Water Use by Saltcedar and by Replacement 

Vegetation in the Pecos River Floodplain 

Between Acme and Artesia, New Mexico 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 491-G 

Prepared in cooperation with the 

U.S. Bureau of Reclamation 

Water Use by Saltcedar and by Replacement 

Vegetation in the Pecos River Floodplain 

Between Acme and Artesia, New Mexico 

By EDWIN P. WEEKS, HAROLD L. WEAVER, GA YLON S. CAMPBELL, 

and BERT D. TANNER 

STUDIES OF EVAPOTRANSPIRATION 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 491-G 

Prepared in cooperation with the 

U.S. Bureau of Reclamation 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1987 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Water use by saltcedar and by replacement vegetation in the Pecos River floodplain between Acme and Artesia, New 

Mexico. 

(U.S. Geological Survey professional paper 491-G) 

Bibliography: p. 

Supt. of Docs. no.: I 19.16:491-G 

1. Tamarix chinensis-Water requirements. 2. Floodplain flora-Pecos River Valley (N.M. and Tex.)-Water requirements. 3. Evapotranspiration-Pecos River Valley (N.M. and Tex.) 4. Plant-water relationships-Pecos River 

Valley (N.M. and Tex.) I. Weeks, Edwin P., 1936- . II. United States. Bureau of Reclamation. III. Title. 

IV. Title: Replacement vegetation in the Pecos River floodplain between Acme and Artesia, New Mexico. V. Series. 

QE75.P9 no. 491-G 557.3 s 85-600364 [QK495.T35] [583'.158] 

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225 
CONTENTS 

Page Page 

Abstract ------------------------------------------------- G1 Instrumentation-Continued 

Introduction ---------------------------------------------

~rpose and scope ------------------------------------

Acknowledgments ------------------------------------

Historical background --------------------------------

Previous work -------------------------------------------

Consumptive use by saltcedar -------------------------

Consumptive use by replacement vegetation -----------

Discussion of previous results -------------------------

Theory -------------------------------------------------- Boundary-layer limitations ---------------------------

The Bowen-ratio method ------------------------------

The eddy-correlation method --------------------------

Instrumentation------------------------------------------

Sonic anemometer - - - - - - - - - - - - - - - - - - - - - - - - - -- - -- - - - - - -

Lyman-alpha hygrometer------------------------------

1 Other instrumentation -------------------------------- G13 

1 Field measurements -------------------------------------- 14 

3 Discussion ------------------------------------------- 15 

3 Energy-budget closure -------------------------------· 17 

·4 Estimates of water use ----------------------------------- 21 

4 Water use by saltcedar ------------------------------- 22 

5 Water use by replacement vegetation ------------------ 24 

7 Comparison with potential evapotranspiration ---------- 24 

8 Annual water use ------------------------------------ 27 

8 Comparison with previous results --------------------- 28 

8 Estimates of evapotranspiration salvage - - - - -- -- - - - - - - - 30 

9 Summary and conclusions -------------------------------- 31 

10 Recommendations for future studies of evapotranspiration -- 32 

11 References ----------------------------------------------- 32 

12 

ILLUSTRATIONS 

Page 

FIGURE 1. Map showing location of stream gages and evapotranspiration measurement sites used in this study ---------------- G2 

2, 3. Graphs showing: 

2. Annual rates of bare-soil evaporation of ground water versus depth to water for various 

evapotranspirometerexperiments ------------------------------------------------------------------ 6 

3. Relationship of evapotranspiration by saltgrass versus depth to water for various 

evapotranspirometer studies ----------------------------------------------------------------------- 7 

4. Diagrams showing the physical characteristics of the original sonic anemometer, the revised sonic anemometer, 

and the Lyman-alpha hygrometer -------------------------------------------------------------------------- 11 

5, 6. Graphs showing daily cycle of energy fluxes 

5. Measured at the wet old-growth site on September 1, 1982 ----------------------------------------------- 15 

6. Measured at the grass and forbs site on June 26, 1982 --------------------------------------------------- 15 

7. Graphs showing summary of estimates of daily evapotranspiration by saltcedar by the energy-budget-residual 

method and the eddy-correlation method ------------------------------------------------------------------- 23 

8. Sample logs showing the nature of the materials from land surface to the water table for each saltcedar 

measurement site----------------------------------------------------------------------------------------- 25 

9-12. Graphs showing: 

9. Summary of estimates of daily evapotranspiration by replacement vegetation by the energy-budgetresidual method and the eddy-correlation method --------------------------------------------------- 26 

10. Estimated annual water use by saltcedar in the Pecos valley, New Mexico, as determined by the energybudget-residual method and the eddy-correlation method -------------------------------------------- 27 

11. Estimated annual water use by replacement vegetation in the Pecos valley, New Mexico, as determined 

by the energy-budget-residual method -------------------------------------------------------------- 28 

12. Graph showing monthly precipitation at Roswell, N. Mex., during the period 1980-82 ----------------- ---- 29 

TABLES 

Page 

TABLE 1. Locations and descriptions of the sites at which evapotranspiration measurements were made ------------------------ G 14 

2. Schedule of eddy-correlation measurements made during the study ------------------------------------------------- 16 

3. Summary of daily sums of energy-density measurements at the various sites for which eddy-correlation data were 

collected---------------------------------------------------------------------------------------------------- 18 

4. Summary of daily water-use estimates at the various sites --------------------------------------------------------- 22 

5. Estimates of the range in annual water use by saltcedar and by replacement vegetation in the Pecos River floodplain 

and of projected water salvage due to saltcedar eradication ----------------------------------------------------- 28 

III 
IV CONVERSION FACTORS 

CONVERSION FACTORS 

For readers who prefer to use inch-pound or other non-SI units, conversion factors for terms used in this report are listed below: 

Multiply By To obtain 

hectare (ha) 

square meter (m2) 

Joule (J) 

Megajoule/square meter (MJm -2) 

watt/square meter (Wm -2) 

Joule/gram (Jg-1) 

watt/meter· Kelvin (Wm-1K-1) 

nanometer (nm) 

micrometer (IJ.m) 

millimeter (mm) 

centimeter (em) 

meter (m) 

kilometer (km) 

gram (g) 

gram/meter2 

kilograrnlmeter3 (kgm -3) 

gram/meter3 (gm - 3) 

watt (W) 

kilopascal (kPa) 

degrees Celsius (°C) 

Kelvin (K) 

meter/second (ms-1) 

cubic meter (m3) 

cubic meter per hectare (m3fha) 

Area 

2.471 

10.76 

Energy 

9.4787x1o-4 

0.2388 

Energy/Area 

88.05 

Energy/Area · Time 

5.2895x10-3 

1.434x1o-3 

Energy/Mass 

0.2390 

0.4303 

Heat coTtductance 

9.629x10-3 

2.387x10-3 

Length 

0.3937 X 10-4 

.397x 10-1 

0.03937 

0.3937 

3.281 

0.621 

Mass 

2.205x10-3 

Mass flux 

17.71 

Mass/Volume 

6.24x1o-2 

0.001 

6.24x10-5 

10-6 

Power 

3.412 

1.340x1o-3 

0.2388 

Pressure 

0.2953 

0.1450 

10 

Temperature 

1.8°C+32 

(K -273.15)1.8+32°F 

Velocity or Rate 

2.237 

Volume 

8.107x1o-4 

Volume/Area 

3.281x10-4 

acre 

square foot (ft2) 

British thermal unit (mean) (Btu) 

calories (cal) 

Btulft2 

Btulft2 · minute 

cal/centimeter2 ·minute (cal 

cm-2 min-1) 

cal/gram (cal g-1) 

Btu/lbm 

Btulft min degree Fahrenheit 

cal/cm · second · Kelvin 

mil 

mil 

inch (in) 

in 

foot (ft) 

mile (mi) 

pounds mass (Ibm) 

pounds mass/ft2/day 

pound mass/foot3 

gramlcm3 (gem - 3) 

pound mass/foot3 

gcm-3 

Btu/hour 

horsepower 

calories/second (cal s-1) 

inches of mercury (in Hg) 

pound/inch2 (psi) 

millibar (mb) 

degrees Fahrenheit (°F) 

mile/hour (mph) 

acre-foot (acre-ft) 

acre-ftJacre 
STUDIES OF EVAPOTRANSPIRATION 

WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION 

IN THE PECOS RIVER FLOODPLAIN BETWEEN 

ACME AND ARTESIA, NEW MEXICO 

By EDWIN P. WEEKS, HAROLD L. WEAVER, GAYLON S. CAMPBELL,l 

and BERT D. TANNER2 

ABSTRACT 

Water-use estimates for saltcedar and for replacement plant communities following root plowing in the Pecos River floodplain between Acme and Artesia, New Mexico, were made by the eddycorrelation technique and a combined eddy-correlation 

energy-budget technique during 1980-82. Twenty-seven measurements of daily water use were obtained for various periods during the 

growing season-17 measurements from four saltcedar thickets and 

10 from three stands of replacement vegetation. Large uncertainties 

exist in these estimates because of problems in extrapolating the 

data seasonally and areally, and because of discrepancies between 

the two methods. Nonetheless, the measurements indicate that annual water use by saltcedar probably is about 0.3 meters greater than 

that by the replacement vegetation. Such reductions in water use 

should have resulted in increased base flow of the Pecos River of 

1.2-2.5X107 cubic meters per year (10,000 to 20,000 acre-feet per 

year). The fact that such gains have not been identified from streamgage records may arise from masking of short-term gains by variations in climate and in ground-water pumpage and from a continuing 

decline in the ground-water contribution to base flow from the shallow aquifer. 

INTRODUCTION 

Saltcedar was cleared, initially (1967) by bulldozing 

and mowing and later (1974) by root plowing, from 

8,700 hectares (21,500 acres) of the floodplain of the 

Pecos River in the reach between Acme and Artesia, 

N.Mex., in anticipation that substantial quantities of 

water would be salvaged from evapotranspiration. An 

unpublished analysis of base flow in the reach (G.E. 

Welder, U.S. Geological Survey (USGS), written commun., 1978) indicated that no readily identifiable gains 

in base flow occurred following either mowing or root 

lProfessor of Soils, Washington State University. 

2Campbell Scientific, Inc. 

plowing, suggesting that the actual salvage of water by 

evapotranspiration was considerably less than anticipated. 

PURPOSE AND SCOPE 

The current study was begun in 1979 to determine 

evapotranspiration from representative plots of 

saltcedar and of replacement vegetation, in order to 

determine whether initial estimates of consumptive 

use by saltcedar were erroneously high and (or) 

whether estimates of use by replacement vegetation 

were erroneously low to the extent that actual salvage 

from evapotranspiration was quite small. This report 

contains a preliminary evaluation of the evapotranspiration measurements and of the suggested magnitude 

of ground-water salvage from evapotranspiration. 

Measurements of evapotranspiration were made periodically during the growing season in 1980-82 using 

an eddy-correlation approach. During 1980 and 1981, 

the measurements were made over three plots (fig. 1)-

one with old-growth saltcedar, one with saltcedar regrowth following mowing, and one with replacement 

vegetation. In 1982, two other saltcedar sites and two 

other replacement vegetation sites were added to gain 

areal coverage. 

