, if the wind during the mass-transfer process 

does not heat or cool and if the air flowing over the 

surface is already saturated, no evaporation will take 

place due to the movement of air. Under such circumstances one could use an energy-budget method pure 

and simple. Under natural conditions, however, one 

must combine the principles of energy budget and 

aerodynamics if one desires to arrive at reliable results. This is especially true in areas where the air is 

dry and the wind very often strong-in other words, 

under conditions typical for the Buckeye Project and 

for the vast majority of areas in the arid and semiarid 

southwest. 

TRANSPIRATION 

If evaporation takes place through the surfaces of 

plants, it is called transpiration. In most plants growing in or under water or growing under very moist 

conditions, much of this transpiration takes place 

through the epidermis and its waxy covering, the cuticle (from the Latin cutis = skin). This water loss 

through the cuticle of stems and leaves is called cuticular transpiration. But in most land plants the outer 

surface is covered with dense hairs, with heavy layers 

of wax or, on stems, with corky bark. Under such circumstances cuticular transpiration accounts for only a 

small fraction of the total. Most transpiration then occurs through little openings in the leaves called 

stomata-plural of the Greek word fTTOJUX. (stoma = 

mouth). 5 It is through these openings that during daylight carbon dioxide is taken up for the process of 

photosynthesis and water is transpired. It should be 

noted that a small exchange of carbon dioxide and 

vapor can also take place through much larger openings (lenticels) in the outer surface of branches and 

stems. 

The size of the stomata varies with the plant species 

but those of saltcedar measure, when open, about 7 x 

20 J.Lm with an open-pore size of about 60J.Lm2 

(Campbell and Strong, 1964). Figure 13 shows schematic drawings of a stoma and a typical distribution of 

stomata over a leaf surface. Unlike most deciduous 

'' Saltcedar can also exude some liquid water through hydathodes at the end of vascular 

bundles in the leaves. Hydathodes in saltcedar usually are found about 1 mm below the 

leaftips !Kisser. 19561. For the salt glands, see section on "Vegetation." 

100#lm 

FIGURE 13.-0ne stoma in surface view (top left) and in 

cross section (top right); view of some stomata distributed over the underside of a leaf (bottom). The dotted 

lines in the cross section (arrow) indicate position and 

shape of the guard cells (a) in closed position. Schematic after Fitting, Schumacher, Harder, and Firbas 

(1951): a, guard cell (one of two); b, epidermis cells with 

cuticle (c); d, nucleus in cell; e, substomatal air space; f, 

parenchyma (thin-walled tissue cells); g, open stomata. 

Dots in cells indicate chloroplasts where water and 

carbon dioxide are combined to form sugars with the 

help of solar energy. 

trees, which usually have their stomata only on the 

underside of the leaves, the scalelike leaves of 

saltcedar have stomata all over the surface. These 

leaves are about 1.6 mm long and somewhat egg 

shaped; the largest diameter is about 0.8 mm. Since 

there are about 250 stomata per mm2 (Tomanek and 

Ziegler, 1960) it follows that at most about 1.5 percent 

of the leaf surface is open for the passage of water 

vapor. 

Stomata react differently to ambient influences, depending on the species of the plant. Some stomata close 

partially or completely at night, others (for instance 

pineapple, Ananas sativas )

6 are wide open during that 

time. In some species they open during periods of high 

6Cacti and other succulents are capable of taking up C02 during the night with little loss 

of water. Then, during times of sunshine, the C02 is photosynthetically combined with 

water and other chemicals in the plant cells to build plant material (Walter, 1973). 
F12 STUDIES OF EVAPOTRANSPIRATION 

~ind, in others they close, but all close when the plant 

IS under water stress, at which time the turgor in the 

guard cells (see fig. 13) diminishes, resulting in closure 

of the openings. 

FLOW RESISTANCE 

One can now visualize a series of resistances against 

the movement of water and vapor. Disregarding the 

resistances against water movement in the soil from 

the soil to the roots, and from the roots throu~h the 

plant, which do not concern us here, it is intuitively 

clear that a resistance develops when water moves 

through the thin-walled parenchyma cells surrounding 

the substomatal cavity and vaporizes. The vapor then 

has to pass through a comparatively small opening to 

reach the ambient air. This is called the stomatal resistance (Monteith, 1963; Smirnov, 1963). Just outside 

the stomata and along the surface of the leaves is a 

layer, a few tens of molecules thick, where the flow is 

only laminar and the diffusion molecular, causing an 

external resistance. Further resistances occur within 

and above the canopy of a vegetation. 

It seems clear from the above that transpiration 

must be considered a phenomenon decidedly different 

from that of evaporation. But because evaporation 

from soil surfaces and from wet leaves (after dew or 

rain) can occur simultaneously with transpiration, the 

term evapotranspiration is commonly used when the 

loss of water from vegetated surfaces is considered. 

In the following pages it will be shown that the inclusion of a resistance term in evapotranspiration 

equations used at the Buckeye project leads to waterloss data that fit the measured ones better than when 

no resistance term is used. 

In the discussion of the methods used to estimate 

evapotranspiration only the principles are considered. 

There are many variations proposed and in use. Only 

those tested at the Buckeye Project will be mentioned 

during the data analysis. 

VAPOR-FLOW METHODS 

Two methods are considered to belong in this group 

and they differ greatly in approach. One, the atmos~ pheric water-balance method, measures the amount of 

vapor that enters and leaves a tall column of air over a 

large area. The other, the eddy-correlation method, 

measures vertical vapor flow as close as possible to the 

evaporating surface. 

THE ATMOSPHERIC WATER BALANCE 

Basically, the estimation of evapotranspiration by 

the atmospheric water balance method is analogous to 

the soil-water balance method. For the latter we have 

the equation: 

P = RO + E ± 118 (1) 

in which P is precipitation, RO is runoff, E is evaporation, and 118 is the change in soil-moisture storage. 

Deep percolation and underground drainage are neglected. 

Instead of considering a block of soil with the surface 

at its upper boundary, one can consider the volume of 

air with the soil surface at its lower boundary and the 

top, say, at the 500 millibar level (50 kPa [kilo pascal J, 

about 5.5 km above sea level). Then 118 becomes a 

change in the quantity of moisture in the air which can 

be due to evaporation from soil or plants (vertical components) or to moisture entering or leaving the air column more or less horizontally. This ((horizontal" component is an advective term and has to be considered 

not only in the atmospheric water balance but also in 

the combination methods to be discussed later. 

With this in mind, one can write: 

E = P- C\l·M ± 11W) (2) 

in which '\1 · M is the difference between the moisture 

in the atmosphere entering and leaving the column. 

The quantity '\1 · M is usually called ((divergence" if it is 

positive and ((convergence" if negative. It can be measured by radiosonde observation. The change in precipitable water (analogous to 118 in equation 1) is denoted by ±11W. Since precipitation in this procedure is 

considered to be a measurable quantity, E can be computed and compared with data derived from more direct measurements such as lysimetry. 

Taumer (1955) was probably the first one to make 

complete investigations over a 10 000 km2 area for individual months and to compare meteorologic results 

with lysimeter data published by Kalweit (1953). More 

recently Rasmusson (1967; 1968) and Malhotra (1969) 

used the dense radiosonde network over the North 

American continent (an area at least 1000 times as 

large as Taumer's) to compute monthly values. It is 

obviously difficult to check the results of this type of 

studies against those obtained from conventional 

methods and one must resort to more esoteric methods 

such as seasonal changes in latitude or in the wobble of 

the earth caused by the movement of large air masses 

(Munk and McDonald, 1960; van Hylckama, 1970b). 

It is therefore not surprising that an attempt to 

apply the principle of equation 2 at the test site failed. 

The distance (92 m between the masts 0 and Min fig. 

3) was far too short and the height of the sensing elements (2 m above the vegetation) was far too small to 

enable the successful integration of windspeed, tern-
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F13 

perature, and vapor-pressure data for the computation 

of an increase in water vapor due to transpiration. 

THE EDDY-CORRELATION METHOD 

Since evaporation and transpiration are basically 

surface phenomena, the first from the surfaces of free 

water and the second from the surfaces of plants, it is 

logical to measure these phenomena as close to the 

surface as possible. But technically this is difficult to 

do. However, during the last 20 years sensing and recording equipment has been greatly improved (L. J. 

Fritschen, oral commun., 1977). Although the Buckeye 

Project was not equipped for this model, it shall be 

mentioned here because of its promising potential. 

The eddy-correlation method can be considered the 

most fundamental approach to the measurement of 

evapotranspiration because the turbulent air 

movements that give rise to eddy diffusion are measured together with the fluctuations in humidity. 

An eddy can probably best be visualized as being a 

parcel or ''glob" of air with a long or short history of 

existence of its own. Brunt (1952, p. 219) did not consider it likely that any definition could be given which 

would be universally acceptable. He wrote, 

The eddies which form the edge of a stream flowing into a mill pond 

are of the nature of vortices with vertical axis, but the name 'eddy' as 

used in discussing motion in the atmosphere is not restricted to circular motions. We can only define eddy as a physical entity which 

disturbs the uniform flow of air and this definition will include rotating eddies, convection currents, and any other type of disturbance. 

This method, also known as the eddy-transfer 

method, rests on the idea that a moving parcel of air 

carries with it, on its journey up, down, or sideways, 

the heat, water vapor, dust, and momentum it contained at the start. That this is not necessarily so is one 

of the difficulties in applying this method and some 

other methods of estimating water losses or evapotranspiration, as will be discussed later on. 

If measured close to the surface and with sufficient 

frequency (of the order of once per second or more often) the mean upward flux of humidity (or any other 

property ofthe atmosphere) per unit mass can be given 

as: 

F = PnWq (3) 

where p11 is the density ofthe air, wits vertical velocity, 

q its humidity, and where the bar indicates the average 

condition during a selected period. Now any change 

can be expressed as an average and the deviation from 

that average during such a period. Hence: 

where a prime gives the instantaneous deviations from 

the mean. Thus we can write: 

F = (pa + p~) (w + w I) (q + q I) 

which can be expanded to: 

F= PaWij+pawq 1 +paw 1

ij +paw 1q 1 

+ p~wij + p~wq I+ p~w I ij + p~w I q I 

(5) 

(6) 

Because the sum of the deviations from the mean is 

zero by definition, all terms in this equation that have 

a single prime go to zero but the product of two such 

primed quantities does not. With this in mind, and also 

neglecting density fluctuations which are small close to 

the ground, we have: 

F = PaWij + PaW 1q 1 (7) 

The effective rate of upward diffusion of water vapor 

past the measuring point is now given by w 1 q 1

, which is 

called the eddy flux, and this can be obtained by separately measuring w and q. These values can be multiplied and integrated over a measuring time period, and 

the product of the individual mean values over that 

period can be subtracted. 

The instrumentation for direct measurements of the 

eddy and its properties has been under development 

since the work of Swinbank (1951). (Also see Perepelkina, 1959.) These instruments use hot-wire 

anemometers and fine-wire dry and wet thermocouples 

and even more sensitive instruments are under study 

(Dyer, 1961; Goltz and others, 1970; Hicks and Goodman, 1971). The fact that enormous quantities of data 

have to be converted into required output over periods 

of 5 10 or more minutes and the resulting requireme~t fo; integrating circuits, analog computers, or digital microprocessor networks, makes this method very 

expensive. 

This discussion of the eddy-correlation method is included here to emphasize its importance. It is the only 

method that is independent of the characteristics of the 

surfaces over which measurements are made. The instrumentation can be easily moved about and installed 

for any desired time over any surface to determine 

evapotranspiration rates and, if suitable, to provide 

ground-truth data for remote-sensing (including satellite) studies. 

BULK-AERODYNAMIC METHODS 

Pn = Pn + p~' w = w + w I' and q = q + q I 

There are two methods that belong in this category: 

(4) the Dalton approach and the profile approach. 
F14 STUDIES OF EVAPOTRANSPIRATION 

THE DALTON APPROACH THE AERODYNAMIC OR PROFILE APPROACH 

This method is so called because it was Dalton (1802) 

who recognized the principles of the atmospheric vapor 

flow close to a surface at which evaporation occurs. He 

reasoned that vapor transfer had to occur along a gradient of moisture concentration and that the efficiency 

of this concentration depends on the turbulence of the 

air (hence on the windspeed) above the surface. ln 

other words, 

(8) 

in which f(u) is a function of the mean windspeed, often 

given as: 

f(u) = a (1 +bu) E9) 

or 

f(u) = cu (10) 

E 0 is the evaporation rate, e0 and ea are respectively the 

pressures at the evaporating surface at height z = 0 

and in the air above it (for practical purposes 1 or 2m), 

u is the average windspeed, and a, b, and c are constants.7 

Several investigators have given numbers to these 

constants, for instance Rohwer (1931) has: 

E 0 = 0.4 (1 + 0.27 u0 ) (e0 - ea) mm/day (11a) 

where u, the mean wind at the surface, is in miles per 

hour and the vapor pressures are in mm Hg. Penman 

(1948) modified this to: 

. - E 0 = 0.4 (1 + 0.17 u2 ) (e0 - ea) mm/day (11b) 

with u at 2 m above the surface. Other variations are 

given by Kuznetsov and Fedorova (1968). Harbeck 

(1962) developed this variation for estimating evaporation from reservoirs: 

(11c) 

where N is a coefficient related to the reservoir's surface area. Since the variables are comparatively easy 

to measure close to the evaporating surfaces, average 

values over periods of half an hour or longer can be 

obtained. The method is an integral part of the combination appraoch which will be discussed later. 

"'The use of two constants in the wind function a( 1 + bii) can be considered an admission of 

the fact that many cup-type anemometers have considerable starting and stalling speeds, 

that is, it takes a comparatively strong wind to get them going and they keep turning for a 

considerable time after the wind has died down <G. E. Harbeck, Jr., written commun., 1955). 

The method is called profile approach because it considers temperature, humidity, and wind speeds at two 

or more levels above the surface, rather than at the 

surface alone. This sounds like a simplification, but the 

shape of the profiles depend so much on the stability or 

instability of the air that, unless the wind profile is 

logarithmic, large deviations from observed evapotranspiration values can be expected. 

In the aerodynamic methods, we distinguish three 

vertical fluxes which result from the turbulent diffusion in the lower layers of the atmosphere. They are 7, 

the flux of momentum, A the flux of sensible heat, and 

V the flux of water vapor. They are expressed as follows: 

7 = P K au a m az 

aT A = PaCp KA az 

and 

(12) 

(13) 

(14) 

The K's are exchange coefficients respectively for mass 

(m), heat (A), and vapor (u ); Cp is the specific heat for 

dry air; T is the temperature; and the other symbols 

are as defined before. 

Any other type of atmospheric flux can be expressed 

in a similar manner. For instance, for carbon dioxide 

flux, Fe, we would have 

Fe = Ke ~; (15) 

where C is the C02 content as mass per volume. Intuitively it seems reasonable to assume that the exchange 

coefficients (K) would be the same for all four equations. That this is not always so causes difficulties in 

applying the aerodynamic approach. For instance, assuming that Km = K,, and that the wind profile is 

logarithmic, Thornthwaite and Holzman (1942) proposed the following equation for evapotranspiration 

(E): 

(16) 

where k is von Karman's coefficient, often taken as 

0.40 or 0.41, (see section on ~~winds"). Later, other 

workers (for instance Pasquill, 1950 and Pierson and 

Jackman, 1975) attempted to correct the aerodynamic 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F15 

equation for conditions of instability. The methods 

then became very complex, requiring very accurate 

and frequent measurements of windspeed, specific 

humidity, and temperature at three of more elevations 

above the surface. Webb (1965) presents a good review 

of this method. 

ENERGY-BUDGET METHOD 

Penman (1956, p. 16) starts his discussion of the energy balance as follows: ((The fundamental basis of the 

energy balance approach is unchallenged: the challenge is to our ability to measure or estimate all the 

quantities needed to exploit the principle of the conservation of energy." 

The energy-budget, or heat-budget method, is one of 

the theoretical methods based on the principle of the 

conservation of energy. Evaporation and transpiration 

are essentially the same thing. In both cases about 

2450 joules (585 calories) are needed to evaporate 1 g of 

water at summer temperature. The energy to do this 

comes directly or indirectly from the sun. 

At the top of the atmosphere the earth receives on 

each square meter perpendicular to the sun's rays 

about 1400 W or 2 cal cm- 2 min- 1, but a large part of 

the radiation is reflected back into space by dust, water 

vapor, and clouds; another part is scattered into the 

air; and a third part heats the soil and therewith the 

air. How much reaches the plants and the ground depends on latitude and season, on weather conditions, 

on elevation of the place, and on its exposure. The 

quantity may vary from practically zero under heavy 

overcast to more than 1000 W m- 2 on high mountains 

above the clouds. 

If we now measure all those different types of radiation, incoming and outgoing, and also the changes in 

temperature in the soil and in the air, and take into 

account that at times considerable quantities of hot air 

are brought into the test site by advection, we can set 

up a balance sheet. On one side we have all the incoming radiation, on the other the sum of all that goes out 

plus what is stored in the soil, in the air, and in the 

vegetation. If we do this, we find that the two sides do 

not balance. The difference is that part of the radiation 

which is used to evaporate water either from the surfaces of lakes or from moisture on and in the soil, or 

from water on and in the plants. Thus we have a 

method of indirectly measuring the amount of water 

transpired, which is the only item in the balance sheet 

that we cannot measure directly. 

If the energies consumed in photosynthesis or liberated by metabolic processes are neglected we can write: 

R 11 = LE +A+ G (17) 

in which Rn is the net radiative flux received by the 

surface, G is the heat flux into or out of the ground, A 

and LE are the sensible heat flux and latent-heat flux 

into the air respectively, and L is the latent heat of 

evaporation. 

In this equation R11 and G can readily be measured, 

but the partitioning of the energy between evaporation 

and heating of the air has to be approached indirectly. 

The ratioA/LE, usually called the Bowen ratio ({3), was 

found by Bowen (1926) to be: 

{3 = AILE = c [ To - Ta J _P_ (18) 

eo - ea 1000 

where c, according to Bowen, varies between 0.58 and 

0.66 depending on the state of the atmosphere. Note 

that this ratio was used to compute evapotranspiration 

over open water. T0 and Ta are the temperatures of the 

water surface and of the air above it; e0 is the saturation vapor pressure at T0 , and ea the actual vapor pressure at the elevation of Ta. Over Lake Hefner (U.S. 

Geological Survey, 1954), where Ta and ea were measured at 2m above the surface, single-day average values were found to vary from -20.02 to +31.50 although most of the time they lay between -0.5 and 

+0.5. 

From the flux-gradient equations discussed (13 and 

14) it follows that, if KA = K" and atmospheric pressure 

is assumed to be constant, {3 can also be expressed as 

f3 aT 

or= y-&} (19) 

where y is the psychrometer constant, which has a 

value of about 0.65 oc- 1• If vapor pressure (e) differences are used instead of specific humidity (q) values 

the dimensions are mbrC. If {3 can be considered constant with height, the energy balance can also be written as: 

E = Rn- G 

L 

f- LE ] = .Rn- G 

LA+ LE J L 

or LE = R ,J ( 1 + {3) if G can be neglected. 

1 (20) 

1 + f3 

Table 4 shows the values of yaT/ae for a 24-hour 

period at five different paired levels. The values are 

quite erratic, although most of the time when they are 

negative or positive together, they indicate some consistency with height. However, there are large deviations: for instance between 0300 and 0400 the values range from -0.78 to -13.65 and between 1400 and 

1500 from -0.07 to +0.33. 
F16 STUDIES OF EVAPOTRANSPIRATION 

TABLE 4.-Values of y ( tJ.T/tJ.e) for a 24-hour period, May 5 and 6, 1966 

[~T and ~e are differences in temperature (°Cl and vapor pressure (mbl between a lower and higher level; y is the psychrometer constant taken to be 0.65 mbfC. The vegetation is 3m tall 

and forms an even stand] 

Start of measuring periods Instrument heights above ground in meters 

(time in hours) 11 and 7 11 and 5 7 and 5 7 and 4 5 and 4 

5 -2.92 -2.60 -1.95 -3.25 -4.55 

6 -2.81 -2.21 -1.30 -2.16 -3.90 

7 1.30 0.65 0 -0.65 -1.30 

8 0.21 0.26 0.33 0.21 0 

9 0.07 0.08 0.09 0.21 0.39 

10 0.37 0.49 0.65 0.81 1.09 

11 0.05 0.25 0.51 0.56 0.65 

12 0.33 0.23 0.12 0.47 1.02 

13 0.33 0.33 0.65 0.31 -0.18 

14 -0.07 0.08 0.26 0.29 0.33 

15 0.52 0.14 -0.33 -0.05 0.39 

16 0 0.17 0.39 0.10 0.21 

17 0.37 0.12 -0.33 -0.49 -0.65 

18 -0.54 -0.59 -0.65 -0.92 -1.63 

19 -0.46 -0.49 -0.52 -0.86 -1.30 

20 -0.65 -1.03 -1.56 -1.54 -1.51 

21 -1.69 -1.37 -0.98 -1.30 -1.73 

22 -0.65 -0.91 -1.30 -0.43 -1.30 

23 -0.65 -1.02 -1.51 -1.78 -2.60 

24 -3.58 -4.11 -5.20 -3.25 -1.30 

1 -2.44 -1.95 -1.46 -1.51 -1.63 

2 -5.20 -3.90 -2.60 -2.81 -3.25 

3 -0.78 -0.92 -1.30 -5.41 -13.65 

4 -6.83 -6.28 -5.20 -3.90 -2.60 

The effect of the value of {3 on the computed LE values can be shown as follows: 

Assume Rn = 1 cal cm-2 min- 1 or 60 cal cm-2 hr- 1 (a 

nice summer mid-day value). Then if 

{3 = -2.0 -1.0 -0.5 -0.1 0 0.1 0.5 1.0 2.0 ' 

LE = -0.1 ? +2.0 1.1 1.0 .091 0.67 0.50 0.33 

where? indicates that the LE value is undetermined. 

Of course, during daylight, positive values are to be 

expected since temperature and vapor-pressure differences are nearly always negative. Nonetheless, as {3 

increases from 0 to 2.0, LE diminishes from 1.0 to 0.33. 

Such differences can be avoided by integrating temperature and vapor pressure data over a large area and 

avoiding instrument bias by frequently changing the 

position of the sensing devices. Also, observations have 

to be made very close to the transpiring surface. Suomi 

and Tanner (1958) (p. 301) present a fascinating description of such instrumentation and mention that ((on 

occasion plants even brushed against the bottom sensors". At the Buckeye Project the system was rigid, and 

the lower sensor was placed 1 m above the top of the 

vegetation. These are the main reasons that the 

energy-budget, using the Bowen ratio, could not be 

used at the Buckeye Project. 

COMBINATION METHODS 

The instrumentation seemed better adapted to the 

use of a combination of the energy-budget and a masstransfer term. Penman (1948) combined the energy 

balance and the aerodynamic balance into the follow~ 

ing equation: 

t,./y Ho + E~ (21) t,.jy + 1 

in which the saturation vapor pressure at the surface 

replaces the actual vapor pressure. The actual vapor 

pressure is very difficult to measure. In this equation, 

E0 is the potential evaporation from open water; H0 is 

the net gain of radiation at a water surface; E* a = f (u) 

(e0 - ea) (see equation 8); and t,. is the slope of the curve 

when saturation vapor pressure is plotted against temperature. Modifying this method, van Bavel (1966) derived the following expression for the instantaneous 

evaporation rate: 

LE 0 = __ t,._/_._y_H~+_L_B_,"--,d"""""'a __ t,.jy + 1 

in which B,, is defined as 

B,. = 

cal em - 2m in_, (22) 

(23) 

where E is the ratio of the molecular weights of water 

and air, (0.622), z0 the roughness parameter, H the sum 

of energy inputs at the surface (except A and LE), and 

da the vapor pressure deficit (e8 - ea), withes the saturation vapor pressure. 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SAL TCEDAR THICKET, ARIZONA F17 

This equation has been applied in the analysis of the 

Buckeye data and often has been given values at variance with the measured ones. Better agreement can be 

obtained if resistance terms are incorporated in the 

equation. Monteith (1963 and 1965) showed that the 

external resistance can be expressed as 

and the stomatal resistance as 

rs = (A/y + 1) (E0 1Ea - l)ra sec cm- 1 (25) 

where Ea is the actual evapotranspiration as measured 

with a lysimeter, for instance. 

Because the above equation contains bothEo andEa, 

the stomatal resistance can be computed from measured values of these terms. The values so obtained can 

then be used to compute the evapotranspiration rates 

according to the following equation (the equation contains both resistance values): 

Eo= 1/L (A/y) H + [(pt:)/p] (dalra) 

A./y + 1 + rslra 

Examples of the results will be given later. 

INSTRUMENTATION 

(26) 

Basically the instrumentation chosen to measure the 

various components in the aerodynamic and energy 

budgets was the same as that used in earlier investigations by the U.S. Geological Survey at Lake Hefner and 

Lake Mead (U.S. Geol. Survey, 1954; and Harbeck and 

others, 1958) and described in detail by Anderson, Anderson, and Marciano (1950). These studies, however, 

dealt with evaporation from open water, and for 

studies over tall vegetation certain adjustments had to 

be made. 

The characteristic high temperatures and strong 

winds of this area frequently produce blowing dust and 

dust devils. This dust made it highly unlikely that reliable results could be obtained by the use of evaporation pans or a Cumming's radiation integrator. These 

instruments, therefore, were not installed. 

Unlike the lake investigation just mentioned, where 

recording milliammeters were used, strip-chart recorders were used which had built-in amplification and 

automatic zero references, so no ice buckets or Dewar 

flasks were necessary. 

The two kinds of recorders used printed a record 

every 60 seconds, sampling in sequence the electrical 

output from a series of 12 or 16 different sensors. An 

example is given in figure 14. 

r I 

I 

I 

I 

I I I I 

I I I 

I 

I 

If I 

I I I 

I 

I· I I 

l 1111 I I Ill. 

FIGURE 14.-Example of a 16-point strip chart about% of actual size, 

with date and times. The numbers, millivolts, must be converted 

into degrees Celsius by means of a template or by graphical means, 

because the relation between millivolts and degrees Celsius is not 

linear. The instrument prints four symbols in four colors (not 

shown here). Examples: Arrows 1 point to brown crosses, the drybulb temperature 11 meters above the ground; arrows 2 point to 

red crosses, the wet-bulb temperature, both recorded as millivolts. 

During the Lake Hefner investigations, anemometers were usually placed on top of the radiation shields. 

In the Buckeye Project, as in the Lake Mead studies, 

cup rotors were placed at the level of the thermocouples. 

Other variations will be mentioned below. 

THE ATMOSPHERE: WIND AND TEMPERATURE 

WIND 

Because 1t was unknown what wind and temperature profiles would look like over the rough surfaces of 

saltcedar thickets, wind speeds and temperatures were 

measured 11, 7, 5, and 4 m above the ground, the average height of the vegetation being 3 m. A fifth 

anemometer was installed underneath the canopy 

about 2m above ground. To the southwest (from which 

the prevailing wind presumably blew8 ) two masts were 

erected (0 and Min fig. 3), each with an anemometer 5 

m above ground. 

The transmitters originally installed were of the 

three-cup variety; they were made of aluminum and 

"To determine the location of the auxiliary masts <0 and Min fig. 3), information on winds 

was obtained from the Litchfield Park Naval Air Facility, 21 km east of the project site, but 

the data proved not to be representative of those at the project. See the section on "The 

Atmospheric Water Balance." 
F18 STUDIES OF EVAPOTRANSPIRATION 

weighed about 140 grams each. The system was connected to counting devices regulated by mechanical 

and magnetic switches as described by Anderson, Anderson, and Marciano (1950). However, the starting 

and stalling speed of these transmitters (explained in 

the section on ((The Dalton Approach") was far too high 

to obtain reliable wind profiles over tall vegetation. 

Moreover, the counting devices broke down frequently, 

because they could not withstand the rigors of the climate. All anemometers, therefore, were replaced with 

a much more sensitive system. A new type of 

transmitter, consisting of three light plastic cups of 5 

em diameter attached to 5 em long arms, weighed only 

32.5 g. The counting devices were driven by photoelectric cells. These sensitive anemometers performed very 

well. Those at the auxiliary masts (0 and Min fig. 3) 

were at times installed inside the vegetation and made 

it possible to construct wind patterns inside the thickets. See the section on ((Winds." 

The counting mechanisms indicated every fifth and 

every fiftieth turn of the rotor on a strip chart. An 

example is given in figure 15. The number oC(pips" per 

hour was manually converted to meters per second 

using a calibration table. 

AIR TEMPERATURE 

Air temperatures were measured with copperconstantan thermocouples connected to the strip recorders: The thermocouples were protected from direct 

sunlight with aluminum shields of the type described 

by Anderson, Anderson, and Marciano (1950); they are 

outlined in figure 16. At this site these ((coolie hats" 

had a tendency to corrode, to lose their reflectivity, and 

to become less effective shields. Covering them with 

adhesive aluminum foil proved to be a very practical 

and lasting remedy. 

Mean hourly temperatures were found by averaging 

the five or six values on the strip charts (see fig. 15) 

over 1-hour periods. Most of the averaging could be 

done graphically. Especially valuable was a commercially available data-reading device connected to a 

counter which greatly facilitated transposing data 

manually from the strip charts to graphs and tables. 

PRECIPITATION AND WATER VAPOR 

RAil\: FALL 

Precipitation was measured with three standardtype recording rain gages, one placed outside the 

saltcedar thickets, one on the 4-m platform, and one 

inside the thickets. Plastic wedge-shaped rain gages 

were installed on all tanks at about 1.50 m above 

ground. 

~ -+----~-.a=-~--*- -- --=- ,_ : --- t ---- -~ -~ 1: ~ 

-- -j- . ~- ~: 

' - FIGURE 15.-Windspeed record for June 21, 1965 between 1200 and 

1600 (marked 6 p.m. and 8 p.m. on this chart). The two columns on 

the left are the windspeed "pips" for the anemometer 11 m above 

ground; the 11th to 14th columns are the records for the anemometers inside the thickets counting five times as fast as the others; 

the four columns on the right give the wind directions, N, E, S, and 

W, respectively. About% actual size. 

Precipitation data show that the rainfall was typical 

of the Sonoran Desert. During the summer, heavy 

thunder storms did occur, and 24-hour rainfalls of 30 

mm and more were not uncommon. The heaviest rainfall during the project period, which resulted from hurricane Helga, was 115 mm in 3 hours, on September 16, 

1966. 

WATER \'APOR 

Water vapor, expressed in millibars (1mb = 100 Pa) 

was computed from the dry and wet bulb thermocouples records. The wet thermocouples were kept 

permanently wet with wicks fed from reservoirs (fig. 

16). All thermocouples above the vegetation had sufficient ventilation due to the system illustrated in figure 

17. Those close to and inside the vegetation were ventilated with fans driven by small electric motors. 

A nomogram was constructed (fig. 18) to convert dry 

and wet bulb temperatures into vapor pressures, under 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SAL TCEDAR THICKET, ARIZONA F19 

F'IGURE 16.-Sketch of the air-temperature sensing system. a, radiation shield; b, plates holding the ventilator systems (see fig. 17); c, 

wet and dry thermocouple assemblies (enlarged below); d, dry and 

wet thermocouples, the wet one covered by wick e, the ends of 

which are floating in the reservoir; f, wire connectors to the recorder; g, vent screw to allow refilling of the reservoir. 

the assumption of a constant atmospheric pressure of 

985 mb (98.5 kPa) (the test site is 260 m above sea 

level). 

RADIATION 

The total hemispherical incoming solar radiation 

was measured with a pyrheliometer designed for 

measuring the intensity of solar radiation from 0.3 11-m 

to 5.0 11-m. Nearly all the solar energy received at the 

earth's surface arrives in these wavelengths. The instrument consists of two concentric rings, the inner one 

is painted with lampblack, the outer one has a cover of 

magnesium oxide which strongly reflects solar radiation. The difference in the temperature of the two rings 

gives a measure of the solar radiation. This sensing 

element is hermetically sealed in a bulb of soda-lime 

glass. The total reflected solar radiation was measured 

with a similar instrument mounted upside down. 

FIGURE 17 .-Schematic diagram of the ventilator system. 

Both dry and wet thermocouples are ventilated; the dry one 

between top and middle plates (see fig. 16), the wet one 

between middle and bottom plates. (From Anderson and 

others, 1950.) 

The total short- and long-wave incoming· and reflected radiation was measured with a radiometer similar to the one used for the lake studies. Unlike that 

instrument which had a reflecting surface on the 

underside, this one was painted black on both surfaces 

and therefore acted as a net radiometer, recording the 

differences between incoming and outgoing radiation. 

The sensing element measured about 100 x 100 mm, 

four times larger than that of the instruments used in 

the lake studies. This plate was ventilated by a fishtail 

nozzle blower to eliminate the effect of gusts of wind on 

the calibration. The accuracy of such a radiometer depends greatly on the uniformity of the blackness on 

both surfaces. Amusingly, the cooling wind was apparently very attractive to doves nesting in the neighborhood and frequent cleaning and repainting of the top 

surface was necessary. Painting the retaining equipment with a sticky substance discouraged undesirable 

visitors with some measure of success. Figure 19 shows 

the radiation mast shortly after installation. 

Net radiometers of the type used here, and also the 

soil heat-flow plates to be discussed later, measure heat 

flow by means of thermopile voltages. The voltage 

needs a correction for temperature of the plate and 

each instrument has its own calibration factor. This 

makes transcribing of heat-flow data tedious and very 

time consuming. An adjustable nomogram was constructed to facilitate this procedure (fig. 20). Details 

and theory are given in Appendix B. 
F20 STUDIES OF EVAPOTRANSPIRATION 

SOIL TEMPERATURE, HEAT FLOW, AND SOIL MOISTURE 

SOIL TE~IPERATURE 

The term G in the energy budget (equation 17) is 

often neglected when data are computed for periods of a 

day or longer. Over bare ground they certainly cannot 

be neglected over shorter periods. However, when the 

soil, as at the Buckeye project, is thoroughly protected 

by a dense stand of tall vegetation, the hourly and even 

daily changes become very small. Nonetheless, to have 

a complete picture of the situation, thermocouples were 

installed at 0,10, 45, and 90 em below the surface in 

tanks number 2 and 6, and thermocouples were also 

installed outside tank 6 at the same depths. If the 

moisture content is known (as it was) and a thermal 

conductivity can reasonably assumed to be known (as 

it could), then measurement of incoming or outgoing 

heat is possible by a graphical analysis to be discussed 

later. 

HEAT-FLOW PLATES 

In addition to the thermocouples, heat-flow plates 

were installed in tanks 2 and 6 at 1 em below the surface. They work on the same principle as the net 

radiometer but are much smaller (about 29 x 86 mm) 

so as to interfere as little as possible with the soilmoisture movement. Because such a small surface may 

not produce representative data, three plates were installed in each tank and connected in such a way that 

the strip-chart recorder printed the mean value of each 

of the triplets. Unfortunately, these plates were very 

difficult to seal against the corrosive action of the soil 

moisture. Hence only a few sets of reliable soil heatflow data could be obtained from these plates. 

Blackwell and Tyldesley (1965, p. 142) report that 

their tests reveal a variation in the calibration of their 

soil heat-flux plates of about ±30 percent, and they 

continue: ('Allowing for the use of unrepresentative 

calibrations, storage errors, 2nd sampling difficulties, 

it would be unrealistic to claim an overall accuracy of 

much better than ± 20 percent." 

The experience at the Buckeye Project was similar. 

Table 5 illustrates some of the difficulties. It gives the 

calibration factors for four plates as provided by the 

manufacturers at time of purchase and at two later 

dates. Neither the drastic change in factors nor the 

drop in significant figures underpins trust in the heatflow records. Luckily, as will be shown later, the flows 

are small anyway and the actual values were of little 

importance in the energy budgets. 

SOIL MOISTURE 

Soil moisture and soil-moisture changes were measured by the neutron-scattering method (van 

~ 

Ta c ! 52 

~ f!J 50 <t 

Tw c 

48 " ea mbar 46 54 6 

44 33 

42 32 

40 31 

38 30 

36 29 

54 

26 

24 

22 

zo 

20 

18 18 

16 16 

14 

14 

12 

II 

10 

10 

HYGROMETRIC CHART 

FOR THE 

BUCKEYE PROJECT 

-2 

FIGURE 18.-Nomogram to convert a given set of dry and wet bulb 

temperatures into vapor pressure expressed in millibars assuming 

a constant atmospheric pressure of985 mb (98.5 kPa). Connect air 

temperature (Tal with a straight line to the wet-bulb temperature 

(T,.) and continue this line to thee" line to read vapor pressure. 

Example: Ta = 28.5, T.,. = 18.0, e" = 14.0. 

Hylckama, 1974, p. E16-E18). Readings were taken at 

least once a month, but during periods of intensive observations, the frequency was increased to two or four 

times per day. 
WEATHER AND EVAPOTRANSPIRATIO N STUDIES IN A SALTCEDAR THICKET, ARIZONA F21 

b 

) 

FIGURE 19.-Radiation mast with pyrheliometers up (a) and down 

(b) and with the net radiometer (c). Mast is 7 m tall or 4 m above 

the mean height of the vegetation. At the time the photograph was 

taken the soil was still bare. Compare with figure 5. 