Weather data, including solar radiation, air temperature, relative humidity, precipitation, windspeed, and 

soil temperatures were collected on a continuous basis 

during the growing season of each year from 1980 to 

1982 at the New Mexico State Wildlife Refuge at Artesia. Other data, including stomatal resistances, plantwater potentials, soil-moisture contents, root densities, 

moisture-characteristic data, and other plant- and soilrelated parameters, also were collected, in order to 

model the evapotranspiration process. 

G1 
G2 STUDIES OF EVAPOTRANSPIRATION 

1W 104o 

~o~--------------------------~----------~~------------------~10~~· 

330 

32° 

0 

I I 

0 10 

NEW 

MEXICO 

10 

I 

I 

20 

Base from U.S. Geological Survey State of New Mexico 

Old-growth saltcedar 

0 

Artesia 

Burned saltcedar 

Study area 

Wet old-growth saltcedar 

Bitter Lake National 

Wildlife Refuge 

Grass and forbs 

a gaging station 

Lake McMillan 

Lake Avalon 

Carlsbad 

~· t.-G,.. 

20 30 MILES 

I I I 

30 KILOMETERS 

Scale 1:1,000,000 M XI CO 

----------------N_E~ ___ E --

A 

• 

II 

EXPLANATION 

Stream gage 

Evapotranspiration 

measuring site 

Study area (Mower and others, 1964) 

FIGURE !.-Location of stream gages and evapotranspiration measurement sites used in this study. 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G3 

ACKNOWLEDGMENTS 

This study was partially financed by the U.S. Bureau 

of Reclamation (USBR). In addition, USBR personnel, 

including Leon Marcell, Kirby Lykins, and Joseph 

Odell, obtained landowner permission for the studies, 

made backhoe excavations for soil sampling, and provided materials and equipment. Personnel of the USGS 

New Mexico District office, including G.E. Welder, 

Paul Davis, and Doug McAda, provided logistical support and assisted at various times. Tinco van Hylckama 

(USGS) provided assistance by identifying the various 

vegetation species at the replacement vegetation site. 

Other USGS personnel who assisted in data collection 

at various times include Rita Carmen, Dwight Hoxie, 

Eric Lappala, David Redinger, and David Stannard. 

Keith Bristow of Washington State University also 

provided advice and assistance in making plant-water 

status and stomatal-resistance measurements. 

HISTORICAL BACKGROUND 

The Roswell basin, which basically includes the 

study reach of the Pecos River, has been extensively 

developed for irrigation. The first irrigation development, which started in the 1890's, involved the use of 

flowing wells that tapped the artesian aquifer that 

underlies the basin. Development of the shallow alluvial aquifer took place primarily during the period 

1935-37. State law ended growth of irrigation development in both aquifers, and in 1967, flow meters were 

placed on all irrigation wells in the basin by order of 

the State Engineer's office. Since that time, pumpage 

from existing wells has been limited to a 1.1X104 m3 

per ha (3.5 acre-ft per acre) annual allocation, and no 

new wells have been permitted. These restrictions have 

resulted in stabilization of water levels in the artesian 

aquifer, but water levels in the shallow aquifer were 

still declining in 1975 (Welder, 1983, fig. 21). 

Saltcedar (Tamarix chinensis) was first noted in the 

basin in about 1912 (Eakin and Brown, 1939); it 

rapidly became the dominant species in the Pecos River 

floodplain, occupying 11,200 ha (28,000 acres) of the 

16,400-ha (41,000-acre) floodplain in 1958 (Mower and 

others, 1964). Saltcedar mainly replaced two phreatophytic grass species, saltgrass (Distichlis stricta) and 

alkali sacaton (Sporobolus airoides ). Saltcedar has a 

substantial reputation as a plant that consumes large 

quantities of water (Robinson, 1965), and the observed 

declines in streamflow in the middle basin of the Pecos 

River (the reach from Alamagordo to Red Bluff Reservoirs) was attributed in part to the saltcedar invasion 

(Thomas, 1963). 

Because of the concern that saltcedar could be a substantial agent in the depletion of the flow of the Pecos 

River, the New Mexico State Engineer requested in 

1956 that the USGS investigate the use of water by 

"nonbeneficial vegetation" in the 80-km (48-mi) reach 

of the river from Acme (now a ghost town) to Artesia, 

N.Mex. This reach was chosen for study because longterm streamflow records were available. In that study 

(Mower and others, 1964), saltcedar was found to cover 

about 11,200 ha (28,000 acres) of the floodplain within 

the study reach, although only 7,200 ha (18,000 acres) 

supported an areal saltcedar density of more than 10 

percent. Also, on the basis of the extrapolation of 

water-use data for saltc~dar developed by Gatewood 

and others (1950) for the Safford Valley, Mower and 

others (1964) estimated that the eradication of 

saltcedar in the reach would result in a reduction in 

consumptive use in the floodplain of 3.5 x 107 m3 

(28,000 acre-ft) per year. 

Blaney (1961) made a separate estimate of water salvage by saltcedar eradication in the reach of the Pecos 

River from Acme to Artesia. He estimated annual salvage of about 2.5 x 107 m3 (20,000 acre-ft). 

In 1967, the U.S. Bureau of Reclamation began a 

program to eradicate saltcedar from about 8, 700 ha 

(21,500 acres) in the Acme-Artesia reach. Initially, the 

trees were felled by bulldozers and burned. Thereafter, 

an attempt was made to control saltcedar regrowth by 

mowing. This technique was not successful, because 

the mown saltcedar regrowth grew back with great 

vigor. Consequently, a program of root plowing saltcedar from the previously bulldozed and mowed areas 

was initiated in 197 4, followed by a maintenance program. These clearing operations included nearly all the 

denser thickets of saltcedar except those in a 15-m-wide 

(50-ft-wide) strip along each bank of the river, which 

meanders for 132 river kilometers (82 mi) within the 

80-km (48-mi) reach. Saltcedar thickets also were retained on the Artesia State Wildlife Refuge and on the 

Bitter Lakes National Wildlife Refuge. About 600 ha 

(1,500 acres) of dense to moderately dense saltcedar 

and about 2,000 ha (5,000 acres) of saltcedar with a 

density of less than 10 percent remain of the saltcedar 

plots mapped by Mower. 

In 1971, G.E. Welder, of the USGS New Mexico District office, began a study of base flow of the Pecos River 

in the Acme-Artesia reach to determine whether the 

expected gains due to saltcedar eradication could be 

detected. His unpublished study indicated that the 

base-flow pickup in the reach declined from about 

3.9x107 m3 (32,000 acre-ft) in 1957 to about 1.8X107 

m3 (15,000 acre-ft) in 1964 (3 years before initial clearing) but then varied between 2.0 x 107 and 3.0 x 107 m3 

(16,000 and 25,000 acre-ft) per year without a particular time trend until the present time (1982). Lack of 

any apparent gain in base flow that could be attributed 

to saltcedar eradication, either following the period of 

mowing in 1967-69 or following root plowing in 1974-
G4 STUDIES OF EVAPOTRANSPIRATION 

76, raised concern that the amount of water that could 

be salvaged by saltcedar eradication had been substantially overestimated. This concern prompted the 

present study. 

In order to select a micrometeorological technique to 

be used in the study, grants were made to Lloyd Gay of 

the University of Arizona and to Leo Fritschen of the 

University of Washington to demonstrate such techniques in the summer of 1979. Gay (1980) made measurements necessary to apply the Bowen-ratio method, 

and Fritschen and others (1980) made measurements 

necessary to apply the eddy-correlation technique. 

Both methods are described in the section on "Theory." 

PREVIOUS WORK 

CONSUMPTIVE USE BY SALTCEDAR 

Perhaps the earliest measurements of water use by 

saltcedar were from tank experiments performed in 

1940 at Carlsbad, N. Mex., by Blaney and others 

(1942). They reported water use of 1.67 m/yr (meters 

per year) from a tank with a 0.6-m depth to water and 

of 1.43 m/yr for a tank with a 1.2-m depth to water. 

An extensive study of water use by saltcedar was 

made in the floodplain of the Gila River in the Safford 

Valley in 1943-44 by Gatewood and others (1950). 

They measured evapotranspiration by saltcedar in 

nine tanks, two of which were 3.05 min diameter and 

seven of which were 1.8 min diameter, with a range in 

depth to water of 1.2 to 2.1 m. However, these tanks 

were located in a natural clearing (Gatewood and 

others, 1950, p. 37) and were undoubtedly subject to 

"oasis effects" (explained in "Discussion of Previous Results"). Water use ranged from about 1.2 m/yr to about 

3m/yr. Gatewood and others also introduced the concept that consumptive use of water by saltcedar is proportional to its volume density. Based on this concept, 

their consumptive use adjusted for 100-percent volume 

density ranged from about 2.1 to 3 m/yr, excluding precipitation. 

Gatewood and others (1950) also measured water use 

from 3,765 ha (9,300 acres) of the Gila River floodplain 

by a water-budget (inflow-outflow) method. These 

measurements indicated an annual water use of about 

0. 73 m over that area, about half of which had a cover 

of saltcedar and the remaining half a cover of seepwillow (Baccharis ), cottonwood (Populus fremontis ), 

mesquite (Prosopis velutina ), undifferentiated brush, 

and bare ground (predominantly sandbars in the river 

channel). 

The U.S. Bureau of Reclamation (1973) measured 

evapotranspiration by saltcedar at a site in the floodplain of the Rio Grande near Bernardo, N.Mex. Measurements were made in six large tanks 93m2 in area by 

3.7 m deep that were planted with saltcedar in 1961 

and 1962. The tanks were operated as constant-waterlevel evapotranspirometers from 1962 to 1968. In 1968, 

saltcedar in two tanks was replaced by Russian olive 

(Elaeagnus augustifolia ). Salt was added at a rate of 

about 5,000 mg/L to the feed water for two other tanks. 

The water level in the remaining two tanks was 

dropped from about 0.90 m to about 2.75 m below land 

surface by cutting off the feed water or by pumping 

water from the tank. Water use by saltcedar in the 

various tanks showed substantial variation, ranging 

from 0.7 m to 1.4 m, including rainfall. Average water 

use during the period of operation with fresh water and 

constant water levels was 1.0 m (including precipitation), with water levels maintained 0.9 to 1.5 m below 

land surface. Lowering the water table from 0.9 to 

2. 7 m initially reduced annual consumptive use by 

0.30-0.45 m, but usage nearly recovered in about 2 yr. 

Hughes (1972) analyzed the U.S. Bureau ofReclamation (1973) data to show that evapotranspiration by 

saltcedar was not linearly related to volume density, 

but instead was almost as large for 50-percent volume 

density as for 100-percent volume density. He also 

demonstrated that water use by saltcedar was less than 

potential evapotranspiration as computed by the Penman (1956) equation, unless corrected downward by 

use of an empirical stomatal-resistance term. 

Van Hylckama (1974) measured water use by 

saltcedar during the period 1962-67 in the floodplain 

of the Gila River near Buckeye, Ariz., using six large 

(9 mx9 mx4.25 m deep) tanks located within a saltcedar thicket and two smaller ( 6 m x 6 m x 2 m deep) 

tanks within a large bare area. In 1965 (the year during which vegetative cover and lysimeter operations 

were most representative of the natural environment), 

water use by saltcedar in three large tanks using fresh 

water ranged from 2.6 m/yr, with a depth to water of 

2.7 m, to 3.4 m/yr, with a depth to water ofl.5 m. Water 

use in one small "oasis" tank previously flushed with 

fresh water was 3.6 m/yr, with the water table maintained at a depth of 1.2 m. Van Hylckama also performed experiments that indicated that a decrease in 

volume density of saltcedar from 100 percent to 50 percent reduced consumptive use by only about 10 percent. 