FIGURE 20.- Adjustable nomogram for the computation of heat flow. 

E, and E2 : milivolt scale (data read from strip charts, see fig. 14); F, 

movable scale to adjust for plate temperature; M, and M2 : heatflow scales to read calories per square centimeter per minute, adjustable to the calibration constant scales C, and C,. See Appendix 

B for details. 

TABLE 5.- Heat-flow plate calibration factors 

Recalibrated on 

Plate Nos. At purchase time 

December 1961 February 1963 

314 0.09666 0.0747 0.072 

315 0.13872 0.0828 0.075 

316 0.12021 0.08365 0.081 

317 0.14259 0.0849 0.080 

Batteries of tensiometers were installed in all the 

tanks at depths commensurate with the water levels in 

the various tanks (fig. 21). Readings were taken 5 days 

a week and at 2-hour intervals during periods of intensive observations. Figure 22 shows such an event at 

night. In this way, periodic changes in soil moisture 

could be detected quickly. 

Directions of soil-moisture movement can also be detected with these instruments by converting the tensiometer readings to manometer readings. Some of the 

results were used in the studies by Ripple, Rubin, and 

van Hylckama (1972), so they will not be discussed 

here. 

WATER-USE MEASUREMENTS 

The instrumentation to measure the water use is described extensively by van Hylckama (1974, p. E7-

FIGURE 21.- Tensiometers installed in one of the tanks. Instruments 

were made at the project from commercially available parts. 
F22 STUDIES OF EVAPOTRANSPIRATION 

FIGURE 22.-Taking tensiometer readings at night during one of the 

intensive observation periods; observer (back to camera) is using a 

headlamp. See the section on "The Special Observation Periods." 

E8.). As mentioned on p. E20- E22 of that report, it was 

possible to analyze the frequency of fillings and to construct curves of water use over 24-hour periods. An 

even more refined analysis could be made if the on-off 

recorders were set for daily instead of weekly charts. 

An example of a daily chart is.given in figure 23. Not 

only could the times of filling be more accurately determined, the duration of a filling could also be known 

and a much more detailed picture could be obtained to 

construct curves of hourly water use. Daily charts were 

always installed during periods of intensive observations when the water meters were read at 2-hour intervals. 

THE MICROCLIMATE OF A SALTCEDAR 

THICKET 

WHY THE MICROCLIMATE? 

To fully understand and appreciate the methods of 

estimating evapotranspiration it is helpful to discuss 

some aspects of the weather inside and above saltcedar 

thickets such as occur near Buckeye, Ariz. After all, a 

saltcedar thicket, or any other flood-plain vegetation, 

comprises a much more complex environment than 

that on which most evapotranspiration equations are 

based. 

For example, Penman (1956, p. 20) defined potential 

evapotranspiration as "the amount of water transpired 

in unit time by a short green crop completely shading 

the ground, of uniform height and never short of water". Only the "never short of water" applies to the 

saltcedar in and around the evapotranspirometers, and 

the evaporating surface is not so well modeled by a 

two-dimensional surface as is a close-cropped lawn (or 

FIGURE 23.-Event recording chart indicating time and duration of 

tank filling. The jagged line near the center indicates water temperature. About Y2 actual size. 

turf as Penman calls it). Thornthwaite and Mather 

(1955, p. 20) specify a number of conditions necessary 

to fit the definition of potential evapotranspiration: 

"First, the albedo of the evaporating surface must be a 

standard. Second, the rate of evapotranspiration must 

not be influenced by the advection of moist or dry 

air****." Certainly these conditions are not met in any 

riparian saltcedar thicket or flood-plain vegetation. 

Furthermore, temperatures, vapor pressures, and 

wind profiles inside the thickets may affect the evapotranspiration rates just as much as do those above the 

vegetation where they are usually measured. Also, the 

roughness length is often assumed to be constant but 

can, as will be shown, vary greatly with changing wind 

speeds and even seasonally. 

THE SPECIAL OBSERVATION PERIODS 

The instruments described earlier, by and large, 

worked faithfully, but did not always record truthfully. 

Bees and sometimes ants, discovering a source of 

moisture in the wicks of the hygrometers, caused 

strange fluctuations in the wet-bulb temperatures. So 

did dust devils which in the course of a few minutes 

could turn the wicks from white to brown. The actions 

of doves changing the reflectivity of the net-radiometer 

plate was mentioned already. Finally, the very sensitive anemometer system was open to undesirable infestation by spiders and other "bugs." 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F23 

At this point an explanation is in order on the kind of 

recording used at the Buckeye Project (see fig. 14). As 

already mentioned in the section on ((Instrumentation," one item is recorded every minute, which means 

that on a 16-point recorder one phenomenon (for instance, the dry-bulb temperature at 11 m) is recorded 

less than 4 times per hour. On a 12-point recorder it is 

recorded 5 times per hour. There is, therefore, a time 

lapse of 16 or 12 minutes between readings of each 

transmitter. During the morning and evening hours 

quite a few nthings" change rather rapidly. At such 

times it is easily possible that anomalous data show up 

caused by infrequency of recording rather than by instrument failures or unusual happenings in the atmosphere. 

For long-term averages all this is not serious because 

such anomalies are easily spotted and can be eliminated before monthly, weekly, or even daily means are 

computed. However, to compute potential evapotranspiration from energy-budget and mass-transfer equations, short-term, even instantaneous, values are 

needed. 

For this reason special observation periods were organized. Each period lasted 3 days, during which time 

.(I) 

w~ 

a:-

~~ 

';;( ~ 30 

a:(J) 

ww 

c..w 

:?:a: 

w t:J20 1-w 

a:o 

;;;:~ A • 5 

,. "\' o ( \._:/. 

all instruments were checked every 2 hours around the 

clock (see fig. 22) and records were kept of those instruments that did not record automatically, such as 

tensiometers and, most important, watermeters on the 

tanks. Before such periods, all instruments were 

checked for proper calibration, cleaned and adjusted if 

necessary, and coworkers were instructed into the 

proper methods of checking and recording. Also clocks 

were set to true local solar time so that it was exactly 

1200 (noon) when the sun was exactly at 180°. The 

hourly averages of the data so collected are presented 

in Appendix A for the convenience of those who desire 

to analyze the information further. In addition, 15 

5-day periods, selected from the records for their 

likelihood of accuracy and completeness, are preserved 

for future reference. 

DAILY CYCLES OF TEMPERATURE AND HUMIDITY 

Figure 24 shows the cycles of temperature anc.i 

humidity. The vegetation was, on the average, 3 m tall, 

so the lower readings were taken inside the vegetation. 

It is clear that these readings were generally higher 

during the day and very much lower during the night; 

in other words, the vegetation was exposed to greater 

,'o 

· .. ·. 

May 1966 •• 

8 

i~; :~ B ~~JV' ~~~~~_;/~~-l 

c:~0~:~~~s:~ ;~;~0J.-j;~;;~(~;. ~ 

!;r: D<~~ 051(5) 091(6) 131(7) ), 

171(5) 

I~J)<,~oc>( 211(6) 01(8) 051 ~~ 091 131 (~\ 171 21, 01 

6 1 2 18 24 6 1 2 24 6 1 2 18 24 

TRUE·SOLAR TIME 

FIGURE 24A.-Some microclimatic characteristics of the Buckeye project site, May 1966. A, March of 

temperature (degrees Celsius\, 11m (open circles), 5 m (solid line), and 2m (dots) above ground; B, 

Windspeeds (meters per second) 7 m above ground; C, March of vapor pressure (millibars) for the 

same days 11m (open circles), 5 m (solid line), and 2m (dots) above ground; D, (left) six temperature 

profiles at 0500, 9000,1300, 1700, 2100, and O!OD hours (true local time) on da.tes indicated by the ~.o 

number in parentheses; ~,(right\ s1x vapor-pressure profiles for the same times. 
F24 

.en 

UJ:::::> 

~ Cii 30 I- ...I 

<l:~ O::en UJw 

~ :i! 20 

~~ a:: a 

:;;{ ~ 10 A 

3 

w· ~ 30 a:-

~~ 

<l:~ 

a: en 20 ww 

c..w 

:!:a: 

UJ<..:l 

1-w 

a: 0 10 

:;;:~ E 

7 

G 

STUDIES OF EVAPOTRANSPIRATION 

TRUE SOLAR TIME 

6 12 18 24 

... 

TRUE SOLAR TIME 

6 12 18 24 

.. June1966 

6 

July 1966 

10 

FIGURE 24B.-Some microclimatic characteristics of the Buckeye project site, June and July 1966. A and E, March of temperature (degrees 

Celsius) 11 m (open circles), 5 m (solid lines), and 2m (dots) above ground; B and F, Windspeeds (meters per second) 7 m above ground; C 

and G, March of vapor pressure (millibars) from the same days, 11 m (open circles), 5 m (solid lines), and 2 m (dots above ground; D and 

H, (left) six temperature profiles at 0500, 9000, 1300, 1700, 2100,. and 0100 hours (true local time) on dates indicated by the number in 

parentheses; D and H, (right) six vapor-pressure profiles for the same times. 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F25 

TRUE SOLAR TIME 

6 12 18 24 6 12 18 24 6 12 18 24 .(/) 

W:J 

a:-

:>~ 

1- w 40 <(u a:(/) 

ww 

c..w 

~ a: 30 ~--~ 

a:o 

;;t~ 

11 

2 

A . 3 

12 18 

o 0 o• 

... ... .... 

12 24 

TRUE SOLAR TIME 

r 

\ 

01 

.. ~ 

Sept. 1966 • 

11 

6 12 18 24 

FIGURE 24C.-Some microclimatic characteristics of the Buckeye project site, August and September 

1966. Legend see figure 24B. 

temperature extremes than the air above it. The 

humidity was also much higher under than above the 

canopy, as is to be expected from a vegetation transpiring at the potential rate. September shows a decrease 

in vapor pressure inside the vegetation, and so does 

March, as can be expected. However, May 1967 shows a 

much lower vapor pressure inside the vegetation than 

May 1966. This lower vapor pressure was due to an 

increase in salinity of the ground water in the evapotranspirometers; it thus indicates a lower evapotranspiration rate. 

Figure 24 also shows selected temperature and vapor 

profiles illustrating better than the graphs the difference in steepness of gradients between the values in-
F26 STUDIES OF EVAPOTRANSPIRATION 

w 

LlJ 3 30 a: - :::>~ 1-w 

~ ~ 20 Ww 

c.. w 2:a: 

w<.:l 

~ ~ 10 

~~ 

11 

6 .en c:oOO: 

<( z ~ 7 t-6w 

Ia:2: 5 ~<.:JZ w -

I 

2 

w· 3 30 a:-

:::>r.n 

1- uj 

~ ~ 20 Ww 

c.. w 2:a: 

w<.:l 

1- w 10 a:o 

~~ 

A 

23 

E 

12 

... 

... 

... 

·····:~ .0 

• 0 

Oo 0 

. \;:~< ... 

TRUE SOLAR TIME 

... ... .. 

March 1967 • • 

26 

···. oO 

·.OOOoooJ~ .. 00 I ~0 • (100 •• \ 

• • \ 0 0 0 

··.· ·.~/ 

May 1967 •. 

15 

TRUE SOLAR TIME 

FIGURE 24D.-Some microclimatic characteristics of the Buckeye project site, March and May 1967. 

Legend see figure 24B. 

side· arid those· above the vegetation. Some anomalies. 

should be pointed out: for instance at 0100 on Septel}lber 11 there is a large increase in vapor between 4 

and '5 m. This increase seems impossible because it 

would indicate a downward movement of moisture and 

possibly dew formation when the air temperature inside the vegetation is 20°C! Similar but smaller 

anomalies.can be observed in the March and May 1967 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SAL TCEDAR THICKET, ARIZONA F27 

profiles and can probably be explained only by some 

malfunctioning of the instruments. 

WINDS 

Figure 24 further illustrates the fluctuations of wind 

speeds 7 m above the ground (about 4 m above the 

vegetation). The curves show that during the observation periods wind speeds were in the light to gentle 

breeze range on the Beaufort scale (less than 4 m/s). 

Notable exceptions occurred in the afternoons of July 8 

and August 3 when wind increased to a moderate 

breeze of more than 6 m/s, and another wind, perhaps 

more unexpectedly, increased to 6 m/s, in the evening 

of August 5. 

The profiles of the winds above the thickets will be 

discussed presently. What concerns us first is what 

happens inside the thickets. 

In September 1966 two anemometers were installed 

0. 5 and 1 m above ground inside the saltcedar on tank 

6. By marking the values for the windspeeds at appropriate heights for each hour, one can draw lines of 

equal windspeed (isotachs). Together they give a clear 

picture of what happens during a day. Isotach curves 

peaking upward indicate that winds are slowing down; 

those pointing down indicate that winds increase. Most 

interestingly during some times of the day winds of 

higher velocity seem to ~~tunnel" through the vegetation. Figure 25 shows five examples. Another example 

was given by van Hylckama (1970a). Unfortunately, 

related data such as vertical windspeeds and vapor 

pressure fluctuations were not observed. The tunneling 

phenomenon is true through, and has been reported 

elsewhere (in particular by Allen, 1968). Such ventilation makes it possible for large quantities of sensible 

heat to be removed, and thus may well affect the rate of 

evapotranspiration. 

Nearly all energy-budget and mass-transfer 

methods, or a combination of these, assume that the 

plots of instantaneous windspeeds (or of those averaged 

over periods of 4 hours or less) versus the logarithm of 

height lie on straight lines. It was therefore desirable 

to analyze the wind data to verify the occurrence of 

such conditions at the project site. About 2000 profiles 

were studied using border-punch cards of which figure 

26 is an example (Casey and Perry, 1951). This system 

allows rapid selection of information by grouping profiles according to windspeed, date, temperature, 

roughness length (z0), friction velocity (u*), stability 

ratios (SR), and zero displacement (d). 

For each profile the stability ratio was determined, 

expressed as (Deacon, 1953) 

2 

12 

10 

8 

6 

4 

(/) 

0: 

w 

~ 

w 12 

:2: 

~ 10 

6 

z 8 :::> 

0 

0: 6 (.!J 

w 

> 4 

0 

IIJ 

<( 2 

~ 

I 

~ 12 w 

I 

10 

8 

4 6 8 1 0 1 2 14 16 18 20 22 24 2 4 

TRUE SOLAR TIME 

FIGURE 25.-Distribution of windspeeds (isotachs) within and above 

a stand of saltcedar near Buckeye, Ariz. The vegetation is about 3 

SR or 2 u5 (27) m tall. The numbers near the isotachs indicate windspeeds in meters per second. Isotachs above 11 m are extrapolated. 
F28 STUDIES OF EVAPOTRANSPIRATION 

BUCKEYE PROJECT 

WIND ANALYSIS 

SOB/PENTAD ]J1:- I 

196___6 Hour # Z 'l. 

Calendar date 7 - 6 

WINDSPEEDS em/sec 

u 18 'I u 11 7 l.98 

u 'l £11_ u 5 4 

u1.5 /2... 

TEMP. Dl FFERENCES 

T11-4 2. ~ J,j 

T7-5 ----

WIND RATIOS 

J . .s-1 

R5/4 /, 3 () 

STABILITY RATIOS 

SR5 ___ _ 

zo '""' () 7J3 

em 

J ']_/ 

---===- em/sec 

oc 

WIND DIRECTION _ _..W~---

d =o 

FIGURE 26.-Example of a border-punched card used for the windprofile analyses at the Buckeye Project. 

in which the subscripts give the heights above ground 

(m), T is temperature (°C), and u is, windspeed (m/s). 

SR is indicative of the stability of the air. Negative 

numbers occur when the temperature at high elevation 

is lower than below, creating unstable conditions if the 

difference is larger (more negative) than the lapse rate. 

The latter, however, is of the order of 0.06°C per 6 m, 

and usually, can be neglected. Strong inversions 

usually connected with low windspeeds give rise to 

high stability ratios. More than 50 percent of all cases 

showed an SR between -0.10 and +.0.10 with about 

half of them negative and half positive. 

The variables needed to describe the logarithmic 

profile are the roughness length and the friction velocity. Both can be determined graphically; Lemon (1963) 

and Sellers (1965) explain the procedure. 

If neutral stability conditions exist, that is, when SR 

is close to zero, windspeed increases linearly with the 

logarithm of height and we have (Sellers, 1965), 

b.u b.u ------ = constant, (28) ln (z2/z1) 

in which b.u is the difference between wind speeds at 

heights z 2 and z 1• However, there is an elevation at 

which the wind speed becomes zero due to friction between the air and the vegetation or the soil. This 

height is called the roughness length (z 0 ), and we have 

Uz = (constant) ln (z/z0 ), (29) 

where Uz is the wind speed at height z. If the plots of 

windspeed fall on a straight line, the roughness length 

is easily determined graphically by extending this line 

to zero. Figure 27 shows a roughness length of0.2 m for 

the windspeeds plotted on line C. 

The nconstant" referred to is only constant for one 

particular profile, but increases with an increase of b.u. 

The flatter the profile, that is, the smaller the angle 

between the profile line and the abscissa, the larger the 

constant becomes. Laboratory experiments of turbulent flow in pipes show that this constant depends on 

the rate at which momentum is transferred (which is 

fast for a ttflat" profile) and on the air density. Thus 

constant = -- -- 1 [ - = _¥_ T J 1/") U· 

k Pa k (30) 

where T is the vertical transfer of momentum, Pa is the 

air density, u* is called <tfriction velocity," and k a constant of proportionality. The constant was named after 

von Karman who first studied this phenomenon. 

The von Karman constant actually varies with the 

temperature gradient. Sheppard (1946) shows that k 

varies from 0.32 at a positive gradient of 0.56° to 

0.61 °C/m at a negative gradient of 1.66°C/m. The gradient at the project varied from -0.34° to +0.56°C/m, 

but two-thirds of the observations showed a negative 

gradient and the bulk of these was on the order of 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F29 

VJ 

0: 

w 

1-

w 

~ 

~ 

6 

z 

::::> 

0 

0: 

(.!) 

w 

> 

0 

co 

<( 

II 

(.!) 

w 

I 

2 4 6 

WINDSPEEDS,IN METERS PER SECOND 

FIGURE 27.-Graphical determination of roughness length (z 0 ), friction velocity (u* ), and zero displacement (d). Curve A is a 

hypothetical set of windspeeds at 11, 7, 5, and 4 m extended to 0 

m/s. Line B is derived by plotting the indicated speeds of curve A 

1.5 m (d) below those "observed" on that curve, giving z0 = 0.5 m. 

Line C connects another set of hypothetical windspeeds at indicated elevations. This line is straight and the extension gives z0 = 

0.2 m. The angle (a) is about 49°, hence u* = lOOk/tan 49 = 35.6, 

k being von Karman's constant, taken as = 0.41. 

-0.10° to -0.20°C/m. Thus, use of an average value of 

0.41 seems reasonable. Monteith and Szeicz (1960) and 

van Bavel (1966) used the same value. 

With this in mind we can now write 

u = .!!::£_ ln z 

k Z 0 

(31) 

The friction velocity can be obtained from the slope 

of the wind profile. As an example, line C in figure 27 

makes an angle of 49° with the abscissa, hence we have 

u = 100 k/tan 49 = 35.6, remembering that In z 0 is a 

constant. 

Actually, z0 is the integration constant for 

8ii/8z = ujk z (32) 

the differential equation of the profile of mean wind 

velocity above ground. This equations strictly holds 

over flat surfaces such as ice and snow or very short 

smooth vegetation. Under such conditions z 0 can beconsidered to be an invariable property of the surface. 

Tall vegetation waving in the breeze does not offer 

such a convenient roughness length. Equation 31 

therefore has been empirically modified to 

u = u* In z - d (33) 

k Zo 

in which d is the zero-plane displacement. It can be 

regarded as a measure of the wind penetrating the 

vegetation or as a level above which the normal turbulent exchange takes place. Sutton (1953) points out 

that, if d and z 0 are regarded as independent arbitrary 

constants, equation 33 cannot be derived from the original differential equation 32 because that is an equation of the first order whose solution must involve not 

more than one arbitrary constant. 

A zero displacement also can be determined graphically as illustrated in figure 27. Assume a curve A 

drawn through 4 points at which windspeeds are 

measured; continuing this curve to the zero line would 

indicate that at 2 m above the ground the velocity is 

zero. By lowering the points, we find by trial and error 

that if each of the speeds are plotted at 1.5 m below the 

original elevation, the points do lie on a straight line 

and form a logarithmic profile, with a Z0 at 0.5 m and 

1.5 is the zero-plane displacement (d). The d values for 

the Buckeye Project data, however, are so variable and 

random and even negative (when a curve of type A in 

figure 27 is convex instead of concave) that, as Sellers 

(1965, p. 150) states: ~~it is almost impossible to attach 

any real physical significance to them" and ~~in practically all cases and within the accuracy of the measurements the wind profile above the vegetation can be 

described just as well by equation 31 as by equation 

33." This is not surprising, remembering that d is an 

empirical constant. 

The profile above tall vegetation, however, is affected by that vegetation, and Z0 actually becomes an 

adjusted roughness length. Such an adjusted roughness length changes with the steepness of the profile 

and also with the stability ratio. For instance, by 

grouping wind ratios u7/u4 (the ratio of the wind speed 

at 7 m over that at 4 m) and evaluating the modified 

roughness length in each of these groups (which can be 

accomplished quickly by sorting the border-punch 

cards), one can plot the means so obtained as shown in 

figure 28. Similarly, the modified roughness length can 

be plotted against stability ratios, as shown in figure 

29. These are the roughness lengths that have been 

used in the mass-transfer and combination equations 

to be discussed later. 
F30 STUDIES OF EVAPOTRANSPIRATION 

2.00 

1 

~ 1.80 ...._ ·' I' 

:::s 

CJ) 

0 1.60 

i= 

<( 

CI: 1.40 0 

z 

~ 1.20 

1.00 

40 80 100 120 140 160 180 200 

MODIFIED ROUGHNESS LENGTH (z0 ), IN CENTIMETERS 

FIGURE 28.-Relation between modified roughness length and the 

ratio of windspeeds at 7 m to those at 4 m. Only the means of z 11 and 

u1/u4 for each stability group are plotted. 

N I' 

:::s ...._ 

E:0 I 

.... 

...:-

CJ) 

0 

i= 

<( 0.4 CI: 

>-

1- 0.2 

-' 

co 

<( 

1- 0 CJ) 

-0.2 

-0.4 

-0.6 

' ' ' \ 

' \ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

100 150 200 

ROUGHNESS LENGTH z0 , IN CENTIMETERS 

FIGURE 29.-Modified roughness length versus stability ratio. A 

dot on a horizontal short line indicates the mean z 0 for each 

stability group. A dot on a vertical short line indicates the mean 

SR for each z 0 group. The length of the lines indicates the standard deviation. Numbers on the inside of the axes indicate the 

groupings of the ranges of z 0's and SR's for which means were 

computed using the border-punch cards (see fig. 26). 

RADIATION 

Figure 30 shows the march of radiation and heat flow 

for the seven 3-day periods mentioned earlier. The 

curves of the total short-wave (direct and diffuse) solar 

radiation show that most of the time there was a clear 

sky. Variable cloudiness occurred on July 7; August 3 

was overcast as was March 24. Reflected radiation is 

also quite regular, but the change in reflectivity with 

the change in cloudiness is clearly shown in the record. 

Net radiation, consisting of the total short- and longwave radiation coming in, minus that going out, follows the march of the total short-wave radiation quite 

faithfully, except on May 5, 1966, where between 1200 

and 1500 an unusual pattern can be observed which is 

not easily explainable. A warm rain could cause this 

kind of record, but there was no rain during any of the 

observation periods; nor do temperature and wind profiles show anything unusual. 

SOIL HEAT FLUX 

The soil heat-flux term appears in the energy budget 

because part of the incoming radiation is used to heat 

the soil during the daylight hours. Conversely, at night 

the soil loses heat by outgoing long-wave radiation and 

by conduction. 

The soil heat flux is considered positive when heat 

moves through the soil toward the surface and negative when it moves from the surface into the soil. Figure 30 shows that the night-time flows are quite regular but during the day they are not. Some of this irregularity is undoubtedly due to a variation in shading, for instance the increase in flow at 1400 on all 

three days in June 1966 and also at 0700 on September 

8, 9, and 10. Other sudden changes may well be due to 

temporary accumulations of moisture on the heat-flow 

plates. 

Measuring this flux is troublesome because the sensing elements must be small in order to allow water and 

vapor movements which otherwise may affect the temperature of the elements and give wrong flux values. 

Heat flux through a small area, therefore, must represent the soil heat flux through a large area. At the 

Buckeye Project a measuring area of 0.00735 m2 supposedly represented an area of 81m2 , which is the size 

FIGURE 30.-March of radiation and heat flow at the Buckeye project ..... 

for 3-day periods as indicated. Total short-wave radiation (direct 

plus diffuse): open circles (oooo); short-wave radiation reflected by 

soil and vegetation (albedo): dots (. .... ); net short plus long-wave 

radiation (incoming minus reflected and outgoing): xxx; heat flux 

into and out of the soil: ~.:l.:l. 
WEATHER AND EV APOTRANSPIRATlON STCDIES IN A SALTCEDAR THICKET, ARIZONA 

6 

1.2 f.. 3 

1.2 1- 6 

UJ 

1-

::::> 

z 1.2 f.. 3 

~ 

a: 

UJ 

a.. 

CJ) 

>-

UJ 

_J 

t:J 

z 

<.( 

_J 

1.2 f.. 23 

1.2 

6 

12 

12 

18 24 

4 

7 

24 

12 

xo 

18 24 

6 

0 

ox 

0 

ox 

6 

12 

0000 

0 0 

xxxx 

X 

0 o o 0 

X 

xX oXO 

18 

0 

xo 

X X X 0 

00 

0 0 

0 

X X X X 0 

X xo 

xo 

I I 

12 18 

24 

5 

8 

25 

13 

24 

TRUE LOCAL SOLAR TIME 

6 12 

oo 0 0 

0 

xXXX 0 

0 X X ox 

00 

0 0 

0 0 X X X OX X 

X ox 

6 12 

0 

xo 

18 24 

6 

June 1966 

9 

July 1966 

26 

- 10 

- 10 

-

-

-

- 10 

-

-

- 10 

-

-

- 10 

- Mar. 1967 _ 

-

-

14 - 10 

May 1967 0 -

xo -

I 

18 24 

a: 

UJ 

1-

UJ 

~ 

UJ 

a: 

<.( 

::::> 

0 

CJ) 

a: 

UJ 

a.. 

CJ) 

II-

<.( 

s 

0 

UJ 

a: 

0 

z 

::::> 

I 

F31 
F32 STUDIES OF EVAPOTRANSPIRATION 

of one evapotranspirometer. Measurements made on 

bare soil might actually be representative, but on vegetated soil, where sunlight and shade may alternate in 

unpredictable patterns, one can expect large variations 

which even show up in hourly averages. On the other 

hand, when the soil is vegetated and mostly shaded, 

the amounts of incoming and outgoing fluxes are very 

small compared to those of solar radiation; so they are 

often neglected (Penman, 1956). 

If the density of the soil, its heat capacity, and 

moisture content are known, and if temperature records at different depths in the soil are available, it is 

possible to compute the heat flux. We start with the 

equation for heat flux derived from Fourier's conduction equation in one dimension: 

where T =temperature, in degrees Celsius, 

t =time, in seconds, 

z = depth, in meters, 

'YJ = A./ p c = meters squared per second (thermal 

diffusivity), 

A.= thermal conductivity (watts per meter per 

degree Celsius), 

c =heat capacity-(joules per kilogram per degree Celsius)·, 

p =density (kilogram per cubic meter). 

At any given depth (z) the heat flux (Gz in W m-2 ) is: 

G~ =A. aaT ~ z (35) 

or 

p c aT = aGz 

at az (36) 

Following Portman (1954) we can now multiply both 

sides of equation 36 by ?>z and integrate from the surface where z = 0 to a depth z: 

cz aTaz_ G J 0 pc at -Go- z (37) 

If, from temperature gradients, we can determine the 

depth where aT/& approaches 0, we can determine G0 

by evaluating the integral term from the surface to a 

depth where Gz = 0. 

The term pc is the product of density and heat capacity, and both variables change with the moisture 

content of the soil. The moisture content was regularly 

measured and expressed as moisture percent by volume (Mv); Moisture content by weight (Mw) is then: 

M~~_. = 100M,. (38) 

140 + M,. 

where 140 is 100 x the density of the dry soil as determined by numerous weighings of Buckeye soils. 

The specific heat, according to Kersten (1949), varies 

from 0.176 for Northway silt loam to 0.197 for Northway fine sand. Because the texture of the soils at the 

Buckeye Project is slightly sandier than that of Northway silt loam, a value of 0.18 calories gram- 1 

oc- 1(753.5 J kg- 1 oC- 1

) was adopted for dry soil. Hence, 

considering the specific heat for water being unity for 

all practical purposes, the volumetric heat capacity is: 

pc = 1.40 (0.18 + Mw ) cal cm-3 oc- 1 (39) 1 -Mw 

One can now plot two temperature profiles, say, 1 hour 

apart, on each of the layers of soil for which the 

moisture-dependent volumetric heat capacity can be 

considered uniform. The product of depth times a 

change in temperature can then be measured, for 

example, by planimeter, and this product multiplied by 

the pc for the particular depth gives the heat flux in 

calories per cm2 (or joules/m2) per hour. For the example shown in figure 31, the profiles were plotted on DI 

Form 213-F paper where 1 oc was represented by 0.5 

in. and 1 em depth by 0.05 in. Because the planimeter 

measured areas in cm2 , an adjustment factor of6.2 was 

necessary to convert the data to calories em - 2 • 

Figure 32 shows the march of heat flow during one 

day. computed from temperature measurements compared to that from heat-flow plates and also shows the 

24-hour sinusoidal wave of these fluxes. It is immediately striking that these waves have roughly the 

oc 18 20 22 24 26 

en 

a: 

w 2.4 

1-

w 0.2 ~ 

~ 

w· TOTAL FLUX 

u 0.4 -0.4 X 0.276 X f <! 

u. +1.3 X 0.276 X f a: 

:::l +0.7 X 0.426 X f en = 3.3 cal cm-2 hr-1 3: 0.6 

0 

....J 

w 

co 

:r 0.8 

1-

a.. 

w 

0 

1.0 

FIGURE 31.-Graphical computation of heat flow from 0500 (.) to 

0600 (x) hours on May 5, 1966 in the soil of tank No. 6 at the 

Buckeye project. M1,: moisture content by volume (percent); pc; 

volumetric heat capacity (calories per cubic centimeter per degree 

Celsius); and f: factor to adjust units given by the planimeter to 

units used on the graph paper (see text). 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F33 

FIGURE 32.-Heat flow into (negative) and out of (positive) the soil in 

an evapotranspirometer near Buckeye, Ariz., recorded from heatflow plates and computed from temperature records. The dashed 

lines are the curves ofthe first harmonics of the heat-flow and the 

temperature data, respectively. 

same amplitude, but their phase angles are about 4.5 

hours apart. The sinusoidal wave for the fluxes computed from temperature profiles peaks at a little before 

0800 while the one computed from the heat-flow plates 

peaks, as could be expected, at about noon. This discrepancy was quite consistent for all the periods 

analyzed. 

To find if more accurate or closely spaced thermometers could give closer agreement, a set of similar 

data, published by the Water Conservation Laboratory 

of the Agricultural Research Service in Tempe, Ariz. 

(van Bavel, 1967), was analyzed. The results, given in 

figure 33, show the close agreements between heat-flow 

data from heat-flow plates and those computed from 

temperature gradients. However, the temperatures 

were measured at depths 0.5, 1, 2, 4, 8, 16, 32, 64, and 

128 em whereas temperatures at the Buckeye site were 

only measured at 0, 10, 45, and 90 em. Also the Tempe 

thermographs were reported to be accurate to at least 

0.1 °C, while the Buckeye ones were only accurate to 

0.5°C. Notice further that the Tempe fluxes are an 

order of magnitude larger. The data refer not to flow 

into or out of vegetated soil but into or out of bare 

ground. 

Especially the uncertainty as to what happens in the 

top 10 em of the soil during the course of a day must 

lead to grave errors. Hence soil-temperature data, as 

collected at the Buckeye Project, could not be used to 

compute heat fluxes on an hourly or more frequent 

basis. They could, however, be used on a daily basis 

because the sinusoidal waves were very similar in 

shape. Figure 34 shows four examples of waves computed from temperature profiles that have been shifted 

over to coincide in phase with the ones of the heat-flow 

plates. 

-20 

-2 

a: 

w 

1-

-10 w -1 :2: 

a: w 

:::l a: 0 <( 

I :::l 

a: 0 0 0 

w (/) 

a.. a: (/) w 

>- a.. 

w (/) 

...J 10 1-

C) 1-

z <( 

<( s ...J 0 2 w 

20 a: 

0 

z 

:::l 

3 I 

4 8 12 16 20 24 

LOCAL SOLAR TIME 

FIGURE 33.-Heat flow into (negative) and out of(positive) the soil in 

a lysimeter near Tempe, Ariz., recorded from heat-flow plates and 

computed from temperature records. The cl~shed lines are the 

curves of the first harmonics of the heat-flow 8nd the temperature 

data, respectively. (Plate and temperature data from van Bavel, 

1967.) 

TRUE LOCAL SOLAR TIME 

FIGURE 34.-Sinusoidal 24-hour waves of heat flow into (negative) and out of (positive) the soil computed by harmonic 

analysis of heat-flow plates data (dashed lines) and from 

data computed from soil-temperature gradients (solid lines). 
F34 STUDIES OF EVAPOTRANSPIRATION 

More than 20 years ago McCulloch and Penman 

(1956, p. 279) ran into similar difficulties in their attempt to compute heat flow in soils. They emphasized 

the need for precise thermometers and concluded: ~~If 

only one set of observations is to be used, then phases 

are preferred to amplitudes, that is inaccurate thermometers with an accurate clock are better than good 

thermometers with a bad clock." 

SOIL TEMPERATURES 

We have seen that the soil-temperature data of the 

Buckeye Project usually were inadequate for the computation of heat fluxes into or out of the soil. They are, 

however, adequate for computing some soil characteristics, using data derived from the harmonic 

analyses of the temperature waves. 

The method assumes that the data comprise only 

perfect sine curves of known periods P (in this case, 

days) or submultiples of P. Each curve explains a part 

of the variance of the data and it is possible to determine how large a part of the variance is expressed by a 

first, second, etc., sinusoidal curve after the amplitudes 

(a) and phase angles (a) of each curve have been computed (see Panofsky and Brier, 1965). 

Analyses of the temperature data of the Buckeye 

Project showed that nearly always more than 85 percent of the variance could be explained by the first 

harmonic, that is the sine waves with a 24-hour, or 86 

400-second period. Using these phase angles and 

amplitude, one can compute thermal diffusivities CYJ ), 

timelags (t), and other characteristics. 9 If a::: is the 

amplitude at a depth z and a0 the amplitude of the 

surface temperature wave, we have (van Wijk and deVries, 1966): 

a::: = a 0 exp { - z [ 'YJ~J 112} (40) 

where P is the length of the period (in this case 24 

hours or 86 400 seconds.) 

Once "fJ is computed, the time lag between the occurrence of the maximum temperature at the surface and 

the maximum temperature at a distance below the surface can be computed using 

p t = z/2 ( -- ) '/z 

7r'Y] (41) 

This time lag can then be compared with a time lag 

"Thennal diffusivity (71) is defined as K/pc, where K is the the1=al conductivity (cal em-• 

sec -•oc-•), p(g cm-3

) the density ofthe soil, andc (cal g-•o c-•) its heat capacity. It denotes 

the temperature change in any layer of the soil as heat flows into it from an adjacent layer 

(Baver and others, 1972). 

observed between peaks of the sinusoidal graphs or actual curves at the two depths. 

A similar procedure can be followed for other depths, 

such as between z 1 = 10 em and z 2 = 45 em. The term z 

in (40 and 41) then becomes dz = z 2 - z,. 

Van Wijk and deVries (1966) refer to the quantity 

(42) 

as the damping depth. It is the depth at which the 

amplitude is 1/e or 0.37 x the amplitude at the surface. 

The quantity arises as a constant when integrating the 

heat-flux equation (34) under suitable boundary conditions. At a depth of 4.61 Dr1, the amplitude equals only 

0.01 a 0 and the temperature (on a daily basis) can be 

considered constant. 

Table 6 shows the results of harmonic analyses for 

two locations and two 3-day periods using temperature 

data at three depths. With a few exceptions, observational and computational data of the time lags agree 

quite well; this agreement gives some confidence in the 

computations of diffusivities and damping depths. The 

diffusivities increase as the moisture content increases 

with depth, and those computed for the three layers are 

in agreement with those given by van Wijk and de 

Vries (1966) and also by Baver, Gardner, and Gardner 

(1972). 

Figure 35 illustrates how damping depth and depths 

of near-zero amplitude can be obtained graphically by 

plotting the amplitudes computed for 0, 10, and 45 em 

depths on semilogarithmic paper. The short horizontal 

lines indicate the depth of Dd and the arrows are at the 

depth where the amplitude is about equal to 0.05 x the 

amplitude at the surface. 

HEAT FLUX AND WATER USE 

It follows from the first law of thermodynamics that 

if an evapotranspirometer for one reason or another 

uses less water, the energy of vaporization that becomes available has to be used somewhere else. Probably most of it is used to create turbulence in the air, 

and a certain amount will raise the temperature of the 

foliage, the branches, and the stems of the shrubs. 

Though the last three quantities are measureable, they 

were not determined at the Buckeye Project. The first 

quantity would be very difficult to distinguish from 

other turbulences. However, data were collected that 

showed an increase in the amount of heat" stored in 

soils of evapotranspirometers using less water. 

Figure 36 shows the heat flux in two tanks having 

identical vegetation covers. One tank used less water 

than the other because of a difference in depth to 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F35 

TABLE 6.-Some results of harmonic analyses of soil-temperature fluctuations inside and outside tank 6 for means of two 3-day periods 

[Diffusivity between 0 and 10, 0 and 45, and 10 and 45 em, respectively. Damping depth Wd) = ( ~) 112 at which a = 0.37 a.,, between 0 and 10, 0 and 45, and 10 and 45 em) 

First harmonic ( oc I Time lag Depth 

Mean Phase Time of highest 

Depth temperature Amplitude Fluctuations angles temperature Diffusivity Computed Observed Damping a= 0.05 a., 

Tank (em I (oC) (a) accounted (hr;min) (hr; min) (em) for CCI (hr; min) {'J)) mm2s-' (em) 

(%1 

May 3-6, 1966 

Inside 0 30.3 9.92 80 318 13:49 0.21 5:02 4:19 7.6 23 10 27.4 2.66 89 253 18:08 0.45 15:27 12:05 11.1 33 45 24.0 0.17 72 137 01:54 0.59 10:30 7:46 12.7 38 Outside 0 24.6 6.11 79 308 14:27 0.47 3:22 2:49 11.4 34 10 23.6 2.55 89 266 17:16 0.75 11:58 11:14 14.4 43 45 21.6 0.27 89 140 01:46 0.88 8:59 8:25 15.6 47 

July 6-9, 1966 

Inside 0 32.4 6.20 87 314 10 30.2 2.15 87 257 45 27.4 0.23 90 124 Outside 0 33.1 8.53 85 323 10 29.9 2.24 90 262 45 27.4 0.25 91 132 

0.1 0.2 0.4°'6 1 2 4 6 8

10 0.6 

0.2 0.4 2 

10 

68 

en 

a: 

w 

1- 0.2 w 

:::2: 

~ 

w· 0.4 (.) 

~ 

u. 

a: 

::l 0.6 en 

....J 0 0 

en 

w 

J: 0.2 1-

3: 

0 

....J 

w 

0) 0.4 

J: 

1-

a.. 

w 

0 0.6 

0.1 

:7 

:L ?' _.. / !'7 ------/ /o/ ------ / /'/-?"' ---- /"' ------ '/"' /"-?"' 

~ ............ ~ / 

,.. .... 

Tank 6 May 3, 1966 

Near Tank 6 

May 3, 1966 

/ ""/. r- ,.,.. ..... / 

------ / __ .... --.... -- ~ ~ 

~ 

Tank6July 5,1966 

0.2 0.40.6 1 2 4 6 810 0.2 0.40.6 1 2 4 

AMPLITUDES OF DAILY SINUSOIDAL WAVES, 

IN DEGREES CELSIUS 

0.2 

0.4 

0.6 

0 

0.2 

0.4 

0.6 

6 810 

FIGURE 35.-Relations between amplitudes (a) of daily temperature 

waves computed from data at 0, 10, and 45 em depth, damping 

depths (-), and depths of 0.05 a (+--) for two locations and two 

dates near Buckeye, Arizona. 

ground water. The heat flux in the tank using less 

water is larger than that in the other. 

Note that for better comparison, the water use is 

expressed in calories per day per tank or joules per day 

per tank. The slopes of the lines connecting a tank with 

higher water use to one with lower water use indicate 

that each time the water use decreases, the soil heat 

flux increases. The dashed line, which is the mean 

slope of all curves, shows that an increase of 108 

14:06 

17:53 

02:42 

13:28 

17:34 

01:14 

0.32 4:04 3:47 9.4 28 

0.68 12:34 12:36 13.7 41 

0.89 8:33 8:49 15.6 47 

0.20 5:09 4:06 7.4 22 

0.58 13:37 12:46 12.6 38 

0.93 8:22 8:40 16.0 48 

:::£ 108 z JOULES PER DAY PER TANK 

<( 1 2 3 4 5 6 Ia: 10 

w 40 a.. :::£ 

r z <( 8 <( 

0 Ia: 30 a: 

w w a.. a.. 

CJ) 6 

'~ r w <( 

a: 0 

0 

~~ 20 a: ...J w <( 4 a.. 

u CJ) 

CX) w 

0 ...J 

~", 10 ::J 

2 0 

w ...., 

CJ) " CX) 

::J 0 

" a: 

w 0 

I- 0.5 1.0 1.5 

<( 

$ SOIL HEAT FLUX: 108 CALORIES PER DAY 

PER TANK 

FIGURE 36.-Soil heat flux versus water use expressed as heat 

units. Two tanks using different amounts of water in each 

period are compared. 1 :May 1-6, 1963; 2: June 5-10, 1963; 

3 : September 8-13, 1963; 4 : October 8-13, 1963; 5 : May 

5-8, 1966; 6: August 3-6, 1966; 7: September 8-11, 1966. 

Dashed line: mean slope of all curves. 

calories per day per tank is the result of a decrease of 

water use of 6 x 108 calories per day per tank. Obviously considerable energy is used elsewhere, but it was 

impossible to show this transfer inside the thickets at 

the Buckeye Project. 

TURBULENT TRANSPORT 

As noted earlier (see section on !!Empirical 

Methods"), most of the theoretical and even some of the 

empirical equations to determine evapotranspiration 
F36 STUDIES OF EVAPOTRANSPIRATION 

are based on the profile theory. We saw that at the 

Buckeye Project the vast majority of the wind profiles 

could be described by equation 31 with z 0 being the 

ttadjusted" roughness length and d (the zero displacement) suppressed. Also mentioned were the transport 

constants (see equations 12, 13, 14, and 15) for 

momentum, heat, vapor, and carbon dioxide which, depending on the type of turbulence, may or may not be 

the same. If they are the same, temperature and 

vapor-pressure values plotted against windspeeds at a 

number of elevations should fall on straight lines regardless of the shape of the profile (Penman and Long, 

1960). Figure 37 shows examples for 6 of the periods 

used in figures 24 and 30. Note that 4-hour averages 

have been computed to avoid embarrassing anomalies. 

Nonetheless there are some glaring ones, for instance 

period 6 of September 10-11, 1966, which apparently 

can only be explained by instrumental or observational 

errors. Also, the data for March 23-26, 1967, are quite 

erratic. During that period the actual evapotranspiration is low and the transport constants for momentum, 

temperature, and vapor pressure might very well not 

be the same. Swinbank and Dyer (1967) showed that 

the transport constants for temperature and vapor 

pressure are often equal, but they are not equal to the 

momentum transport constant. If this is so, then the 

deviations from a straight line should be the same for 

temperature as well as vapor pressure. This actually 

seems to be the case as shown by the data for March 

1967; wind profiles for that period deviate most from 

the logarithmic ones. 

CARBON DIOXIDE 

During the growing seasons of 1960-61 it was observed that the rate of growth and development of 

saltcedar grown in evapotranspirometers is correlated 

with the use of water by that plant. The less vigorous 

the growth, the less water was U6ed. This raised the 

question whether the growth rate affected the rate of 

transpiration or whether other environmental factors 

affected both. In one attempt to explain the diminishing rates of growth and development, van Hylckama 

(1963) mentioned that maybe the carbon dioxide had 

become the limiting factor, especially at times of low 

windspeeds. If, during times of vigorous assimilation, 

the C02 content were to drop from a mean of about 300 

ppm to 100 ppm, rates of growth and development 

would be diminished. Hence, the use of water might be 

diminished also. Such observations haye been reported 

by Bonner and Galston (1952). 

Also the fact that rates of photosynthesis and of 

transpiration are related is well known. Bierhuizen 

and Slatyer (1965) found that, under laboratory conditions, an increase in carbon dioxide content of the air to 

0.10 percent or more results in less transpiration per 

gram of carbon dioxide assimilated by cotton. One 

could say, therefore, that the plants become more efficient producers. But such conditions do not exist in the 

field, where the C02 content of the air rarely exceeds 

0.02 to 0.04 percent (200 to 400 ppm). Pallas (1965), 

also under laboratory conditions, observed less 

transpiration when the C02 content of the ambient air 

is high. He noticed that the stomata of his plants closed 

when the C02 content went up to 4,000 ppm. One must 

assume that this impaired the passages of both C02 

and vapor. 

Therefore, the C02 content of the air within and 

above the saltcedar thickets was measured with an infrared gas analyzer (van Hylckama, 1969). 

Figure 38 is an example of the results of such measurements. During the day, air turbulence caused mixing and the C02 content remained about the same in, 

as well as over, the thickets. During the night, sharp 

gradients occurred as the C02 content inside the thickets rose at times to more than 380 ppm. The figure also 

shows rather large fluctuations in C02 content at all 

levels and times. Much of this was undoubtedly due to 

turbulence and possibly also to shifts in calibration of 

the analyzer. 

When photosynthesis takes place, one can expect to 

find smaller C02 contents inside, or near, the vegetation than above it. Diffusion and turbulence tend to 

equalize the C02 content of the air at all levels just as 

they equalize temperature, water-vapor content, and 

other properties of the atmosphere. These fluxes can be 

expressed as g m-2 s- 1 or cal m-2 s- 1• There are several 

empirical methods for computing them, and the ones 

used here are: 

F = k 2 (ua - ub) (Ca - Cb) (43) 

c [ln{ (a- Zo) I (b -zo) }F 

for C02 and 

V = k 2 (ua - ub) (ea - eb) (44) 

[ln {(a - z0 ) I (b - Zo) } F 

for water vapor. Fe is the C02 flux, k the von Karman 

constant (here taken as 0.41), Ua and ub (mls) are the 

windspeeds at levels a and b (m), respectively, Ca and 

cb the C02 contents (ppm), Zo is the roughness length 

(m); and V the vapor flux with ea and eb the vapor 

pressures (mb) (Lemon, 1960). To convert to glm3 , it 

was assumed that 1 ppm C02 at 985 mb (98.5 kPa) and 

25°C equaled 1. 77 mglm3 and 1 mb vapor equaled 724 

mglm3 • 

Figure 39 is an example of C02 fluxes during the day. 

Although short-time fluctuations resulting from those 

shown in figure 38 were somewhat smoothed out by 

averaging 3 days of data, there still are considerable 

irregularities. 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SAL TCEDAR THICKET, ARIZONA 

Cl 

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FIGURE 37.-Plots of windspeeds (u) in meters per second versus vapor pressures (e) in millibars and versus temperatures (T) in degrees Celsius at the same levels. Data presented are 4-hour means: 1 = hours 5-8, 2 = hours 9-12, etc. 

Note: e and T scales on the abscissa are sometimes broken (88) to save space. The windspeed scale (ordinate) does not 

change. Thee (mb) and T (°C) values at the top of each box refer to time periods 2, 4 (1700-2000), and 6 (0100-0400), 

those at the bottom to 1, 3 (1300-1600), and 5 (2100-2400). 

F37 
F38 

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STUDIES OF EVAPOTRANSPIRATION 

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FIGURE 37.-Continued. 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F39 

20 25 22 20 23 20 23 

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FIGURE 37.-Continued. 
F40 STUDIES OF EVAPOTRANSPIRATION 

.z 

a: a 380 ~::::i 

LL~ 

0~ 360 

(\ 

Ground level-/ 

I- a: Zw 

Wa_ 340 

I-(/) zlOa: 320 

u<( 

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8~ 300 

c:lcno wa:Z 4 

wwO 

a..f-U 

cnww 3 

0, ~ (/) z a: -zw 

~-a.. 2 

18 6 18 

TRUE SOLAR TIME 

FIGURE 38.-Top. Fluctuation of carbon dioxide content (parts per 

million) in and over a stand of saltcedar near Buckeye, Ariz., June 

3-5, 1966. Carbon dioxide content at 6 m, at the top of the vegetation, 3 m, and at ground level. Bottom: windspeed (meters/second) 

at 7 m above the ground. 

First and third harmonic 

12 18 24 6 

TRUE SOLAR TIME 

FIGURE 39.-Carbon dioxide fluxes in a saltcedar thicket near Buckeye, Ariz. Observed data, mean of June 3-5, 1966, compared with 

the computed first harmonic and with the combined first and third 

harmonic. 

By applying harmonic analysis, the data can be 

smoothed out further. Figure 3_9 shows that the first 

harmonic alone represents the data rather well, although by adding the third (8 hour) harmonic a surprisingly faithful picture is obtained. The reason for 

this has yet to be explained. 

Using first harmonics of both water vapor and C02 

fluxes an interesting curve is obtained showing the 

correlation between the fluxes as exemplified in figure 

40. The vapor flux, except during times of dew formation, would always be negative, but it is more negative 

during daylight hours, when the C02 flux is positive 

and much less so during the night, when C02 flux is 

also negative and large amounts of carbon dioxide are 

formed inside the thickets as was shown in figure 38. 

Figure 40 also makes it possible to estimate the 

water-use efficiency (the amount of carbohydrate produced per unit of water) by considering the amount of 

water transpired per unit of C02 absorbed. Ratios of 

over 600 occur in the early morning when light intensity is low, while values around 160 occur during the 

middle of the day and increase rapidly towards the 

evening. These values agree with data presented by 

Arkley (1963) and Bierhuizen and Slatyer (1965). 

PREDICTING WATER USE 

USE OF LONG-TERM MEANS 

For the purpose of estimating how much water may 

be saved by managing riparian vegetation, it is usually 

and reasonably assumed that riparian vegetation 

transpires at the potential rate. For flood-plain vegetation less close to stream or lake, the situation grows 

a: 

WM 

a_, 

(/)0 

:2:.-

<(X 

a:O 

(.!)Z 

zO _u 

.w 

xcn 

:>a: ..Jw 

u.a.. 

wa: Ow 

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za:: 

o<( 

co:> 

a:O <((/) 

u 

+6 

15 

+4 

+2 

-2 

-'4 

-6 

-60 

3 

2 

-50 -40 -30 -20 -10 

VAPOR FLUX, IN GRAMS PER SQUARE 

METER PER SECOND X 10"2 

FIGURE 40.-Relation between carbon dioxide and water vapor 

fluxes during a day over a stand of saltcedar near Buckeye, 

Ariz., means of 3 days June 3-5, 1966. Ordinate: C02 flux toward the vegetation positive; away from the plant negative. 

Abscissa: water vapor fluxes away from the vegation negative. 

Numbers indicate hours of the days. Flux data are quantities 

resulting from harmonic analysis of the fluxes and represent 

first harmonics only. 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F41 

more complex. Salinity of soil moisture, increasing 

depth to ground water, topography, and exposure affect, and usually diminish evapotranspiration. The 

presumption of potential evaporation is then no longer 

valid. However, in most cases, the approach to the estimation of actual evapotranspiration is through the 

potential. The main interest therefore remains to predict or be able to predict potential evapotranspiration. 

The foregoing description of the microclimate of a 

saltcedar thicket shows it to be a very intricate 

phenomenon. If one wants to know how this ((complicated mess we call a canopy" (Tanner, 1963, p. 148) 

affects evapotranspiration rates, all variables pertinent to the problem will have to be taken into account. 

However, models constructed for the prediction of 

water use often do not include a sufficient number, let 

alone all, of the variables that determine actual or potential evapotranspiration. Many models have been 

used, and even more proposed, that attempt to simplify 

the computation; often only one or two variables are 

used because of a lack of more information on the one 

hand and perhaps an overanxious desire to predict on 

the other. 

A few examples will be presented, and the results 

compared with one another and with data from the 

analyses of water use by two Buckeye evapotranspirometers. No detailed description of the methods are 

given because they are readily available in the literature (see, for instance, the discussions by Cruff and 

Thompson (1967) and by Jensen (1973) ). Table 7 lists 

the references and also the variables used by the various authors. Note that temperature (T) and radiation 

(Rs) are the most frequently chosen variables. Rs, when 

not measured, is supposed to be taken from climatic 

tables. Actual hours of sunshine relative to possible 

sunshine duration for that latitude and time are occasionally used instead of radiation. Vapor pressure (e) 

TABLE 7.-Variables used in some models for predicting potential 

evapotranspiration 

[T: mean _temperature or maximum and minimum; e: vapor pressure, relative humidity, saturatiOn vapor pressures (from T1, wet-bulb depression or saturation vapor density; R_,: solar radiation; Lat: latitude term to indicate duration of sunlight; and u: mean windspeed] 

T 

Lowry-Johnson (19421 ____________ X Eagleman (19671 __________________ X Jensen-Raise (19631 ______________ X Lane (19641 ______________________ X 

Thornthwaite (19481 ______________ X Turc (1961) ______________________ X 

Blaney-Criddle (1962L ____________ X 

Weather Bureau (Kohler and others (19551 ________ X 

Christiansen-Hargreaves (19691 __________________________ x Morton (19751 ____________________ X Papadakis (1966) _________________ _ Makkink (19571 ___________________ _ Hamon (1961) _____________________ _ 

Olivier (1961) ____________________ _ 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

Lat 

X 

X 

X 

X 

X 

u 

X 

X 

Other 

Crop 

Factor 

Albedo 

can be computed from temperature and relative 

humidity records. Sometimes saturation vapor pressures are used; these are obtainable from temperature 

data and vapor-pressure tables. 

Such models are incapable of depicting the (often 

sudden) changes in any of these and other variables in 

and over the vegetation. They are typically applied not 

to short-term periods (although some of their authors 

so do), but to average monthly data, preferably taken 

as means over a series of years. Figure 41 gives the 

mean potential evapotranspiration per year as computed by the models mentioned in table 7; the results 

are compared with measured data taken as the average 

from tanks 3 and 5. These tanks had, between 1961 and 

1965, the smallest depth to water, and, without a 

doubt, evapotranspired at the potential rate, the more 

so, because the soil-moisture salinity was also low during that period. Effect of depth to ground water and of 

soil-moisture salinity will be discussed presently. 

Another way of comparing models with measured 

data is by smoothing out the fluctuations throughout 

the year by using harmonic analyses. This is done for 

the year 1963 as shown in table 8. It is obvious from the 

percent column that the first harmonic describes the 

3000 

a: 

<( 

w 

>-

a: 

w 

a.. 

V'J 

a: 

w 2000 ~ 

w 

~ 

;rc---------------

-e § 104 ~ ·g Me_asured water use-means of 94 98 ~ ·~ 4 years and two tanks 

T --------- - 84¥81 

78-f-

::::i 

..J 

~ 

1000 

681691 65T 

60T T 

55l55 I 50~ T !f- The numbers express the estimated use f48T in percentages of the measured ones 

~ c: ,:,;, ::i :i .::: ~ Q) 52 Q) :i ....:. ..; c: 0: ~ 0 .:.t. .:.t. c: Cl 0 co E _j -- 6 :::::s ~ co -- co c. co (.) co .c ~ cri ..J -; w ~ a.. co ~ (.) cri cri I 

~ ~ 

FIGURE 41.-Water use (millimeters per year) as measured and as 

computed by various models. The horizontal lines indicate the average use computed from 4 years of data and the vertical lines 

indicate the standard deviation. The measured use is computed 

from the means of tanks 3 and 5, which had, from 1961 to 1965, the 

smallest depth to water (1.5 m). The references are: CWT: 

Thornthwaite, 1948; Hamon, 1961; Makk.: Makkink, 1957; L./J.: 

Lowry-Johnson, 1942; Ch./H.: Christiansen-Hargreaves, 1969; 

Oliv.: Olivier, 1961; Turc, 1961; W. B. lake: Kohler and others, 

1955, lake evaporation; B./C.: Blaney-Criddle, 1962; Lane, 1964; 

J./H.: Jensen-Raise, 1963; Eagl.: Eagleman, 1967; Mort.: Morton, 

1975; W. B. pan: Kohler and others, 1955, pan evaporation; Pap.: 

Papadakis, 1966. 
F42 STUDIES OF EVAPOTRANSPIRATION 

TABLE B.-Comparison of different methods of estimating evapotranspiration for 1963 using the harmonic analysis data 

[h =mean anda 1 =amplitude of first harmonic, both in millimeters. c/> 1 =phase angle of first harmonic (in degrees);%= percent of all fluctuations accounted for by first harmonic and peak 

=approximate day of maximum use, computed from c/> 1; total/year= total water use rounded to tens of millimeters. Measured use was taken as the mean of the use measured in tanks 3 and 5 which had the smallest depth to water (1.5 m)] 

Method % cPJ Peak 

Total 

per 

year 

~easured ____________________________ 190 104 82 277 July 8 2280 

1170 

1230 

1350 

1300 

1560 

1590 

1660 

Thornthwaite __________________________ 97 104 87 277 8 

flarnon ________________________________ 102 92 83 271 14 

~akkink ______________________________ 113 57 89 285 June 30 

Lowry-Johnson ____________________________________ No monthly values ___________________________________ _ 

Christiansen-flargreaves ______________ 130 77 90 287 June 28 

Olivier ________________________________ 133 110 85 288 27 

Turc __________________________________ 141 84 86 288 27 

Lake evaporation Kohler and others __________________ 139 

Blaney-Criddle ________________________ 152 

Lane __________________________________ 172 

Jensen-liaise __________________________ 177 

Eaglernan ____________________________ 195 

~orton ________________________________ 195 

Pan evaporation Kohler and others __________________ 204 

Papadakis ____________________________ 254 

90 

68 

114 

128 

141 

105 

126 

140 

fluctuations accurately enough for all practical purposes. The second and third harmonics (not shown in 

the table) contribute from less than 1 to almost 5 percent to the fluctuations. Also the phase angles, and 

hence the time of maximum use, are quite similar for 

both measured and computed use, but there the similarity ends. The means (h) of the Eagleman and 

Morton models are closest to the measured means, but 

the amplitude (a 1 ) of the Eagleman model is much 

larger than the measured ab while the Morton model 

shows an amplitude about equal to the measured one. 

Russian authors are notably absent in table 8 and 

figure 41. There is good reason for this. Such methods 

as proposed by Budagovskii and Savina (1956); Ognev 

and Kozlov (1962); Konstantinov (1963a); and Bavina 

(1967) are all based on the one proposed by Budyko 

(1948; 1956). Budyko's method is basically a combination method similar to Penman's (See sections on 

((Bulk-Aerodynamic Methods" and ((Energy-Budget 

Methods" in this report.) Since such methods are essentially models of instantaneous phenomena, they cannot 

successfully be used for daily or monthly averages, but 

one will be discussed in the section on ((Short-Term 

Predictions." 

USE OF DAILY AVERAGES 

It was earlier explained (see section on ((Water Use 

Measurements") how hourly water use could be obtained from the on-off recorders and the watermeters 

connected to the evapotranspirometers. These data, 

too, can then be smoothed out by harmonic analysis. 

An example is given in table 9. With one exception, all 

first harmonics account for more than 90 percent of the 

91 

89 

90 

90 

76 

89 

91 

84 

291 

272 

278 

276 

264 

282 

286 

269 

24 

July 13 

7 

9 

21 

3 

June 29 

July 16 

1660 

1820 

2070 

2120 

2340 

2350 

2450 

3040 

fluctuations. Tank 4, which before 1966 had the same 

density of stand as tank 2, was thinned in March of 

that year to 50 percent of its original density, and the 

data show the slight effect of this treatm9nt on water 

use, except in March 1967 when instrument failure 

was, if not the likeliest, the easiest explanation for the 

difference. 

Table 10 shows some of the climatic data for the special observation periods mentioned in table 9. The 

daily mean temperatures (Td4 ) did not vary much during each of the 3-day periods, but there are considerable differences in radiation (Rs and Rn) and in 

windspeed. It is also of interest to note that, whereas 

the air temperature during the August period was by 

far the highest, the radiation was lower than in any of 

the earlier months. This explains in part the drastic 

decline in water use during August as compared to 

May, June, and July. Since most of the empirical 

models have mean temperature as their main input, 

one could expect a better fit between measured and 

computed data in that month than in any of the others. 

Table 11 shows this to be the case. Six of the 15 

models in table 8 have been used here to estimate 

evapotranspiration by days because only those models 

were used whose authors have applied them to estimate daily evapotranspiration values. As in table 8 

and in figure 41, the fit is very poor, the least poor one 

being in August. 

The reason for discussing some of the methods for 

estimating evapotranspiration was to emphasize the 

fact that a saltcedar thicket is a complicated entity. It 

is subject to (and reacting to) continuously and at times 

rapidly changing inputs. The three empirical models 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F43 

TABLE 9.-Water use in tanks 2 and 4 during special observation periods with some results of harmonic analyses of the hourly data 

[Means of three day, mm per hour. h =mean; a,= amplitude of the first harmonic (mm per hour);% =percent of total fluctuation due to first harmonic, <f>, =phase angle of first harmonic, 

rounded to whole degrees. Peak = time of maximum use computed from <PJ 

Total 

per 

Period Tank h a, % "'' Peak day 

(mm) 

1966 

May 5-8 --------------------------------------2 0.70 0.47 91 324 13:25 16.8 

4 N 0 D A T A 

June 3-6 --------------------------------------2 0.80 0.54 93 325 13:21 19.3 

4 0.73 0.43 94 324 13:25 17.5 

July 6-9 --------------------------------------2 0.87 0.65 93 341 12:17 21.0 

4 0.86 0.60 92 335 12:41 20.5 

Aug. 3-6 --------------------------------------2 0.51 0.34 90 334 12:45 12.2 

4 0.46 0.25 92 334 12:45 11.1 

Sept. 8-11 ------------------------------------2 0.50 0.44 88 342 12:13 11.9 

4 0.51 0.38 92 339 12:25 12.2 

1967 

Mar 23-26 ------------------------------------2 2.3 

No hourly values available 

4 0.9 

May 11-14 ------------------------------------2 0.36 0.22 95 327 13:13 8.7 

4 0.35 0.24 94 343 12:09 8.4 

TABLE 10.-Some climatic data pertaining to seven special observation periods of 3 days each 

[R,: total incoming short-wave radiation in langleys per day <ly/d), watts per m2 (Wm- 2 ) and millimeters water equivalent per day (mm/d); Rn: same but for net radiation; Td, and Tw,: dry 

and wet bulb temperature at 4 m above the soil (°C, mean of24 hours); e4: vapor pressure at 4 m, determined with fig. 18 (1 kilopascal = 10 millibar); u, windspeed at 4 m (kilometer per day); 

rh: relative humidity (percent)) 

R, Rn Td, T • ., e, u, rh 

Period ly/d wm- 2 mm/d ly/d wm-2 mm/d oc oc kPa km/d % 

1966 

~ay 5-8 ____________________ 1 593 287 10.2 387 187 6.7 28.6 17.0 1.21 144.6 31 

2 641 310 11.0 410 199 7.0 27.7 17.2 1.28 93.2 34 

3 596 289 10.3 374 181 6.4 28.9 16.7 1.13 123.0 28 

June 3-6--------------------1 716 347 12.3 414 200 7.1 25.3 12.5 0.66 121.4 20 

2 713 345 12.2 415 201 7.1 25.5 13.3 0.76 122.7 23 

3 699 338 12.0 416 201 7.1 26.7 14.2 0.85 115.9 24 

July 6-9 --------------------1 668 323 11.5 356 172 6.1 32.6 16.7 0.91 130.4 18 

2 579 280 10.0 342 166 5.9 31.8 19.1 1.42 169.7 30 

3 655 317 11.3 415 201 7.1 32.4 21.0 1.77 140.9 36 

Aug. 3-6 ____________________ 1 417 202 7.2 260 126 4.5 35.1 23.6 2.19 193.6 39 

2 580 281 10.0 382 185 6.6 35.6 22.8 1.97 176.7 34 

3 525 254 9.1 344 167 5.9 35.1 23.6 2.18 170.2 39 

Sept. 8-11 ------------------1 539 261 9.3 275 133 4.7 30.2 19.4 1.57 115.2 37 

2 482 233 8.3 240 116 4.1 30.3 19.1 1.50 126.3 35 

3 480 232 8.3 224 108 3.9 31.4 18.7 1.37 101.1 30 

1967 

Mar. 22-26 __________________ 1 421 204 7.2 179 87 3.1 20.8 10.2 0.58 113.0 24 

2 460 223 7.8 178 86 3.0 20.9 10.5 0.62 198.1 25 

3 508 246 8.7 251 122 4.3 16.2 8.7 0.65 106.2 35 

May 11-14 __________________ 1 685 332 11.7 385 186 6.6 20.2 10.8 0.71 133.7 30 

2 662 321 11.3 375 182 6.4 20.5 10.7 0.67 162.3 28 

3 685 332 11.7 358 173 6.1 19.9 9.9 0.60 187.5 26 

using monthly mean values that give estimates close to 

SHORT-TERM PREDICTIONS the measured values were also the ones that used four 

instead of two or three variables as inputs (fig. 41), but For short-term predictions the above mentioned emthese three models are not supposed to be valid for pirical models have no fit. On the other hand, models 

short-term periods; hence, they were not tested against capable of predicting water use for hourly or even 

daily water-use data. shorter periods should not be applied to monthly or 
F44 STUDIES OF EVAPOTRANSPIRATION 

TABLE 11.-Water use during special observation periods of 72 hours each as measured in tanks 2 and 4 and estimated according to various 

equations 

[Water level in tanks 2.1 m below the surface. Means of 3 days, millimeters per day] 

Method May 

5-8 

Measured in tank 2 ----------------------------------16.8 Measured in tank 4 -------------------------------------- Olivier ----------------------------------------------9.0 Jensen-Haise ----------------------------------------8.3 Christiansen-Hargreaves ------------------------------5.5 Thornthwaite ----------------------------------------5.7 Hamon ----------------------------------------------5.1 Makkink --------------------------------------------4.8 

weekly periods by inserting into the equations mean 

monthly or weekly inputs such as mean temperature, 

mean windspeed, etc. They can, of course, give good 

results by adding 24 hourly values to give daily quantities of water use, and a compilation of 30 qaily values 

will give a good monthly value. 

The better a prediction equation represents the actual proceedings in and over a saltcedar thicket, the 

better the accuracy of prediction. The principles and 

theories of the eddy-diffusion, the aerodynamic, and 

the combination methods were discussed earlier (see 

the section on ((Evapotranspiration and its Calculation"). Since no instrumentation for it was available, 

the eddy-diffusion method could not be tested at the 

Buckeye Project. Also energy-budget methods that do 

not include advective terms are omitted in the following discussion because, as was shown in the sections on 

((Climate" and ((The Microclimate of a Saltcedar Thicket", advection is a very important and persistent part of 

the proceedings in and over any riparian or flood-plain 

vegetation in a dry and therefore usually windy climate. Finally, the Bowen ratio could not be tested because of its erratic nature, as was discussed in the section on the ((Energy-Budget Method." 

Three methods for predicting short-term water use 

will be discussed: one by Konstantinov (1963b), one by 

Rider (1957), and one by van Bavel (1966) which is 

modified by the inclusion of resistance terms as described by Monteith (1963; 1965). 

Konstantinov's 10 method is based on aerodynamic 

approach including gradients of windspeed and vapor 

pressure and a Richardson number defined as 

Ri = -0 078 Tn.'2- T'2.o · Cu2.o - Un.2) 2 (45) 

This is similar to the stability ratio of equation 27. The 

subscripts indicate the height (in meters) of the in-

"' The reason for including this model in the present discussion is that Konstantinov 

considers his methods to be an improvement to be used with the official GGO manual of 

1957 (Giavnaya Geophysicheskaya Observatoriya = Main Geophysical Laboratory! for 

making observations by the gradient method. 

1966 1967 

June July Aug. Sept. Mar. May 

3-6 6-9 3-6 8-11 22-26 11-14 

19.3 21.0 12.2 11.9 2.3 8.7 

17.5 20.5 11.1 12.2 0.9 8.4 

10.4 11.1 9.2 7.8 6.3 6.3 

8.9 9.7 8.5 7.3 4.4 6.8 

6.1 6.5 5.9 4.7 3.5 5.4 

5.1 6.9 7.0 5.8 2.3 2.8 

4.8 6.9 7.2 4.8 2.4 3.2 

5.3 4.7 4.3 3.5 2.3 4.9 

struments above the soil. These heights are much too 

low for saltcedar studies, but Konstantinov gives instructions on how to reduce higher observations to 

lower ones. To compute windspeeds, for instance, we 

have 

Uz'2 - Uzl 

log (z2/z 1) U2.o (46) - Uo.2 = 

where z2 and z1, are elevations other than 2.0 and 0.2 

m. Of course, such a transform is only admissable if 

there exists a logarithmic wind profile throughout. 

Such a profile may not always exist over a saltcedar 

thicket. 

The computations give results that, hour by hour, 

deviate considerably from the measured values. For 

4-hour periods the outcome is a little better, but still 

leaves a lot to be desired. Figure 42 is a small example. 

The actual temperatures, windspeeds, and vapor pressures were taken from the 7 and 5 m levels and reduced, according to Konstantinov's instructions, to 2 

and 0.2 m. This procedure may not be valid where tall 

vegetation is involved and may well be the reason for 

the poor fit. The daily totals for the day, illustrated in 

figure 42, are 24 mm for the Konstantinov model and 

20 mm as measured. The correlation coefficient, 0. 72, is 

only significant at the 5 percent level. 

In 1957, Rider published a study of water losses from 

various land surfaces in which he used the 

aerodynamic method for predicting evapotranspiration. His model was based on the ThornthwaiteHolzman (1942) equation as modified by Pasquill 

(1949): 

E _ 3.46 x 10- 5 Ce1 - e2) Cu2 - u1) 

n - T [In z2 - d ]2 

Zt- d 

(47) 

A comparison with equation 16 shows that the 

modification consists in the introduction of a zero-plane 

displacement, d, (discussed in the section on ((Winds"), 

air temperature (T, in degrees Kelvin) instead of air 

density, and vapor pressure (e) instead of specific humidity (q). 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SAL TCEDAR THICKET, ARIZONA F45 

CJ) 6 a: 

::> .4 0 

I 

qa: 5 w 

Q.. 

CJ) 

a: 

w 

1-

w 4 

~ 

:::i 

...J 

~ 3 ~ 

w· 

CJ) 

::> 

a: 2 w 

1-

<( 

$ 

Q 

w 

1-

() 

Q 

w 

a: 

Q.. 0 

0 2 3 4 5 

MEASURED WATER USE, IN MILLIMETERS PER 4 HOURS 

FIGURE 42.-Water use as predicted by the Konstantinov model (ordinate) versus the one measured in tank 2 on June 3-4, 1966. 

Correlation coefficient = 0. 72 *. Regression y = 0. 76 + 0.88x. The 

numbers at the dots give the 4-hour periods 1 = 5-8, 2 = 9-12, etc. 

Rider determined hourly values for 18 days for which 

he had measured hourly water-use values available. 

The fits are only fair, the main reason being, by Rider's 

own admission, the quality of his records. Also equation 4 7 as well as equation 16 are strictly valid only 

under natural conditions of atmospheric stability. 

The often successful method of smoothing out 

erraticism is to use sums of a certain number of values 

and (or) to use means of a certain number of days. This 

was done here for 4-hour sums and 3-day means. Two 

examples are presented at the top of figure 43. The 

correlation coefficients are at the 1 percent level of 

confidence and the regression lines show a close to 1 : 1 

relationship. 

The two curves at the bottom of figure 43 show the 

result of computations for only one day. On the right 

the air temperatures and humidity data are taken from 

instruments 7 and 4 m above the ground ( 4 and 1 m 

above the vegetation). On the left, the data at 11 and 5 

m were used (8 and 2 m above the vegetation). The 

method is rather sensitive to the height at which the 

observations were taken, but both curves show a fairly 

good fit. Nonetheless, the instrumentation used at the 

Buckeye Project was not accurate or sensitive enough 

for a successful application of the aerodynamic model. 

The combination method promised to give a better 

prediction, therefore, it was subjected to more elaborate testing. The model itself has been presented in the 

section on ~(Combination Methods" in this report. 

CJ) 

a: 

::> 

0 

I 

qa: 

w 

Q.. 

CJ) 

a: 

w 

1-

w 

~ 

:::i 

...J 

~ 

~ 

w· 7 CJ) 

::> 

a: 6 w 

1-

<( 5 

$ 

Q 4 w 

1-

() 3 

Q 

w 

a: 2 Q.. 

2 3 4 5 6 2 3 4 5 6 

MEASURED WATER USE,IN MILLIMETERS PER 4 HOURS 

FIGURE 43.-Water use predicted by the Rider model versus that 

measured in tank 2. Gradients: the numbers indicate the elevation above ground from which the input data were taken. 