Gay and Fritschen (1979) determined evapotranspiration from a saltcedar thicket at the Bernardo, 

N. Mex. lysimeter site during a 5-day (d) period in June 

1977 using an energy-balance (Bowen-ratio) technique. 

They measured rates of evapotranspiration of 7.4 and 

9.0 mm/d (millimeters per day) at two locations about 

75 m apart, which compared favorably with measurements in four lysimeters that varied from 6.4 to 9.2 

mm/d during the same period. 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N. MEX. G5 

Fritschen and others (1980) measured evapotranspiration from two thickets of saltcedar in the Lake 

McMillan delta area of the Pecos River near Artesia, 

N.Mex., by the eddy-correlation method. One thicket 

constituted regrowth from a fairly recent burn, and the 

other constituted old-growth saltcedar. Water use during an 8.5-h (hour) period in July was 2.3 mm for the 

regrowth and 1.9 mm for the old growth. The day was 

cloudy, and hence the results probably are not representative of water use by saltcedar. Gay (1980) measured water use of 1. 7 mm during an 8-h period on the 

same day by the same regrowth plot measured by 

Fritschen and others (1980), using a Bowen-ratio technique. 

Van Hylckama (1980) made micrometeorological 

measurements at the Buckeye site in Arizona. The 

measurements were made during 3-d periods about one 

month apart from May to September in 1966 and in 

March and May 1967. The measurements included drybulb and wet-bulb temperature measurements and 

windspeed at five levels, net radiation, and soil-heat 

flux. These measurements should be adequate to estimate evapotranspiration by the energy-budget 

(Bowen-ratio) method. However, Bowen ratios computed using various pairs of wet- and dry-bulb temperature sensors indicated significant scatter in the computed Bowen ratio. These variations, and the fact that 

bias among the various sensors was not eliminated, as 

is generally recommended for Bowen-ratio studies 

(Suomi and Tanner, 1958, for example), caused van 

Hylckama to reject the energy-budget approach. 

Our examination of van Hylckama's data suggests 

that the results are better than he allowed, particularly 

when examined in the context of the results of the 

study by Culler and others (1982) and of our study. For 

example, a cursory examination of the hourly average 

temperature and humidity data at various heights 

above land surface tabulated by van Hylckama (1980, 

p. F62-F69) indicate that, for the measurements at 

4-5-m and 5-7 -m heights, the Bowen ratio is typically 

near zero during the hours of peak radiation. Based on 

the net-radiation and soil-heat-flux measurements 

made by van Hylckama (1980, p. F70-F73), evapotranspiration would have been about 8 mm/d, compared 

with values of as much as 20 mm/d measured from van 

Hylckama's tanks. The Bowen-ratio estimates are in 

relatively good agreement with ours, and with those of 

Culler and others (1982) described below, whereas the 

evapotranspiration-tank data are not. 

Leppanen (1981) applied the Bowen-ratio method to 

the measurement of evapotranspiration from rapidly 

growing young saltcedar near the upstream end of San 

Carlos Reservoir on the Gila River in Arizona. His 

measurements covered a 48-d period extending from 

August 17 to October 3, 1971. Evapotranspiration averaged 5.8 mm/d during this period, and 7.0 mm/d during the last 15 days of August. Depth to water at the 

site varied greatly because of reservoir-level fluctuations, but soil water content was high throughout the 

period of measurement. 

Culler and others (1982) measured evapotranspiration by the water-budget method from a floodplain area 

of 2,190 ha along the Gila River between the town of 

Bylas and the backwater of San Carlos Reservoir in 

Arizona. The dominant riparian vegetation is saltcedar 

(1,540 ha) and mesquite (600 ha), with about 40 ha 

mapped as not having phreatophyte cover. Before 

clearing, consumptive use (excluding precipitation) 

from this area averaged about 0.83 m annually. Water 

use from reach 3, which had a cover of about 97 percent 

saltcedar, was about 1.0 m/yr. 

CONSUMPTIVE USE BY 

REPLACEMENT VEGETATION 

Water use by the various plant communities that 

have replaced saltcedar in the Pecos River floodplain 

has been less intensively studied than water use by 

saltcedar. Nonetheless, some studies are available. 

The most comparable data were collected by Culler 

and others (1982). They measured evapotranspiration 

from vegetation that replaced saltcedar following root 

plowing in the floodplain of the Gila River. The 

saltcedar in the floodplains of some reaches was cleared 

by root plowing in 1967, and other reaches were cleared 

during 1968-71. Water-use data on replacement vegetation were collected from 1968 to 1971. These measurements indicate that evapotranspiration from the 

patches of bermuda grass, annual weeds, and bare 

ground that replaced the saltcedar averaged 0.64 rnlyr, 

including 0.28 m of annual precipitation. 

Patches of bare soil remain in some of the cleared 

areas of the Pecos River floodplain, and a possible 

water-management scheme is to maintain the soil bare 

over large areas of the floodplain. Thus, an estimate of 

bare-soil evaporation would be desirable. 

Evaporation from bare soil underlain by a shallow 

water table has been studied extensively, using theory, 

laboratory columns, and field evapotranspirometer 

tanks. Only the evaporation from bare soil measurements derived from evapotranspirometer studies will 

be reviewed here. Blaney and others (1930) and Blaney 

(1933) determined evaporation at Santa Ana, Calif., 

from nine 0.60-m-diameter tanks, three containing 

repacked Hanford fine sandy loam and six containing 

"undisturbed" soil of the same composition. Results of 

their studies, as summarized by Blaney (1933), in 
G6 STUDIES OF EVAPOTRANSPIRATION 

C/) 120 

a: 

w 

1-

w 

~ 

1901 

+ 

Blaney ( 1933) 

Undisturbed 

fine sandy loam 

McDonald and 

/ Hughes ( 1968) 

Sandy Silt 

1964 

ctata 

van Hylckama (1974) 

X..- Coarse-textured 

alluvium 

~ 60@--

0 Gatewood 

1- and others 

~ (1950) 

fu Gravel 

0 

'~ r;J~~' ! J.s. Bureau of Blaney ( 1933) 

Reclamation ( 1973) Disturbed 

Sandy loam fine sandy loam 

U.S. Bureau of 

Reclamation (1973) ~ 

Sand 

Robinson (1970) 

Sandy silt 30 

CONSUMPTIVE USE, IN CENTIMETERS PER YEAR 

FIGURE 2.-Annual rates of bare-soil evaporation of ground water versus depth to water for various evapotranspirometer experiments. 

terms of annual water use versus depth to water, are 

shown in figure 2. Gatewood and others (1950) conducted experiments on 12 tanks, each 1.2 min diameter, to determine evaporation from bare soil in the Safford Valley near Glendale, Ariz. Six of the tanks were 

filled with sand and gravel, two with gravel, and four 

with clay loam. Results for those tanks for which fullyear data are available are shown in figure 2. Large 

(3x3 to 12x12 m) plastic-lined tanks have been used 

near Yuma, Ariz. (McDonald and Hughes, 1968), near 

Buckeye, Ariz. (van Hylckama, 1974), near Winnemucca, Nev. (Robinson, 1970), and near Bernardo, 

N.Mex. (U.S. Bureau of Reclamation, 1973), to measure evaporation from bare soil underlain by a constant 

shallow water table. The tanks near Yuma and Winnemucca were filled with sandy silt, while those near 

Buckeye contained a coarse-textured alluvial soil but 

with more clay and silt than the Yuma soil (van Hylckama, 197 4). Three tanks at Bernardo were filled with 

sand and silt with clay lenses of various thickness, and 

one was filled with sand. The results are summarized 

in figure 2. Veihmeyer and Brooks (1954) measured 

evaporation from 1935 to 1939 from bare soil near 

Davis, Calif., in 38 tanks-13 tanks 0.64 min diameter 

filled with Yolo fine sandy loam and 25 tanks 0.7 min 

diameter filled with Yolo silt loam. They show an average monthly water use in August of about 2.5 em. 

The Pecos River has developed a natural levee, and 

the depth to water landward of the levee is about 1.5 to 

3 m, with an average depth to water of about 2 m. The 

materials above the water table are typically sandy 

loam to clay loam in texture. None of the tank experiments involved depth to water greater than 1.2 m, but 

the data shown in figure 2 suggest that evaporation 

discharge of ground water should be no more than 

about 0.1-0.2 m/yr from bare soil in the Pecos River 

floodplain. 

Young and Blaney (1942) cite measurements of 

water use by various species of weeds grown in tanks 

having a shallow water table. Of the weeds grown, only 

Russian thistle (Salsola kali) was common to the Pecos 

River floodplain. The Russian thistle used about 0.60-

0.65 m of water during the period May 3 to September 17, 1932, with a depth to water of 0.30-0.45 m. 

Blaney and Hanson (1965) quote water use for alkali 

sacaton, a species prevalent in the Pecos River flood-
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G7 

ci 0.90 

w 

1-

< 

3: 

g 

~ 0.60 0.. 

w 

0 

0.30 

+ 

+ ISLET A, N. MEX. 

$ BERNARDO, N. MEX. (U.S. Bureau 

of Reclamation, 1973) 

0 ESCALANTE, UTAH 

A MESSILA, N. MEX. 

• OWENS VALLEY, CALIF. 

X SANTA ANA, CALIF. 

(!) LOS GRIEGOS, N. MEX. 

- YOUNG AND BLANEY 

INTERPRETATION 

o~--~~--~~----~-----1~.2o _____ 1J.s_o----~1.so 

CONSUMPTIVE USE, IN CENTIMETERS PER YEAR 

FIGURE 3.-Relationship of evapotranspiration by saltgrass versus 

depth to water for various evapotranspirometer studies (modified 

from Young and Blaney, 1942). 

plain, that was grown in tanks at Carlsbad, N.Mex. 

Annual water use by sacaton was 1.2 m with a 0.60-m 

depth to water, and 1.05 m with a 1.2-m depth to water. 

Average daily use by sacaton in the tank with the 1.2-

m depth to water was 5.6 mm in June and 6.6 mm in 

August 1940. 

Evapotranspiration by saltgrass has also been measured at several locations using tanks. Results of most of 

these studies have been summarized by Young and 

Blaney (1942). In addition, the U.S. Bureau of Reclamation (1973) has measured water use by saltgrass at 

Bernardo, N.Mex. The results (fig. 3) show substantial 

scatter, and annual consumptive use ranged from 

about 0.25 m to 1.20 m. However, for water-table 

depths of a meter or more, the range in water use by 

saltgrass was 0.3 to 0.8 m, with climate a major factor. 

Although the authors observed little saltgrass in the 

study area, Mower and others (1964) report that saltgrass was the dominant species in about half of the 

Pecos River floodplain prior to the saltcedar infestation. Hence, saltgrass may again become prevalent in 

the floodplain as a regrowth species as time progresses. 