Equations 22 to 26 are especially pertinent. A large 

part of the computations were done by longhand, using 

graphs and nomograms. Later a small programmable 

desk calculator was available. In both cases computations were simplified by assuming a constant air pressure of 98.3 kPa. In that case, the fl/y term in equation 

22 is dependent on air temperature alone and it was 

found that 

fl/y = exp 0.045 Ta - 0.32 (48) 

was a very good approximation. Similarly, the first factor in equation 23 could be made dependent on air temperature alone and 

~= 36 4 x 10--6 -7- cr + 273) 983 . a (49) 

proved to be of excellent accuracy. 

As van Bavel (1966) points out, the expression for B1• 

in equation (23) is based upon standard wind profile 

theory and applies strictly to adiabatic conditions only. 

Under such conditions logarithmic wind profiles obtain. Analyses of the wind data from the Buckeye Project revealed that within the limits of accuracy of the 

anemometers the profiles were nearly always 
F46 STUDIES OF EVAPOTRANSPIRATION 

logarithmic but a variable modified roughness length 

had to be taken into account as was discussed in the 

section on ((Winds." Inserting this modified roughness 

length, and using temperature and wind data from the 

4-m level, gives computed values that are reasonably 

close to those measured. 

The results of two computations, using the combination equation and covering 72-hour periods in May and 

June 1966, are displayed in figure 44 together with the 

water-use data. As can be seen, the computed potential 

evapotranspiration rate not only fluctuates much more 

than the one measured for tank 2 but, especially during the daylight hours, often seems excessively high. 

That the measured data do not fluctuate as much as the 

theoretical ones is partly due to the slow response of 

the vegetation to comparatively sudden changes in the 

ambient parameters and also due to the assumption 

under which the hourly water use in the tanks was 

computed; namely, a constant rate of water use between one filling and the next. 

A perusal of these and other data shows clearly that 

large deviations occur when windspeeds are more than 

about 2 m/s and during periods of high temperature. 

This suggests the existence of a significant 

aerodynamic and stomatal or leaf resistance (see the 

sections on ((Flow Resistance" and ((Combination 

Methods" in this report, especially equations 24, 25, 

and 26). 

The aerodynamic resistance (ra) can readily be computed if windspeeds and roughness lengths are known. 

The stomatal resistance, however, depends not only on 

ra but also on /1/y and on the ratio of potential over 

actual evapotranspiration. Stomatal resistance can be 

measured directly on broad-leafed plants with a 

porometer (Stiles, 1970; Byrne and others, 1970) but 

2.0 

a: 

:J 1.5 

0 

I 1.0 a: 

UJ 0.5 a.. 

(/) 

a: 

UJ 

f- 2.0 UJ 

::?! 1.5 :::i 

_I 

::?! 1.0 

0.5 

6 1 2 1 8 24 6 1 2 18 24 6 1 2 18 24 

TRUE SOLAR TIME 

FIGURE 44.-Hourly rates of evapotranspiration for two 3-day 

periods near Buckeye, Ariz. Measured water use in tank No.2, 

compared to that computed with equations 22 and 23. 

these instruments cannot be used on the scalelike 

leaves of a saltcedar twig.ll However, Eo can be computed with equation 22 and Ea can be measured. Thus 

values for the stomatal resistance (r8 ) can be obtained 

by the use of equation 25. This was done for one set of 

data, and it was found that rs was highly correlated 

with windspeeds and also, but much less so, with temperatures and humidity. 

Equation 25 shows that negative values appear 

when E0 < Ea. This can happen if (1) the graphical 

analysis of hourly water use results in a highEa, (2) an 

erratic set of (say) temperature records gives a low Eo, 

or (3) if Ea is actually larger than the potential value. 

The data analysis showed that vapor pressure deficit 

affected rs very little, the effect probably being overshadowed by that of temperature and wind. It was 

found that 

-1.26 + 0.013 T 7 + 0.5 u7 (50) 

It may seem strange that there is a negative constant 

in the equation but this is the result of the fact that T 

and u values were taken at 7 m above ground and not 

at the surface of the leaves where temperatures are 

higher (Gates, 1963) and windspeeds lower (Monteith, 

1965). Thus, equation 50 is purely empirical, being derived from the 3-day data of August and then applied to 

other 3-day periods. Within the range of temperatures 

and windspeeds used in the regression .analysis, rs 

ranges from -0.7 s/cm for T7 =soc and u7 = 0.9 m/s to 

2.9 s/cm for T7 = 42°C and u7 = 7.3 m/s. 

Hughes (1972), who used average monthly data, reports slightly higher rs values and finds that rs = 0.128 

u.5 t. 92/Tl· 73 , but restricts his analyses to temperatures 

between 12° and 24°C and windspeeds up to 3 m/s only. 

His relationship thus shows that rs increases with increasing windspeed, as in the present study, but decreases with increasing temperatures (at least in the 

quoted temperature range) in contrast with the present 

findings. 

Whether the use of porometer data or of data obtained by other means of actually measuring the resistance term would give better results is a matter of 

speculation, but the data computed by equations 26 

and 50 and illustrated in figure 45 seem quite acceptable with the glaring exception of those from the period 

May 11-14, 1967, shown at the bottom of the figure. 

Here salinity had a decisive effect. 

In 197 4 van Hylckama discussed the effect of salinity 

on rate of water use and explained how some of the 

"D. C. Davenport (oral and written communs., 1977) reported successful use of the 

porometer on saltcedar twigs by careful standardization of twig length and volume, but 

admits that the method is rather subjective and chances of discrepancies increase if different people use the method. 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET. ARIZONA F47 

a: 

:J 

0 

I 

a: 

w 

a.. 

(/) 

a: 

w 

Iw 

~ 

::::i 

..J 

~ 

2.0~~~~~~~~~~~~~~~~~~~~~ 

1.5 

1.0 

2.0 

1.5 

1.0 

0.5 

2.0 

1.5 

1.0 

0.5 

2.0 

1.5 

1.0 

2.0 

1.5 

1.0 

0.5 

12 24 12 24 12 24 

FIGURE 45.-Computed evapotranspiration, using equation 26, 

versus that measured in tank 2 at the Buckeye project in millimeters per hour for five periods of 3 days each. The oooo (adjusted) line on the bottom graph compensates for salinity buildup 

(see text). 

tanks were flushed in 1965 and 1966 to reduce the salinity of the soil moisture and the ((ground water." In 

1967 the tanks were given no such treatment. The records showed that the specific conductance in tank 2 

increased between September 1966 and May 1967 from 

18 to 32 millimhos per em at 25°C. Such an increase 

would result in a decrease of water use by about 50 

percent. Therefore the measured water use, indicated 

by the solid line in figure 45, was doubled as depicted 

by open circles. Although the corrected values do not fit 

ideally, they obviously compensate for drastic effects of 

soil-moisture salinity on water use. 

It was mentioned (see section on ((Short-Term Predictions") that models capable of predicting water use 

for hourly periods should not be applied to longer 

periods by inserting long-time means into the equations. An example of the result of such faulty procedure 

is given in table 12. Daily totals of evapotranspiration 

computed as sums of hourly values fit the measured 

ones quite well, whereas those computed using daily 

means of radiation, winds, etc., are much too large. 

DISCUSSION 

A few important conclusions can be drawn from the 

foregoing. 

Flood-plain vegetation in the arid and semi-arid 

southwest is characterized by a very complex set of 

interactions between inputs of ambient weather, soil, 

soil-moisture characteristics, and plant reactions to 

such inputs. 

The instrumentation at the Buckeye Project had 

been used with adequate, if not excellent, results when 

applied to measuring evaporation from open water, but 

it was less adequate to cope successfully with the complexity of a riparian or flood-plain vegetation and its 

evapotranspiration. 

For a rough estimate of yearly water use, the Weather Bureau's pan evaporation method (Kohler and 

others, 1955), and the Eagleman (1967) and the Morton 

(1975) models seem adequate. That Kohler's panevaporation, rather than lake-evaporation method, 

produced values closer to the measured quantities emphasizes the oasitic characteristics of riparian vegetation where the advective term is very important, especially when, as is often the case, wind speeds are high 

and humidity is low. 

For detailed estimates or predictions, the combination model (equations 22-26) gives the best results although such estimates often will be too high if aerodynamic and stomatal resistances are neglected. Riparian and flood-plain vegetation simply are not sets of 

wicks that release water on demand. 

The disadvantages of the use of closed-bottom, 

plastic-lined tanks for measuring evapotranspiration 

rates have been discussed elsewhere (van Hylckama, 

1974, p. E29). Better lysimeters would not only be 

flood-prone, if they were properly located, that is, 

within the riparian vegetation, but they would be very 

TABLE 12.-Water use (millimeters/day) by saltcedar for 3 days in 1966 

Date Measured Computed as the Deviation Computed with Deviation sum of 24 hourly from eq. 26 from mean from values (eq. 26) measured daily values measured 

April 9 ----------------------------------10.4 11.5 + 1.1 14.4 +4.0 April 28 __________________________________ 15.4 15.6 +0.2 18.2 +2.8 ~ay 3 ____________________________________ 13.5 12.6 -0.9 15.8 +2.3 
F48 STUDIES OF EVAPOTRANSPIRATION 

expensive. Moreover, like the plastic tanks, they would 

provide data that, strictly speaking, would be applicable only to areas quite similar in ecology. Such similarity is difficult to ascertain unequivocally. 

The present studies have shown that even under the 

very complicated circumstances of riparian and floodplain vegetation, comparatively simple instrumentation can provide fairly adequate data for the purpose of 

estimating evapotranspiration using the combinationtype model. Improved sensors and recorders, especially 

those of humidity and radiation, would undoubtedly 

provide more reliable results. 

However, although not tested at the Buckeye Project, the eddy-correlation model and its instrumentation seems to be the most promising of all models and 

methods in use so far because of the fact that the 

method is independent of the height or exposure of the 

vegetation and independent of lapse rates or temperature and other profiles. By contrast, most aerodynamic 

or energy-budget models are valid only under adiabatic 

conditions. 

In future evapotranspiration research, the eddycorrelation methods and its further instrumentation 

development should have the predominant attention of 

hydrologists, botanists, and other researchers in the 

Water Resources Division and, actually, in all other 

Federal, State, and private organizations studying 

plant-soil-water relationships. 

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F50 STUDIES OF EVAPOTRANSPIRATION 

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van Bavel, C. H. M., 1966, Potential evaporation-the combination 

concept and its experimental verification: Water Resources Research v. 2, no. 3, p. 455-467. 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F51 

---1967, Surface energy balance of bare soil as influenced by 

wetting and drying: U.S. Dept. of Agriculture, Water Conserv. 

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27-58. 

APPENDICES 
F54 STUDIES OF EVAPOTRANSPIRATION 

TABLE Al-L-Hourly averages of windspeeds and wind direction at the Buckeye Project during selected periods of 3 days each 

[Windspeeds in meters per second (m s- 1 ) at indicated levels (meters) above ground. Clocks set to true local solar time: 1200 (noon) when sun exactly at 180°] 

1-hr. period 

beginning 11m 

5 ________________________________________________ 2.97 

6 ________________________________________________ 3.33 

7 ________________________________________________ 3.33 

8 ________________________________________________ 3.64 

9 ________________________________________________ 5.45 

10 ________________________________________________ 5.81 

11 ________________________________________________ 3.64 

12 ________________________________________________ 3.65 

13 ________________________________________________ 2.53 

14 ________________________________________________ 2.90 

15 ________________________________________________ 2.53 

16 ________________________________________________ 2.65 

17 ________________________________________________ 2.94 

18 ________________________________________________ 2.72 

19 ________________________________________________ 2.23 

20 ________________________________________________ 1.98 

21 ________________________________________________ 3.09 

22 ________________________________________________ 5.45 

23 ________________________________________________ 2.11 

24 ________________________________________________ 2.49 

1 ________________________________________________ 3.06 

2 ________________________________________________ 2.35 

3 ________________________________________________ 1.18 

4 ________________________________________________ 1.93 

5 ________________________________________________ 1.24 

6 ________________________________________________ 2.48 

7 ________________________________________________ 1.69 

8 ________________________________________________ 2.12 

9 ________________________________________________ 2.00 

10 ________________________________________________ 2.12 

11 ________________________________________________ 1.75 

12 ________________________________________________ 1.92 

13 ________________________________________________ 2.12 

14 ________________________________________________ 2.04 

15 ________________________________________________ 1.99 

16 ________________________________________________ 2.47 

17 ________________________________________________ 1.74 

18 ________________________________________________ 2.30 

19 ________________________________________________ 2.41 

20 ________________________________________________ 1.27 

21 ________________________________________________ 0.71 

22 ________________________________________________ 0.85 

23 ________________________________________________ 1.95 

24 ________________________________________________ 1.33 

1 ________________________________________________ 3.52 

2------------------------------------------------1.93 3 ________________________________________________ 1.69 

4 ________________________________________________ 1.69 

5------------------------------------------------2.65 6 ________________________________________________ 1.67 

7 ________________________________________________ 1.28 

8 ________________________________________________ 1.36 

9 ________________________________________________ 1.80 

10 ________________________________________________ 1.10 

11 ________________________________________________ 1.38 

12 ________________________________________________ 1.10 

13_--------------------------------------- ________ 1. 7"6 

13 ________________________________________________ 1.93 

15 ________________________________________________ 2.88 

16 ________________________________________________ 3.52 

17 ________________________________________________ 4.37 

18 ________________________________________________ 3.76 

19 ________________________________________________ 3.41 

20 ________________________________________________ 3.38 

7m 

5-6 May 1966 

2.15 

1.92 

2.62 

3.16 

4.90 

4.75 

3.09 

3.11 

2.28 

2.45 

2.28 

2.24 

2.41 

2.36 

1.73 

1.64 

2.53 

4.34 

1.77 

1.66 

2.17 

1.48 

1.05 

1.74 

6-7 May 1966 

0.98 

1.84 

1.23 

1.87 

1.73 

1.86 

1.54 

1.63 

1.81 

1.78 

1.72 

2.17 

1.49 

1.79 

1.79 

0.92 

0.61 

0.88 

1.54 

1.06 

0.60 

1.52 

1.35 

1.28 

7-8 May 1966 

1.94 

1.40 

0.95 

1.07 

1.51 

0.90 

1.21 

0.92 

1.51 

1.69 

2.40 

2.88 

3.58 

3.15 

2.72 

2.51 

5m 

1.54 

1.69 

2.04 

2.59 

3.80 

3.96 

2.66 

2.65 

2.00 

2.12 

1.95 

2.04 

2.11 

1.86 

1.33 

1.23 

2.07 

3.59 

1.51 

1.22 

1.60 

1.11 

1.10 

1.07 

1.38 

1.07 

1.56 

1.51 

1.69 

1.38 

1.42 

1.63 

1.60 

1.48 

1.88 

1.27 

1.48 

1.24 

0.72 

0.92 

0.90 

1.28 

0.88 

1.87 

1.20 

1.08 

0.98 

1.54 

1.14 

0.98 

1.07 

1.36 

0.96 

1.08 

1.02 

1.36 

1.41 

2.00 

2.41 

2.73 

2.59 

2.16 

2.00 

Wind 4m 2m direction 

1.01 0.13 

1.27 0.57 

1.79 0.72 

2.16 1.10 

3.06 1.31 

3.33 1.46 

2.29 1.16 

2.30 1.00 

1.79 0.86 

1.88 0.88 

1.69 0.88 

1.73 0.76 

1.73 0.61 

1.46 0.68 

0.90 0.40 

0.84 0.24 

1.75 0.46 

3.15 0.81 

1.25 0.28 

0.90 0.09 

1.19 0.12 

0.85 0.09 

0.88 0.07 

0.98 0.10 

0.86 0.07 

1.08 0.28 

0.86 0.32 

1.35 0.73 

1.32 0.66 

1.41 0.90 

1.26 0.85 

1.30 0.88 

1.41 0.86 

1.41 0.93 

1.24 0.82 

1.60 0.76 

1.14 0.62 

1.20 0.32 

0.89 0.09 

0.55 0.09 

0.76 0.09 

0.68 0.09 

1.00 0.13 

0.72 0.13 

1.46 0.13 

0.93 0.09 

0.83 0.07 

0.63 0.07 

1.19 0.08 

0.93 0.06 

0.75 0.23 

0.85 0.47 

1.10 0.72 

0.84 0.57 

0.96 0.64 

0.84 0.46 

1.24 0.72 

1.24 0.86 

1.73 0.77 

1.92 0.67 

2.18 0.75 

2.16 0.75 

1.73 0.51 

1.60 0.32 

E 

E 

NE 

NE 

NE 

NE 

E 

E 

NE 

SE 

SE 

SE 

NE 

NE 

NE 

NE 

sw 

sw 

w 

w 

w 

w 

w 

N 

N 

SE 

SE 

NE 

NE 

NE 

NE 

NE 

NE 

NE 

NE 

N 

N 

N 

N 

N 

N 

N 

w 

w 

w 

NE 

NE 

NE 

NE 

SE 

SE 

SE 

NE 

NE 

NE 

N 

N 

NW 

w 

w 

w 

w 

w 

w 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F55 

TABLE A1-1.-Hourly averages of windspeeds and wind direction at the Buckeye Project during selected periods of 3 days each-Continued 

1-hr. period 

beginning 11m 7m 

7-8 May 1966-Continued 

21 ________________________________________________ 3.26 2.27 

22 ________________________________________________ 4,48 3.54 

23 ________________________________________________ 5,20 4.14 

24 ________________________________________________ 2,86 2.30 

1------------------------------------------------3.64 2.85 

2------------------------------------------------2.65 1.99 3 ________________________________________________ 1,55 1.21 

4------------------------------------------------1.33 1.08 

3-4 June 1966 

5------------------------------------------------2.79 2.05 

6------------------------------------------------1.50 0.90 

7------------------------------------------------1.95 1.35 8 ________________________________________________ 2,41 2.01 

9------------------------------------------------2.59 2.23 10 ________________________________________________ 1,89 1.66 

11 ________________________________________________ 1.92 1.66 

12 ________________________________________________ 1,98 1.73 

13 ________________________________________________ 2,59 2.29 

14 ________________________________________________ 3,77 3.27 

15 ________________________________________________ 3,84 3.33 

16 ________________________________________________ 3,90 3.33 

17 ________________________________________________ 4,34 3.45 

18 ________________________________________________ 4,14 3.39 

19 ________________________________________________ 3.15 2.53 

20 ________________________________________________ 3.21 2.23 

21------------------------------------------------3.45 2.41 22 ________________________________________________ 1.31 1.28 

23------------------------------------------------1.35 1.10 24 ________________________________________________ 1,16 0.98 

1------------------------------------------------1.60 1.38 

2------------------------------------------------2.11 1.55 

3------------------------------------------------0.98 1.23 4 ________________________________________________ 1,60 1.54 

4-5 June 1966 

5------------------------------------------------2.11 1.35 

6------------------------------------------------2.17 1.63 

7------------------------------------------------2.32 1.86 

8------------------------------------------------2.85 2.41 

9------------------------------------------------2.78 2.53 10 ________________________________________________ 2.04 1.86 

11 ________________________________________________ 2,41 2.11 

12 ________________________________________________ 2,35 2.01 

13 ________________________________________________ 3,33 2.91 

14 ________________________________________________ 3,96 3.39 

15 ________________________________________________ 3,36 2.88 

16------------------------------------------------3.54 3.06 17 ________________________________________________ 4,08 3.45 

18 ________________________________________________ 3,77 3.24 

19 ________________________________________________ 3,39 2.62 

20 ________________________________________________ 3.12 2.17 

21 ________________________________________________ 3,09 2.11 

22 ________________________________________________ 1,98 1.16 

23 ________________________________________________ 1.69 1.57 

24 ________________________________________________ 1.66 1.25 

1------------------------------------------------0.95 0.76 

2------------------------------------------------1.16 1.05 

3------------------------------------------------1.89 1.63 

4------------------------------------------------1.73 1.20 

5------------------------------------------------2.56 

6------------------------------------------------1.28 

7------------------------------------------------2.04 

8------------------------------------------------2.35 

9------------------------------------------------2.11 

10------------------------------------------------2.23 11 ________________________________________________ 2.23 

5-6 June 1966 

1.95 

0.98 

1.69 

2.16 

1.98 

1.94 

1.98 

5m 

1.65 

2.91 

3.45 

1.89 

2.27 

1.52 

0.98 

0.88 

1.54 

0.91 

1.16 

1.66 

1.86 

1.48 

1.51 

1.63 

1.98 

2.85 

2.85 

2.85 

3.03 

2.78 

2.07 

1.73 

1.82 

1.38 

1.10 

1.04 

1.23 

1.16 

1.09 

1.35 

1.07 

1.41 

1.54 

1.98 

1.98 

1.60 

1.79 

1.69 

2.47 

2.91 

2.44 

2.53 

2.85 

2.72 

2.17 

1.66 

1.57 

0.99 

1.22 

1.14 

0.85 

1.10 

1.38 

1.04 

1.48 

0.98 

1.48 

1.73 

1.60 

1.69 

1.70 

Wind 4m 2m direction 

1.33 0.32 w 

2.48 0.55 sw 

2.97 0.86 sw 

1.53 0.67 sw 

1.90 0.63 sw 

1.17 0.12 w 

0.77 0.12 NW 

0.77 0.12 N 

1.12 0.12 SE 

0.70 0.06 SE 

0.91 0.38 s 

1.35 0.67 NE 

1.51 0.79 NE 

1.25 0.73 NE 

1.25 0.79 SE 

1.41 0.79 v 

1.79 0.85 sw 

2.47 0.91 sw 

2.41 0.98 sw 

2.47 0.73 sw 

2.59 0.64 sw 

2.26 0.67 w 

1.60 0.55 w 

1.25 0.10 w 

1.41 0.11 w 

1.01 0.04 w 

0.79 0.04 w 

0.76 0.04 w 

0.83 0.05 w 

0.88 0.07 s 

0.76 0.06 s 

0.94 0.10 s 

0.71 0.14 s 

1.09 0.14 s 

1.22 0.64 NE 

1.60 0.79 NE 

1.69 0.91 NE 

1.38 0.85 NE 

1.63 0.76 v 

1.48 0.83 NW 

2.11 0.83 sw 

2.50 0.90 sw 

2.17 0.87 sw 

2.17 0.79 w 

2.35 0.77 w 

2.32 0.61 w 

1.76 0.38 w 

1.21 0.10 w 

1.13 0.10 w 

0.66 0.06 w 

0.88 0.06 v 

0.90 0.06 NE 

0.62 0.03 w 

0.76 0.04 w 

0.98 0.28 NW 

0.75 0.13 N 

1.10 0.21 E 

0.72 0.09 NE 

1.16 0.63 NE 

1.37 0.79 NE 

1.37 0.76 NE 

1.48 0.85 NE 

1.51 0.85 NE 
F56 STUDIES OF EVAPOTRANSPIRATION 

TABLE A1-1.-Hourly averages of windspeeds and wind direction at the Buckeye Project during selected periods of 3 days each-Continued 

1-hr. period 

beginning 11m 7m 

5-6 June 1966-Continued 

12------------------------------------------------2.91 2.53 13 ________________________________________________ 3.45 2.91 

14 ________________________________________________ 3.84 3.45 

15 ________________________________________________ 4.08 3.58 

16 ________________________________________________ 3.96 3.54 

17------------------------------------------------3.58 3.09 18 ________________________________________________ 2.85 2.23 

19------------------------------------------------2.59 1.86 20 ________________________________________________ 2.59 1.66 

21 ________________________________________________ 2.53 1.66 

22----------------------------------------- _______ 1.69 1.13 23 ________________________________________________ 0.73 0.83 

24 --------------------------------- _______________ 1.19 1.01 

1 ---------------------------------------------- __ 1.54 1.28 

2------------------------------------------------2.50 1.86 3 ________________________________________________ 1.51 0.91 

4------------------------------------------------1.95 1.38 

6-7 July 1966 

5------------------------------------------------1.24 0.86 6 ________________________________________________ 1.43 0.73 

7 ________________________________________________ 1.35 1.08 

8 ------------------------------------------------1.17 0.98 

9------------------------------------------------1.23 1.00 

10-----------------------------------_____________ 1.23 1.02 

11 ----------------------------------------------- _1.80 1.57 

12 ----------------------------------------------- _2.19 1.91 

13 ------------------------------------------------3.13 2.68 14 ________________________________________________ 4.46 3.84 

15 ________________________________________________ 4.95 4.17 

16 ________________________________________________ 4.58 3.87 

17 --------------- ________________________________ A.14 3.49 18 ________________________________________________ 4.83 3.73 

19 ________________________________________________ 4.58 3.71 

20 ________________________________________________ 3.60 2.65 

21 ________________________________________________ 4.08 2.99 

22 ________________________________________________ 3.84 2.98 

23 ________________________________________________ 1.22 1.06 

24 ________________________________________________ 2.50 1.80 

1------------------------------------------------1.93 1.33 

2 ------------------------------------------------1.19 0.99 3 ________________________________________________ 1.40 1.31 

4------------------------------------------------1.65 1.27 

7-8 July ,1966 

5 ----------------------------------------------- _1.24 1.04 

6------------------------------------------------1.56 1.07 7 ________________________________________________ 1.53 1.24 

8 ________________________________________________ 5.50 4.46 

9------------------------------------------------5.28 4.48 10 ________________________________________________ 5.49 4.71 

11 ________________________________________________ 4.74 4.18 

12 ________________________________________________ 3.25 2.89 

13 ________________________________________________ 3.64 3.21 

14 ________________________________________________ 6.80 5.37 

15 ________________________________________________ 7.25 6.03 

16------------------------------------------------5.26 4.55 

17------------------------------------------------4.24 3.45 

18 ------------------------------------- ___________ 3.12 2.53 19 ________________________________________________ 2.93 2.39 

20 ________________________________________________ 2.97 2.24 

21------------------------------------------------2.60 1.87 

22----------------------------------------------- _1.54 1.19 23 ________________________________________________ 2.11 1.41 

24 ________________________________________________ 2.39 1.64 

1------------------------------------------------3.15 2.20 

2------------------------------------------------3.60 2.75 

3------------------------------------------------2.02 1.50 4 ________________________________________________ 2.59 2.17 

5m 

2.29 

2.47 

2.91 

3.09 

3.03 

2.59 

1.73 

1.48 

1.16 

1.15 

0.98 

1.04 

1.00 

1.24 

1.48 

0.94 

1.03 

0.98 

1.07 

1.08 

0.98 

1.06 

1.07 

1.48 

1.74 

2.38 

3.09 

3.45 

3.23 

2.97 

3.15 

3.15 

2.23 

2.44 

2.49 

0.83 

1.32 

1.03 

0.99 

1.13 

1.20 

0.95 

0.98 

1.14 

3.84 

3.75 

4.08 

3.67 

2.47 

2.73 

4.63 

5.12 

3.86 

2.78 

1.94 

2.02 

1.80 

1.49 

0.98 

1.20 

1.22 

1.63 

2.15 

1.13 

1.75 

Wind 

4m 2m direction 

1.98 0.91 v 

2.11 0.90 sw 

2.59 0.87 sw 

2.68 0.97 sw 

2.53 0.91 sw 

2.17 0.73 w 

1.41 0.34 w 

1.07 0.11 w 

0.76 0.11 w 

0.78 0.09 w 

0.71 0.09 w 

0.73 0.05 w 

0.71 0.05 w 

0.98 0.07 v 

1.01 0.13 E 

0.61 0.07 E 

0.66 0.09 E 

0.71 0.06 s 

0.83 0.06 s 

0.89 0.24 s 

0.76 0.24 v 

0.89 0.32 NE 

0.94 0.38 v 

1.24 0.62 w 

1.50 0.73 v 

2.04 0.78 sw 

2.52 0.93 w 

2.86 0.93 w 

2.68 0.77 sw 

2.47 0.70 sw 

2.62 0.45 w 

2.66 0.52 sw 

1.81 0.32 sw 

2.03 0.24 sw 

1.91 0.12 w 

0.73 0.06 v 

1.00 0.13 NW 

0.81 0.10 NW 

0.85 0.07 NE 

0.76 0.08 NE 

0.72 0.09 NE 

0.78 0.07 NE 

0.70 0.09 NE 

0.88 0.09 v 

3.28 0.72 sw 

3.17 0.70 sw 

3.45 0.75 sw 

3.00 0.98 w 

2.11 0.79 sw 

2.37 0.72 sw 

3.96 1.00 sw 

4.50 1.05 sw 

3.22 0.84 w 

2.24 0.58 NW 

1.49 0.61 NW 

1.60 w 

1.45 w 

1.19 sw 

0.78 sw 

0.85 v 

0.96 NW 

1.22 w 

1.67 w 

0.85 w 

1.43 sw 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SAL TCEDAR THICKET, ARIZONA F57 

TABLE A1-1.-Hourly averages of windspeeds and wind direction at the Buckeye Project during selected periods of 3 days each-Continued 

1-hr. period 

beginning 11m 

5 ________________________________________________ 1.86 

6 ________________________________________________ 1.87 

7 ________________________________________________ 2.06 

8 ________________________________________________ 2.76 

9 ________________________________________________ 2.24 

10 ________________________________________________ 2.36 

11 ________________________________________________ 2,72 12 ________________________________________________ 2.55 

13 ________________________________________________ 3.25 

14 ________________________________________________ 2.41 

15 ________________________________________________ 3.33 

16 ________________________________________________ 3.54 

17 ________________________________________________ 4.08 

18 ________________________________________________ 4.10 

19 ________________________________________________ 3.18 

20 ________________________________________________ 2.99 

21 ________________________________________________ 2.87 

22 ________________________________________________ 3.39 

23 ________________________________________________ 3.64 

24 ________________________________________________ 3.45 

1 ________________________________________________ 2.98 

2 ________________________________________________ 3.22 

3 ________________________________________________ 3.21 

4 ________________________________________________ 0.98 

5 ________________________________________________ 3.09 

6 ________________________________________________ 2.79 

7 ------------------------------------------- _____ 1.10 8 ________________________________________________ 1.75 

9 ________________________________________________ 2.63 

10 ________________________________________________ 2.50 

11 ________________________________________________ 2.51 

12 ________________________________________________ 2.91 

13 ________________________________________________ 3,34 

14 ________________________________________________ 3.97 

15 ________________________________________________ 9.05 

16 ________________________________________________ 9.17 

17 ________________________________________________ 8.55 

18 ________________________________________________ 7.06 

19 ________________________________________________ 4.95 

20 ________________________________________________ 4,21 

21 ________________________________________________ 3.55 

22 ________________________________________________ 3,26 

23 ________________________________________________ 3.98 

24 ________________________________________________ 4.08 

1------------------------------------------------2.08 2 ________________________________________________ 2.62 

3 ________________________________________________ 2,13 

4 ________________________________________________ 2.42 

5 ________________________________________________ 2.21 

6 ________________________________________________ 0.99 

7 ________________________________________________ 1.55 

8 ________________________________________________ 4,12 

9 ________________________________________________ 2.45 

10 ________________________________________________ 2.09 

11 ------------------------------------------------1.62 12 ________________________________________________ 1.45 

13 ------------------------------------------------1.96 14 ________________________________________________ 3.33 

15 ________________________________________________ 3.58 

16 ________________________________________________ 3.09 

17 ________________________________________________ 3.58 

18 ________________________________________________ 3.62 

19 ________________________________________________ 3.21 

20 ________________________________________________ 2.99 

21 ________________________________________________ 3.06 

7m 

8-9 July 1966 

1.32 

1.23 

1.73 

2.42 

1.95 

2.14 

2.32 

2.28 

2.78 

2.17 

2.91 

3.06 

3.45 

3.51 

2.60 

2.07 

2.07 

2.63 

2.97 

2.66 

2.14 

2.52 

2.49 

0.73 

3-4 August 1966 

2.38 

2.07 

0.92 

1.61 

2.23 

2.00 

2.17 

2.53 

2.94 

3.33 

7.18 

7.31 

6.94 

5.57 

3.90 

3.31 

2.79 

2.57 

3.12 

3.14 

1.62 

2.06 

1.62 

1.64 

4-5 August 1966 

1.65 

0.68 

1.28 

3.55 

1.99 

1.73 

1.38 

1.32 

1.70 

2.75 

2.99 

2.69 

3.03 

2.96 

2.50 

2.13 

2.35 

5m 

1.10 

1.10 

1.51 

1.98 

1.76 

1.82 

2.05 

1.95 

2.25 

1.92 

2.43 

2.61 

2.94 

2.98 

2.18 

1.60 

1.54 

2.16 

2.42 

2.25 

1.75 

2.04 

2.12 

0.84 

1.90 

1.74 

0.85 

1.33 

1.94 

1.66 

1.90 

2.18 

2.52 

2.85 

5.94 

6.06 

5.69 

4.58 

3.21 

2.72 

2.25 

2.11 

2.40 

2.51 

1.36 

1.61 

1.20 

1.43 

1.19 

0.56 

1.07 

2.95 

1.69 

1.52 

1.29 

1.22 

1.51 

2.29 

2.50 

2.27 

2.59 

2.49 

1.89 

1.58 

1.87 

4m 

0.82 

0.78 

1.20 

1.61 

1.52 

1.55 

1.77 

1.67 

2.00 

1.69 

2.05 

2.11 

2.49 

2.49 

1.76 

1.27 

1.21 

1.69 

1.98 

1.88 

1.40 

1.75 

1.78 

0.67 

1.56 

1.35 

0.72 

1.07 

1.61 

1.41 

1.80 

1.92 

2.25 

2.47 

5.20 

5.32 

4.95 

3.96 

2.67 

2.30 

1.90 

1.69 

1.98 

2.04 

1.06 

1.22 

0.86 

1.05 

0.83 

0.27 

0.90 

2.45 

1.45 

1.32 

1.19 

1.14 

1.40 

1.94 

2.13 

1.93 

2.27 

2.18 

1.54 

1.24 

1.55 

2m 

0.56 

0.48 

0.68 

0.78 

0.70 

0.73 

0.78 

0.64 

0.58 

0.78 

0.15 

0.15 

0.30 

0.40 

0.99 

0.44 

0.56 

0.46 

0.63 

0.50 

0.63 

0.57 

0.66 

0.71 

1.35 

1.35 

1.19 

0.99 

0.66 

0.74 

0.53 

0.47 

0.58 

0.58 

0.46 

0.57 

0.28 

0.30 

0.27 

0.17 

0.32 

0.73 

0.67 

0.67 

0.61 

0.61 

0.62 

0.71 

0.66 

0.63 

0.64 

0.67 

0.40 

0.07 

0.46 

Wind 

direction 

sw 

w 

w 

w 

sw 

sw 

v 

sw 

v 

sw 

v 

sw 

sw 

sw 

sw 

w 

sw 

w 

w 

w 

sw 

sw 

sw 

sw 

w 

w 

w 

w 

w 

w 

sw 

sw 

sw 

sw 

sw 

sw 

sw 

sw 

sw 

s 

s 

s 

SE 

SE 

E 

NE 

NE 

NE 

NE 

NE 

NE 

SE 

NE 

v 

s 

s 

v 

NW 

w 

w 

sw 

w 

w 

sw 

sw 
F58 STUDIES OF EVAPOTRANSPIRATION 

TABLE A1-1.-Hourly averages of windspeeds and wind direction at the Buckeye Project during selected periods of 3 days each-Continued 

1-hr. period 

beginning 11m 7m 

4-5 August 1966-Continued 

22 ________________________________________________ 3.11 

23 ________________________________________________ 2.20 

24 ________________________________________________ 4.22 

1------------------------------------------------3.35 2 ________________________________________________ 2.11 

3 ________________________________________________ 2.40 

4 ________________________________________________ 1.49 

5------------------------------------------------1.23 6 ________________________________________________ 1.24 

7------------------------------------------------1.00 8 ________________________________________________ 1.20 

9------------------------------------------------1.76 10 ________________________________________________ 1.73 

11 ________________________________________________ 3.08 

12-----------------------------------------------~2.31 13 ________________________________________________ 2.18 

14 ________________________________________________ 3.68 

15 ________________________________________________ 3.75 

16 ________________________________________________ 3.46 

17 ------------------------------------------------3.15 18 ________________________________________________ 5.42 

19 ________________________________________________ 7.26 

20 ________________________________________________ 7.99 

21 ________________________________________________ 6.55 

22 ________________________________________________ 5.13 

23 ________________________________________________ 5.94 

24 ________________________________________________ 5.29 

1------------------------------------------------2.85 2 ________________________________________________ 3.58 

3 ________________________________________________ 1.07 

4 ________________________________________________ 1.41 

2.49 

1.74 

3.31 

2.62 

1.63 

1.79 

1.05 

5-6 August 1966 

0.76 

0.84 

0.88 

1.13 

1.50 

1.54 

2.48 

2.04 

1.93 

2.91 

3.03 

3.01 

2.69 

4.31 

5.63 

6.36 

5.39 

4.25 

4.95 

4.39 

2.23 

2.85 

0.75 

0.85 

8-9 September 1966 

5 ________________________________________________ 1.79 

6 ________________________________________________ 2.05 

7------------------------------------------------2.11 8 ________________________________________________ 3.75 

9 ________________________________________________ 4.50 

10 ________________________________________________ 4.73 

11 ________________________________________________ 4.00 

12 ________________________________________________ 2.89 

13 ________________________________________________ 2.38 

14 ________________________________________________ 2.70 

15 ________________________________________________ 3.52 

16 ________________________________________________ 2.87 

17------------------------------------------------2.57 18 ________________________________________________ 2.04 