Fritschen and others (1980) measured water use by 

kochia (Kochia scoparia) at the forbs site described below in July 1979, by an eddy-correlation technique. 

(The listed common name for Kochia scoparia is 

summer-cypress, but the weed is locally known as 

"kochia" in the Roswell basin area and kochia will be 

used as its common name in this report.) Gay (1980) 

made simultaneous measurements of water use by the 

same stand at an adjacent site using a Bowen-ratio 

technique. Both methods indicated water use of about 

3 mm/d. 

DISCUSSION OF PREVIOUS RESULTS 

Most rates of evapotranspiration from saltcedar have 

been made in the arid Southwest in areas having relatively similar climates. Nonetheless, the measured values vary by a factor of about six, with summertime 

latent heat flux, defined as the evapotranspiration 

rate, E, multiplied by the latent heat of vaporization of 

water, "A, or AE, ranging from about 0. 7 times to more 

than 4 times the energy available from solar and thermal radiation, Rn. Conditions that produce values of 

AE!Rn greater than unity are commonly called "oasis 

effects" and indicate a locally greater supply of water 

compared with closely adjacent areas. The most extreme values of AE IR n have been obtained from evapotranspirometer tanks, suggesting that these tanks 

were strongly influenced by oasis effects. The tanks 

used by Gatewood and others (1950) were located at the 

edge of a saltcedar thicket, and the potential for an 

oasis effect at that site is apparent. The six large tanks 

used by van Hylckama (1974) were located within a 

saltcedar thicket, however, and the occurrence of very 

localized oasis conditions is less easily inferred. The 

temperature profile data collected by van Hylckama 

(1980) indicate that oasis conditions did not generally 

prevail for the thicket as a whole, so that local oasis 

effects must have occurred within the vegetation in the 

tanks. Possibly the shallow water table in the tanks 

and the greater vigor of the relatively newly planted 

saltcedar combined to allow those saltcedar to use 

water more freely and to thus create oases within the 

thicket. In fact, the saltcedar outside the tanks appears 

to have been deprived of a significant source of water 

by a dike constructed in November 1964 to divert floods 

in Waterman Wash from the area. Following construction of the dike, saltcedar in the vicinity of the tanks 

began to die during periods of prolonged drought (van 

Hylckama, 197 4, p. E29), indicating that the saltcedar 

underwent substantial moisture stress when deprived 

of soil moisture. 

The occurrence of localized oases is also suggested by 

the work of Gay and Fritschen (1979). One of their 

Bowen-ratio towers demonstrated the occurrence of a 

significant oasis effect, while the other, 75 m distant, 

did not. However, the saltcedar cover in that study area 

reportedly is relatively homogeneous. Thus the indicated oasis effect is not readily explainable. 
G8 STUDIES OF EVAPOTRANSPIRATION 

Results of the other cited studies, including the tank 

studies at Bernardo (U.S. Bureau of Reclamation, 

1973) as well as our own results, indicate that water 

use by saltcedar is approximately equal to or less than 

net radiation, suggesting that oasis conditions generally do not prevail over large sal tcedar thickets or 

within the floodplain environment as a whole. Thus, 

extrapolation of the tank experiment data on water use 

to natural floodplain environments may lead to erroneous conclusions concerning water use in the natural 

environment. 

Historically, tank-derived evapotranspiration values 

have been extrapolated using the volume-density technique developed by Gatewood and others (1950). This 

method relies on the assumption that evapotranspiration is proportional to the volume density of plant foliage, an assumption that probably is valid when 

strongly oasitic conditions prevail. However, when 

such conditions do not prevail, water use may be relatively constant for a variety of situations, from full 

areal vegetative cover down to some fraction of full 

cover, as noted, for example, by Hughes (1972, fig. 1) 

and by van Hylckama (1974, p. E13-E16). 

Culler and others (1982) developed an empirical relationship indicating that evapotranspiration from plots 

with less than 100-percent volume density was proportional to the fractional volume density raised to the 

0.75 power. This relationship is more linear than that 

determined by Hughes (1972) or van Hylckama (197 4). 

However, the average volume densities of their reaches 

1, 2, 2a, and 3 were 0.41, 0.54, 0.43, and 0.81, respectively (Culler and others, 1982, table 10). Data from 

reach 3, the only one showing substantially different 

cover, were obtained for only two periods in the summer of 1965. During those periods, water use per unit 

area from this reach was about the same as that from 

reach 2 but was significantly larger than that from 

reach 1. It is possible that the differences in water use 

among reaches were due to saltcedar vigor or other 

factors rather than to volume density. 

The above data suggest that, under conditions of 

varying cover, net radiation will be the main energy 

source for evapotranspiration, with local advection of 

energy from areas between plants increasing evapotranspiration by individual plants in less dense stands. 

Thus, the volume-density approach may result in an 

underestimate of evapotranspiration under conditions 

of less than full cover. However, estimates of floodplain 

water use based on too-high water-use rates with overcompensating plant-density-effect estimates may result in overall projections that are nearly correct for an 

entire floodplain area. Nonetheless, grossly erroneous 

estimates would result from the volume-density technique for individual thickets, and use of these values to 

estimate total water consumption resulting from increases in saltcedar density in recently colonized areas 

would also be overemphasized. 

THEORY 

Several micrometeorological techniques have been 

developed to determine evapotranspiration from meteorological measurements made in the turbulent 

boundary layer above a plant canopy or other surface. 

Of these, the Bowen-ratio technique has been widely 

applied to measure evapotranspiration from floodplain 

vegetation in previous studies, and the eddycorrelation technique was applied in this study. The 

theory of both techniques is briefly described below, 

following a discussion of boundary-layer limitations 

that are applicable to both methods. 

BOUNDARY -LAYER LIMITATIONS 

Application of both the Bowen-ratio and eddycorrelation methods is based on the assumption that 

horizontal gradients in the vertical fluxes of heat, 

vapor, and momentum are zero. However, as air moves 

horizontally from a surface of a given roughness, wetness, and temperature to another surface having significantly different properties, the air-velocity, vapordensity, and air-temperature profiles (and the 

associated fluxes) must be in equilibrium with the new 

surface. This adjustment is completed within a 

boundary layer over the new surface that grows in 

thickness with distance from the upwind edge of the 

surface. A rule of thumb, based on review of windtunnel data and on various theoretical studies, indicates that the micrometeorological measurements 

should be made at a height above the canopy of no more 

than 0.01 times the "fetch," or distance from the upwind edge of the canopy (Tanner, 1967, p. 547). 

Local inhomogeneities in the canopy surface also 

may cause horizontal gradients in the vertical fluxes. 

Effects of such local inhomogeneities can be minimized 

by increasing the height of the instruments above the 

canopy. However, the use of this approach is limited by 

the above requirement that the instruments be at a 

height no greater than 0.01 times the fetch. 

THE BOWEN-RATIO METHOD 

The Bowen-ratio method (Tanner, 1967) relies on the 

measurement of the energy budget (net radiation and 

soil-heat flux) and of temperature and vapor density or 

vapor pressure at two or more heights above an evapotranspiring surface. Heat and vapor are assumed to be 

transported by an eddy-diffusion process, and the eddy 

diffusivities are assumed to be equal for heat and 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N. MEX. G9 

vapor. Under these assumptions, the ratio of sensible to 

latent heat flux is given by the Bowen ratio: 

where 

tl.T R=HI'AE="VtJ J tl.p ' 

~=Bowen ratio, dimensionless; 

H=sensible heat flux, Wm-2; 

A.=latent heat of vaporization, Jg-1; 

(1) 

E =evapotranspiration rate or vapor flux, gm-2s-1; 

-y=psychrometer constant, gm-3oc-1; 

tl.T =difference in temperature at two vertically displaced measuring points; oc; and 

tl.p=vapor-density difference measured across the 

same vertical interval, gm -3. 

Evapotranspiration is then computed by the equation 

(2) 

where 

Rn =net radiation, wm-2; and 

G =soil-heat flux, wm-2. 

Both R n and G are readily measured using commercially available instrumentation, as described by 

Fritschen and Gay (1979). This method also requires 

the measurement of temperature and of vapor pressure 

or vapor density at two heights above the canopy or 

exchange surface. Because of the instrument height-tofetch requirement and the need to make measurements 

far enough above the exchange surface to avoid the 

effects of surface heterogeneities, the temperature and 

vapor sensors in the Bowen-ratio setup must be quite 

closely spaced, commonly about 1 m apart. Under these 

conditions, the temperature and vapor differences are 

quite small, and slight biases in the temperature and 

humidity sensors, easily tolerated for most point measurements, can cause a significant error in the measured Bowen ratio. One common approach to avoiding 

this bias is to switch sensor positions periodically, so 

that each sensor is in the upper position half the time 

and in the lower position the other half. Bowen ratios 

are calculated for intervals including a full sequencing 

of sensor positions. This method was used by Gay 

(1980) and by Gay and Fritschen (1979). Switching sensor positions is a practical and proven method for avoiding bias-type errors in sensors if data are collected at 

only two heights. However, the Bowen-ratio technique 

can be made more reliable by collecting data at three or 

more heights. A ploy used in these circumstances to 

avoid bias in sensor readings is to sequentially bring 

air from different levels past the same sensor and then 

to average several readings to avoid the effects of not 

having simultaneous readings. LepJ:.•nen (1981) used 

this approach. 

THE EDDY-CORRELATION METHOD 

The eddy-correlation method for measuring the flux 

of heat and water vapor from a surface is based on the 

concept (van Hylckama, 1980, p. F13) that a moving 

parcel of air carries with it, on its journey up, down, or 

sideways, the heat and water vapor that it contained at 

the start. Thus, if heat and water vapor are being carried upward in the atmosphere from a warm, moist 

surface or vegetation canopy, the updrafts will be 

warmer and wetter than the downdrafts. Hence, if the 

vertical wind vector, air temperature, and water-vapor 

density can be measured at high frequency within the 

turbulent boundary layer, both sensible and latent 

heat fluxes can be measured directly (Priestley, 1959). 

The vertical wind in turbulent eddies that transport 

the heat and vapor may be separated into two components, a mean velocity, w, and a fluctuating instantaneous deviation from the mean velocity, w ', such that 

w=w+w' , 

where w is vertical windspeed, ms-1. 

Similarly, temperature, T, and water vapor density, 

p, can be separated into mean and fluctuating components: 

T=T +T' 

and 

p=p+p' ' 

where 

T =temperature, oc; and 

p=vapor density, gm-3. 

In this study, air temperature was measured with a 

fine-wire thermocouple as the difference between the 

junction temperature and that of a reference junction 

in contact with a large thermal mass. Thus, the temperature equation becomes 

T=T -T0+T' , 

where T 0 is the reference-junction temperature. 

Assuming that the heat and vapor fluxes are indeed 

due mainly to convection by the turbulent eddies, as 

opposed to thermal conduction or molecular diffusion, 

and that fluctuations of air density with temperature 

are small, the heat-flux equation can be written 
G10 STUDIES OF EVAPOTRANSPIRATION 

Products of a mean and a fluctuating quantity become 

nearly zero when averaged over an appropriate time 

interval, and the mean vertical wind is assumed equal 

to zero. Hence, the equation reduces to 

(4) 

where 

H=sensible heat flux, Wm- 2; 

Pa =mean density of moist air, gm - 3; 

Cp =heat capacity of air at constant pressure, 

Jg-1oc-1; 

and the overbar signifies the time average of the quantity. 

Webb and others (1980) show that the assumption 

that Pa is constant is adequate in computing sensible 

heat flux. 

By a similar process, the vapor flux can be shown to 

be given by the equation (Webb and others, 1980) 

E=(l+M)w'p' , (5) 

where 

E =evapotranspiration, gm-2s-1; and 

M =ratio of moles of water vapor to moles of dry air 

in a unit volume of moist air, dimensionless. 

However, Webb and others (1980) show that, for vaporflux measurements, the effects of temperature on air 

density cannot be ignored and the vapor flux must be 

corrected by using the equation 

E= (l+M)[w'p'+(pJi\) w'T'] , (6) 

where 

Tk =temperature, °K. 

This correction term can add up to 10 percent of the 

vapor flux when sensible heat flux is large. The vapor 

flux can be converted to a latent heat flux by multiplying the evapotranspiration rate by the latent heat of 

vaporization of water, A, where A is in Jg- 1. 

The eddy-correlation fluxes represent one measure of 

the energy budget: 

where 

Rn =net radiation, wm-2; 

G =soil-heat flux, wm-2; 

(7) 

as =rate change in heat storage in the vegetation 

canopy during the time ofinterest, wm-2; and 

P=energy flux used in photosynthesis, Wm-2• 

In general, as and P are only a few percent of the 

energy budget and are ignored. Hence, 

Measurements of Rn and G thus provide a means of 

checking fluxes measured by eddy correlation. 

In practice, vertical wind, temperature, and vapor 

density are sampled a finite number of times during a 

given sampling period, and the sensible heat flux is 

computed as 

(9) 

where N is the total number of samples taken during 

the sampling period. 

Likewise, the latent heat flux, L, is computed as 

INSTRUMENTATION 

The instrumentation used to measure eddycorrelation fluxes included CSI (Campbell Scientific, 

Incorporated)3 CA 27-T sonic anemometers equipped 

with fine-wire thermocouples, CSI Lyman-alpha hygrometers, Fritschen (1965) net radiometers, homemade soil-heat flux plates (Tanner, 1963), and thermocouples for measuring soil temperature. Mean 

temperatures and mean vapor densities were measured 

using a Delta T aspirated wet-bulb dry-bulb psychometer, a model 201 Physchem probe that measures temperature with a thermistor and relative humidity with 

a polystyrene resistance chip, and (or) a manually read 

Assman wet-bulb dry-bulb psychrometer. Windspeed 

and direction were measured using a Met-One 

anemometer and a Met-One wind-direction indicator. 

The eddy-correlation data were obtained, processed, 

and recorded on a CSI CR-5 data logger that contained 

special hardware for 10-Hz sampling and special software to output sensible and latent heat fluxes. The net 

radiometers, soil-heat flux plates, and soil-temperature 

thermocouples were read by integrator modules in the 

CR-5 data logger, and other instrument responses 

were obtained using a CSI CR-21 data logger. 

3The use of trade names in this report is for identification purposes only 

(8) and does not imply endorsement by the U.S. Geological Survey. 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G11 

Sonic 

transponder 

I 

~ 

So urce 

tu be 

~ 0. 

De 

10 em ~4 em~-++ Fine-wire 

I thermocouple 

8 em (typical) 

To 

electronic 

Electronics case 

A 

Level bubble 

I 

8 

tu 

Electronics case 

c 

tector 

be 

10 em 

FIGURE 4.-Physical characteristics of (A) the original sonic anemometer, (B) the revised sonic 

anemometer, and (C) the Lyman-alpha hygrometer. 

SONIC ANEMOMETER 

The sonic anemometers used in this study determine 

windspeed by measuring the phase shift between a 

transponder and a receiver resulting from the upward 

or downward displacement of the emitted sound waves 

as they are transported by the vertical wind. This 

model is an improved version of that described in detail 

by Campbell and Unsworth (1979). The original model 

was updated in 1980 to increase its operational temperature range and sensitivity and again in 1982 to improve its cosine response. The transducers in this sonic 

anemometer (figs. 4A, 4B) have 10-cm separation. 

Hence, the velocity of eddies smaller than 10 em in size 

will be underestimated owing to space-averaging. The 

CA-27T sonic anemometer is factory calibrated to generate 1.00 V (volt) in a 1-ms-1 wind, and in the 

anemometer-data logger system resolution is 0.005 

ms- 1. A wind-tunnel calibration in 1982 indicated that 

the factory calibration is accurate to about ± 7 percent 

for the units used in this study. 

High-frequency temperature fluctuations can also be 

measured with a sonic anemometer. However, the 

Campbell-Unsworth design did not function well as a 
G12 STUDIES OF EVAPOTRANSPIRATION 

thermometer, and, instead, the CA-27T anemometer 

has a built-in 12.5-J.Lm chromel-constantan thermocouple mounted 4 em laterally from the center of the sonic 

path. This thermocouple has a time constant of about 

0.01 s. The reference junction for the thermocouple is 

located in the metallic mount for the anemometer and 

has a time constant of about 20 min. The thermocouple 

signal is amplified to provide an output of 0.25 vrc. 

Output from the sonic anemometer, V8 , and from the 

fine-wire thermocouple, V T' is in volts. The fluctuating 

component of the vertical wind, w ', is w - w, so 

(11) 

where 

w{=the fluctuating component of wind for sample i, 

ms- 1; 

C8 =sonic anemometer calibration, ms-1V- 1; 

Vsi=sonic anemometer volt reading for sample i, V; 

and 

V8 =mean sonic anemometer reading for sampling 

N 

period, ~ V8 )N 

i=1 

Likewise, 

(12) 

where 

T{=fluctuating temperature component, oc; 

Cr =thermocouple signal calibration, oc v- 1

; 

Vri=amplified thermocouple output for sample i, V; 

and 

Vr=mean amplified thermocouple output, V. 

For the instruments used here, C8 =1 ms- 1V- 1 and 

Cr=4°C v- 1. Substituting these expressions into the 

eddy-correlation equation (eq. 4) for sensible heat, 

. (13) 

LYMAN-ALPHA HYGROMETER 

High-frequency humidity measurements were obtained with a Lyman-alpha hygrometer (fig. 4C), 

which measures the attenuation by water vapor of ultraviolet radiation having a wavelength of about 

121.56 nm (the Lyman-alpha spectral line). Water vapor has a conveniently high attenuation coefficient at 

this wavelength, while the coefficient for oxygen is 

about an order of magnitude smaller. Radiation at this 

wavelength is generated by a hydrogen-glow discharge 

lamp and is detected by a nitric oxide ion chamber 

(Buck, 1976). Magnesium fluoride windows set the 

lower wavelength limit of detected radiation at about 

115 nm. The ion chamber itself imposes the upper 

wavelength limit at about 135 nm. Buck (1976) has 

poineered the development of Lyman-alpha hygrometers and provides further description of their design 

and operation. 

For monochromatic radiation, the relationship between transmitted radiation and water-vapor density 

is provided by a form of Beer's law (Campbell, 1977, 

p. 48): 

Vn=V0 exp (-kpx) (14) 

where 

V H =hygrometer voltage, which is proportional to 

the transmitted light intensity; 

V0=voltage proportional to the emitted light intensity; 

k =attenuation coefficient, m2g-1; 

p=water-vapor density, gm-3; and 

x =path length, m. 

The hydrogen-ion glow tubes in the Lyman-alpha 

hygrometers emit a low-level radiation continuum in 

addition to the spectral line at 121.6 nm, which causes 

departures from the simple Beer's law attenuation. 

These departures are satisfactorily accounted for by 

assuming a modified form of Beer's law that includes a 

vapor-density-dependent attenuation coefficient. The 

attenuation coefficient was found by laboratory calibration assuming 

kx=a+bp (15) 

so that 

Vn=V0 exp [ -(a+bp)p] . (16) 

The values for a among the different instruments, as 

determined by calibration in an environmental chamber or by pumping air of known vapor density through 

the light path, ranged from 0.200 to 0.235 m3g-1; b 

ranged from -0.0027 to -0.0043 m6g-2 for different 

hygrometers but did not shift greatly for a given hygrometer with time. 

Noting that Pi =pi-p, substituting Pi and pinto equation 16, taking logarithms of both sides, and subtracting, yields 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G13 

(17) 

where 

N 

VH= L VHi/N. 

i=1 

If fluctuations of V Hi about V H are small, In V H/V H 

may be approximated by V H/V H, and equation 17 can 

be rearranged 

vHi -vH 

Pi - VH(a+2bp) 

Recalling that w'=Cs(Vsi-V8 ), 

cs c---) v .v .-vv -VH(a+2bp) st Ht s H 

(18) 

(19) 

Substitution of equation 16 into the eddy-correlation 

equation for evapotranspiration and multiplying by A. 

to obtain latent heat flux yields 

The voltage output from the sonic anemometer, the 

fine-wire thermocouple, and the Lyman-alpha hygrometer are integrated over 0.1-s intervals, and the integrated voltages for alternate 0.1-s intervals are accumulated in the microprocessor to produce means and 

covariances. The following block averages over 2.5-min 

(before June 1981) or 5.0-min (during and after June 1981) 

intervals were accumulated: 

N N N 

N 

_LvsiVriiN- _Lvsi _LvrJN2 , 

and 

These were stored by the microprocessor, accumulated, 

and averaged for a longer (typically 30-min) userselected interval in the data logger. At the end of the 

interval, the longer term averages were printed on 

paper tape and recorded on magnetic cassette tape. 

Computation of sensible heat flux from the recorded 

data-logger counts requires that the counts be multiplied by the density of air, which is a function of air 

temperature and of vapor density. However, the actual 

air temperature is not provided by the thermocouple 

output. Also, latent heat flux computations need to be 

corrected using the actual mean vapor density. The 

Lyman-alpha hygrometers show substantial slow diurnal drift about a persistent mean, the drift mainly 

caused by variations in the intensity of the emitted 

Lyman-alpha radiation. Consequently, the hygrometer 

provides a relatively reliable measure of water-vapordensity fluctuations but not of absolute vapor density. 

Because of these factors, temperature and water-vapor 

densities were measured independently using the instrumentation described above to provide 30-min averages of air temperature and vapor density simultaneously with the eddy-correlation data on other 

cassette tapes. 

The 30-min average values of the eddy-correlation 

sensor outputs of temperature and vapor density, and 

of net radiation and soil heat flux, were read from the 

cassette tapes into a mainframe or microcomputer, 

where the appropriate multipliers were computed to 

provide sensible and latent heat flux in wm-2• The 

data were also used to compute the energy-budget closure, 

(21) 

The energy-budget closure provides a measure of the 

accuracy of the flux measurements, as described below. 

OTHER INSTRUMENTATION 

Net radiation was measured using commercially 

available miniature net radiometers (Fritschen, 1965) 

manufactured by Micromet Instruments, Bothell, 

Wash. These net radiometers have an output of about 

5 f.Lvw- 1m2 (microvolts per watt per square meter) of 

net radiation. Output from these net radiometers was 

read out directly in terms ofwm-2 through a millivoltintegrating module in the data logger. 