19 ----------------------------------------------- _1.64 20 ________________________________________________ 0.60 

21------------------------------------------------1.45 22 ________________________________________________ 1.57 

23 ________________________________________________ 2.25 

24 ________________________________________________ 2.76 

1------------------------------------------------2.81 

2------------------------------------------------2.81 

3------------------------------------------------2.30 4 ________________________________________________ 2.63 

1.39 

1.55 

1.63 

2.81 

3.45 

3.74 

3.40 

2.47 

2.00 

2.21 

2.90 

2.49 

2.08 

1.51 

1.19 

0.48 

1.41 

1.41 

1.70 

1.93 

1.70 

1.81 

1.55 

1.76 

9-10 September 1966 

5------------------------------------------------3.11 6 ________________________________________________ 2.89 

7 ________________________________________________ 2.92 

8 ________________________________________________ 2.97 

9 ________________________________________________ 3.58 

10 ________________________________________________ 4.52 

11 ________________________________________________ 3.33 

2.01 

2.20 

2.25 

2.40 

2.95 

3.48 

2.88 

5m 

2.05 

1.49 

2.69 

2.13 

1.23 

1.33 

0.93 

0.65 

0.84 

0.82 

0.96 

1.23 

1.33 

2.05 

1.86 

1.74 

2.35 

2.54 

2.53 

2.28 

3.58 

4.62 

5.27 

4.54 

3.58 

4.21 

3.71 

1.89 

2.35 

0.92 

0.95 

1.14 

1.15 

1.30 

2.13 

2.65 

3.03 

2.92 

2.15 

1.79 

1.93 

2.45 

2.06 

1.69 

1.22 

0.86 

0.68 

1.20 

1.21 

1.32 

1.54 

1.48 

1.49 

1.24 

1.43 

1.73 

1.70 

1.70 

2.00 

2.40 

3.18 

2.45 

4m 

1.64 

1.17 

2.21 

1.76 

0.99 

1.03 

0.67 

0.28 

0.58 

0.74 

0.88 

1.07 

1.25 

1.73 

1.72 

1.63 

1.98 

2.11 

2.18 

2.10 

3.07 

3.98 

4.54 

3.88 

3.10 

3.64 

3.22 

1.52 

2.11 

0.45 

0.26 

0.78 

0.85 

1.05 

1.67 

2.19 

2.59 

2.65 

1.89 

1.54 

1.62 

2.17 

1.82 

1.48 

0.95 

0.62 

0.34 

0.85 

0.84 

0.94 

1.17 

1.04 

1.07 

1.08 

1.03 

1.27 

1.27 

1.38 

1.65 

2.06 

2.64 

2.11 

2m 

0.58 

0.44 

0.73 

0.57 

0.46 

0.36 

0.21 

0.21 

0.17 

0.24 

0.47 

0.55 

0.85 

0.66 

0.62 

0.61 

0.64 

0.63 

0.58 

0.55 

0.85 

1.04 

1.17 

0.90 

0.70 

0.82 

0.68 

0.47 

0.34 

0.21 

0.14 

0.09 

0.09 

0.38 

0.77 

1.06 

1.18 

1.03 

0.81 

0.64 

0.75 

0.93 

0.73 

0.62 

0.17 

0.17 

0.66 

0.24 

0.17 

0.17 

0.36 

0.76 

0.34 

0.52 

0.67 

0.89 

1.03 

0.89 

Wind 

direction 

v 

NE 

s 

s 

NE 

NE 

NE 

NE 

N 

N 

NE 

N 

v 

v 

v 

sw 

NW 

w 

w 

w 

sw 

sw 

sw 

sw 

sw 

sw 

sw 

sw 

sw 

w 

w 

E 

NE 

NE 

NE 

NE 

NE 

NE 

v 

v 

v 

NE 

NE 

NE 

N 

N 

N 

NE 

NE 

v 

NE 

NE 

NE 

E 

NE 

E 

NE 

NE 

NE 

NE 

NE 

v 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F59 

TABLE A1-1.-Hourly averages of windspeeds and wind direction at the Buckeye Project during selected periods of 3 days each-Continued 

1-hr. period 

beginning 11m 7m 

9-10 September 1966-Continued 

12 ________________________________________________ 3.75 

13 ________________________________________________ 2.53 

14 ________________________________________________ 3.10 

15 ________________________________________________ 3.60 

16 ________________________________________________ 3.88 

17 ________________________________________________ 3.09 

18 ________________________________________________ 2.76 

19 ________________________________________________ 1.88 

20 ________________________________________________ 1.82 

21 ________________________________________________ 1.87 

22 ________________________________________________ 2.95 

23 ________________________________________________ 2.29 

24 ________________________________________________ 2.80 

1 ________________________________________________ 2.27 

2------------------------------------------------1.57 3 ________________________________________________ 2.45 

4 ________________________________________________ 3.30 

3.04 

2.23 

2.57 

2.92 

3.20 

2.55 

2.08 

1.44 

1.42 

1.17 

1.77 

1.70 

2.04 

1.51 

1.31 

2.05 

2.08 

10-11 September 1966 

5 ________________________________________________ 2.60 

6 ________________________________________________ 2.57 

7 ________________________________________________ 2.35 

8 ________________________________________________ 1.19 

9 ----------------------------------------------- _1.15 10 ________________________________________________ 1.77 

11 ________________________________________________ 2.29 

12 ________________________________________________ 3.97 

13 ________________________________________________ 2.55 

14 ________________________________________________ 2.07 

15 ________________________________________________ 2.47 

16 ----------------------------------------------- _1.48 

17 ------------------------------------------------1.39 18 ________________________________________________ 1.73 

19 ________________________________________________ 2.29 

20 ________________________________________________ 3.13 

21 ________________________________________________ 3.71 

22 ________________________________________________ 3.35 

23 ________________________________________________ 2.15 

24 ________________________________________________ 1.80 

1 ________________________________________________ 2.29 

2 ________________________________________________ 2.43 

3 ________________________________________________ 2.20 

4 ________________________________________________ 1.86 

5 -------------------------------------------------- 6 ---------------------------------------------- ---- 7 ---------------------------------------------- ---- 8 ________________________________________________ 2.62 

9 ________________________________________________ 2.97 

10 ________________________________________________ 3.14 

11 ________________________________________________ 1.60 

12 ________________________________________________ 1.59 

13 ________________________________________________ 1.48 

14 ________________________________________________ 2.07 

15 ________________________________________________ 3.09 

16 ________________________________________________ 2.47 

17 ________________________________________________ 1.60 

18 ________________________________________________ 1.51 

19 ________________________________________________ 1.81 

20 ________________________________________________ 1.89 

21 ________________________________________________ 3.30 

22 ________________________________________________ 4.55 

23 ________________________________________________ 4.11 

24 ________________________________________________ 2.20 

1 ________________________________________________ 1.79 

2 ________________________________________________ 1.28 

3 ________________________________________________ 1.17 

4 ________________________________________________ 1.36 

1.69 

1.85 

1.57 

1.06 

1.00 

1.48 

2.00 

3.45 

2.15 

1.78 

2.16 

1.26 

1.18 

1.24 

1.73 

2.47 

2.85 

2.72 

1.55 

1.13 

1.64 

1.67 

1.54 

1.22 

23-24 March 1967 

2.07 

2.53 

2.60 

1.30 

1.45 

1.32 

1.76 

2.35 

2.00 

1.21 

1.05 

1.42 

1.48 

2.44 

3.06 

2.85 

1.60 

1.48 

1.10 

1.10 

1.13 

5m 

2.54 

1.98 

2.27 

2.36 

2.69 

2.08 

1.54 

1.04 

1.10 

1.05 

1.52 

1.36 

1.54 

1.32 

1.03 

1.28 

1.74 

1.38 

1.30 

1.31 

0.95 

1.03 

1.30 

1.77 

2.93 

1.90 

1.57 

1.75 

1.10 

1.00 

1.10 

1.38 

2.02 

2.24 

2.18 

1.11 

0.86 

1.22 

1.17 

1.07 

1.07 

1.63 

2.29 

2.21 

1.04 

1.31 

1.20 

1.54 

1.89 

1.73 

0.99 

0.84 

1.76 

1.99 

1.54 

1.14 

1.18 

1.05 

4m 

2.17 

1.80 

1.95 

1.99 

1.82 

1.20 

0.86 

0.89 

0.72 

1.02 

1.02 

1.18 

0.92 

0.90 

0.85 

1.22 

0.98 

0.84 

0.99 

0.88 

1.00 

1.17 

1.66 

2.65 

1.64 

1.41 

1.58 

0.92 

0.88 

0.85 

1.08 

1.64 

1.82 

1.90 

0.83 

0.58 

0.85 

0.77 

0.71 

0.77 

1.40 

2.04 

1.95 

1.38 

1.26 

1.17 

1.43 

1.52 

1.51 

0.80 

0.63 

0.85 

0.95 

1.42 

1.25 

1.00 

0.80 

1.07 

0.93 

0.89 

0.94 

2m 

0.79 

0.76 

0.78 

0.85 

1.04 

0.77 

0.42 

0.17 

0.11 

0.11 

0.11 

0.48 

0.66 

0.11 

0.11 

0.11 

0.11 

0.13 

0.13 

0.13 

0.38 

0.34 

0.51 

0.72 

0.83 

0.66 

0.66 

0.73 

1.23 

0.30 

0.17 

0.17 

0.36 

0.42 

0.48 

0.58 

0.21 

0.13 

0 

0 

0 

0.75 

1.74 

1.60 

0.90 

0.76 

0.70 

0.57 

0.71 

0.56 

0.67 

0.61 

Wind 

direction 

v 

v 

v 

NE 

NE 

NE 

N 

N 

N 

N 

NE 

NE 

NE 

NE 

v 

NE 

NE 

E 

E 

NE 

NE 

NE 

SE 

v 

SE 

SE 

v 

NE 

NE 

N 

N 

NE 

v 

sw 

sw 

w 

w 

w 

w 

NW 

v 

N 

NE 

NE 

NE 

s 

SE 

w 

NW 

NW 

N 

NW 

w 

w 

sw 

sw 

sw 

sw 

NW 

NW 

NE 

NE 
F60 STUDIES OF EVAPOTRANSPIRATION 

TABLE A1-l.-Hourly averages ofwindspeeds and wind direction at the Buckeye Project during selected periods of 3 days each-Continued 

1-hr. period 

beginning 11m 

5 ________________________________________________ 1.96 

5------------------------------------------------1.96 6 ________________________________________________ 2.15 

7 ________________________________________________ 1.59 

8 ________________________________________________ 1.87 

9 ________________________________________________ 1.60 

10 ________________________________________________ 3.44 

11 ________________________________________________ 4.58 

12 ________________________________________________ 4.58 

13 ________________________________________________ 4.34 

14 ________________________________________________ 5.20 

15 ________________________________________________ 5.57 

16 ________________________________________________ 6.48 

17 ________________________________________________ 5.69 

18 ________________________________________________ 6.55 

19 ________________________________________________ 4.70 

20 ________________________________________________ 2.97 

21 ________________________________________________ 2.85 

22 ________________________________________________ 3.21 

23 ________________________________________________ 3.77 

24 ________________________________________________ 5.07 

1 ________________________________________________ 5.51 

2 ________________________________________________ 3.21 

3 ________________________________________________ 2.23 

4 ________________________________________________ 1.86 

5 ________________________________________________ 1.38 

6 ________________________________________________ 2.48 

7------------------------------------------------1.98 8 ________________________________________________ 1.58 

9 ________________________________________________ 1.17 

10 ________________________________________________ 1.35 

11 ------------------------------------------------1.73 12 ________________________________________________ 2.12 

13 ________________________________________________ 3.09 

14 ________________________________________________ 3.21 

15 ________________________________________________ 2.54 

16 ________________________________________________ 2.62 

17------------------------------------------------1.73 18 ________________________________________________ 1.48 

19 ------------------------------------------------1.18 20 ________________________________________________ 0.84 

21 ________________________________________________ 0.88 

22 ________________________________________________ 2.23 

23 ________________________________________________ 2.75 

24 ________________________________________________ 2.92 

1 ________________________________________________ 2.41 

2 ________________________________________________ 2.26 

3 ________________________________________________ 1.78 

4 ________________________________________________ 2.18 

5 ________________________________________________ 2.11 

6 ________________________________________________ 1.53 

7 ________________________________________________ 0.77 

8 ________________________________________________ 1.20 

9 ________________________________________________ 2.38 

10 ___________________________________________________ _ 11 ___________________________________________________ _ 12 ___________________________________________________ _ 13 ________________________________________________ 3.99 

14 ________________________________________________ 4.34 

15 ________________________________________________ 3.71 

16 ________________________________________________ 4.58 

7m 

24-25 March 1967 

1.58 

1.58 

1.71 

1.30 

1.45 

1.30 

2.57 

3.04 

3.36 

2.97 

3.84 

4.76 

4.51 

3.69 

4.37 

3.50 

2.23 

2.12 

2.45 

2.98 

3.51 

4.21 

2.59 

1.83 

1.68 

25-26 March 1967 

1.13 

1.69 

1.52 

1.13 

1.05 

1.28 

1.51 

1.69 

2.70 

2.75 

2.22 

2.14 

1.53 

1.17 

0.95 

0.68 

0.90 

1.84 

1.98 

2.24 

1.98 

1.58 

11-12 May 1967 

1.28 

1.00 

0.62 

1.04 

2.04 

2.38 

2.85 

2.97 

3.30 

3.65 

3.05 

3.84 

5m 

1.30 

1.30 

1.08 

2.54 

2.96 

2.95 

3.06 

3.74 

4.24 

4.34 

3.90 

4.27 

2.96 

1.63 

1.70 

1.89 

2.35 

2.94 

2.65 

2.04 

1.55 

1.03 

0.70 

1.03 

1.07 

1.07 

0.96 

1.22 

1.45 

1.73 

2.38 

2.33 

1.91 

1.86 

1.23 

0.98 

1.46 

1.48 

1.77 

1.55 

4m 

1.15 

1.15 

1.17 

0.85 

0.96 

0.96 

2.09 

3.02 

2.95 

2.95 

3.48 

3.90 

3.96 

3.65 

4.15 

2.75 

1.30 

1.40 

1.61 

2.05 

2.72 

3.10 

1.79 

1.35 

0.94 

0.94 

0.95 

0.80 

1.07 

0.93 

1.18 

1.39 

1.60 

2.21 

2.17 

1.76 

1.66 

1.30 

0.86 

0.77 

0.67 

0.85 

1.18 

1.24 

1.41 

1.35 

1.03 

0.85 

0.99 

0.78 

0.55 

0.42 

0.67 

1.54 

1.89 

1.78 

1.82 

2.38 

2.56 

2.23 

2.72 

2m 

0.84 

2.17 

1.14 

1.15 

1.20 

1.32 

1.57 

1.58 

1.32 

1.39 

0.79 

0.90 

1.22 

0.67 

0.67 

0.85 

0.64 

1.02 

1.06 

0.81 

0.84 

0.21 

0.46 

0.63 

0.73 

0.85 

0.78 

0.77 

0.76 

0.73 

Wind 

direction 

v 

v 

sw 

w 

NE 

NE 

sw 

SW 

sw 

sw 

SW 

sw 

sw 

sw 

w 

w 

w 

SW 

sw 

sw 

sw 

sw 

sw 

s 

sw 

sw 

N 

NE 

NE 

NE 

sw 

s 

SW 

sw 

sw 

sw 

SW 

w 

NW 

w 

NW 

w 

NE 

E 

E 

E 

E 

E 

E 

N 

N 

N 

N 

sw 

sw 

sw 

sw 

sw 

w 

w 

w 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F61 

TABLE A1-1.-Hourly averages of windspeeds and wind direction at the Buckeye Project during selected periods of 3 days each-Continued 

1-hr. period 

beginning 11m 7m 

11-12 May 1967-Continued 

17------------------------------------------------2.65 18 ________________________________________________ 2.94 

19 ________________________________________________ 4,37 

20 ________________________________________________ 4,88 

21 ________________________________________________ 4,03 

22 ________________________________________________ 2.85 

23 ________________________________________________ 1.95 

24 ________________________________________________ 2.11 

1------------------------------------------------1.49 

2------------------------------------------------1.15 3 ________________________________________________ 1.96 

4 ________________________________________________ 1.60 

5 ________________________________________________ 2.95 

6 ________________________________________________ 1.55 

7 ________________________________________________ 1.89 

8 ________________________________________________ 1.22 

9 ________________________________________________ 1,04 

10 ------------------------------------------------1.41 11 ________________________________________________ 2.68 

12 ________________________________________________ 4,80 

13 ________________________________________________ 4,50 

14 ________________________________________________ 5.05 

15 ________________________________________________ 5,12 

16 ________________________________________________ 5.32 

17------------------------------------------------5.87 18 ________________________________________________ 4,88 

19 ________________________________________________ 4,95 

20 ________________________________________________ 3.08 

21 ________________________________________________ 2.60 

22 ________________________________________________ 2.62 

23 ________________________________________________ 3.37 

24 ________________________________________________ 3,07 

1 ________________________________________________ 2,76 

2 ________________________________________________ 2.57 

3 ________________________________________________ 3.77 

4 ________________________________________________ 3.27 

5 ________________________________________________ 3,16 

6 ________________________________________________ 3.08 

7 ________________________________________________ 2.47 

8 ________________________________________________ 4.78 

9 ________________________________________________ 3,70 

10 ________________________________________________ 3.71 

11 ________________________________________________ 2.94 

12 ________________________________________________ 5.57 

13 ________________________________________________ 3.84 

14 ________________________________________________ 5.50 

15 ________________________________________________ 5.69 

16 ________________________________________________ 5.20 

17 ________________________________________________ 5.30 

18 ________________________________________________ 5.05 

19----------------------~---~---------------------4.92 20 ________________________________________________ 3.98 

21 ________________________________________________ 3.61 

22 ________________________________________________ 3,80 

23 ________________________________________________ 3.70 

24 ________________________________________________ 3.33 

1------------------------------------------------1.60 

2------------------------------------------------1.73 3 ________________________________________________ 1,40 

4 ________________________________________________ 1.41 

2.35 

2.41 

3.50 

3.96 

3.15 

2.24 

1.49 

1.73 

1.18 

1.10 

1.41 

1.25 

12-13 May 1967 

2.05 

1.11 

1.80 

1.48 

1.15 

1.35 

2.35 

3.94 

3.64 

4.27 

4.27 

4.46 

4.88 

3.96 

3.88 

2.39 

1.76 

1.90 

2.42 

2.52 

2.03 

1.93 

2.79 

2.48 

13-14 May 1967 

2.51 

2.40 

1.93 

3.87 

2.94 

3.21 

2.36 

4.34 

3.20 

4.14 

4.60 

4.40 

4.46 

4.60 

4.08 

3.27 

2.97 

3.08 

2.97 

2.47 

1.26 

1.39 

1.13 

1.35 

5m 

1.99 

2.91 

3.28 

2.47 

1.71 

1.11 

1.44 

0.94 

0.92 

1.11 

0.94 

1.52 

0.98 

1.60 

1.43 

1.03 

1.13 

2.03 

3.28 

3.21 

3.65 

3.58 

3.71 

4.12 

3.39 

3.16 

1.87 

1.21 

1.33 

1.79 

1.92 

1.57 

1.49 

2.24 

1.98 

2.01 

1.95 

1.61 

3.25 

2.45 

2.85 

1.90 

3.71 

2.75 

3.19 

3.71 

3.84 

3.84 

3.88 

3.45 

2.82 

2.50 

2.54 

2.40 

1.83 

1.02 

1.13 

0.98 

1.05 

4m 

1.60 

1.76 

2.47 

2.78 

2.05 

1.45 

0.92 

1.24 

0.79 

0.85 

0.85 

0.82 

1.15 

0.79 

1.22 

0.88 

0.93 

1.13 

1.90 

2.97 

2.78 

3.20 

3.27 

3.33 

3.51 

2.95 

2.72 

1.58 

0.88 

1.06 

1.28 

1.57 

1.23 

1.17 

1.87 

1.61 

1.69 

1.63 

1.41 

2.78 

1.99 

2.58 

1.63 

3.08 

2.39 

2.60 

3.22 

3.45 

3.39 

3.33 

3.00 

2.44 

2.11 

2.16 

2.03 

1.53 

0.94 

0.92 

0.86 

0.92 

2m 

0.55 

0.44 

0.70 

0.77 

0.53 

0.21 

0.09 

0.13 

0.05 

0.09 

0.46 

0.61 

0.62 

0.55 

0.73 

0.89 

0.88 

0.88 

0.90 

0.92 

0.90 

0.66 

0.61 

0.17 

0.66 

0.58 

0.66 

0.67 

1.06 

0.86 

0.93 

0.95 

0.92 

0.82 

0.82 

0.68 

0.45 

0.30 

0.32 

0.28 

Wind 

direction 

w 

w 

sw 

sw 

sw 

sw 

NE 

NE 

NE 

E 

E 

SE 

s 

SE 

SE 

SE 

SE 

v 

v 

sw 

sw 

sw 

sw 

sw 

w 

w 

sw 

sw 

w 

w 

w 

sw 

sw 

w 

w 

w 

w 

sw 

sw 

w 

w 

w 

sw 

NW 

NW 

w 

w 

w 

w 

w 

w 

w 

w 

w 

w 

w 

w 

v 

E 

E 
F62 STUDIES OF EVAPOTRANSPIRATION 

TABLE A2-1. Hourly averages of air temperature and vapor pressure at the Buckeye Project during selected periods 'of 3 days each 

(Air temperature in degrees Celsius roC), vapor pressure in millibars (mb)* at indicated levels (meters) above ground. 1mb= 100 pascal (Pal] 

1-hr period 11m 7m 5m 4m 2m 

beginning oc mb oc mb oc mb oc mb oc mb 

5-6 May 1966 

5 __________ 18.9 11.3 18.0 11.5 17.7 11.6 17.0 11.7 14.3 12.6 

6 __________ 20.8 11.0 19.5 11.3 19.1 11.5 18.5 11.6 15.5 13.3 

7 __________ 22.6 11.7 22.8 11.8 22.8 11.9 22.6 12.0 21.4 13.8 

8 __________ 26.2 12.1 26.3 12.4 26.4 12.6 26.4 12.7 26.4 14.8 

9 __________ 29.5 10.7 29.6 11.6 29.7 12.3 30.0 12.8 30.6 14.8 

10 __________ 31.6 10.6 32.0 11.3 32.5 11.8 33.0 12.1 34.7 14.8 

11 __________ 33.4 11.3 33.5 12.5 34.2 13.4 34.7 13.9 37.4 15.8 

12 __________ 34.4 10.0 35.1 11.4 35.3 12.5 36.4 13.2 38.6 15.5 

13 __________ 35.1 10.2 35.3 11.6 36.3 12.6 36.1 13.3 38.3 15.7 

14 __________ 35.8 9.2 35.6 11.0 36.2 12.5 36.7 13.5 37.3 16.4 

15 __________ 36.1 10.5 36.9 11.5 36.5 12.3 36.8 12.8 37.7 14.5 

16 __________ 36.4 10.3 36.4 11.6 37.0 12.6 36.7 13.5 38.2 15.4 

17 __________ 35.9 10.6 36.3 11.3 36.1 11.7 35.7 12.1 35.7 15.0 

18 __________ 34.7 11.0 34.2 11.6 33.7 12.1 33.2 12.3 32.3 15.1 

19 __________ 32.6 11.2 32.1 11.9 31.7 12.4 30.9 12.8 27.7 16.4 

20 __________ 31.2 10.8 30.5 11.5 29.3 12.0 28.6 12.3 23.8 15.9 

21 __________ 31.0 10.8 29.7 11.3 29.1 11.7 28.3 12.0 24.6 14.9 

22 __________ 31.1 9.2 30.8 9.5 30.4 9.7 30.6 9.8 28.5 12.4 

23 __________ 27.2 10.4 26.8 10.8 26.1 11.1 25.7 11.2 23.8 13.4 

24 __________ 25.9 10.6 24.8 10.8 24.0 10.9 23.8 11.0 19.0 14.4 

1 __________ 25.9 10.3 24.4 10.7 23.5 11.1 23.0 11.3 17.9 13.4 

2 __________ 24.4 10.4 22.8 10.6 22.0 10.8 21.5 10.9 16.3 13.2 

3 __________ 24.4 10.5 23.8 11.0 23.4 11.2 21.3 11.3 15.5 12.5 

4 __________ 22.3 10.6 20.2 10.8 19.4 10.9 19.0 11.0 14.9 12.6 

6-7 May 1966 

5 __________ 21.3 10.7 19.7 10.8 19.1 10.9 18.2 11.0 13.6 12.2 

6 __________ 19.8 9.9 19.3 10.1 18.8 10.2 18.5 10.3 14.8 12.0 

7 __________ 23.1 11.0 22.7 11.8 23.0 12.4 22.4 12.8 20.9 15.4 

8 __________ 24.3 12.8 24.1 13.4 24.5 13.9 23.9 14.2 24.3 16.1 

9 __________ 26.9 10.9 27.1 11.4 27.2 11.8 27.3 12.1 28.4 14.5 

10 __________ 29.2 10.7 29.7 11.6 29.4 12.3 30.3 12.7 31.2 15.0 

11 __________ 31.9 11.8 31.6 12.8 32.1 13.5 32.2 13.9 34.1 16.7 

12 __________ 33.8 11.0 34.2 12.2 34.4 13.2 33.8 13.9 35.9 18.0 

13 __________ 34.8 10.2 35.6 11.4 36.1 12.2 36.3 12.8 38.1 16.4 

14 __________ 36.3 10.4 35.7 11.9 35.7 12.4 36.5 13.7 38.0 17.9 

15 __________ 36.4 9.0 36.5 10.8 36.7 12.2 36.9 13.2 37.7 16.6 

16 __________ 36.5 10.5 36.8 11.7 36.5 12.2 36.1 13.4 37.2 16.8 

17 __________ 36.5 10.8 36.8 11.9 36.3 12.7 35.7 13.2 35.9 17.1 

18 __________ 35.6 11.5 35.5 13.1 35.1 14.4 34.3 15.2 32.4 20.0 

19 __________ 34.2 12.1 33.2 13.3 32.4 14.2 31.7 14.8 26.3 19.5 

20 __________ 31.9 12.1 30.0 13.1 29.3 14.0 29.0 14.4 22.4 17.0 

21 __________ 30.4 11.0 29.6 11.6 29.0 12.0 28.5 12.4 20.4 15.8 

22 __________ 29.2 11.3 27.1 11.9 25.7 12.3 24.3 12.6 18.9 15.4 

23 __________ 26.8 11.1 23.9 11.6 23.1 12.0 22.3 12.3 17.8 14.6 

24 __________ 23.9 12.5 22.8 13.0 22.2 13.3 22.3 13.5 17.3 14.8 

1 __________ 27.1 10.0 26.0 10.4 24.8 10.6 23.8 10.8 18.6 13.8 

2 __________ 23.6 10.3 21.9 10.8 21.8 11.2 22.9 11.4 18.1 13.3 

3 __________ 22.6 10.2 21.5 10.5 20.4 10.7 19.7 10.8 15.7 13.0 

4 __________ 21.1 10.4 19.1 10.6 18.6 10.8 16.9 10.9 14.1 12.5 

7-8 May 1966 

5 __________ 21.9 9.4 20.4 9.6 19.7 9.8 19.1 9.9 14.5 11.8 

6 __________ 21.3 9.4 19.4 10.0 19.2 10.4 18.0 10.7 15.2 12.7 

7 __________ 21.0 12.0 20.9 12.6 20.9 13.1 20.4 13.4 20.1 15.3 

8 __________ 23.6 11.8 23.8 12.8 23.9 13.6 23.5 14.1 23.6 16.3 

9 __________ 27.7 11.1 27.8 12.3 27.7 13.3 27.3 13.9 27.9 18.3 

10 __________ 30.7 10.5 31.2 12.0 30.9 13.1 30.4 13.8 31.2 18.7 

11 __________ 33.3 10.9 33.0 11.9 33.5 12.5 32.7 13.0 33.6 18.3 

12 __________ 35.0 10.5 34.2 12.5 34.8 14.0 33.9 15.0 35.0 16.5 

13 __________ 35.6 9.7 36.3 11.0 36.3 11.8 36.5 12.5 35.8 19.9 

14 --------- _36.4 9.3 37.6 10.5 37.9 11.4 37.0 12.0 39.3 18.0 

15 __________ 36.9 9.9 36.6 11.3 37.1 12.4 36.8 13.1 37.5 20.0 

16 __________ 37.3 8.6 36.6 10.1 36.8 11.3 36.0 12.1 36.3 16.8 

17 __________ 37.1 9.3 36.3 10.4 36.0 11.3 36.2 11.8 35.9 16.7 

18 __________ 35.6 9.3 35.1 10.6 34.8 11.6 34.3 12.3 33.2 16.4 

19 __________ 33.2 11.4 32.8 12.0 32.3 12.4 32.0 12.7 29.8 16.0 

20 __________ 31.3 10.8 30.3 11.4 30.0 11.9 29.5 12.3 26.4 15.6 

21 __________ 30.3 10.3 28.8 10.9 28.0 11.3 27.4 11.6 24.1 14.0 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SAL TCEDAR THICKET, ARIZONA F63 

TABLE A2-l. Hourly averages of air temperature and vapor pressure at the Buckeye Project during selected periods of 3 days each.-Continued 

1-hr period 11m 7m 5m 4m 2m 

beginning oc mb oc mb oc mb oc mb oc mb 

7-8 May 1966-Continued 

22 __________ 31.2 7.9 30.9 8.3 30.2 8.6 30.1 8.8 27.7 11.6 

23 __________ 30.7 7.5 30.4 8.0 30.3 8.4 30.0 8.7 28.2 11.5 

24 __________ 28.5 7.9 27.9 8.4 27.6 8.7 27.3 9.0 25.3 11.5 

1 --------- _28.0 8.1 27.3 8.4 27.0 8.7 26.6 8.8 24.3 11.0 

2 __________ 26.7 8.2 25.5 8.7 24.7 9.1 24.5 9.3 21.0 12.0 

3 __________ 24.5 9.7 23.1 9.9 22.4 10.0 22.7 10.1 19.1 12.8 

4 __________ 25.4 11.0 23.1 11.6 21.8 12.0 21.3 12.3 18.4 14.4 

3-4 June 1966 

5 __________ 18.8 6.1 17.2 6.4 16.5 6.6 15.4 6.7 8.8 8.0 

6 __________ 16.4 6.0 16.0 6.4 15.5 6.7 14.6 6.9 10.9 8.4 

7 __________ 18.1 6.8 18.3 7.3 18.3 7.7 17.7 8.0 16.9 9.3 

8 __________ 21.7 6.6 22.0 7.4 22.0 8.0 21.8 8.4 22.2 10.2 

9 __________ 25.3 6.6 25.6 7.5 25.7 8.1 25.6 8.5 26.2 9.5 

10 __________ 28. 7 6.3 29.0 7.4 28.9 8.3 29.0 8.8 29.6 9.6 

11 __________ 31.2 5.4 31.3 6.2 31.7 6.8 31.4 7.2 31.9 9.4 

12 __________ 33.1 4.9 33.0 6.0 33.5 6.8 33.2 7.4 34.6 10.1 

13 --------- _34.3 3.3 34.1 4.6 34.5 5.5 34.6 6.2 35.3 8.7 

14 __________ 34.5 3.2 34.5 4.4 34.6 5.1 34.9 5.7 35.4 8.5 

15 __________ 34.9 3.3 35.3 4.3 35.7 5.1 35.2 5.6 35.8 7.8 

16 __________ 35.0 3.1 35.2 4.6 35.8 5.7 35.5 6.4 35.4 8.1 

17 __________ 35.2 3.0 35.6 4.1 35.4 5.1 35.3 5.6 34.5 7.0 

18 __________ 34.7 3.5 34.9 4.9 34.8 5.9 34.1 6.5 32.4 7.4 

19 __________ 32.5 4.6 32.2 5.9 32.1 6.9 31.0 7.5 28.0 8.0 

20 __________ 28.5 5.2 27.4 6.5 26.6 7.5 25.9 8.1 20.5 9.8 

21 __________ 27.3 5.6 25.7 6.0 25.0 6.3 24.2 6.4 17.9 8.7 

22 __________ 24.3 5.6 23.0 6.1 21.6 6.5 20.8 6.7 15.9 8.1 

23 __________ 23.9 6.1 22.3 6.3 20.4 6.5 19.7 6.6 13.8 7.8 

24 __________ 23.5 5.4 21.8 5.6 20.4 5.8 18.8 5.9 11.3 9.1 

1 __________ 22.7 4.3 22.0 4.5 20.8 4.7 19.3 4.8 10.0 7.4 

2 --------- _22. 7 4.3 20.1 4.4 18.8 4.5 18.5 4.6 11.0 6.2 

3 __________ 21.4 4.4 20.0 4.6 18.2 4.8 16.8 4.9 9.8 6.4 

4 --------- _21.2 4.4 19.8 4.7 16.2 5.5 14.6 5.8 10.0 7.3 

4-5 June 1966 

5 __________ 19.5 4.9 18.3 5.3 17.3 5.6 16.4 5.8 8.8 8.4 

6 __________ 17.7 5.7 16.7 6.4 16.8 6.9 16.2 7.3 10.6 8.2 

7 --------- _18.1 7.3 18.2 7.9 18.1 8.3 17.7 8.6 17.6 9.6 

8 __________ 21.7 7.0 22.0 7.7 22.0 8.3 21.8 8.7 22.1 9.6 

9 __________ 25.6 6.9 25.9 7.5 25.8 8.0 25.5 8.3 26.7 9.9 

10 --------- _28. 7 6.2 29.1 7.7 29.1 8.8 29.0 9.6 29.9 10.9 

11 __________ 31.2 6.9 31.4 8.3 30.8 9.2 31.0 9.9 32.5 10.1 

12 - _________ 32.5 6.2 32.1 7.1 32.8 7.7 32.7 8.2 34.2 9.1 

13 --------- _33.6 4.8 34.0 6.2 34.7 7.3 34.4 8.0 35.9 8.4 

14 __________ 34.8 4.4 34.7 6.0 35.7 7.2 35.1 8.0 36.5 10.4 

15 --------- _36.0 3.8 35.6 5.4 35.9 7.1 35.3 8.1 36.4 8.1 

16 __________ 35.7 3.7 36.1 5.5 36.3 6.8 35.7 7.7 36.0 10.3 

17 __________ 35.5 3.3 36.4 5.0 36.3 6.4 35.5 7.3 35.1 7.9 

18 __________ 35.2 4.4 35.5 5.5 35.4 6.4 34.5 6.9 32.8 8.0 

19 --------- _33.2 5.3 32.9 6.1 32.7 6.8 31.7 7.1 28.3 8.4 

20 __________ 31.3 4.8 29.8 5.8 28.7 6.7 27.6 7.2 22.1 8.9 

21 ----- _____ 28.3 5.4 26.5 6.3 25.3 7.0 23.9 7.4 18.2 9.6 

22 __________ 25.1 7.2 23.4 7.4 22.8 7.5 21.5 7.6 15.6 9.2 

23 __________ 24.9 5.3 24.2 5.9 21.1 6.4 20.0 6.7 14.8 8.3 

24 --------- _24.2 6.7 20.9 7.0 19.9 7.1 19.0 7.4 13.5 8.5 

1 ---- ______ 23.2 6.3 20.5 6.9 19.0 7.3 17.6 7.4 12.3 9.0 

2 __________ 23.1 4.3 21.6 4.8 18.5 5.2 17.1 5.5 11.1 9.0 

3 __________ 21.5 6.0 18.1 6.2 16.7 6.4 15.3 6.5 11.9 7.8 

4 __________ 21.9 5.7 20.7 6.0 19.3 6.2 17.4 6.3 10.7 8.7 

5-6 June 1966 

5 --------- _23.0 6.4 20.7 6.6 19.3 6.7 18.4 6.8 10.6 8.8 

6 --------- _20.6 8.1 17.0 8.3 16.1 8.5 15.5 8.6 11.8 9.8 

7 --------- _20.5 8.6 20.4 9.2 20.3 9.7 19.7 10.0 19.2 11.5 

8 __________ 23.3 8.4 23.6 9.1 23.5 9.6 23.1 9.9 23.6 10.8 

9 --------- _27 .4 8.3 27.7 9.4 27.4 10.2 27.2 10.7 28.0 12.8 

10 __________ 29.7 9.1 29.8 10.0 30.0 10.1 30.1 11.0 30.7 12.1 

11 __________ 31.3 8.0 32.0 9.4 31.7 10.4 31.6 11.0 33.0 14.4 

12 --------- _33. 7 8.2 34.1 9.0 34.8 9.5 34.7 9.9 35.6 11.1 
F64 STUDIES OF EVAPOTRANSPIRATION 

TABLE A2-1. Hourly averages of air temperature and vapor pressure at the Buckeye Project during selected periods of 3 days each.-Continued 

1-hr period Urn 7m 5m 4m 2m 

beginning oc mb oc mb oc mb oc mb oc mb 

5-6 J one 1966-Continued 

13 -- ________ 35.3 5.7 35.4 7.0 36.1 8.1 35.3 8.7 33.7 12.7 

14 __________ 36.1 4.0 36.1 5.6 36.4 6.9 36.4 7.7 37.4 10.8 

15 __________ 36.5 4.1 36.7 5.4 36.8 6.7 36.3 7.5 37.4 10.2 

16 -- ________ 36.9 4.1 36.6 5.5 36.7 6.5 36.4 7.2 37.0 8.0 

17 --- _______ 36.4 3.5 36.5 5.5 36.6 6.9 35.8 8.0 35.2 9.2 

18 __________ 35.1 5.3 35.2 6.5 35.0 7.4 33.8 7.9 31.8 9.5 

19 __________ 32.5 7.5 31.9 8.5 31.6 9.0 30.6 9.6 26.6 11.4 

20 __________ 31.2 7.4 29.8 8.1 29.3 8.5 28.4 9.0 21.4 11.8 

21 __________ 28.7 6.9 27.2 7.6 26.0 8.1 25.1 8.5 18.6 11.1 

22 __________ 26.7 7.2 25.1 7.7 24.1 8.1 23.4 8.3 16.6 10.9 

23 __________ 25.2 7.3 24.5 7.6 23.7 7.9 23.0 8.1 15.8 9.6 

24 __________ 25.1 7.0 24.0 7.1 21.8 7.3 20.7 7.4 13.9 9.5 

1 __________ 23.5 6.5 22.7 6.8 22.3 6.9 20.4 7.1 13.0 8.7 

2 __________ 22.8 5.2 21.1 5.8 20.3 6.1 18.9 6.4 13.5 7.7 

3 __________ 21.2 5.8 18.9 6.3 17.7 6.7 16.7 6.9 11.8 8.7 

4 __________ 21.6 6.0 20.0 6.5 19.4 6.9 18.7 7.1 11.3 9.1 

6-7 July 1966 

5 __________ 20.6 5.3 18.3 6.2 18.7 5.2 16.9 6.2 12.2 7.5 

6 __________ 19.7 6.6 18.7 6.6 18.5 6.8 17.6 7.3 14.4 8.1 

7 __________ 22.5 8.7 22.4 8.3 22.3 8.4 22.0 8.9 21.2 10.1 

8 --- _______ 27.8 7.1 27.6 7.9 27.8 7.9 27.3 9.3 26.8 10.0 

9 --- _______ 32.0 5.9 31.7 6.4 32.3 6.5 31.0 8.3 31.5 9.6 

10 __________ 35.5 7.3 35.5 8.0 35.8 8.2 35.5 9.0 35.5 10.2 

11 __________ 38.6 4.8 38.6 5.9 38.9 6.6 38.4 8.4 39.2 9.7 

12 --------- _40.4 2.5 40.6 4.5 40.8 5.6 40.9 7.6 42.3 8.6 

13 _________ _42.2 . 5.0 40.2 6.0 42.5 6.7 41.9 7.5 43.9 6.9 

14 _________ _41.8 4.4 42.0 5.7 42.7 7.0 43.1 7.8 45.3 6.2 

15 _________ _42.5 4.2 42.8 5.8 43.3 7.0 43.1 8.5 44.7 6.8 

16 _________ _42.6 6.0 42.8 7.2 43.2 8.0 42.7 9.0 42.2 8.0 

17 _________ _42.2 6.4 42.5 7.4 42.4 7.5 42.3 8.7 41.1 8.6 

18 _________ _41.4 5.9 41.2 7.6 41.0 8.0 40.9 9.4 38.5 9.3 

19 _________ _40.1 5.8 39.8 7.1 39.4 8.0 38.9 8.7 36.0 8.3 

20 __________ 38.7 5.7 37.4 7.3 36.7 7.9 36.2 8.6 32.7 8.1 

21 __________ 37.4 6.7 36.0 7.8 35.4 8.1 34.8 8.6 30.2 8.8 

22 __________ 37.1 6.6 35.8 7.5 35.2 7.9 34.7 8.6 30.8 9.1 

23 ---- ______ 30.6 8.0 29.6 9.0 28.5 9.4 27.9 10.1 25.0 10.4 

24 __________ 30.4 9.0 28.9 9.7 27.9 9.8 27.0 10.1 21.9 10.6 

1 __________ 28.7 11.2 27.5 11.3 26.5 11.4 26.0 11.6 22.4 12.1 

2 __________ 28.0 13.4 26.9 13.0 26.0 12.8 25.7 12.4 20.8 12.0 

3 __________ 27.1 13.5 26.6 12.7 25.4 12.0 23.6 11.6 19.5 12.1 

4 __________ 26.9 12.8 25.3 12.5 23.4 12.3 22.9 12.1 18.8 12.8 

7-8 July 1966 

5 __________ 25.4 12.8 24.0 14.0 23.1 13.0 22.7 13.1 18.5 13.8 

6 __________ 26.0 13.3 24.5 14.1 24.2 14.9 23.5 15.3 21.0 16.4 

7 __________ 29.2 16.1 28.6 16.8 28.3 17.1 7.0 31.4 17.0 31.4 

8 __________ 31.3 17.0 31.3 17.0 31.4 17.0 31.4 17.0 29.8 17.9 

9 __________ 31.9 15.0 31.9 16.2 32.0 17.1 31.8 17.5 30.7 18.3 

10 __________ 33.8 15.9 34.4 16.2 34.9 16.3 34.7 16.5 34.7 18.6 

11 __________ 35.0 10.5 35.5 11.7 35.8 13.5 36.5 14.4 38.2 15.6 

12 __________ 37.1 11.0 37.9 12.6 38.4 13.5 38.1 14.5 39.6 15.9 

13 __________ 38.3 10.9 38.5 12.1 39.0 12.8 38.9 13.1 39.2 14.1 

14 __________ 38.6 10.5 39.9 11.4 39.9 12.0 39.8 12.5 41.0 13.0 

15 __________ 38.1 11.2 38.2 12.5 38.3 12.9 38.7 13.7 40.9 14.0 

16 __________ 38.1 9.4 38.2 10.9 38.2 11.5 38.2 12.3 38.9 13.0 

17 __________ 37.7 9.0 37.9 10.6 37.5 11.6 36.7 12.4 35.2 12.4 

18 __________ 37.6 9.8 37.4 11.0 37.2 11.1 36.4 12.5 35.1 12.3 

19 __________ 36.0 10.5 36.0 11.5 35.8 11.9 35.0 12.3 32.8 13.0 

20 __________ 34.1 9.7 33.5 10.4 33.0 10.7 32.2 11.2 29.1 12.1 

21 __________ 34.9 10.0 34.0 10.8 33.5 11.2 33.0 11.6 29.0 11.4 

22 __________ 31.9 11.0 30.9 11.6 30.1 11.7 29.5 12.1 25.3 13.9 

23 __________ 30.9 12.1 29.5 12.5 28.9 12.8 28.0 13.0 35.1 13.8 

24 __________ 29.2 11.9 28.0 12.4 27.0 12.9 26.2 13.1 22.5 14.0 

1 --------- _29.5 10.9 28.1 11.4 27.1 11.8 26.1 12.2 21.1 13.4 

2 __________ 27.3 13.9 26.9 14.6 26.3 14.9 25.9 15.1 23.3 15.1 

3 __________ 27.2 19.5 26.4 19.4 26.1 19.5 25.8 19.3 23.6 19.5 

4 ------ ____ 27.2 19.6 27.2 19.6 26.6 19.9 26.1 19.8 23.7 19.9 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F65 

TABLE A2-l. Hourly averages of air temperature and vapor pressure at the Buckeye Project during selected periods of 3 days each. -Continued 

1-hr period 11m 7m 5m 4m 2m 

beginning oc mb oc mb oc mb oc mb oc mb 

8-9 July 1966 

5 __________ 26.9 20.4 26.4 20.2 25.9 20.1 25.5 19.9 23.1 20.0 

6 __________ 26.4 20.5 25.7 20.4 25.2 20.3 24.7 20.0 22.6 19.4 

7 __________ 26.9 20.6 26.9 20.7 27.1 21.0 26.6 21.6 26.0 23.4 

8 __________ 28.8 20.2 28.4 20.3 28.7 20.8 29.0 21.0 28.0 23.1 

9 __________ 30.2 19.0 30.5 19.6 30.7 20.4 30.7 21.0 30.1 22.6 

10 __________ 31.7 18.1 31.8 18.9 32.2 19.2 32.1 19.5 32.5 21.4 

11 __________ 32.7 18.5 32.8 18.8 33.4 19.0 33.9 19.5 35.4 19.5 

12 __________ 34.0 18.2 34.4 18.3 35.5 18.5 35.2 18.7 37.0 20.9 

13 __________ 35.3 14.8 35.6 15.6 36.0 15.9 35.5 16.5 38.7 18.7 

14 __________ 36.7 14.5 36.9 14.7 37.3 14.8 37.1 15.3 39.0 16.8 15 __________ 36.5 13.5 36.9 14.4 37.2 15.4 37.6 15.8 39.7 16.0 

16 __________ 37.2 13.9 38.2 14.3 38.3 14.8 38.0 15.1 38.4 16.4 

17 __________ 37.4 13.4 38.0 14.1 38.1 14.9 37.6 15.2 37.3 16.1 

18 __________ 37.2 14.2 37.6 14.6 37.8 14.7 37.2 14.8 35.8 16.4 

19 __________ 36.5 14.0 31.4 14.9 36.1 15.1 35.2 15.7 33.1 16.6 

20 __________ 34.9 14.4 34.2 15.0 33.6 15.2 32.9 15.5 29.0 16.6 

21 __________ 36.0 13.8 34.5 14.1 33.5 14.6 32.7 14.9 27.5 16.1 

22 __________ 33.5 14.7 33.1 15.0 32.6 15.6 31.6 15.9 28.3 16.6 

23 __________ 32.4 14.5 32.6 15.7 32.1 16.1 31.3 17.0 29.0 16.6 

24 __________ 31.4 16.2 31.3 16.9 30.8 17.1 30.3 17.3 28.2 17.6 

1 __________ 31.6 17.3 30.9 17.8 30.7 18.0 30.1 18.1 27.2 18.4 

2 __________ 31.6 17.2 31.2 18.0 31.1 18.2 30.6 18.6 28.3 18.4 

3 __________ 30.6 18.7 30.3 19.0 30.2 19.4 29.9 19.8 28.7 19.0 

4 --------- _29.4 19.2 28.6 19.1 28.0 19.0 27.8 18.9 26.1 19.3 

3-4 August 1966 

5 __________ 31.2 20.0 30.7 20.5 30.3 21.1 30.0 21.7 26.5 21.4 

6 __________ 31.8 20.5 31.7 21.1 31.6 21.5 31.3 21.8 28.3 22.1 

7 __________ 33.4 22.1 33.4 22.7 33.7 23.3 33.1 23.5 31.9 24.4 

8 __________ 34.8 21.3 35.2 21.8 35.4 22.3 35.3 22.5 34.5 25.0 

9 __________ 35.4 22.1 35.7 22.7 36.0 23.0 36.7 23.3 35.8 25.2 

10 __________ 37.7 21.0 37.8 21.8 38.5 22.7 38.6 22.9 38.9 24.0 

11 __________ 38.8 20.1 39.4 21.0 39.8 21.5 40.8 22.0 41.6 24.0 

12 __________ 39.4 20.4 39.7 20.9 40.0 21.4 40.6 21.7 41.4 23.3 

13 __________ 39.9 19.1 40.6 20.1 41.3 21.6 41.5 21.1 43.0 23.5 14 _________ _40.6 17.7 40.8 19.0 41.6 19.5 42.2 20.4 43.4 22.2 

15 __________ 39.7 20.4 40.1 20.5 40.3 20.8 40.6 21.0 41.4 22.0 

16 __________ 38.3 23.5 38.5 23.7 38.9 23.9 39.0 24.2 39.9 25.0 

17 __________ 37.3 22.5 37.9 22.6 37.8 23.0 38.0 23.2 38.1 24.2 

18 __________ 37.4 22.0 37.9 22.2 37.9 22.4 37.6 22.5 37.5 23.0 

19 __________ 36.8 21.1 37.1 21.3 37.1 21.6 36.9 21.8 36.1 22.4 

20 __________ 35.9 21.0 36.3 21.2 36.2 21.5 35.6 22.0 34.5 22.6 

21 __________ 35.4 21.8 35.2 22.0 35.1 22.4 34.8 22.6 33.7 22.8 

22 __________ 34.5 21.3 34.1 21.4 33.9 21.6 33.7 21.8 32.7 23.0 

23 __________ 32.8 20.1 32.8 20.3 32.5 20.4 32.4 20.5 31.4 22.1 

24 __________ 32.3 19.5 32.2 19.7 31.9 19.8 31.8 19.9 30.9 21.3 

1 __________ 31.5 20.1 31.3 20.2 31.1 20.3 30.8 20.4 29.7 22.1 

2 __________ 28.8 20.7 28.7 21.1 28.3 21.7 28.1 21.8 28.0 22.4 

3 __________ 28.1 20.9 27.7 21.1 27.6 21.3 27.3 21.6 26.8 22.8 

4 __________ 27.3 21.4 27.0 21.7 26.6 21.7 26.5 21.8 25.1 22.9 

4-5 August 1966 

5 __________ 26.4 21.5 26.2 22.0 26.1 22.3 26.0 22.7 25.0 22.8 

6 __________ 26.4 21.5 26.4 21.8 26.1 22.0 26.0 22.0 24.5 23.0 

7 __________ 27.9 22.6 27.8 22.3 28.1 22.7 27.8 23.0 27.1 24.8 

8 __________ 31.2 19.8 32.0 19.9 31.7 21.2 31.6 22.3 32.1 22.5 

9 __________ 32.9 20.6 33.9 21.7 33.8 22.0 33.7 22.5 35.1 23.4 

10 __________ 34.4 20.5 35.8 20.8 35.9 21.2 36.3 21.5 37.1 24.0 

11 __________ 37.2 20.2 37.7 21.0 37.8 21.4 38.1 21.5 39.9 23.8 

12 __________ 38.5 20.0 38.9 20.3 39.5 20.6 40.1 20.8 41.9 22.7 

13 __________ 39.6 19.0 40.2 19.8 41.3 20.3 41.9 20.6 43.9 22.7 

14 _________ _40.8 18.6 41.5 19.0 42.0 19.6 42.7 19.9 44.3 22.9 

15 _________ _41.4 17.0 42.4 17.4 42.7 17.8 43.5 18.0 45.6 20.9 

16 __________ 41.7 16.5 43.0 17.2 43.4 17.8 43.5 18.0 45.3 19.1 

17 _________ _41.8 14.9 42.8 15.8 42.7 16.4 43.2 16.8 43.2 20.8 

18 _________ _41.5 15.4 42.0 15.6 42.3 16.0 42.1 16.5 40.4 20.3 

19 --------- _40.3 15.5 39.8 16.6 39.8 17.9 39.0 17.4 35.8 21.5 

20 ------- ___ 40.2 14.8 38.8 15.7 38.1 16.2 37.4 16.6 32.6 20.0 

21 __________ 39.5 15.9 38.6 16.2 38.1 16.7 37.6 17.1 34.8 18.6 
F66 STUDIES OF EVAPOTRANSPIRATION 

TABLE A2-1. Hourly averages of air temperature and vapor pressure at the Buckeye Project during selected periods of 3 days each. -Continued 

1-hr period 11m 7m 5m 4m 2m 

beginning oc mb oc mb oc mb oc mb oc mb 

4-5 August 1966-Continued 

22 __________ 36.4 16.9 35.3 17.2 34.9 17.5 35.0 17.8 32.1 20.6 

23 __________ 34.9 17.8 34.8 18.3 34.6 18.5 34.5 18.8 32.8 20.4 

24 __________ 34.8 17.7 34.2 18.1 34.5 18.4 33.9 18.6 32.1 20.0 

1 __________ 34.5 17.9 34.2 18.0 34.0 18.3 33.8 18.6 32.6 20.3 

2 __________ 31.7 19.2 31.3 19.5 31.2 19.5 30.9 19.5 29.0 21.5 

3 __________ 29.0 20.4 28.8 20.5 28.7 20.6 28.4 20.7 27.5 21.9 

4 __________ 27.8 20.4 27.5 20.5 27.4 20.7 27.1 21.0 25.4 22.4 

5-6 August 1966 

5 __________ 26.8 21.3 26.5 21.4 26.3 21.4 25.9 21.5 24.1 21.9 

6 __________ 27.0 21.5 26.5 21.7 26.4 21.8 26.3 21.9 24.2 22.6 

7 __________ 29.2 21.8 28.2 22.0 28.8 22.7 28.8 23.2 28.1 24.3 

8 __________ 31.7 20.8 32.0 22.4 31.1 23.0 31.1 23.5 31.8 24.5 

9 __________ 33.9 21.5 34.0 22.2 34.6 22.4 34.8 25.2 35.8 25.4 

10 __________ 36.9 19.6 37.1 20.7 37.4 21.5 37.5 21.8 38.8 24.1 

11 __________ 37.3 20.9 38.0 ,21.4 39.1 21.8 39.4 22.0 41.3 23.8 

12 __________ 38.7 18.4 39.4 18.9 40.2 20.0 40.6 20.1 43.3 22.0 

13 __________ 39.9 18.3 41.1 19.6 42.1 19.8 41.8 20.1 44.4 22.5 

14 _________ _40.2 16.7 41.2 17.1 41.9 17.8 42.6 18.6 44.7 21.4 

15 __________ 40.8 15.4 41.7 16.0 42.1 16.8 42.6 17.4 43.7 21.5 

16 __________ 41.8 15.8 42.0 16.4 42.5 16.7 42.8 17.3 43.5 19.9 

17 __________ 41.3 15.9 41.9 16.0 42.4 17.2 42.4 17.6 41.7 20.4 

18 _________ _40.3 19.0 40.8 19.5 40.8 18.2 41.1 19.6 39.9 22.5 

19 __________ 38.6 21.6 38.4 21.9 38.5 22.0 38.7 22.0 38.0 24.1 

20 __________ 36.1 21.8 36.5 22.5 36.3 24.2 36.3 22.3 35.8 24.1 

21 __________ 35.3 22.0 35.2 22.1 35.1 22.2 35.0 22.3 34.5 23.6 

22 __________ 33.7 22.3 33.8 22.4 33.9 22.4 33.6 22.5 33.2 24.3 

23 __________ 32.6 22.3 32.9 22.6 32.6 22.6 32.5 22.9 32.1 24.1 

24 __________ 31.2 23.0 31.4 23.0 31.4 23.0 31.5 22.9 31.1 23.7 

1 __________ 30.5 23.9 30.6 23.5 30.4 23.6 30.2 23.7 29.7 25.0 

2 __________ 30.1 24.4 30.1 24.3 30.2 24.4 29.9 24.5 29.1 25.4 

3 --------- _29.1 24.8 29.3 25.3 29.3 25.1 29.1 24.7 28.0 25.6 

4 __________ 28.9 25.0 28.4 24.9 28.1 24.9 28.0 24.9 26.3 25.4 

8-9 September 1966 

5 __________ 24.2 14.3 23.3 14.4 22.6 14.5 22.0 14.6 18.2 14.8 

6 __________ 23.8 14.2 22.5 14.4 21.8 14.5 21.4 14.6 17.2 15.0 

7 __________ 24.3 15.0 23.9 15.2 23.6 15.3 23.3 15.4 20.1 16.0 

8 __________ 27.5 15.8 27.4 16.0 27.5 16.2 27.3 16.3 26.3 16.5 

9 __________ 30.2 16.3 30.4 16.4 30.6 16.5 30.7 16.7 30.8 16.8 

10 __________ 32.2 17.1 32.6 17.2 133.1 17.3 33.4 17.4 34.2 17.6 

11 __________ 35.0 15.0 35.4 15.5 35.4 15.8 36.3 16.5 37.4 15.3 

12 __________ 36.7 14.4 37.5 14.6 37.7 15.0 38.0 15.5 39.7 14.5 

13 __________ 37.8 14.5 38.1 15.0 138.5 15.4 38.4 15.5 40.7 14.9 

14 ---~------38.3 12.8 38.9 13.2 139.5 14.0 39.6 14.6 41.4 13.6 

15 __________ 37.6 14.2 38.1 14.4 !38.9 14.6 39.6 15.0 40.7 13.9 

16 __________ 38.2 13.4 38.5 13.6 38.9 13.8 39.1 14.1 39.8 13.3 

17 __________ 38.1 13.8 38.5 14.0 38.7 14.2 38.1 14.5 38.2 13.3 

18 __________ 37.6 15.2 37.4 15.4 37.6 15.6 36.7 15.8 33.9 16.6 

19 __________ 35.3 15.8 33.8 16.8 33.2 17.6 32.5 18.2 27.7 20.0 

20 __________ 32.8 15.6 31.9 16.6 131.5 17.3 31.1 17.8 24.7 18.6 

21 __________ 32.8 14.4 31.4 14.7 30.1 15.0 29.0 15.4 23.8 16.9 

22 __________ 31.2 13.6 29.6 13.7 128.5 13.9 27.6 14.3 22.6 16.8 

23 __________ 27.3 15.3 26.1 15.5 25.6 15.7 25.0 16.0 21.9 16.6 

24 __________ 26.6 15.0 25.9 15.2 25.5 15.3 25.1 15.5 22.1 16.1 

1 __________ 25.4 16.4 24.6 16.4 24.2 16.5 23.6 16.5 21.5 16.8 

2 __________ 24.4 15.8 23.3 15.8 22.9 16.0 22.5 16.0 20.2 16.3 

3 __________ 24.4 15.5 23.3 15.5 22.7 15.5 22.1 15.4 19.1 15.8 

4 __________ 24.1 14.6 22.7 14.7 22.1 14.6 21.5 14.6 18.7 15.2 

9-10 September 1966 

5 __________ 23.1 13.8 22.1 14.0 21.8 14.2 21.5 14.1 18.7 14.8 

6 __________ 23.3 14.0 22.4 14.0 22.2 14.1 21.7 14.0 19.2 14.6 

7 ------- ___ 23.3 14.8 22.9 14.9 23.0 15.2 22.8 15.2 21.5 15.4 

8 __________ 26.3 15.7 26.4 15.6 26.5 15.8 16.6 16.0 25.6 16.8 

9 __________ 30.5 15.5 30.7 15.5 30.8 15.8 31.0 15.9 31.6 15.3 

10 __________ 34.0 13.6 34.3 14.5 34.6 15.2 34.7 15.7 35.6 14.3 

11 __________ 35.9 13.9 36.4 14.0 36.6 14.1 37.0 14.2 38.9 14.3 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F67 

TABLE A2-1. Hourly averages of air temperature and vapor pressure at the Buckeye Project during selected periods of 3 days each.-Continued 

1-hr period 11m 7m 5m 4m 2m 

beginning oc mb oc mb oc mb oc mb oc mb 

9-10 September 1966-Continued 

12 __________ 37.5 13.3 37.8 13.8 38.0 14.1 38.4 14.3 40.0 13.6 

13 __________ 38.6 13.0 39.0 13.0 39.3 13.2 39.8 13.4 41.4 13.5 

14 __________ 39.2 13.5 39.3 13.5 39.9 13.6 40.0 13.6 41.7 13.2 

15 __________ 38.5 13.5 38.6 13.8 39.2 14.0 39.0 14.5 39.7 14.7 

16 __________ 37.5 14.1 37.6 14.8 37.8 15.2 38.2 15.5 38.6 15.0 

17 __________ 37.7 14.9 38.0 15.0 38.3 15.2 38.4 15.5 38.6 15.0 

18 __________ 37.3 15.1 36.7 15.4 37.0 15.6 36.5 15.8 35.0 16.0 

19 __________ 34.8 15.5 34.1 15.6 33.6 15.8 33.2 16.0 28.0 17.6 

20 __________ 33.5 15.2 31.1 15.4 29.9 15.8 29.5 16.1 24.9 17.5 

21 __________ 30.6 15.0 28.7 15.2 28.1 15.4 27.5 15.5 23.1 16.6 

22 __________ 29.5 15.1 27.9 15.2 27.3 15.3 26.8 15.3 22.2 16.4 

23 __________ 29.1 15.5 28.2 15.5 28.0 15.6 27.3 15.6 24.0 15.6 

24 __________ 26.3 15.8 25.9 15.7 25.7 15.7 25.3 15.6 23.5 16.0 

1 __________ 24.7 15.6 24.0 15.8 23.7 15.8 23.2 15.8 21.2 16.0 

2 __________ 26.2 13.7 25.4 14.0 24.3 14.6 23.0 15.0 19.2 15.6 

3 __________ 26.1 12.5 24.2 13.0 23.6 13.6 22.8 13.8 19.0 14.4 

4 __________ 25.1 12.3 24.0 12.4 23.4 12.6 22.6 12.8 18.8 13.4 

10-11 September 1966 

5 __________ 23.6 12.5 22.2 12.5 21.6 12.6 21.0 12.7 17.8 13.7 

6 __________ 25.0 12.0 23.1 12.1 22.0 12.2 21.7 12.2 16.8 13.3 

7 --------- _25.1 12.5 24.2 12.5 24.3 12.6 23.7 12.8 19.8 13.8 

8 __________ 27.7 14.1 27.5 14.5 27.8 14.9 27.1 15.7 25.3 15.0 

9 __________ 32.0 13.4 31.5 14.0 31.4 14.3 31.3 14.7 29.3 14.6 

10 __________ 33.9 12.9 34.4 12.9 34.5 15.0 34.3 15.1 32.9 14.9 

11 __________ 35.3 13.4 36.1 13.5 36.8 14.3 37.0 14.6 36.6 14.0 

12 __________ 36.6 13.6 37.0 14.0 37.5 14.2 37.9 14.5 39.8 14.4 

13 __________ 38.3 12.0 38.0 12.4 38.2 13.0 38.9 13.4 41.0 13.8 

14 __________ 38.7 13.3 38.4 13.5 39.7 13.8 39.5 14.0 40.6 13.7 

15 __________ 38.9 13.5 38.2 13.6 38.3 14.0 37.8 14.1 38.1 12.9 

16 __________ 38.3 13.0 37.8 13.1 38.5 13.3 38.0 13.7 36.5 13.8 

17 __________ 38.0 13.0 38.1 13.5 37.8 13.7 37.7 13.8 34.6 15.3 

18 __________ 37.7 13.8 37.1 14.0 36.8 13.8 36.5 14.3 32.4 16.0 

19 __________ 35.6 13.9 34.6 13.8 34.2 13.9 33.6 14.1 27.8 16.8 

20 __________ 33.8 13.6 32.8 13.6 32.1 13.8 31.8 13.8 27.2 14.9 

21 __________ 35.2 12.0 34.6 12.3 33.7 12.6 33.6 11.0 31.4 10.5 

22 __________ 33.6 11.6 32.8 10.0 32.2 12.5 31.9 10.4 29.2 11.0 

23 __________ 32.0 11.5 31.4 10.9 31.0 13.0 30.7 11.3 27.2 13.0 

24 __________ 30.4 12.5 28.9 12.5 28.3 14.5 28.0 12.6 24.3 13.4 

1 __________ 30.5 12.2 29.5 11.6 28.7 13.8 28.8 12.0 24.1 14.0 

2 __________ 27.4 14.9 26.8 13.5 26.2 15.1 25.6 14.4 22.1 15.2 

3 __________ 27.3 17.5 25.9 16.5 24.8 17.8 24.1 16.7 20.6 16.1 

4 __________ 25.6 17.1 24.0 16.6 23.3 17.5 23.0 16.5 20.0 16.7 

23-24 March 1967 

5 ---------- ---- 6 ---------- 8.5 4.5 7.8 5.0 6.5 5.0 5.8 4.8 2.8 5.3 

7 ---------- 9.5 4.5 8.5 4.8 8.2 4.4 8.0 4.5 4.8 5.6 

8 __________ 12.0 5.5 12.0 5.6 12.0 5.5 12.0 5.4 10.5 6.0 

9 __________ 16.5 6.1 16.8 6.4 16.8 5.9 16.8 6.4 17.0 6.5 

10 __________ 20.0 6.0 20.8 6.2 21.0 6.1 21.2 7.1 22.5 6.3 

11 __________ 24.0 5.3 24.5 6.6 24.5 6.2 25.2 6.6 26.5 6.2 

12 __________ 27.0 5.9 27.5 6.2 28.0 5.9 27.8 7.0 30.2 6.2 

13 __________ 29.0 5.8 30.0 6.8 30.0 5.9 30.2 5.8 32.0 5.0 

14 --------- _30.0 4.3 30.5 5.6 30.8 5.8 30.5 5.3 32.0 4.6 

15 --------- _29.5 6.3 29.8 6.9 30.0 6.8 30.8 7.6 31.5 7.0 

16 __________ 30.0 5.1 29.8 6.1 30.0 6.0 30.2 6.7 30.5 7.0 

17 __________ 29.5 6.0 29.8 6.5 29.8 6.0 29.8 6.5 29.2 6.9 

18 ------- ___ 29.0 5.0 29.0 6.3 28.8 5.8 28.5 6.0 25.8 8.2 

19 __________ 26.5 6.2 26.2 7.5 26.0 6.8 25.5 6.9 21.2 7.5 

20 __________ 26.5 5.8 25.2 7.0 24.5 6.6 24.2 6.4 18.5 8.5 

21 __________ 26.0 5.0 24.2 5.6 23.5 5.7 23.2 5.9 17.5 7.6 

22 __________ 25.8 5.1 25.0 5.2 24.8 5.3 24.5 5.5 22.2 5.4 

23 --------- _24.2 4.6 23.8 4.8 23.2 5.2 23.0 5.3 20.5 5.4 

24 __________ 22.0 5.1 21.0 5.6 20.5 5.7 21.0 5.1 17.2 5.3 

1 __________ 18.2 5.4 15.8 7.0 15.2 6.5 15.0 6.6 13.2 6.5 

2 __________ 18.5 5.9 17.0 6.3 15.5 6.4 14.5 6.6 12.5 6.6 

3 __________ 19.2 5.8 18.0 6.2 16.2 6.3 16.0 6.0 12.8 6.4 

4 __________ 18.0 6.2 17.0 6.5 16.5 6.5 16.5 6.1 13.5 6.6 
F68 STUDIES OF EVAPOTRANSPIRATION 

TABLE A2-1. Hourly averages of air temperature and vapor pressure at the Buckeye Project during selected periods of 3 days each.-Continued 

1-hr period 11m 7m 5m 4m 2m 

beginning oc mb oc mb oc mb oc mb oc mb 

24-25 March 1967 

5 __________ 20.2 5.2 18.8 6.1 18.2 5.7 17.5 5.9 14.5 6.4 

6 __________ 19.0 6.0 18.5 6.3 17.8 5.7 16.8 6.4 14.5 6.7 

7 __________ 17.8 6.0 15.2 7.4 14.2 7.1 13.8 7.1 11.0 7.8 

8 __________ 15.8 7.0 15.8 7.3 15.8 7.0 15.8 7.3 15.2 8.4 

9 __________ 18.2 7.9 18.8 8.3 19.0 7.8 19.2 8.8 19.2 8.8 

10 __________ 22.8 6.5 23.0 6.9 23.0 6.9 23.8 7.0 24.8 7.5 

11 __________ 24.2 6.4 25.0 6.3 25.2 6.1 26.0 6.8 28.0 7.2 

12 __________ 25.5 7.1 25.8 7.0 26.0 7.1 27.2 7.7 28.8 7.5 

13 __________ 25.8 6.6 26.2 6.5 26.2 6.7 26.8 7.1 28.5 7.7 

14 __________ 26.5 6.3 27.5 6.3 27.8 6.0 28.0 6.9 29.8 6.0 

15 __________ 27.0 5.0 27.5 5.1 28.2 5.1 28.5 5.3 30.8 5.1 

16 __________ 27.2 5.3 28.0 5.3 28.2 5:4 28.8 5.9 30.5 5.6 

17 __________ 27.0 5.9 27.5 5.9 27.8 5.6 27.8 6.0 27.8 5.7 

18 __________ 26.0 5.6 26.0 6.0 26.0 5.6 26.0 6.5 25.5 6.0 

19 __________ 23.2 4.0 23.2 4.4 23.0 4.2 23.0 4.5 22.0 4.5 

20 __________ 21.2 3.9 21.0 4.4 20.8 4.2 20.5 4.4 18.8 4.6 

21 __________ 20.0 3.2 19.5 3.9 19.0 3.9 19.0 3.8 16.8 3.9 

22 __________ 20.2 3.4 19.8 4.0 19.2 4.0 19.0 4.2 15.8 4.3 

23 __________ 19.0 4.2 18.5 4.5 18.2 4.4 18.2 4.4 15.0 4.7 

24 __________ 17.8 4.9 17.5 5.1 17.2 5.3 17.0 5.9 16.2 4.9 

1 __________ 16.8 5.9 16.8 5.9 16.5 6.1 16.5 6.1 15.8 6.1 

2 __________ 16.0 6.4 15.8 6.5 15.5 6.4 15.2 6.5 14.0 6.7 

3 __________ 14.8 6.5 14.8 6.5 14.5 6.6 14.5 6.6 12.8 6.4 

4 __________ 14.2 6.8 13.8 6.8 13.2 6.8 12.8 7.0 10.8 7.0 

25-26 March 1967 

5 __________ 13.5 7.0 12.5 7.3 11.8 7.0 10.8 6.6 7.5 7.0 

6 __________ 10.2 7.0 8.0 7.3 7.0 7.0 7.0 7.3 6.0 7.0 

7 ---------- 9.8 7.0 8.0 7.3 7.2 7.2 7.0 7.0 5.8 7.1 

8 ----------11.5 7.5 11.8 7.6 12.0 7.2 12.0 7.5 11.2 7.7 

9 __________ 15.2 8.0 15.5 9.1 16.0 8.3 15.8 8.4 16.0 8.3 

10 __________ 17.8 7.8 18.2 8.6 18.2 7.9 18.2 8.4 19.5 8.5 

11 __________ 19.8 7.4 20.2 8.6 20.5 7.5 20.8 8.2 21.5 8.5 

12 __________ 21.5 6.3 22.0 7.7 22.5 6.7 23.0 7.2 24.8 7.6 

13 __________ 22.2 5.8 23.0 6.4 23.2 6.2 24.2 6.7 27.2 6.9 

14 __________ 23.5 6.1 24.0 6.2 24.2 6.1 25.0 6.2 27.5 6.6 

15 __________ 24.8 5.3 25.5 6.0 25.5 5.6 25.5 6.4 28.2 5.8 

16 __________ 24.5 5.1 24.8 5.2 24.5 6.2 24.8 6.0 25.8 5.7 

17 __________ 24.5 5.1 24.8 6.0 24.8 5.7 24.5 6.2 25.0 5.9 

18 __________ 24.0 5.7 24.0 6.9 24.0 6.5 23.5 6.9 22.2 6.9 

19 __________ 22.2 6.2 21.8 7.6 21.0 6.9 20.5 6.9 16.0 7.5 

20 __________ 20.8 5.9 19.2 7.2 19.2 6.9 18.2 6.1 12.5 7.2 

21 __________ 19.2 5.8 18.0 6.6 16.8 6.4 15.8 6.2 10.8 7.4 

22 __________ 17.0 4.8 16.0 5.0 15.0 4.8 14.0 5.0 10.2 5.5 

23 __________ 15.8 4.2 15.0 5.0 14.2 5.3 13.8 4.8 9.5 5.4 

24 __________ 14.0 5.0 13.2 5.1 12.5 5.0 12.2 5.2 8.5 5.3 

1 ----------12.8 5.1 12.0 5.4 11.2 5.1 11.0 5.0 8.2 5.6 

2 __________ 12.2 5.7 10.8 6.1 10.5 6.0 10.0 6.0 6.8 6.2 

3 __________ 11.0 6.2 9.8 6.0 8.2 5.8 7.5 6.3 5.2 6.0 

4 ---------- 9.8 5.2 8.2 6.2 7.5 6.0 7.0 6.1 4.8 6.6 

11-12 May 1967 

5 __________ 14.5 7.4 11.0 7.6 10.3 7.7 9.5 8.2 8.3 7.7 

6 __________ 13.0 7.6 10.3 7.8 10.3 8.0 9.3 8.0 7.5 7.9 

7 __________ 17.0 8.6 16.0 9.0 16.5 9.4 15.3 9.8 12.8 10.6 

8 __________ 19.0 8.9 19.0 9.5 19.3 9.8 18.5 10.0 17.5 10.6 

9 __________ 20.5 7.0 20.8 7.7 20.8 8.5 20.8 9.3 20.8 10.5 

10 __________ 21.8 6.4 22.3 6.8 22.3 7.2 22.5 7.9 23.8 9.8 

11 __________ 22.8 5.7 23.3 5.9 23.5 6.5 23.3 7.7 25.8 8.2 

12 __________ 24.0 5.5 24.5 5.7 24.8 6.0 24.8 6.9 27.3 8.1 

13 __________ 25.3 5.4 25.5 5.6 26.3 5.9 26.3 7.1 28.8 8.9 

14 __________ 25.5 6.0 26.3 6.2 26.8 6.5 27.0 7.0. 29.0 9.1 

15 __________ 26.3 5.9 27.0 6.2 27.3 6.8 27.3 7.3 28.8 8.0 

16 __________ 27.0 5.9 27.3 6.1 27.3 6.3 27.3 6.5 28.3 7.9 

17 __________ 27.3 6.0 27.8 6.2 28.0 6.4 27.3 7.3 27.0 8.4 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F69 