Soil-heat flux was measured using soil-heat flux 

plates and (or) thermocouple pairs. The soil-heat flux 

plates were constructed and calibrated by one of the 

authors (Weaver) according to the method of Tanner 

( 1963 ). They were constructed by winding copperconstantan thermopiles on glass plates covered by electrically insulated aluminum. The thermal conductivity 
G14 STUDIES OF EVAPOTRANSPIRATION 

of the plates is about 0.8 wm-1K-1, which is about the 

same as that for fairly dry soil. 

The data loggers, sonic anemometers, and Lymanalpha hygrometers all operate with very low current 

drain and were powered in the field using eight D-cell 

batteries for each data logger and a 12-V lantern battery for each sonic anemometer-Lyman alpha hygrometer pair. The ability to operate the systems with small 

DC power sources, rather than AC power, greatly enhanced the portability of the systems and minimized 

power supply maintenance. 

FIELD MEASUREMENTS 

Three sites were initially chosen for eddy-correlation 

measurements of evapotranspiration: a thicket of oldgrowth saltcedar at the New Mexico State Wildlife 

Refuge at Artesia; a thicket of saltcedar regrowth after 

mowing located about 1 km to the south; and a site with 

replacement vegetation, including annual weeds and 

alkali sacaton grass, located about 5 km southsoutheast of the old-growth site. Evapotranspiration 

from these sites was sampled repeatedly during the 

growing seasons in 1980 and 1981. In 1982, measurements were made at these and at four other sites within 

the study area in order to determine the areal representativeness of the three sites that had intensive measurements. The type of vegetative cover for all seven 

sites is given in table 1, and the locations are shown in 

figure 1. 

Typically, evapotranspiration measurements were 

made over saltcedar by mounting the eddy-correlation 

sensors and net radiometer on scaffolding or on a television antenna tower at a height of about 1 m above the 

average height of the cover. Such a height left the instruments about level with an occasional protruding 

TABLE !.-Locations and descriptions of the sites at which evapotranspiration measurements were made 

Saltcedar 

Site Location 

Old growth -- --- --------------------- 300 m NNW of SE corner of 

Artesia Wildlife Refuge, 

NWl/4 of SEl/4 of sec. 35, 

T. 16 S., R. 26 E. 

Mowed regrowth --------------------- 300m WSW by W ofSE corner of Artesia Wildlife 

Refuge, SWl/4 of SEl/4 of 

sec. 35, T. 16 S., R. 26 E. 

Wet old growth ---------------------- 150m east of Pecos River 

bank, about 900 m NNE 

of El Paso Pipeline river 

crossing; NWl/4 of SEl/4 of 

sec. 1, T. 8 S., R. 25 E. 

Burned regrowth --------------------- SEl/4 of NEl/4 of sec. 13, T. 17 

S., R. 26 E. 

Water 

table 

depth 

(meters) 

3.4 ± 0.3 

3.3 ± 0.3 

0.6-1.0 

1.5-2.0 

Approximate 

height of 

vegetation 

(meters) 

3 

2.5 

5 

2.5 

Remarks 

Mowed in 1977. 

Burned in about 197 4. 

Replacement vegetation 

Site Location 

Grass and forbs ------------------- Near the highway crossing of 

the Pecos River east of 

Artesia, SEl/4 of SEl/4 of 

sec. 12, T. 17 S., R. 26 E. 

Forbs ----- -------- ------------- --- About 1.5 km west of Lea 

Lake in the SEl/4 of 

NWl/4 of sec. 36, T. 11 S., 

R. 25 E. 

Grass----------------------------- On Bitter Lakes National 

Wildlife Refuge, SEl/4 of 

SWl/4 of sec. 34, T. 9 S., 

R. 25 E. 

Dominant 

species 

Water 

table 

depth 

(meters) 

Alkali sacaton 1.7 ± 0.6 

grass, kochia, 

and seepweed 

l{ochia 1.5 

Alkali sacaton 1.6-2.1 

grass 

Approximate 

height of 

vegetation 

(meters) 

0.2 

0.1 

0.3 

Remarks 

Root plowed in 

1974. 

Root plowed in 

1974. 

Root plowed in 

1974 or 1975. 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G15 

dead branch or extra-tall plant. However, the fetch requirement that the instruments be no higher above the 

canopy than .01 times the horizontal distance to the 

upwind limit of the thicket precluded placing the instruments at greater heights above the vegetation. 

Soil-heat flux was measured using a soil-heat flux 

plate inserted about 1 em below land surface and by one 

or two sets of thermocouples placed 2 and 10 em into the 

soil. The soil-heat-flux measurements are necessarily 

point measurements and are subject to substantial 

variation owing to alternate sunning and shading of 

the soil at the sampling point and to spatial variability 

in the thermal properties of the soil. 

At the replacement vegetation sites, the instrumentation was mounted on tripods or on guyed 1.9-cm (3/4-

in) pipes at heights ranging from 1.6 to 2.0 m above the 

land surface and at least 1 m above the taller plants in 

the vicinity of the setup. Soil-heat flux was measured 

in a manner similar to that for saltcedar. 

Measurements were made for approximate 5-d periods in June and October 1980 and in May, June, and 

September 1981, and for approximate 10-d periods in 

June and August 1982. For most of the measurements, 

at least two, and as many as four, sets of instruments 

were in operation. However, in August 1980 and again 

in August 1981, rain-associated problems caused both 

of the available sets of instruments to fail early in the 

tests, and insufficient data were collected at either time 

to allow a valid analysis. Schedules of successful runs 

at each site, along with a key identifying the instruments used for the measurements and the approximate 

depth to water during the run, are listed in table 2. 

a: 700 w 

1-

w 

::E 600 --NET RADIATION 

w ----SENSIBLE HEAT FLUX 

a: ------LATENT HEAT FLUX c 500 -·-·-SOIL HEAT FLUX 5 

0 

a: 

~ 

0 

1-

1-

c 

~ 

~ /;_;'\'(! >. ~/ \.___- 1-

a; ......._ __.../ '-.J I\\ \ z 

w lj / ""' 0 J / /\._. .""~ X "'/ . ~ 

::;:, 

.J 

1.1.. '~ 

-100 

0 4 8 12 16 20 24 

TIME (MOUNTAIN DAYLIGHT TIME) 

FIGURE 5.-Daily cycle of energy fluxes measured at the wet oldgrowth site on September 1, 1982. 

DISCUSSION 

Typical sets of daily measurements are shown for 

saltcedar in figure 5 and for replacement vegetation in 

figure 6. Radiation flux is considered positive toward 

the canopy or soil surface, and the other fluxes are 

considered positive away from the soil or canopy surface. 

The net-radiation curves are typical of those measured on sunny summer days at this latitude. Time is 

shown as mountain daylight time, and the locations are 

slightly east of the 105th meridian, so that peak radiation occurs at about 1300 hours. Maximum net radiation is greater for the saltcedar than for the replacement vegetation because of slightly lower shortwave 

reflectance and because of substantially lower surface 

temperatures, which reduce emitted longwave radiation. Net radiation becomes negative at night as shortwave radiation goes to zero and the outgoing longwave 

radiation exceeds that incoming. 

Sensible heat flux typically is negative at night as 

heat moves from the air to the cool radiating surface, 

but becomes positive during the day, when the surface 

becomes warmer than the air. This behavior was followed in all the measurements made over saltcedar and 

over replacement vegetation. However, a day's run 

over an alfalfa field located near the burn site showed 

a negative sensible heat flux throughout the day as 

heat moved from the air to the evaporatively cooled 

alfalfa-an example of the oasis effect. 

Latent heat flux generally is near zero at night for 

the vegetative communities studied in this investiga600 

a: 

w 

1- --NET RADIATION w 

:::E ---- SENSIBLE HEAT FLUX 

------LATENT HEAT FLUX w -·-·-SOIL HEAT FLUX a: 

c( 

::;:, 

0 

0 

a: 

w 

~ 

C/) 

1-

1-

c( 

~ 

~ 1~:'--v, ~ 

~ 

~ i/:j'~~~f~ z 

w .->-/-· \, ' 0 I' • _..-

X 

::;:, / ...I 

1.1.. 

-100 

0 4 8 12 16 20 24 

TIME (MOUNTAIN DAYLIGHT TIME) 

FIGURE 6.-Daily cycle of energy fluxes measured at the grass and 

forbs site on June 26, 1982. 
G16 STUDIES OF EVAPOTRANSPIRATION 

TABLE 2.-Schedule of eddy-correlation measurements made during the study 

[Lw, Lyman-alpha hygrometer-Washington State Univ.; Lc, Lyman-alpha hygrometer-property of Campbell Scientific, Inc. (CSI); Un), Lyman-alpha hygrometer no. n-property of 

USGS; He, Sonic anemometer-property ofCSI; H(n), Sonic anemometer no. n-property of USGS; Hw, Sonic anemometer, property of Washington State Univ.; G, Soil heat flux plate; 

Gt, Soil thermocouple pair; NR, Net radiometer; sc, saltcedar; dashes indicate no data] 

Beginning Beginning Period Number of Depth to 

Site date time of record Instrument integrations water 

(hours) (meters) 

June 1980 

Grass & forbs 24 1940 34 Hl, L2, 2 NR's 2 1.9 

H3, Ll, H2, L3, G 

Old-growth se 25 1000 52 H3, L2, 2 NR's 2 3.7 

Saltcedar Hl, Ll 

H2, L3, G 

Mowed sc 26 1830 19 Hl, Ll 1 3.5 

October 1980 

Grass & forbs 21 1030 27 Hl, L2, Gt, NR 4 1.7 

H2, Ll, Gt 

Old-growth sc 22 1700 43 Hl, Ll, Gt, NR 2 3.3 

Mowed se 22 1700 43 H2, L2, Gt 2 3.1 

May 1981 

Grass & forbs 11 1100 26 H2, L2, Gt, NR 1 1.7 

Old-growth se 12 1600 25 H2, L2, Gt, NR 1 

Mowed sc 13 1700 24 H2, L2, Gt, NR 1 

June 1981 

Grass & forbs 22 1600 38 H2, L2, G, NR 2 1.4 

Hl, Ll, G, NR 2 

Old-growth se 24 1030 45 H2, L2, Gt, NR 12 3.3 M~wed se 24 1030 45 Hl, Ll, Gt, NR 2 3.0 

September 1981 

Old-growth se 14 0830 57 Hl, Ll, Gt, NR •a 3.2 

Mowed sc 14 0830 80 H2, L2, G, NR 123 

He, Lc 123 

Grass & forbs 16 2030 20 Hl, Ll, G, NR 11 

He, Lc 11 1.2 

June 1982 

Grass & forbs 6-22-82 1430 112 H3, Ll, G, NR 7 2.1 

Hl, L2 

Old-growth se 6-24-82 0800 35 H4, L2, G, NR 2 3.3 

Burnedsc 6-26-82 0700 55 H4, L2, G, NR 3 2.1 

Wet old-growth se 6-28-82 1300 53 H3, Ll, G, NR 1 1.0 

Grass 6-29-82 0900 15 H4, L2, G, NR 31 2.1 

August 1982 

Grass & forbs 8-24-82 0830 478 H4, Lw, G, NR56 1 2.1 

Old-growth se 8-25-82 1000 122 H3, L2, G, NR 2 3.3 

Burned sc 8-26-82 1230 54 He, Le, NR7 2 

Mowed sc 8-28-82 1000 56 Hw,Lw, G, NR 2 

Wet old-growth se 8-29-82 92 H3, L2, G, NR 3 0.6 

Grass 8-29-82 1900 88 H4, L4, G8 3 2.5 

He, L2, G,NR 2 

Forbs 8-31-82 1100 51 Hw,Lw, G,NR 2 1.7 

120-hr period; 4-hr shortfall at night. 5After 0800, 8-26-82, Rn was reconstructed from R8, T8 , T0 , and Pa· (Abbreviations 2Two 16-hr periods; 8-hr shortfall at night. explained in text.) 39-hr shortfall, mainly at night. 6L3 replaced Lw at 1330, 8-25-82. 4Record not continuous. 7No G measured; assumed G=0.07 Rn. 