TABLE A2-1. Hourly averages of air temperature and vapor pressure at the Buckeye Project during selected periods of 3 days each. -Continued 

1-hr period 11m 7m 5m 4m 2m 

beginning oc mb oc mb oc mb oc mb oc mb 

11-12 May 1967-Continued 

18 __________ 27.0 6.2 27.3 6.4 27.0 6.6 26.0 7.3 24.5 7.8 

19 --------- _26.0 6.1 25.8 6.2 25.5 6.3 24.8 6.5 23.5 5.8 

20 __________ 24.8 6.5 24.5 6.6 24.3 6.7 23.5 6.8 22.5 6.0 

21 __________ 23.8 6.4 23.5 6.2 23.3 6.0 22.3 6.0 21.3 5.2 

22 __________ 21.8 5.5 21.3 5.7 20.8 5.9 20.0 6.0 17.3 6.3 

23 __________ 16.0 6.8 14.8 6.9 15.0 7.0 14.0 7.0 13.0 6.9 

24 __________ 16.8 6.4 15.0 6.7 14.3 6.7 13.3 6.7 10.8 7.0 

1 __________ 15.5 6.0 14.0 6.2 12.0 6.3 11.8 6.4 8.3 7.1 

2 __________ 17.3 4.9 15.5 5.3 14.0 5.6 12.8 5.8 6.8 7.1 

3 __________ 15.8 4.9 14.8 5.1 14.0 5.4 13.5 5.8 6.5 7.0 

4 __________ 14.8 5.5 13.5 5.6 13.0 5.6 12.8 6.0 6.3 7.1 

12-13 May 1967 

5 ----------15.0 6.1 14.0 6.1 13.3 6.1 12.8 6.1 7.5 7.0 

6 __________ 13.5 5.7 11.5 5.9 11.0 6.2 10.5 6.3 7.8 7.5 

7 __________ 12.3 6.7 12.0 6.9 11.8 7.1 11.5 7.2 10.3 8.0 

8 __________ 15.5 6.7 15.5 6.9 15.5 7.1 15.3 8.0 15.5 8.2 

9 ----------19.3 7.6 19.5 7.8 19.8 8.1 19.8 8.5 20.0 9.0 

10 __________ 22.3 7.6 22.3 7.8 22.8 8.1 22.5 8.6 23.3 9.4 

11 __________ 24.3 7.1 24.8 7.4 25.0 8.0 25.3 8.4 26.3 8.8 

12 __________ 26.0 6.5 26.5 6.6 26.5 7.0 27.0 8.4 28.8 8.9 

13 ----------26.8 6.4 27.0 6.5 27.5 6.6 27.8 8.2 30.5 8.7 

14 __________ 28.3 6.2 28.5 6.5 28.5 6.9 28.8 7.5 31.0 8.4 

15 __________ 28.5 6.5 29.0 6.8 29.3 7.0 29.0 7.5 31.0 8.7 

16 __________ 28.8 6.7 29.0 6.8 29.3 6.9 29.0 5.0 30.0 7.6 

17 ---- ______ 29.0 6.2 29.0 6.3 29.3 6.4 28.5 6.5 28.5 6.1 

18 --------- _28.0 6.0 28.0 4.0 28.0 4.8 27.3 6.0 26.3 5.5 

19 __________ 27.8 5.4 27.5 3.5 27.3 4.5 26.5 5.4 25.0 4.1 

20 --------- _26.3 5.1 25.5 3.8 25.0 4.1 24.0 5.0 21.3 4.5 

21 __________ 23.0 5.4 21.3 5.0 20.8 4.9 20.0 5.7 15.3 6.1 

22 __________ 20.5 5.5 19.0 5.0 17.8 5.0 17.0 5.5 11.5 6.4 

23 __________ 20.3 5.1 18.0 5.6 17.5 5.5 16.8 6.0 11.8 6.1 

24 --------- _20.0 5.7 18.3 5.4 17.8 5.3 17.0 5.8 13.0 6.0 

1 ------- ___ 19.3 5.8 17.8 5.3 17.0 5.1 15.5 5.8 11.0 6.0 

2 __________ 17.3 5.3 15.8 5.2 14.5 6.5 13.5 5.7 9.0 6.6 

3 --------- _16.3 5.2 15.4 4.6 15.0 5.0 14.3 5.5 10.8 5.7 

4 ---- ____ 15.3 5.1 17.0 3.5 14.0 5.0 13.3 5.4 10.3 5.8 

13-14 May 1967 

5 __________ 14.8 5.1 14.0 5.2 13.3 5.3 12.8 5.5 9.0 6.3 

6 __________ 15.0 5.0 14.3 5.0 13.8 5.1 13.3 5.5 10.0 5.6 

7 __________ 17.0 5.5 16.8 5.6 16.8 5.6 16.3 6.6 14.0 7.4 

8 __________ 19.3 5.8 19.0 6.0 19.3 6.2 19.0 6.6 18.3 7.5 

9 __________ 21.0 6.6 21.0 6.6 21.3 6.7 21.3 7.9 21.5 9.4 

10 __________ 23.3 7.0 23.8 7.0 23.8 7.1 24.0 8.5 25.0 9.0 

11 ----------25.3 6.5 25.5 6.5 25.8 6.6 26.0 8.5 27.8 9.5 

12 __________ 26.3 5.9 27.0 6.1 27.8 6.6 27.8 7.4 29.5 7.6 

13 __________ 26.8 5.5 27.3 5.7 28.3 6.2 29.0 6.6 30.8 6.8 

14 __________ 27.3 5.3 28.0 6.0 28.8 6.2 28.8 6.4 30.8 7.6 

15 __________ 27.3 6.0 28.0 6.1 28.5 6.4 28.5 6.5 30.5 6.1 

16 __________ 27.0 5.9 27.8 6.1 28.0 6.4 27.8 6.5 29.0 6.2 

17 __________ 27.5 6.4 27.5 4.4 27.5 5.1 27.5 5.9 27.5 5.9 

18 __________ 27.0 6.2 27.0 4.6 27.0 5.5 26.5 6.2 26.0 5.0 

19 __________ 24.8 6.5 24.8 5.0 24.8 5.3 24.0 6.2 23.0 5.3 

20 __________ 22.0 6.2 22.0 4.7 21.8 4.6 21.0 5.4 20.0 5.0 

21 __________ 19.5 5.6 19.3 4.7 19.0 5.0 18.5 5.3 16.8 5.0 

22 __________ 20.0 5.3 19.3 4.1 19.0 4.3 18.3 4.9 16.5 4.5 

23 __________ 16.8 5.2 16.3 4.6 16.0 4.8 15.5 5.1 15.0 4.4 

24 __________ 17.5 5.2 16.5 4.1 16.0 4.4 15.0 5.0 12.3 5.1 

1 __________ 17.3 4.6 15.3 4.5 14.3 4.5 13.3 4.8 9.5 5.4 

2 __________ 13.3 4.8 10.5 5.7 9.8 5.2 9.0 5.0 6.3 5.6 

3 ----------12.8 4.3 11.0 4.2 9.0 4.5 8.0 4.5 4.3 5.6 

4 __________ 13.5 4.4 11.3 4.0 10.3 4.0 7.5 4.0 3.8 4.9 
~ 

-:) 

TABLE A3-1.-Total solar short-wave radiation, albedo, total long- and short-wave net radiation, and soil heatflow 0 

[Solar radiation, R,; albedo, Rr; net radiation, Rn; and soil heatflow, S; all in milliwatts per square centimeter] 

R, Rr Rn s 1-hr. period 

beginning Day: 5/6 617 7/8 m 5/6 617 7/8 m 5/6 6/7 7/8 m 5/6 617 7/8 m 

5-8 May 1966 5 --------------- 0 0 0 0 0 0 0 0 -4.18 -4.18 -4.88 -4.41 0.77 0.63 0.91 0.77 

6 4.18 5.58 5.58 5.11 -1.39 -1.39 -1.39 -1.39 -0.70 -2.79 -2.09 -1.86 0.42 0.91 0.63 0.65 

7 20.92 22.31 25.80 23.01 -4.88 -5.58 -6.28 -5.58 9.76 9.76 12.55 10.69 -0.41 -0.28 -0.28 0.33 

8 ----------------- 41.84 41.84 33.47 39.05 -9.06 -9.06 -7.67 -8.60 25.10 25.00 20.22 23.71 -1.81 -1.74 -1.12 -1.56 

9 61.36 61.36 62.76 61.83 -11.16 -11.16 -11.85 -11.39 41.84 43.23 43.23 42.77 -3.83 -3.48 -3.21 -3.51 

10 ------------ 76.70 77.40 77.40 77.17 -12.55 -12.55 -13.25 -12.78 57.18 56.48 57.18 56.95 -3.48 -3.48 -2.44 -3.13 

11 87.16 86.47 85.77 86.47 -13.25 -13.25 -13.25 -13.25 65.55 63.45 68.34 65.78 -2.86 -3.00 -4.11 -3.32 

12 ----------- 92.74 92.74 72.52 85.77 -12.55 -13.25 -11.85 -12.55 68.33 78.79 52.99 66.94 -2.37 -2.58 -1.88 -2.30 

13 92.74 92.04 92.74 92.74 -12.55 -13.25 -13.95 -13.25 52.30 76.01 78.79 69.03 -1.46 -1.67 -2.16 -1.74 

14 ----------------- 57.18 83.68 87.86 76.01 -8.37 -12.55 -13.25 -11.16 76.70 64.15 69.03 69.73 -1.74 -2.44 -2.23 -2.16 

15 46.72 71.12 55.78 57.88 -11.85 -11.16 -10.46 -11.16 55.78 51.60 43.23 50.21 -2.51 -1.53 -2.09 -2.02 

16 ------------ 57.18 53.69 46.02 52.30 -9.76 -9.76 -9.06 -9.76 38.35 36.26 29.98 34.87 -1.04 -0.98 -1.18 -1.04 

17 36.26 36.96 33.47 35.56 -7.67 -7.67 -6.28 -6.97 17.43 18.13 16.04 17.43 -0.91 -0.84 -1.05 -0.91 

18 11.16 15.34 11.16 12.55 -2.09 -4.18 -1.39 -2.79 -1.39 2.79 2.79 1.39 -0.48 -0.42 -0.56 -0.49 

19 -------------- 3.49 4.18 2.79 3.49 -0.70 -1.39 -0.70 -0.70 -4.88 -4.18 -3.49 -4.18 0.07 0.28 -0.21 0.07 

20 0 0 0 0 0 0 0 0 -5.58 -5.58 -6.28 -5.58 0.49 0.77 0.07 0.42 

21 ----------------- 0 0 0 0 0 0 0 0 -3.49 -3.49 -5.58 -4.18 0.49 0.84 0.42 0.56 rJJ 

22 0 0 0 0 0 0 0 0 -9.06 -4.18 -6.28 -6.28 0 0.91 0.28 0.42 ...., 

23 --------------- 0 0 0 0 0 0 0 0 -6.97 -4.18 -6.97 -6.28 0.28 0.84 0.21 0.42 c 24 ------------------ 0 0 0 0 0 0 0 0 -4.88 -4.18 -8.37 -5.58 0.77 0.84 0 0.56 u 1 -------------- 0 0 0 0 0 0 0 0 -4.88 -4.18 -7.67 -5.58 0.77 0.63 0.07 0.49 ;;; 2 . ---------------- 0 0 0 0 0 0 0 0 -4.18 -4.88 -4.88 -4.88 0.84 0.56 0.42 0.63 

3 0 0 0 0 0 0 0 0 -4.18 -4.18 -2.79 -3.49 0.91 0.91 0.56 0.77 rJJ --------------

4 ------------ 0 0 0 0 0 0 0 0 -4.18 -4.18 -0.70 -2.79 0.84 0.91 0.63 0.77 0 Total ___________________ 689.63 800.50 693.12 709.15 -117.84 -126.21 -120.63 -120.63 449.76 476.26 434.42 453.94 -16.31 -13.45 -17.98 -16.03 ..., 

M 

< 

> 

0 

...., 

"' 

:A:l 

> z 

rJJ 

"' 1-hr. period R, R, Rn s ;; 

beginning Day: 3/4 4/5 5/6 m 3/4 4/5 5/6 m 3/4 4/5 5/6 m 3/4 4/5 5/6 m > ...., 

4-6 June 1966 0 5 ------------------- 0 0 0 0 0 0 0 0 -5.58 -5.58 -5.58 -5.58 1.25 1.25 1.05 1.18 z 6 -------------------- 9.06 7.67 9.06 8.37 -2.09 -1.39 -1.39 -1.39 -0.70 0 0 0 0.84 0.91 0.77 0.84 7 ----------- __ 29.98 27.89 27.89 28.59 -7.67 -6.97 -6.97 -6.97 13.95 12.55 13.25 13.25 0.14 0 0 0.07 8 49.51 48.11 48.81 48.81 -1Ll6 -11.16 -11.16 -11.16 29.98 29.29 29.98 29.98 -0.35 -0.42 -0.42 -0.42 

9 ---------------- 69.03 68.34 67.64 68.34 -13.25 -13.95 -13.25 -13.25 46.02 46.02 46.02 46.02 -1.25 -1.81 -1.81 -1.60 10 84.37 83.68 82.28 83.68 -14.64 -14.64 -13.95 -14.64 60.67 61.36 59.27 60.67 -1.74 -1.67 -1.60 -1.67 11 --------------- 94.14 92.74 92.04 92.74 -15.34 -15.34 -15.34 - 15.34 70.43 70.42 70.42 70.43 -1.74 -2.16 -2.02 -1.95 

12 - ----------- 99.71 98.32 97.62 98.32 -16.04 -16.04 -16.04 -16.04 73.91 73.91 73.22 73.91 -2.16 -2.44 -2.30 -2.30 13 99.71 98.32 98.32 99.02 -16.04 -16.04 -16.04 -16.04 73.22 72.52 73.22 73.22 -1.25 -1.25 -1.39 -1.32 14 ---------------- 91.35 90.65 90.65 90.65 -15.34 -15.34 -15.34 -15.34 72.92 72.52 65.55 70.42 -2.93 -3.62 -3.62 -3.42 

15 - 78.79 83.68 76.01 79.49 -13.95 -13.95 -13.95 -13.95 55.09 53.69 54.39 54.39 -1.25 -1.39 -1.60 -1.39 16 60.67 60.67 62.06 61.36 -11.85 -11.85 -11.85 -11.85 38.35 38.35 38.35 38.35 -0.91 -0.98 -1.05 -0.98 17 ---------------- 39.75 41.84 41.84 41.14 -9.76 -9.76 -9.76 -9.76 20.22 19.52 20.22 20.22 -0.63 -0.77 -0.91 -0.77 18 21.62 22.31 13.95 19.52 -6.28 -6.28 -3.49 -5.58 2.79 2.79 0 2.09 0 -0.56 -0.56 -0.35 

19 ----------- 4.88 4.88 4.18 4.88 -1.39 -1.39 -0.70 -1.39 -9.06 -9.06 -4.18 -7.67 0.07 -0.21 0 -0.07 

20 ------------------- 0 0 0 0 0 0 0 0 -9.76 -9.76 -0.70 -6.97 0.49 0.35 0.49 0.42 

21 ---------- 0 0 0 0 0 0 0 0 -8.37 -7.67 -6.28 -7.67 0.77 0.70 0.70 0.70 

22 ------------- 0 0 0 0 0 0 0 0 -6.97 -6.97 -4.88 -6.28 0.91 0.98 0.91 0.91 

23 ----------------- 0 0 0 0 0 0 0 0 -6.28 -6.28 -4.18 0.77 1.05 0.91 0.91 

24 ---------------------- 0 0 0 0 0 0 0 0 -6.28 -6.28 -4.88 -6.28 1.18 0.98 0.98 1.05 

1 ------------------- 0 0 0 0 0 0 0 0 -5.58 -4.88 -5.58 -5.58 1.25 1.05 1.05 1.12 2 ------------------ 0 0 0 0 0 0 0 0 -6.28 -4.88 -6.28 -5.58 1.12 1.12 0.98 1.05 

3 ----------- 0 0 0 0 0 0 0 0 -4.88 -5.58 -4.88 -4.88 1.12 1.05 1.05 1.05 

4 -------------------- 0 0 0 0 0 0 • 0 0 -5.58 -4.88 -4.88 -4.88 1.12 1.05 1.05 1.05 Total ____________________ 832.58 829.09 812.35 824.91 -154.80 -154.10 -149.22 -152.71 481.83 481.14 495.78 487.41 -3.35 -6.83 -7.39 -5.92 
TABLE A3-1.-Total solar short-wave radiation, albedo, total long- and short-wave net radiation, and soil heatflow-Continued 

R, R, R., 

1-hr period 

beginning Day: 617 7/8 8/9 iii. 6/7 7/8 8/9 m 617 7/8 8/9 iii 617 7/8 

7-9 July 1966 

5 0.70 0 0 0 0 0 0 0 -4.88 -3.49 -5.58 -4.88 1.12 0.63 6 7.67 7.67 2.79 6.28 -1.39 -1.39 -0.70 -1.39 -0.70 -0.70 -2.09 -1.39 0.91 0.35 7 26.50 26.50 23.71 25.80 -6.97 -6.28 -5.58 -6.28 11.16 12.55 11.85 11.85 0.14 -0.14 8 ----- 46.92 37.65 42.54 42.54 -11.16 -9.06 -9.76 -9.76 27.89 17.43 27.89 24.41 -0.42 -0.63 9 - 65.55 29.29 60.67 51.60 -13.25 -6.28 -11.85 -13.95 43.93 25.10 43.93 37.65 -2.65 -0.63 10 - ----- 80.19 82.28 76.01 79.49 -14.64 -13.95 -13.95 -13.95 56.48 56.48 57.88 57.18 -1.39 -0.91 11 89.29 87.16 86.47 87.86 -16.04 -15.34 -15.34 -15.34 65.55 66.94 69.03 66.94 -1.67 -1.53 12 ---- 94.14 89.25 90.65 91.35 -16.74 -12.55 -15.34 -14.64 69.73 70.43 71.82 89.25 -1.95 -1.74 

13 --- - 94.14 63.45 91.35 82.98 -16.04 -12.55 -16.04 -14.64 68.34 48.81 71.82 62.76 -1.60 -1.05 14 ------------ 86.47 89.95 85.77 87.16 -18.13 -16.74 -15.34 -16.74 61.36 66.94 66.24 64.85 -2.37 -2.30 15 ---------------- 70.43 73.91 74.61 73.22 -12.55 -13.95 -13.95 -13.95 47.42 52.99 55.78 52.30 -2.16 -1.32 16 -------------------- 50.21 52.99 59.97 54.39 -8.37 -10.46 -11.16 -9.76 19.52 27.19 39.75 28.59 -1.18 -0.84 17 ------------ 39.75 15.34 36.26 30.28 -9.06 -3.49 -8.37 -6.97 16.03 2.79 19.52 12.55 -1.12 -0.42 

18 ---------------------- 19.52 12.55 23.01 18.13 -5.58 -2.09 -6.28 -4.88 2.09 2.79 8.37 4.18 -0.84 -0.28 19 ---------------- 5.58 4.88 6.97 5.58 -1.39 -0.70 -2.09 -1.39 -7.67 -2.79 -2.79 -4.18 -0.49 -0.28 

20 -------------------- 0 0 0 0 0 0 0 0 -10.46 -6.28 -6.97 -7.67 -0.28 -0.07 

21 -------------------------- 0 0 0 0 0 0 0 0 -9.76 -4.18 -5.58 -6.28 -0.07 0.07 

22 ------ ------------------- 0 0 0 0 0 0 0 -9.76 -4.18 -4.88 -6.28 -0.14 0.28 

23 --- ---------------------- 0 0 0 0 0 0 0 0 -6.97 -3.49 -6.97 -5.58 0.14 0.28 

24 ------------ ------------- 0 0 0 0 0 0 0 0 -4.88 -4.88 -6.28 -5.58 0.49 0.49 

1 ------------- ------------ 0 0 0 0 0 0 0 0 -4.88 -4.18 -4.88 -4.88 0.42 0.49 

2 ----------------- 0 0 0 0 0 0 0 0 -4.88 -5.58 -4.88 -4.88 0.49 0.28 

3 ----- ----------. 0 0 0 0 0 0 0 0 -4.88 --5.58 -5.58 -5.58 0.63 0.14 

4----- 0 0 0 0 0 0 0 0 -4.88 -5.58 -4.18 -4.88 0.63 0.21 Total ___________________ 776.79 672.89 760.75 737.05 -151.31 -124.82 -145.74 -142.95 414.89 399.55 483.23 450.46 -13.59 -8.78 

R, R, R., 

1-hr period 

beginning Day: 3/4 4/5 5/6 iii 3/4 4/5 5/6 iii 3/4 4/5 5/6 iii 3/4 4/5 

4-6 August 1966 

5 -------- ------------ 0 0 0 0 0 0 0 0 -4.88 -4.18 -3.49 -4.18 0.07 0.21 6 ----------------- 2.79 2.09 2.79 2.79 -5.58 -0.70 -2.09 -2.09 -1.39 -0.70 -1.39 0 0.28 7 ---------- 23.01 17.43 18.13 19.52 -6.97 -4.88 -4.88 -5.58 12.55 9.06 9.76 10.46 -0.49 0 8 - ------------------------ 30.68 38.35 36.26 34.87 -8.37 -9.76 -8.37 -9.06 20.92 25.10 21.62 22.31 -0.77 -0.49 9 -------------------------- 31.38 56.48 56.48 48.11 -11.16 -11.85 -11.85 -11.85 29.29 42.54 41.14 37.65 -1.12 -1.46 10 -------------------------- 50.90 70.43 68.34 63.45 -9.06 -13.25 -12.55 -11.85 48.81 56.48 52.30 52.30 -1.60 -1.88 11 -------------------------- 55.09 78.79 82.98 72.52 -9.06 -13.95 -14.64 -12.55 44.63 64.85 65.55 58.57 -1.46 -1.67 12 -------------------------- 51.60 80.19 85.77 72.52 -7.67 -14.64 -15.34 -12.55 36.26 66.24 68.35 57.18 -1.53 -1.67 13 -------------------------- 73.22 86.47 76.01 78.79 -12.55 -13.95 -14.64 -13.95 50.21 68.34 65.55 61.36 -1.67 -1.88 14 -------------------------- 52.99 78.79 63.45 65.85 -11.16 -12.55 -11.85 -11.85 42.54 63.45 49.51 51.60 -1.67 -1.95 15 ------------------------ 41.84 65.55 41.84 49.51 -8.37 -10.46 -8.37 -9.06 33.47 50.21 28.59 37.65 -1.32 -1.81 16 -------------------------- 39.75 50.90 36.96 42.54 -8.37 -7.67 -7.67 -7.67 29.98 35.56 26.50 30.68 -1.18 -1.53 17 -------------------------- 18.13 32.77 24.41 25.10 -4.18 -4.16 -5.58 -4.88 9.76 20.22 15.34 15.34 -0.84 -1.32 18 - ------------------------ 12.55 14.64 14.64 13.95 -3.47 -0.70 -3.49 -2.79 4.88 4.18 6.28 4.88 -0.63 -0.98 19 -------------------------- 0.70 1.39 2.09 1.39 -0.70 0 -3.49 -1.39 -4.18 -4.88 2.09 -2.09 -0.49 -0.63 20 ------------------------------ 0 0 0 0 0 0 -0.70 0 -6.97 -5.58 -4.18 -5.58 -0.28 -0.14 21 ------------------------------ 0 0 0 0 0 0 0 0 -6.28 -6.28 -6.97 -6.28 -0.28 -0.21 22 ------------------------------ 0 0 0 0 0 0 0 0 -5.58 -4.88 -6.97 -5.58 -0.14 0 23 ------------------------------ 0 0 0 0 0 0 0 0 -6.28 'm5.58 -4.88 -5.58 0 -0.14 24 ------------------------------ 0 0 0 0 0 0 0 0 -6.28 -6.97 -4.88 -6.28 0 0 1 ------------------------------ 0 0 0 0 0 0 0 0 -4.88 -6.97 -4.88 -5.58 0.07 -0.07 2 ------------------------------ 0 0 0 0 0 0 0 0 -4.88 -5.58 -4.88 -4.88 0.07 0.07 3 -------- ...... __________________ 0 0 0 0 0 0 0 0 -4.18 -''4.88 -4.88 -4.88 0.14 0.07 4 ------------------------------ 0 0 0 0 0 0 0 0 0 -4.18 -4.18 -4.88 0.21 0.21 -4.18 -14.92 -17.01 Total ________________________ 484.62 674.29 610.14 589.92 -106.69 -118.54 123.42 -117.15 302.63 444.88 400.95 383.52 

s 

8/9 

0.21 

0.28 

-0.07 

-0.28 

-1.74 

-0.84 

-1.32 

-1.60 

-0.98 

-2.09 

-1.46 

-0.77 

-0.63 

-0.56 

-0.28 

0 

0.14 

0.07 

0 

0.07 

0.07 

0.07 

0 

0.14 -11.57 

s 

5/6 

0.35 

0.28 

-0.07 

-0.42 

-1.32 

-1.60 

-1.53 

-1.60 

-1.74 

-1.67 

-1.53 

-1.32 

-1.18 

-0.98 

-0.98 

-0.70 

-0.42 

-0.28 

-0.14 

-0.07 

0 

0.07 

0.07 

0.07 -16.73 

iii. 

0.63 

0.49 

0 -0.42 

-1.67 

-1.05 

-1.53 

-1.75 

-1.18 

-2.23 

-1.67 

-0.91 

-0.70 

-0.56 

-0.35 

-0.07 

0.07 

0.07 

0.14 

0.35 

0.35 

0.28 

0.28 

0.35 -11.08 

iii 

0.21 

0.21 

-0.21 

-0.56 

-1.32 

-1.67 

-1.53 

-1.60 

-1.74 

-1.74 

-1.53 

-1.32 

-1.12 

-0.84 

-0.70 

-0.35 

-0.28 

-0.14 

-0.07 

0 

0 

0.07 

0.07 

0.14 -16.03 

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TABLE A3-1.-Total solar short-wave radiation, albedo, total long- and short-wave net radiation, and soil heat/low-Continued 

R. Rr Rn s 1-hr period 

beginning Day: 8/9 9/10 10/11 ffi 8/9 9/10 10/11 m 8/9 9/10 10/11 m 8/9 9/10 

9-11 September 1966 

5 ------ -- ----------------- 0 0 0 0 0 0 0 0 -5.58 -6.28 -6.97 -6.27 0.91 0.77 6 -------------------------- 1.39 0.70 0.70 0 0 0 0 0 -4.88 -5.58 -6.28 -5.58 0.91 0.70 

7 -------------------------- 11.85 6.97 11.85 10.46 -4.18 -2.09 -4.18 -3.49 -2.79 -1.39 2.09 -2.09 0.42 0.42 8 -------------------------- 32.77 30.68 32.77 32.08 -9.06 -8.37 -9.06 -9.06 16.04 16.74 18.13 16.74 0.49 -0.42 9 -------------------------- 52.30 50.21 51.60 51.60 -11.85 -11.85 -11.85 -11.85 32.08 30.68 36.96 33.47 -0.84 -0.70 

10 -------------------------- 67.64 66.24 67.64 66.94 -13.25 -13.25 -13.25 -13.25 46.02 46.02 52.30 48.11 -2.16 -2.23 

11 -------------------------- 78.10 78.10 77.40 78.10 -13.95 -13.95 -13.95 -13.95 55.09 55.78 62.06 57.88 -1.32 -1.46 12 -------------------------- 82.98 78.10 82.98 81.58 -13.95 -13.95 -14.64 -13.95 58.57 55.09 68.34 60.67 -1.67 -1.53 

13 -------------------------- 82.28 75.31 82.28 80.19 -13.95 -13.95 -14.64 -13.95 59.27 58.57 68.34 62.06 -2.02 -2.16 

14 -------------------------- 73.22 68.34 70.43 70.43 -13.25 -11.85 -11.16 -11.85 52.99 47.42 49.51 50.21 -1.81 -1.81 

15 -------------------------- 62.06 41.84 37.65 47.42 -11.85 -8.27 -7.67 -9.06 42.84 25.80 31.38 32.77 -1.18 -1.12 

16 -------------------------- 67.72 32.08 22.31 33.47 -9.76 -6.97 -3.49 -6.97 25.80 19.52 5.58 16.74 -0.91 -0.84 

17 -------------------------- 24.41 24.41 15.34 21.62 -6.97 -7.67 -3.49 -6.28 7.67 12.55 3.49 7.67 -0.56 -0.63 

18 -------------------------- 8.37 7.67 4.88 6.97 -2.39 -2.79 -2.09 -2.79 -4.18 -4.18 -4.88 -4.18 -0.14 -0.14 

19 ---------------------------- 0 0 0 0 0 0 0 0 -6.97 -8.37 -7.67 -7.67 0.42 0.35 20 ---------------------------- 0 0 0 0 0 0 0 0 -6.28 -6.97 -8.37 -6.97 0.77 0.77 21 -- ---- ------ -- -- 0 0 0 0 0 0 0 0 -6.28 -6.28 -11.16 -7.67 0.84 0.84 22 ----------------- 0 0 0 0 0 0 0 0 -5.58 -6.97 -10.46 -7.67 0.91 0.84 23 ---------------------------- 0 0 0 0 0 0 0 0 -5.58 -7.67 -8.37 -6.97 0.91 0.63 

24 ---------------------------- 0 0 0 0 0 0 0 0 -6.97 -8.37 -4.88 -6.97 0.77 0.56 1 --------- ------ --- ---- 0 0 0 0 0 0 0 0 -6.28 -6.97 -4.88 -6.28 0.70 0.77 

2 ---------------------------- 0 0 0 0 0 0 0 0 -6.97 -6.28 -6.28 -6.28 0.84 0.98 

3 ------- -------------------- 0 0 0 0 0 0 0 0 -6.28 -6.28 -6.97 -6.28 0.98 0.91 

4 -- ------------------------- 0 0 0 0 0 0 0 0 -6.28 -6.97 -6.28 ---- 6.28 0.91 0.91 Total __ ----------------------624.08 560.63 557.84 581.55 -124.82 -115.05 -109.48 -116.45 314.48 279.62 304.72 299.84 -1.88 -3.62 

R, Rr Rn s 1-hr period 

beginning Day 23/24 24/25 25/26 m 23/24 24/25 25/26 iii 23/24 24/25 25/26 iii 23/24 24/25 

24-26 March 1967 

5 ------------------------------ 0 0 0 0 0 0 0 0 -4.88 -7.67 -6.28 0.56 6 ------------------------------ 0 0 0 0 0 0 0 0 -6.28 -6.97 -7.67 -6.97 1.18 0.63 7 -------------------------- 3.49 3.49 6.28 4.18 -1.39 -0.70 -1.39 -1.39 -3.49 -2.79 -2.79 -2.79 0.77 0.63 8 -------------------------- 20.92 21.62 23.71 22.31 -5.58 -6.28 -6.97 -6.28 6.97 13.95 8.37 9.76 0 -0.14 9 -------------------------- 48.81 51.60 44.63 48.11 -10.46 -110.46 -11.16 -10.46 29.29 27.19 25.80 27.19 -0.56 -0.63 10 -------------------------- 62.76 62.76 62.76 62.76 -12.55 -11.85 -12.55 -12.55 41.14 41.84 42.54 41.84 -0.84 -0.77 11 -------------------------- 76.01 78.79 76.01 76.70 -13.95 -13.95 -13.95 -13.95 52.99 57.18 54.39 55.09 -1.32 -1.25 12 -------------------------- 81.58 60.67 82.98 75.31 -13.95 -9.76 -13.95 -12.55 58.57 36.26 60.67 51.60 -3.83 -1.95 13 -------------------------- 61.36 56.48 83.68 66.94 -11.16 -9.06 -13.95 -11.16 40.44 36.26 62.06 46.02 -2.44 -1.67 14 -------------------------- 47.42 61.36 67.64 58.57 -9.06 -11.85 -11.85 -11.16 32.08 44.63 57.18 44.63 -1.74 -2.23 15 -------------------------- 43.23 61.36 66.24 57.18 -8.37 -11.16 -12.55 -10.46 24.41 35.56 45.32 34.87 -1.32 -1.32 16 -------------------------- 24.41 46.02 41.84 37.65 -4.18 -9.06 -9.06 -7.67 10.46 27.89 22.31 20.22 -0.70 -0.77 17---------------------------- 13.95 22.31 24.41 20.22 -2.79 -5.58 -5.58 -4.88 2.79 3.49 5.58 4.18 -0.28 0.07 18 -------------------------- 4.88 7.67 9.06 6.97 -0.70 -2.79 -2.79 -2.09 -3.49 -6.28 -4.18 -4.88 0.21 0.28 19 ------------------------------ 0 0 0.70 0 0 0 0 0 -6.97 -11.16 -9.76 -9.06 0.63 0.56 20 ------------------------------ 0 0 0 0 0 0 0 0 -6.28 -9.76 -8.37 -8.37 0.70 0.70 21 ------------------------------ 0 0 0 0 0 0 0 0 -9.06 -6.97 -5.58 -6.97 0.70 0.70 22 ------------------------------ 0 0 0 0 0 0 0 0 -11.85 -9.06 -6.97 -9.06 0.21 0.77 23 ------------------------------ 0 0 0 0 0 0 0 0 -11.16 -10.46 -6.97 -9.76 0.28 0.70 24 ------------------------------ 0 0 0 0 0 0 0 0 -9.06 -11.16 -7.67 -9.06 0.63 0.63 1 ------------------------------ 0 0 0 0 0 0 0 0 -7.67 -11.16 -7.67 -9.06 0.91 0.63 2 ------------------------------ 0 0 0 0 0 0 0 0 -5.58 -9.76 -6.28 -6.97 0.84 0.77 3 ------------------------------ 0 0 0 0 0 0 0 0 -4.88 -9.06 -6.28 -6.97 0.77 0.91 4 ------------------------------ 0 0 0 0 0 0 0 0 -4.88 -8.37 -5.58 -6.28 0.56 0.98 Total ___ ----- ________________ 488.81 534.13 589.92 536.92 -94.14 -102.50 -115.75 104.60 208.49 206.40 290.77 232.90 -4.67 -1.25 

10/11 

0.98 

1.05 

0.56 

-0.56 

-0.56 

-1.60 

-1.25 

-1.60 

-2.23 

-1.60 

-0.98 

-0.91 

-0.28 

0.07 

0.49 

0.63 

0.14 

0.21 

0.35 

0.56 

0.56 

0.77 

0.84 

0.84 

-3.55 

25/26 

1.25 

1.18 

0.98 

-0.14 

-0.21 

-0.35 

-0.77 

-2.51 

-2.44 

-2.16 

-1.39 

-0.42 

0.14 

0.35 

1.05 

1.25 

1.12 

1.18 

1.12 

0.98 

0.98 

0.98 

1.18 

1.12 

-4.75 

m 

0.91 

0.91 

0.49 

-0.14 

-0.70 

-2.02 

-1.32 

-1.60 

-2.16 

-1.74 

-1.12 

-0.91 

-0.49 

-0.14 

0.42 

0.70 

0.63 

0.63 

0.63 

0.63 

0.70 

0.84 

0.91 

0.91 

-3.07 

iii 

0.91 

0.98 

0.77 

0 

-0.49 

-0.63 

-1.12 

-2.79 

-2.16 

-2.02 

-1.32 

-0.63 

0 

0.28 

0.77 

0.91 

0.84 

0.70 

0.70 

0.77 

0.84 

0.84 

0.98 

0.91 

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TABLE A3-l.-Total solar short-wave radiation, albedo, total long- and short-wave net radiation, and soil heatflow-Continued 

1-hr period 

beginning Day 

R 

11112 

5 ------ ------------------ 0 6 4.88 

7 --------------- ---------- 24.41 8 -------------- ------ 46.02 

9 -------------------------- 66.24 10 ---------------------- 83.68 11 ----------- 94.14 

12 --------100.41 13 --------- 99.71 14 ----- 87.86 

15 ------- 73.22 

17 ----------- -------------- 59.97 18 ----------------- 38.35 18 ----------------- 14.64 

19 ------------ ------------- 2.09 20 ---- ------------------ 0 21 -------------- 0 

22---- 0 23-- 0 

24--- 0 

1 --------------- -------------- 0 2 ----------- -------------- 0 3 ----------- ----------- 0 4 ------------ 0 Total ______ _ __________ 795.62 

12/13 

0 

4.88 

10.46 

43.23 

66.24 

80.89 

91.35 

98.32 

98.32 

89.25 

76.01 

58.57 

34.87 

12.55 

4.88 

0 

0 

0 

0 

0 

0 

0 

0 

0 769.92 

13/14 iii 

0 0 4.88 4.88 

23.71 19.52 43.93 42.93 

64.85 65.55 80.89 81.58 

91.35 92.04 

98.32 99.02 98.32 99.02 

89.95 89.25 77.40 75.31 59.97 59.27 39.75 37.65 

18.83 15.34 

3.49 3.49 0 0 0 0 0 0 

0 0 0 0 0 0 0 0 0 0 0 0 795.62 785.86 

R, 

11/12 12/13 13/14 

0 0 0 -1.39 -0.70 -1.39 

-6.28 -2.09 -6.28 -11.16 -12.55 -11.16 

-14.64 -14.64 -14.64 -15.34 -15.34 -15.34 

-16.04 -16.04 -16.04 -17.43 -17.43 -17.43 -17.43 -17.43 -17.43 

-16.74 -16.04 -16.74 

-14.64 -14.64 -15.34 

-13.95 -13.25 -13.95 

-11.16 -10.46 -11.16 -5.58 -3.49 -6.97 -1.39 -2.09 -1.39 

0 0 0 0 0 0 

0 0 0 0 0 0 

0 0 0 

0 0 0 0 0 0 

0 0 0 0 0 0 -163.87 -156.20 -165.26 

R, s 

rn. 11112 12/13 13/14 rn. 11/12 12/13 

12-14 May 1967 

0 -6.27 -6.97 -7.67 -6.97 1.12 1.05 -1.39 -2.09 -2.79 -4.18 -2.79 1.12 0.91 -4.88 9.76 1.39 9.06 6.97 0.42 0.56 -11.85 28.59 25.10 25.80 26.50 -0.21 -0.14 -14.64 46.72 44.63 43.93 45.32 -1.05 -1.12 -15.34 62.76 59.97 58.57 60.67 -0.70 -1.05 

-16.04 71.82 69.73 69.03 70.43 -0.77 -1.05 

-17.43 76.01 74.61 71.82 73.91 -1.39 -1.46 -17.43 73.22 73.91 70.43 72.52 -1.05 -1.39 -16.74 62.02 66.24 64.15 64.15 -1.95 -2.09 -14.64 50.21 57.18 51.60 52.99 -0.63 -1.12 

-13.95 37.65 36.96 36.26 36.96 -0.49 -0.63 

-11.16 18.83 18.83 18.13 18.83 -0.35 -0.49 -5.58 0.70 -2.09 0 -0.70 -0.70 -0.14 -1.39 -10.46 -8.37 -11.16 -9.76 0.14 -0.07 

0 -11.85 -10.46 -12.55 -11.85 0.21 0.28 0 -11.85 -8.37 -11.16 -10.46 0.28 0.84 

0 -9.76 -7.67 -11.16 -9.76 0.35 1.12 0 -7.67 -8.37 -10.46 -9.06 0.91 0.91 u -6.97 -8.37 -9.06 -8.37 1.12 0.70 

0 -6.28 -6.97 -7.67 -6.97 1.18 0.91 

0 -5.58 -6.28 -6.28 -6.28 1.25 1.05 0 -6.28 -7.67 -6.28 -6.97 1.25 0.63 0 -6.28 -7.67 -5.58 -6.28 1.18 0.70 -162.47 446.97 436.51 415.59 433.02 2.02 -1.12 

13/14 m. 