8Until1400, 8-31-82, Rn was reconstructed from R8, T8, T"' and Pa· 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G17 

tion, as the plant stomates tend to close and the vapor 

deficit in the canopy becomes small. Typically, the latent heat flux starts positively upward about sunrise 

and returns to near zero about sunset. 

Soil-heat flux is generally positive (heat moving 

from the surface into the soil) during the day and is 

negative at night. Because the measurement is made at 

a point beneath partial vegetative cover, the measured 

soil-heat flux fluctuates during the day as the plate is 

alternately in sun and shade. Soil-heat flux is quite 

large at the replacement-vegetation sites, which typically have sparse vegetative cover, and maximum daytime soil-heat flux is typically about one-third of maximum net radiation. For the saltcedar sites, on the other 

hand, maximum soil-heat flux is commonly about 10 

percent of maximum net radiation. 

ENERGY-BUDGET CLOSURE 

Throughout the measurements, the eddy-correlation 

fluxes have been qualitatively correct. Positive sensible heat flux was consistently indicated whenever upward air temperature gradients were measured or 

whenever surface temperatures, as measured by an 

infrared thermometer, exceeded air temperature. Likewise, negative sensible heat fluxes always coincided 

with downward air-temperature gradients or with surface temperatures cooler than air temperatures. Measured vapor fluxes also always appeared to be qualitatively consistent, showing expected patterns in almost 

all cases. 

Despite the good qualitative behavior of the eddycorrelation fluxes, it is obvious from figures 5 and 6 

that the magnitude of the sum of the sensible and latent heat fluxes is substantially less than the net radiation minus soil-heat flux. For example, the daily sums 

of the energy-flux densities at the wet old-growth 

saltcedar site for September 1, 1982, included 14.9 

MJm-2 of net radiation minus 1.1 MJm-2 of soil-heat 

flux, or 13.8 MJm- 2• The daily sum of sensible and 

latent heat fluxes measured by the eddy-correlation 

system was 8.4 MJm-2, or 0.61 of the energy budget. 

Similarly, for the grass and forbs site, net radiation 

minus soil-heat flux was 12.8 MJm - 2 on June 26, 1982, 

whereas the sum of sensible and latent heat fluxes was 

8.6 MJm - 2, or 0.67 of the energy budget. Thus, a substantial portion of the energy budget is missed by the 

eddy-correlation system, as will be discussed in greater 

detail below. 

On a half-hourly basis, the energy-budget error is 

largest in the morning and becomes progressively 

smaller in the afternoon. This probably occurs because 

of heat storage in the soil above the soil-heat flux plate 

and in the biomass, and can be minimized by integrating all flux measurements over a 24-h period. The need 

for such integration is particularly important because 

of the point-measurement character of the soil-heat 

flux. Collection of data for short-term analysis would 

have required the measurement of soil-heat flux by an 

array of sensors, as recommended by Tanner (1963) and 

by Fritschen and Gay (1979). 

During each run, eddy-correlation data generally 

were collected at each site for at least 24 h, and sometimes for more than 100 h. Results of these runs were 

integrated over 24-h periods to minimize diurnal variations in soil and biomass heat storage. If data were 

collected for other than 24 h, the data commonly were 

integrated twice, once starting with the beginning time 

and again starting with a later time that was an even 

multiple of 24 h previous to the end time. This procedure was followed to ensure full use of the collected 

data. Fluxes measured by eddy correlation and the 

energy-budget components measured at the various 

sites are presented in table 3, along with the fraction of 

the energy budget measured by the eddy-correlation 

system. Although this fraction ranges from 0.6 to 1.0, it 

frequently is in the 0.6 to 0. 7 range for all sites except 

the old-growth saltcedar site, where closure of the 

energy budget is consistently better. 

Failure to close the energy budget has been a matter 

of persistent concern since the start of the study, and 

several attempts have been made to identify the cause 

and correct the problem. Duplicate systems were operated side by side repeatedly throughout the project, and 

the measured fluxes almost always agree within 10 

percent, suggesting that individual sensors and data 

loggers are consistent. Careful calibration of the 

Lyman-alpha hygrometers in an environmental chamber in September 1980 and by pumping air containing 

a known vapor content in January 1983 led to nearly 

identical calibrations. The good agreement achieved by 

the careful calibrations suggests that no systematic 

error due to calibration shifts occurred. 

The calibrations indicated that the voltage output of 

the anemometers deviated by as much as ± 7 percent 

from the factory calibration, suggesting that little bias 

in the estimates due to the use of the factory calibrations should have occurred. 

The shortwave response of the net radiometers was 

determined periodically by simultaneously shading 

and exposing both the net radiometer and a precision 

Eppley pyranometer, and calibrating the net radiometer accordingly. These calibrations showed some deterioration in the response of the net radiometers as the 

polyethylene shields became scratched, but the original factory sensitivity was restored when new shields 
G18 STUDIES OF EVAPOTRANSPIRATION 

TABLE 3.-Summary of daily sums of energy-density measurements at the various sites for which eddy-correlation data were collected 

[Energy density in megajoules per square meter. H, sensible heat flux; Lee, latent energy flux by the eddy-correlation technique; Leb, latent energy flux as the residual of Rn -G-H; 

G, soil heat flux; Rn, net radiation; Rec, recovery ratio, or fraction of energy budget measured by the eddy-correlation system] 

Date H Lee Leb G Rn Rec 

Old-growth saltcedar site 

1980 

June 25 ------------------------------------- 10.2 2.7 4.5 1.7 16.4 .88 

11.2 4.1 5.5 1.7 18.4 .92 

10.1 2.5 4.8 1.9 16.8 .85 

26 ------------------------------------- 9.7 2.4 4.1 3.0 16.8 .88 

10.0 2.4 4.2 2.7 16.9 .87 

Oct. 22 ------------------------------------- 3.7 3.9 4.2 -0.3 7.6 .96 

23 ------------------------------------- 4.7 3.9 4.8 -1.0 8.5 .91 

1981 

May 12 ------------------------------------- 5.3 7.5 11.8 0.6 17.7 .75 

June 24 ------------------------------------- 7.6 1.7 3.9 0.6 12.1 .81 

25 ------------------------------------- 13.4 1.9 2.2 0.5 116.1 .98 

Sep. 14 ------------------------------------- 3.2 10.8 10.8 0.4 114.4 1.00 

15 ------------------------------------- 2.2 9.2 12.3 -0.4 14.1 .79 

1982 

June 24 ------------------------------------- 10.9 3.2 7.9 0.8 19.6 .75 

25 ------------------------------------- 12.7 2.9 5.2 1.4 119.3 .87 

Aug. 28 ------------------------------------- 6.2 3.2 7.6 1.8 15.6 .68 

29 ------------------------------------- 7.9 3.1 6.1 2.0 16.0 .79 

Mowed saltcedar site 

1980 

June 26 ------------------------------------- 4.4 2.0 210.0 

Oct. 22 ------------------------------------- 2.4 3.8 4.6 0.6 7.6 .89 

23 ------------------------------------- 3.8 3.0 5.5 -0.8 8.5 .73 

1981 

May 13 ------------------------------------- 4.8 5.4 11.7 0.0 16.5 .62 

June 24 ------------------------------------- 1.1 8.8 14.6 0.9 16.6 .63 

25 ------------------------------------- 2.3 8.4 15.0 0.9 18.2 .62 

Sep. 14 ------------------------------------- 3.4 5.8 10.5 1.1 15.0 .66 

15 ------------------------------------- 2.3 5.7 10.7 0.8 13.8 .62 

16 ------------------------------------- 3.2 7.3 11.0 1.4 115.6 .74 

1982 

Aug. 28 ------------------------------------- 3.0 4.8 10.2 1.7 14.9 .59 

29 ------------------------------------- 3.0 5.9 10.8 1.7 15.5 .64 

Bum site 

1982 

June 26 ------------------------------------- 6.9 5.0 11.4 1.5 19.8 .65 

27 ------------------------------------- 6.0 4.1 8.3 1.1 15.4 .71 

7.7 4.7 10.6 1.4 19.7 .68 

Aug. 27 ------------------------------------- 34.2 36.6 8.9 1.4 14.5 .82 

28 ------------------------------------- 34.6 37.1 6.8 1.3 12.7 1.03 

Wet old-growth site 

1982 

June 27 ------------------------------------- 1.0 7.1 12.3 1.9 15.2 .61 

28 ------------------------------------- 0.3 12.4 15.6 41.8 17.7 .80 

29 ------------------------------------- 0.9 12.0 16.1 41.9 18.9 .76 

Aug. 30 ------------------------------------- 2.1 5.8 10.3 0.9 13.3 .64 

31 ------------------------------------- 2.8 5.5 11.1 0.5 14.4 .60 

Sep. 1 ------------------------------------- 3.2 5.2 10.6 1.1 14.9 .61 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G19 

TABLE 3.-Summary of daily sums of energy-density measurements at the various sites for which eddy-correlation data were collectedContinued 

Date H Lee Leb G Ree 

Grass and forbs replacement-vegetation site 

June 24 

Oct. 21 

May 11 

June 22 

1980 

1981 

23 -------------------------------------

Sep. 16 -------------------------------------

June 22 

23 

24 

25 

26 

Aug. 24 

27 

28 

1982 

6.0 

3.1 

3.4 

6.0 

8.1 

6.9 

7.9 

6.4 

1.5 

4.8 

4.6 

4.7 

5.2 

5.7 

5.1 

2.9 

2.6 

4.4 

2.5 

2.3 

2.5 

3.5 

1.7 

1.0 

1.5 

0.9 

2.6 

3.2 

3.3 

3.1 

4.1 

3.6 

3.5 

3.9 

5.5 

3.5 

4.7 

2.2 

2.9 

7.2 

1.6 

3.3 

4.3 

1.1 

5.4 

8.5 

7.7 

7.5 

8.1 

8.7 

7.7 

7.2 

7.3 

5.3 

4.0 14.7 .79 

0.3 5.6 1.02 

0.5 6.8 0.94 

0.9 14.1 .72 

0.7 10.4 1.01 

0.6 10.8 .77 

0.7 12.9 .77 

0.9 8.4 .97 

-0.8 16.1 .59 

1.8 15.1 .60 

1.1 13.4 .64 

1.3 13.5 .64 

2.2 15.5 .70 

1.9 16.3 .65 

2.6 15.4 .67 

2.1 512.2 .67 

3.1 613 .75 

3.3 613 .59 

Grass site 

June 27 

Aug. 30 

31 

1982 

Sep. 1 - - - - - - --- - - - - - - - - - - - - - - - - - - - --- - - - - - -

6.6 

4.1 

2.9 

3.8 

3.1 

4.2 

1.4 

4.5 

3.4 

3.5 

4.2 

4.2 

7.6 

7.2 

7.2 

0.6 714.8 .56 

2.2 8 

3.0 8 

2.1 13.1 .66 

4.4 813.1 .84 

2.7 14.1 .74 

Forbs site 

1982 

Aug. 31 ------------------------------------- 3.6 

Sep. 1 ------------------------------------- 4.0 

iLess than 24-hr periOd; shortfall at night. 