0.77 0.98 0.63 0.91 0.14 0.35 

-0.35 -0.21 -1.12 -1.12 

-1.05 -0.91 -1.05 -0.98 -1.39 -1.39 -1.32 -1.25 

-1.95 -2.02 -0.84 -0.84 -0.49 -0.56 

-0.48 -0.35 -0.14 -0.14 

0.21 0.07 0.49 0.35 0.70 0.63 

0.56 0.70 

0.70 0.84 0.77 0.84 

1.05 1.05 

1.32 1.18 1.39 1.12 1.39 1.12 

0.14 0.35 

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F74 STUDIES OF EVAPOTRANSPIRATION 

TABLE A4-1.-Windspeed, temperature, and vapor pressure at five elevations above ground during seven periods of 3 days each at the Buckeye 

Project 

[Windspeed, u in m s- 1

, temperature, T in oc; vapor pressure, e in kPa; and elevations, z in m] 

Day 1 Day 2 Day 3 Mean 

Hrs u T u T u T u T 

5-8 May 1966 

5-8 3.32 2.21 1.15 1.88 22.1 1.11 1.74 22.0 1.06 2.31 22.1 1.11 11 

2.46 21.6 1.18 1.48 21.4 1.15 1.34 21.1 1.12 1.76 21.4 1.15 7 

1.96 21.5 1.19 1.27 21.4 1.18 1.18 20.9 1.17 1.47 21.3 1.18 5 

1.56 21.1 1.20 1.04 20.8 1.21 0.93 20.2 1.20 1.18 20.7 1.20 4 

0.63 19.4 1.36 0.35 18.4 1.39 0.21 18.4 1.40 0.40 18.7 1.38 2 

9-12 4.64 32.2 1.06 1.95 30.4 1.11 1.34 31.7 1.08 2.64 31.4 1.08 11 

3.96 32.6 1.17 1.69 30.6 1.20 1.14 31.6 1.22 2.26 31.6 1.20 7 

3.27 32.9 1.25 1.50 30.8 1.27 1.10 31.7 1.32 1.96 31.8 1.28 5 

2.74 33.5 1.30 1.32 30.9 1.32 0.94 31.1 1.39 1.67 31.8 1.34 4 

1.23 35.3 1.52 0.82 32.4 1.60 0.60 31.9 1.80 0.88 33.2 1.64 2 

13-16 2.65 35.8 1.00 2.16 36.0 1.00 2.52 36.6 0.94 2.44 36.1 0.98 11 

2.31 36.0 1.14 1.87 36.2 1.14 2.12 36.8 1.07 2.10 36.3 1.12 7 

2.03 36.5 1.25 1.65 36.2 1.22 1.80 37.0 1.17 1.83 36.6 1.21 5 

1.77 36.6 1.33 1.42 36.4 1.33 1.53 36.6 1.24 1.57 36.5 1.30 4 

0.84 37.9 1.55 0.84 '37.8 1.69 0.76 37.2 1.87 0.81 37.6 1.70 2 

17-20 2.47 33.6 1.09 1.93 34.6 1.16 3.73 34.3 1.02 2.71 34.2 1.09 11 

2.05 33.3 1.16 1.50 33.9 1.28 2.99 33.7 1.11 2.18 33.6 1.18 7 

1.63 32.7 1.20 1.18 33.3 1.38 2.37 33.3 1.18 1.73 33.1 1.25 5 

1.23 32.1 1.24 0.94 32.7 1.44 1.92 33.0 1.23 1.36 32.6 1.30 4 

0.48 29.9 1.56 0.28 29.2 1.84 0.58 31.3 1.62 0.45 30.1 1.67 2 

21-24 3.28 28.8 1.02 1.21 27.6 1.15 3.95 30.2 0.84 2.81 28.9 1.00 11 

2.58 28.0 1.06 1.02 25.8 1.20 3.06 29.5 0.89 2.22 27.8 1.05 7 

2.10 27.4 1.08 1.00 25.0 1.24 2.48 29.0 0.92 1.86 27.1 1.08 5 

1.76 27.1 1.10 0.79 24.4 1.27 2.08 28.7 0.95 1.54 26.7 1.11 4 

0.41 24.0 1.38 0.11 18.6 1.52 0.60 26.3 1.22 0.37 23.0 1.37 2 

1-4 2.13 24.2 1.04 2.21 23.6 1.02 2.29 26.2 0.92 2.21 24.7 0.99 11 

1.61 22.8 1.08 1.69 22.1 1.06 1.78 24.8 0.96 1.69 23.2 1.03 7 

1.22 22.1 1.10 1.28 21.4 1.08 1.41 24.0 1.00 1.30 22.5 1.06 5 

0.98 2.12 1.11 0.96 20.8 1.10 1.15 23.8 1.01 1.03 21.9 1.07 4 

0.10 16.2 1.29 0.09 16.6 1.32 0.25 20.7 1.26 0.15 17.8 1.29 2 

3-6 June 1966 

5-8 2.16 18.8 0.64 2.36 19.2 0.62 2.06 21.8 0.79 2.19 19.9 0.68 11 

1.58 18.4 0.69 1.81 18.8 0.68 1.70 20.4 0.83 1.70 19.2 0.73 7 

1.32 18.1 0.72 1.50 18.6 0.73 1.42 19.8 0.86 1.41 18.8 0.77 5 

1.02 17.4 0.75 1.16 18.0 0.76 1.09 19.2 0.88 1.09 18.2 0.80 4 

0.31 14.7 0.90 0.43 14.8 0.90 0.43 16.3 1.02 0.39 15.3 0.94 2 

9-12 2.10 29.6 0.58 2.40 29.5 0.66 2.37 30.5 0.84 2.29 29.9 0.69 11 

1.82 29.7 0.68 2.13 29.6 0.76 2.11 30.9 0.94 2.02 30.1 0.79 7 

1.62 30.0 0.75 1.76 29.6 0.84 1.82 31.0 1.02 1.73 30.2 0.87 5 

1.36 29.8 0.80 1.54 29.6 0.90 1.58 30.9 1.06 1.49 30.1 0.92 4 

0.78 30.6 0.96 0.84 30.8 1.00 0.84 31.8 1.26 0.82 31.1 1.07 2 

13-16 3.52 34.7 0.32 3.55 35.0 0.42 3.83 36.2 0.45 3.63 35.3 0.40 11 

3.06 34.8 0.45 3.06 35.1 0.58 3.37 36.2 0.59 3.16 35.4 0.54 7 

2.63 35.2 0.54 2.59 35.6 0.71 2.88 36.5 0.70 2.70 35.8 0.65 5 

2.28 35.0 0.60 2.24 35.1 0.80 2.48 36.1 0.78 2.33 35.4 0.73 4 

0.87 35.5 0.83 0.85 36.2 0.93 0.91 36.4 1.04 0.88 36.0 0.93 2 

17-20 3.71 32.7 0.41 3.59 33.8 0.44 2.90 33.8 0.59 3.40 33.4 0.48 11 

2.90 32.5 0.54 2.87 33.6 0.56 2.21 33.4 0.72 2.66 33.2 0.61 7 

2.40 32.2 0.64 2.35 33.3 0.66 1.74 33.1 0.80 2.16 32.9 0.70 5 

1.92 31.6 0.69 1.91 32.3 0.71 1.35 32.2 0.86 1.73 32.0 0.75 4 

0.49 28.8 0.80 0.46 29.6 0.83 0.32 28.8 1.05 0.42 29.1 0.89 2 

21-24 1.82 24.8 0.57 2.10 25.6 0.62 1.54 26.4 0.71 1.82 25.6 0.63 11 

1.44 23.2 0.60 1.52 23.8 0.66 1.16 25.2 0.75 1.37 24.1 0.67 7 

1.34 21.8 0.63 1.23 22.3 0.70 1.07 23.9 0.78 1.21 22.7 0.70 5 

0.99 20.9 0.64 0.89 21.1 0.73 0.73 23.0 0.81 0.87 21.7 0.73 4 

0.06 14.7 0.84 0.07 15.5 0.89 0.07 16.2 1.03 0.07 15.5 0.92 2 

1-4 1.57 22.0 0.44 1.43 22.4 0.56 1.88 22.3 0.59 1.63 22.2 0.53 11 

1.42 20.5 0.46 1.16 20.2 0.60 1.36 20.7 0.64 1.31 20.5 0.57 7 

1.21 18.5 0.49 1.09 18.4 0.63 1.17 19.9 0.66 1.16 18.9 0.59 5 

0.85 17.3 0.50 0.78 16.8 0.64 0.82 18.7 0.69 0.82 17.6 0.61 4 

0.07 10.2 0.68 0.12 11.5 0.86 0.09 12.4 0.86 0.09 11.4 0.80 2 

6-9 July 1966 

5-8 1.30 22.6 0.69 2.46 28.0 1.48 2.14 27.1 2.02 1.97 25.9 1.40 11 

1.02 21.8 0.72 1.95 27.1 1.55 1.68 26.8 2.04 1.55 25.2 1.44 7 

1.03 21.8 0.71 1.73 26.8 1.55 1.42 26.7 2.05 1.39 25.1 1.43 5 

0.80 21.9 0.79 1.41 26.4 1.57 1.10 26.4 2.06 1.10 24.6 1.47 4 

0.15 18.6 0.89 0.24 23.8 1.70 1

0.20 24.9 2.15 0.20 22.4 1.58 2 
WEATHER AND EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F75 

TABLE A4-1.-Windspeed, temperature, and vapor pressure at five elevations above ground during seven periods of 3 days each at the Buckeye 

Project. -Continued 

Da,Y 1 Day 2 Day3 Mean 

Hrs u T u T u T u T 

6-9 July 1966-Continued 

9-12 1.61 36.6 0.51 4.69 34.4 1.31 2.47 32.2 1.84 2.92 34.4 1.22 11 

1.42 36.6 0.62 4.06 34.9 1.42 2.17 32.4 1.89 2.55 34.6 1.31 7 

1.34 37.0 0.67 3.49 35.3 1.51 1.90 33.0 1.92 2.24 35.1 1.36 5 

1.14 36.4 0.83 2.93 35.3 1.57 1.63 33.0 1.97 1.90 34.9 1.46 4 

0.51 37.1 0.95 0.80 35.8 1.71 0.57 33.8 2.11 0.63 35.6 1.59 2 

13-16 4.28 42.3 0.49 5.74 38.3 1.05 3.13 36.6 1.41 4.38 39.1 0.98 11 

3.64 42.0 0.62 4.79 38.7 1.17 2.73 36.9 1.48 3.72 39.2 1.09 7 

3.04 43.0 0.72 4.08 38.8 1.23 2.30 37.1 1.52 3.14 39.6 1.16 5 

2.52 42.7 0.82 3.51 38.9 1.29 1.96 37.4 1.57 2.66 39.7 1.23 4 

0.85 44.0 0.70 0.90 40.0 1.35 0.75 39.0 1.70 0.83 41.0 1.25 2 

17-20 4.29 40.6 0.60 3.32 36.4 0.98 3.59 36.7 1.40 3.73 37.9 0.99 11 

3.40 40.0 0.74 2.65 36.2 1.09 2.91 36.6 1.46 2.99 37.6 1.10 7 

2.88 39.9 0.79 2.14 35.9 1.13 2.42 36.4 1.50 2.48 37.4 1.15 5 

2.39 39.6 0.88 1.70 35.2 1.21 2.00 36.3 1.52 2.03 37.0 1.20 4 

0.50 37.1 0.86 0.60 33.0 1.24 0.54 33.8 1.64 0.55 34.6 1.25 2 

21-24 2.91 33.9 0.76 2.16 31.7 1.12 3.34 34.2 1.48 2.80 33.3 1.12 11 

2.21 32.6 0.85 1.53 30.6 1.18 2.58 32.9 1.54 2.11 32.0 1.19 7 

1.77 31.8 0.88 1.22 29.9 1.20 2.09 32.2 1.59 1.69 31.3 1.22 5 

1.42 31.1 0.94 0.94 29.2 1.24 1.69 31.5 1.63 1.35 30.6 1.27 4 

0.14 27.0 0.97 10.21 25.5 1.33 0.28 28.2 1.67 0.21 26.9 1.32 2 

1-4 1.54 27.7 1.28 2.84 27.8 1.61 2.60 30.8 1.81 2.33 28.8 1.57 11 

1.22 26.6 1.25 2.16 27.2 1.62 1.97 30.2 1.85 1.78 28.0 1.57 7 

1.09 25.3 1.21 1.66 26.5 1.65 1.69 30.0 1.86 1.48 27.3 1.57 5 

0.78 24.6 1.20 1.29 26.0 1.66 1.40 29.6 1.88 1.16 26.7 1.58 4 

0.11 20.4 1.22 1

0.11 22.9 1.70 1

0.11 27.6 1.88 0.11 23.6 1.60 2 

3-6 August 1966 

5-8 2.18 32.8 2.09 2.22 28.0 2.14 1.17 28.7 2.14 1.86 29.8 2.12 11 

1.74 32.8 2.15 1.79 28.1 2.16 0.90 28.3 2.22 1.48 29.7 2.18 7 

1.45 32.8 2.20 1.44 28.0 2.20 0.74 28.0 2.24 1.21 29.6 2.21 5 

1.18 32.4 2.24 1.11 27.8 2.22 0.62 28.0 2.25 0.97 29.4 2.24 4 

0.61 30.3 2.32 0.37 27.2 2.23 0.27 27.0 2.33 0.42 28.2 2.29 2 

9-12 2.63 37.8 2.09 1.90 35.8 2.03 2.22 36.7 2.01 2.25 36.8 2.04 11 

2.23 38.1 2.16 1.60 36.6 2.07 1.89 37.1 2.12 1.91 37.3 2.12 7 

1.92 38.6 2.22 1.43 36.8 2.13 1.62 37.8 2.18 1.66 37.7 2.18 5 

1.68 39.2 2.25 1.28 37.0 2.16 1.44 38.1 2.24 1.47 38.1 2.22 4 

0.58 39.4 2.41 0.64 38.5 2.35 0.67 39.8 2.38 0.63 39.2 2.38 2 

13-16 6.38 39.6 2.02 2.99 40.9 1.78 3.27 40.7 1.66 4.21 40.4 1.82 11 

5.19 40.0 2.08 2.53 41.8 1.84 2.72 41.5 1.73 3.48 41.1 1.88 7 

4.34 40.5 2.14 2.14 42.4 1.89 2.29 42.1 1.78 2.92 41.7 1.94 5 

3.81 40.8 2.17 1.85 42.9 1.91 1.98 42.4 1.84 2.55 42.0 1.97 4 

1.02 41.9 2.32 0.66 44.8 2.14 0.62 44.1 2.13 0.77 43.6 2.20 2 

17-20 6.19 36.8 2.16 3.35 41.0 1.52 5.96 39.1 1.97 5.17 39.0 1.88 11 

4.93 37.3 2.18 2.66 40.8 1.59 4.75 39.4 2.00 4.11 39.2 1.92 7 

4.05 37.2 2.21 2.14 40.7 1.64 3.94 39.6 2.03 3.71 39.2 1.96 5 

3.47 37.0 2.24 1.81 40.4 1.68 3.42 39.6 2.04 2.90 39.0 1.99 4 

0.90 36.6 2.30 0.44 38.0 2.06 0.90 38.8 2.28 0.75 37.8 2.21 2 

21-24 3.72 33.8 2.09 3.15 36.4 1.71 5.73 33.2 2.25 4.20 34.5 2.02 11 

2.90 33.6 2.08 2.47 35.7 1.74 4.74 33.3 2.25 3.37 34.2 2.02 7 

2.32 33.4 2.10 2.02 35.5 1.78 4.01 33.2 2.26 2.78 34.0 2.05 5 

1.90 33.2 2.12 1.64 35.2 1.81 3.46 33.2 2.26 2.33 33.9 2.06 4 

0.54 32.2 2.23 0.55 33.0 1.99 0.78 32.7 2.42 0.62 32.6 2.21 2 

1-4 2.31 28.9 2.08 2.34 30.8 1.95 2.23 29.6 2.45 2.29 29.8 2.16 11 

1.58 28.7 2.10 1.77 30.4 1.96 1.67 29.6 2.45 1.67 29.6 2.17 7 

1.40 28.4 2.12 1.40 30.3 1.98 1.53 29.5 2.45 1.44 29.4 2.18 5 

1.05 28.2 2.14 1.11 30.0 2.00 1.08 29.3 2.44 1.08 29.1 2.19 4 

0.40 27.4 2.26 0.40 28.6 2.15 0.29 28.3 2.54 0.36 28.1 2.32 2 

8-11 September 1966 

5-8 2.42 24.9 1.48 2.97 24.0 1.46 2.18 25.3 1.28 2.52 24.7 1.41 11 

1.87 24.4 1.50 2.22 23.5 1.48 1.54 24.3 1.29 1.88 24.1 1.42 7 

1.43 23.6 1.51 1.78 23.4 1.48 1.24 23.9 1.31 1.48 23.6 1.43 5 

1.09 23.5 1.52 1.39 23.2 1.48 0.92 23.4 1.34 1.13 23.4 1.44 4 

0.33 20.4 1.56 0.57 21.2 1.54 0.19 19.9 1.39 0.36 20.5 1.50 2 

9-12 3.88 33.5 1.57 3.79 34.5 1.41 2.29 34.7 1.33 3.32 34.2 1.44 11 

3.27 34.0 1.59 3.09 34.8 1.44 1.98 34.8 1.36 2.78 34.5 1.46 7 

2.89 34.4 1.62 2.64 35.5 1.48 1.76 35.0 1.44 2.43 34.9 1.50 5 

2.33 34.6 1.65 2.25 35.4 1.50 1.60 35.1 1.47 2.06 35.0 1.54 4 

1.02 35.5 1.61 0.89 36.5 1.44 0.60 37.2 1.45 0.84 35.8 1.50 2 

13-16 2.87 38.0 1.37 3.28 38.4 1.35 2.77 38.5 1.32 2.97 38.3 1.35 11 

2.40 38.4 1.41 2.73 38.6 1.36 2.39 38.4 1.33 2.51 38.5 1.37 7 

2.06 38.9 1.44 2.33 39.0 1.40 2.04 38.7 1.35 2.14 38.9 1.40 5 
F76 STUDIES OF EVAPOTRANSPIRATION 

TABLE A4-l.-Windspeed, temperature, and vapor pressure at five elevations above ground during seven periods of 3 days each at the Buckeye 

Project -Continued 

Day 1 Day 2 Day 3 Mean 

Hrs u T u T u T u T 

8-1.1 September 1996-Continued 

1.79 39.2 1.48 1.99 39.2 1.42 1.82 38.6 1.38 1.87 39.0 1.43 4 

0.76 40.6 1.39 0.86 40.3 1.46 0.72 39.0 1.36 0.78 40.0 1.40 2 

17-20 1.71 36.0 1.51 2.39 35.8 1.52 2.01 35.8 1.36 2.04 35.9 1.46 11 

1.32 35.4 1.57 1.87 35.0 1.54 1.66 35.6 1.37 1.62 35.3 1.49 7 

1.11 35.2 1.62 1.44 34.7 1.56 1.37 35.2 1.38 1.31 35.1 1.52 5 

0.85 34.6 1.66 1.19 34.4 1.59 1.11 34.9 1.40 1.05 34.6 1.55 4 

0.62 31.1 1.71 0.37 31.6 1.65 0.25 30.5 1.58 0.4~ 31.1 1.65 2 

21-24 2.01 29.4 1.47 2.48 28.9 1.53 2.75 32.8 1.19 2.25 29.2 1.50 11 

1.61 28.2 1.48 1.70 27.7 1.54 2.06 31.9 1.09 1.65 28.0 1.51 7 

1.32 27.4 1.50 1.37 27.0 1.55 1.60 31.3 1.31 1.35 27.2 1.52 5 

0.95 26.7 1.54 0.98 26.7 1.55 1.28 31.0 1.13 1.97 26.7 1.54 4 

0.33 22.6 1.66 0.34 23.2 1.61 0.42 28.0 1.20 0.33 22.9 1.63 2 

1-4 2.64 24.6 1.56 2.40 25.5 1.35 2.20 27.7 1.54 2.52 25.0 1.46 11 

1.71 23.5 1.56 1.74 24.4 1.38 1.52 26.6 1.46 1.72 24.0 1.47 7 

1.41 23.0 1.57 1.34 23.6 1.42 1.13 25.8 1.60 1.38 23.3 1.49 5 

1.06 22.4 1.56 0.97 22.9 1.44 0.78 25.4 1.49 1.01 22.7 1.50 4 

0.24 19.9 1.60 0.11 !9.6 1.48 0.03 21.7 1.55 0.18 19.7 1.54 2 

23-26 March 1967 

5-8 2.62 10.0 0.48 1.69 18.2 0.60 1.86 11.2 0.71 2.06 13.1 0.60 11 

2.07 9.4 0.51 1.51 17.1 0.68 1.37 10.1 0.74 1.65 12.2 0.64 7 

1.63 8.9 0.50 1.30 16.5 0.64 0.98 9.5 0.71 1.30 11.6 0.61 5 

1.40 8.6 0.49 1.03 16.0 0.67 0.94 9.2 0.71 1.12 11.3 0.62 4 

0.75 5.2 0.56 13.6 0.73 7.6 0.72 0.75 8.8 0.67 2 

9-12 2.42 21.9 0.58 3.30 23.0 0.70 1.59 18.6 0.76 2.40 21.2 0.68 11 

1.97 22.4 0.63 2.55 23.1 0.71 1.38 19.0 0.85 1.97 21.5 0.73 7 

1.71 22.6 0.60 2.38 23.3 0.70 1.34 19.3 0.76 1.81 21.7 0.69 5 

1.66 22.8 0.68 2.26 24.0 0.76 1.22 19.7 0.80 1.71 22.2 0.75 4 

1.22 24.0 0.63 1.32 25.2 0.78 0.71 20.4 0.82 1.98 23.2 0.74 2 

13-16 2.28 29.6 0.54 5.40 26.6 0.58 2.86 23.8 0.56 3.51 26.7 0.56 11 

1.86 30.0 0.63 4.02 27.3 0.58 2.45 24.3 0.60 2.78 27.2 0.60 7 

1.59 30.2 0.61 3.84 27.6 0.58 2.12 24.3 0.60 2.52 27.4 0.60 5 

1.41 30.4 0.63 3.57 28.0 0.63 1.95 24.9 0.63 2.31 27.8 0.63 4 

0.64 31.5 0.59 1.42 29.9 0.61 0.93 27.2 0.62 1.00 29.5 0.61 2 

17-20 1.70 27.9 0.58 4.98 24.3 0.49 1.31 22.9 0.57 2.66 25.0 0.55 11 

1.29 27.5 0.68 3.70 24.4 0.52 1.08 22.4 0.69 2.03 24.8 0.63 7 

0.92 27.3 0.63 3.19 24.4 0.49 1.10 22.2 0.65 1.74 24.5 0.59 5 

0.81 27.0 0.64 2.96 24.3 0.54 0.90 21.7 0.65 1.56 24.3 0.61 4 

26.2 0.79 1.17 23.5 0.52 18.9 0.69 1.17 22.9 0.67 2 

21-24 3.54 24.5 0.50 3.72 19.2 0.39 2.19 16.5 0.49 3.15 20.1 0.46 11 

2.49 23.5 0.53 2.77 18.8 0.44 1.74 15.5 0.54 2.33 19.3 0.50 7 

1.61 23.0 0.55 2.22 18.4 0.44 1.57 14.6 0.54 1.80 18.7 0.51 5 

1.12 22.9 0.55 1.95 18.3 0.46 1.42 13.9 0.53 1.50 18.4 0.51 4 

0.64 19.4 0.59 0.90 15.9 0.45 9.8 0.59 0.77 15.0 0.54 2 

1-4 1.40 18.5 0.56 3.20 15.4 0.64 2.16 11.4 0.55 2.29 15.1 0.58 11 

1.20 17.0 0.65 2.58 15.3 0.64 1.78 9.4 0.59 1.85 13.9 0.63 7 

1.12 15.9 0.64 1.82 14.9 0.65 1.55 -9.3 0.57 1.50 13.4 0.62 5 

13.0 0.65 1.22 13.3 0.66 6.2 0.59 1.12 10.8 0.63 2 

11-14 May 1967 

5-8 1.40 15.9 0.81 1.90 14.1 0.63 3.37 16.5 0.53 2.22 15.5 0.66 11 

0.98 14.1 0.85 1.61 13.2 0.64 2.68 16.0 0.54 1.76 14.4 0.68 7 

0.80 14.1 0.87 1.38 12.9 0.66 2.21 15.8 0.55 1.46 14.3 0.69 5 

0.61 13.2 0.90 1.01 12.5 0.69 1.88 15.3 0.61 1.17 13.7 0.73 4 

0.21 11.5 0.92 0.53 10.3 0.77 0.66 12.8 0.67 0.47 11.5 0.79 2 

9-12 3.04 22.3 0.62 2.48 23.0 0.72 3.98 24.0 0.63 3.17 23.1 0.66 11 

2.56 22.7 0.65 2.20 23.4 0.74 3.21 24.3 0.65 2.66 23.5 0.68 7 

2.16 22.8 0.70 1.87 23.5 0.78 2.73 24.4 0.68 2.25 23.6 0.72 5 

1.76 23.2 0.80 1.73 23.6 0.84 2.32 24.8 0.81 1.94 23.9 0.82 4 

0.67 24.4 0.92 0.70 24.6 0.90 0.74 26.0 0.89 0.70 25.0 0.90 2 

13-16 4.15 26.0 0.58 5.00 28.1 0.64 5.06 27.1 0.57 4.74 27.1 0.60 11 

3.46 26.5 0.60 4.16 28.4 0.66 4.08 27.8 0.60 3.90 27.6 0.62 7 

2.84 26.9 0.64 3.54 28.7 0.69 3.37 28.4 0.63 3.25 28.0 0.65 5 

2.47 27.0 0.72 3.14 28.7 0.75 2.92 28.5 0.65 2.84 28.1 0.71 4 

0.76 28.7 0.85 0.90 30.6 0.84 0.92 30.3 0.67 0.86 29.9 0.79 2 

17-20 3.71 26.3 0.62 4.70 27.8 0.57 4.81 25.4 0.63 4.41 26.5 0.61 11 

3.05 26.4 0.64 3.58 27.5 0.44 4.10 25.4 0.47 3.58 26.4 0.52 7 

2.46 26.2 0.65 3.16 27.4 0.50 3.50 25.3 0.54 3.04 26.3 0.56 5 

2.15 25.4 0.70 2.69 26.6 0.57 3.04 24.8 0.59 2.63 25.6 0.62 4 

0.62 24.4 0.70 0.59 25.3 0.51 0.69 24.1 0.53 0.63 24.6 0.58 2 

21-24 2.74 19.6 0.63 2.92 21.0 0.54 3.61 18.4 0.53 3.09 19.7 0.57 11 

2.15 18.7 0.64 2.15 19.2 0.52 2.87 17.8 0.44 2.39 18.6 0.53 7 
WEATHER A:"JD EVAPOTRANSPIRATION STUDIES IN A SALTCEDAR THICKET, ARIZONA F77 

TABLE A4-1.-Windspeed, temperature, and vapor pressure at fi~e elevation.s above ground during seven periods of 3 days each at the Buckeye 

ProJect -Contmued 

Day 1 Day 2 Day 3 Mean 

Hrs u T u T u T u T 

11-14 May 1967-Continued. 

1.68 18.0 0.64 1.56 18.5 0.52 

1.42 17.4 0.64 1.18 17.7 0.58 

0.24 15.9 0.63 - 12.9 0.62 

1-4 1.55 15.8 0.53 3.09 17.0 0.54 

1.24 14.4 0.56 2.31 16.0 0.46 

0.98 13.2 0.57 1.82 15.1 0.54 

0.83 12.7 0.60 1.47 14.2 0.56 

0.07 7.0 0.71 10.3 0.60 

1 Estimated from mean of other days. 

APPENDIX B 

EXPLANATION OF THE NOMOGRAM OF FIGURE 

20 

Whenever the energy-balance method is used, heat 

flow is often measured by means of a thermopile voltage as in heat-flow plates and net radiometers. 

The voltage needs a correction for temperature of the 

plate, and the instrument has a calibration factor (K) 

so that: 

where His the heat flow, 

E is the voltage, 

CA-l) 

F1 is the correction factor for temperature. 

When records are analysed nonautomatically the 

process of converting voltage into heat-flow units becomes quite tedious, especially if data are recorded on 

strip charts. 

Equation CA-l) suggests the possibility of a nomogram to convert voltage directly into heat flow. Taking 

logarithms, 

log H =logE +log F1 -log K. (A-2) 

The graphs for temperature correction are usually 

identical for sets of instruments used in any one heat 

flow study, which makes it possible to draw a scale 

converting thermocouple voltage directly into F1• The 

calibration factor, K, varies from instrument to instrument. 

Choosing the geometric logarithmic moduli, m, n, 

and k for the variables logE, log F1, and log H +log K, 

the geometry of the nomogram can be expressed by the 

determinant: 

-m km logE 1 

n kn log F; 1 = 0. (A-3) 

0 [ kmn (log H + log K)] m + n 

A nomogram is possible if we can insert the correction 

fa~to:, K, conveniently. This can be accomplished by 

shifting the H scale up or down according to size of K. 

2.32 17.5 0.46 1.85 18.0 0.54 5 

1.96 16.8 0.51 1.55 17.3 0.58 4 

0.30 15.2 0.48 0.27 14.7 0.58 2 

1.54 14.2 0.45 2.06 15.7 0.51 11 

1.28 12.0 0.46 1.61 14.1 0.49 7 

1.04 11.1 0.46 1.28 13.1 0.52 5 

0.91 9.4 0.46 1.07 12.1 0.54 4 

6.0 0.54 0.07 7.8 0.62 2 

Trial shows that a convenient modulus for the F1 line 

is 125 em, but theE and H lines need two moduli each 

to accommodate the range of readings, namely 12.5 

and 25 em forE and 11.375 and 20.833 em for the H. 

The distances between E and H are 2. 727 and 5 em, 

respectively, and betweenH andF1, 27.273 and 25 em. 

The H scales, which slide in grooves, can be set according to the size of K (which has the same modulus as 

H). A rack and pinion slides a transparent arm along 

the F1 scale on the right, (fig. 20). Sliding the other end 

of the arm over the E scale at the left, the heat flow in 

proper units can be read immediately on the H scales 

which are the second and third lines from the left. The 

H scales are pasted on steel strips held in plac{! by 

magnets. The nomogram is mounted on a drawing 

board standing upright behind the data pad of the 

analyst. 

APPENDIX C 

SYMBOLS AND DIMENSIONS 

The numbers in parentheses refer to the equations 

where the symbol first appeared. 

Dimensions: H = heat, L = length, M = mass, 

T = time, 8 = temperature, 0 = dimensionless. 

Symbol Dimensions Description 

a 0 empirical constant (9) 

a L amplitude (page 20) 

b 0 empirical constant (9) 

c 0 empirical constant (10) 

c L 2 T-2 e- 1 specific heat (34) 

(or H M- 1 e- 1) 

Cv do specific heat at 

constant pressure (13) 

d L zero-plane displacement (33) 

da M L - 1 T- 2 vapor-pressure deficit (22) 

= es - ea 

e do vapor pressure (8) 
F78 STUDIES OF EVAPOTRANSPIRATION 

Symbol Dimensions Description Symbol Dimensions Description 

ea do e in the air above the surface K L2 T-1 eddy transfer coefficient 

(8) KA do K for sensible heat (13) 

eo do e at the water surface (8) Kc do K for carbon dioxide (15) 

es do saturation vapor pressure, See Km do K for momentum (12) 

da Kl, do K for water vapor (13) 

k 0 von Karman constant (16) L L2 T-2 latent heat of vaporization 

p M L-1 T-2 atmospheric pressure (18) (or H M-1) (17) 

q MM-1 humidity of the air (3) M L moisture in the atmosq' do deviation from mean humidity phere (2) 

(4) \l·M L divergence or convergence of 

r T L-1 evapotranspiration resis- M (2) 

tance (24) Mv M L-3 moisture content of the soil by 

ra do external r (24) volume (38) 

rs do stomatal r (25) Mw MM-1 moisture content of the soil by 

r 0 correlation coefficient (fig. 43) weight (38) 

t T time (34) N 0 empirical constant (11 c) 

u L T-1 horizontal windspeed (8) p L precipitation (1) 

u* do friction velocity (30) p T period (40) 

w do vertical speed of air movement Ri L2 T-2 Richardson number ( 45), sta-

(3) bility ratio in equation 45 

w' do eddy component of w (4) Rn M T-3 net radiation (17) 

z L vertical height or depth (12, (or M L - 2 T-1) 

(35) RO L runoff (1) 

Za do z in the air above the surface s L soil moisture (1) 

(23) ~s L change inS (1) 

Zo do roughness length (23) SR 8 L-2 T-2 stability ratio (27) 

A M T-3 flux of sensible heat (13) T 8 temperature (13) 

(or H L - 2 T-1) v M L-2 T-1 water-vapor flux (14) 

Bv M T-3 the advective energy part in w L precipitable water in the atequation 22 (23) mosphere (2) 

c M L-3 carbon dioxide content (15) dW L change in W (2) 

Dd L damping depth ( 42) a angle phase angle (page 20) 

E L evaporation (1) f3 0 Bowen ratio (18) 

E M L-2 T-1 evaporative flux (16) y M L - 1 T-2 8-1 psychrometer constant (19) 

Ea L actual or measured evapo- d M L - 1 T-2 8-1 slope of the saturation vapor 

transpiration (25) pressure versus the temperE* a L f(u) (eo - ea) (21) ature curve (de8 /d°C) (21) 

Eo L evaporation from open water E 0 water-air molecular ratio (23) 

(8) YJ L2 T-1 thermal diffusivity of soils 

F M L-2 T-1 flux (3) (34) 

Fe do flux of carbon dioxide (15) A H L - 1 8-1 T-1 Thermal conductivity of soils 

G H L-2 T-2 heat flow into or out of the (or M L 8-1 T-3 ) (34) 

(or M T-3 ) ground (17) p M L-3 density 

Gz do heat flow at depth z (35) Pa do density of air (3) 

H H L-2 T-2 sum of energy input at the p~ do deviations from mean density 

(or M T-3 ) surface, except A and LE (4) 

(22) T M L-1 T-2 shearing stress or momentum 

Ho do net gain of radiation at the transfer (12, 20) 

water surface (21)