2Less than 24-hr period; Lee three times that at old-growth site. 

3Includes Lee of about 3 MJfm2 and H of -0.9 MJ/m2 that are night time anomalies. Data 

not included in evapotranspiration or other analyses. 

4G not measured, but estimated as 0.1 Rn. 

5Jntegration from 0800 to 1800. 

were installed. Field calibrations were made frequently enough so that no systematic problem due to 

calibration drift should have arisen. 

The total response of the net radiometers was also 

checked in 1982 by independently measuring the components making up net radiation, as given by the equation 

(22) 

where 

Rsd =shortwave incoming radiation; 

3.0 

6.4 

7.6 

1.2 

1.9 

11.2 

13.5 .60 

6Estimates based on measurements of solar radiation, albedo, surface temperature, and 

air temperature resulted in computed net radiation of 19.5 MJ on Aug. 27, and of 16.8 MJ 

on Aug. 28. These values are implausibly large and were replaced by the rounded values for 

net radiation measured at the grass and the forbs sites a few days hence. 

7Integration for 19-hr period; Lee for 16-hr period, including entire daylight interval. 

SJntegration for 21-hr period; Rn not available or indirectly estimated. 

R su =shortwave outgoing radiation; 

Ld = longwave incoming radiation; and 

Lu = longwave outgoing radiation. 

R sd was measured using a Kipp pyranometer pointed 

upward. The same pyranometer was periodically inverted to obtain the shortwave reflectance of the 

canopy. Longwave outgoing radiation was computed 

from the surface temperature of the canopy measured 

with the infrared thermometer using the equation 

(23) 
G20 STUDIES OF EVAPOTRANSPIRATION 

where 

e = longwave emissivity of the surface, dimensionless; 

cr=Stefan-Boltzman constant, 5.67 x lo-s 

Wm - 2K-4; and 

T8 =surface temperature, K. 

E was assumed to be 0.98 for the surface in each case, as 

almost all vegetative and bare-soil surfaces have an 

emissivity ranging from 0.95 to 1.00. One estimate of 

Ld was obtained using air temperature in the equation 

(Montieth, 1973, p. 34) 

(24) 

where Ea =longwave emissivity of the atmosphere, computed by the empirical equation 

(25) 

where Ta=air temperature, K. 

A second estimate of Ea was obtained using the vapor 

density of air at a height of approximately 1 m above 

land surface in the equation (Brutsaert, 1975) 

e=0.58 p117 , (26) 

where p=water vapor density, in gm-3• 

These checks consistently indicated a net radiation 

that is about 10 percent less than that measured by the 

Fritschen (1965) net radiometer. These estimates suggest that the longwave calibration for the Fritschen net 

radiometer may be in error, and that net radiation is 

slightly overestimated on an algebraic basis. However, 

the error would appear to be substantially smaller than 

the energy-budget-closure error. 

Another potential source of error involves the frequency cutoff for high-frequency eddies that arises because of limits on the rate of sampling imposed by the 

response time of the sensors or by the processing speed 

of the data logger. The response times for the fine-wire 

thermocouple and the infrared hygrometer are on the 

order of 0.01 s. Response time of the anemometer is 

difficult to determine, because it can result from averaging the velocity of a small eddy over the 10-cm sonic 

path length, as well as the speed of response of the 

electronics. Nonetheless, it almost certainly is substantially less than 0.1 s. The data logger samples by simultaneously integrating each signal for 0.1 s. This integration smoothes and attenuates high-frequency 

signals, so that 98 percent of a sinusoidal signal with a 

period of 1 s, 76 percent of a sinusoidal signal with a 

period of 0.25 s, and none of a sinusoidal signal with a 

period of 0.1 sis measured. Moreover, the covariances 

of water-vapor density versus vertical wind and air 

temperature versus vertical wind are attenuated by 

the product of the attenuations of the two signals. 

Hence, sampling by integration results in an effective 

sampling frequency cutoff of about 3-4 Hz. Tests were 

conducted to determine whether a significant portion of 

the fluxes were not being sampled because of this highfrequency cutoff. In one test, the sensors were instantaneously sampled at about 100 Hz using a microcomputer as a data logger. In other tests, data were 

recorded at heights varying by a factor of 2 to make use 

of the tendency of the eddy frequency to decrease with 

height above land surface according to the relationship 

u f oc-z ' 

where 

f =frequency of eddy, s-1; 

u=mean horizontal windspeed, ms-I; and 

z =height above land surface or zero-displacement 

plane, m. 

In no case was energy-budget closure improved. 

Other tests were made to determine whether some of 

the flux was not being measured because of the lowfrequency cutoff imposed by averaging the fluxes for 

time blocks of2.5 or 5.0 min. However, the change from 

2.5- to 5.0-min averages showed no effect on budget 

closure. Another test in 1979 included side-by-side 

comparison of monitoring systems with 2.5-min and 

variable 5- to 30-min block averages. In general, the 

two systems behaved identically. Occasionally, however, the system for which 15-min or 30-min averages 

were obtained showed a digression in the measured 

fluxes to give combined flux values that substantially 

exceeded the energy budget. These excursions were assumed to represent correlations in wind, temperature, 

and vapor density that do not represent eddy flux. Finally, the fact that systems operating simultaneously 

at different heights gave flux values that agreed as 

closely as side-by-side systems suggests that the measured fluxes were not being attenuated by the lowfrequency cutoff either. 

Another hypothesis concerned possible interference 

with the vertical wind component by the underlying 

base of the original sonic anemometer. It was felt that 

the base could attenuate updrafts but have a smaller 

effect on downdrafts. One experiment performed in a 

strong wind during the winter of 1981 at the Denver 

Federal Center, Colo. suggested that inverted and upright sonic anemometers gave somewhat different sensible heat fluxes. Consequently, the sonic anemometer 

was redesigned (fig. 4B) to eliminate interference by 

the base. No difference in budget closure was detected 

during the subsequent 1982 measurements, which 

were made under much calmer conditions. 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G21 

Another consideration was the relative spacing of 

the fine-wire thermocouple and of the position of the 

Lyman-alpha hygrometer in relation to the sonic path 

of the anemometer. Both temperatures and vapor density were measured at points spatially removed from 

the point at which wind was measured. This separation 

results in some spatial averaging of the correlated 

wind-temperature and wind-vapor measurements, 

which in turn would result in at least minor attenuation of the measured fluxes. The fine-wire thermocouple is physically part of the anemometer and is located 

4 em from the sonic path on a perpendicular through 

the midpoint between the sonic transducers. The 

Lyman-alpha hygrometer is mounted separately, and 

it usually was positioned so that the vapor-density 

measurements were made about 15 em from the sonic 

path. Thus, the vertical wind-vapor density correlation 

should be at least somewhat more attenuated by spaceaveraging than the vertical wind-temperature correlation. However, an experiment was conducted at the 

Nevada Test Site in 1983 in which one Lyman-alpha 

hygrometer was positioned to make vapor-density 

measurements about 5 em from the sonic path, while 

another was positioned to make measurements 15 em 

from the same anemometer. The flux measurements 

determined from measurements from the two hygrometers were almost identical. 

The inherent patchiness of the wild land vegetation 

was also considered a possible cause of the lack of 

energy-budget closure. Theoretically, this patchiness 

could cause horizontal gradients in the vertical flux of 

heat and vapor, at variance with the assumptions made 

in making the measurements. Deviations in the measured flux due to such surface heterogeniety should decrease as instrument height above the surface increases. However, as mentioned above, instruments 

operated at different heights produced nearly identical 

results. Also, experiments were performed over a bare 

field at a lysimeter installation at Davis, Calif.; in alfalfa fields near Artesia, N. Mex., Logan, Utah, and 

Kimberley, Idaho; a field with a dense wheat-stubble 

cover at Kimberley; and over parking lots at Washington State University in Pullman and at the Denver 

Federal Center, Colo. In all of these cases, the surface 

for exchange of heat and vapor areally was quite uniform, but the magnitude of departures in the energybudget closures were similar to those obtained in the 

Pecos River floodplain. 

ESTIMATES OF WATER USE 

Although the lack of energy-budget closure casts 

doubt on the validity of the eddy-correlation measurement of vapor flux, the data can be used to establish 

lower and upper limits on evapotranspiration. The 

lower limit is established by assuming that the latent 

energy flux, AE, measured by eddy correlation (eq. 20) 

is correct. Such an assumption implies that all the 

eddy-correlation error occurs in the sensible heat-flux 

measurement (H, eq. 13). The upper limit of evapotranspiration is obtained by assuming that the sensible 

heat-flux measurements are correct and that all of the 

error occurs in the vapor-flux measurement (AE in 

eq. 20). Under this assumption, the latent heat flux is 

computed as the residual of the energy budget minus 

sensible heat flux: 

(27) 

where His as measured by equation 13. 

The contention that Leb represents a maximum estimate of evapotranspiration is based on the assumption 

that sensible heat flux is not overestimated by the 

eddy-correlation measurements. This is a reasonable 

assumption, in that most of the recognized potential 

sources of error discussed earlier result in underestimates of flux. Also, during the tests performed over 

parking lots, latent heat flux was nearly zero yet the 

eddy-correlation-measured sensible heat flux was less 

than the measured energy budget. These tests thus 

show that sensible heat flux is at least somewhat 

underestimated by the eddy-correlation measurements. 

The eddy-correlation values of latent heat flux, symbolized as Lee, differ greatly from the energy-budget 

residual (Leb) values. Several arguments can be made 

in favor of the energy-budget values: 

1. The output of the fine-wire thermocouple is stable, and the theoretical calibration is well known, suggesting that the sensible heat flux is well determined. 

On the other hand, the Lyman-alpha hygrometer must 

be calibrated experimentally. Although careful calibrations performed in June and September 1980 and in 

January 1983 are in good agreement, the recommended practice of frequent Lyman-alpha calibration 

(Redford and others, 1980) was not followed. Moreover, 

the output of the Lyman-alpha hygrometer shows substantial diurnal drift. It has been assumed that this 

drift affects the offset, but not the slope, of the voltagevapor density curve, and that the flux measurements 

are little affected by the drift. Nonetheless, additional 

uncertainty is added to the vapor-flux determinations. 

Finally, when relative humidity is high, current leakage sometimes occurs along the outer surface of the 

detector tube in the Lyman-alpha hygrometer, creating 

an additional voltage offset that affects the mean voltage, and possibly the calibration of the hygrometer. 

This mean voltage is used as a divisor in computing 
G22 STUDIES OF EVAPOTRANSPIRATION 

vapor flux. Also, as discussed above, the vapor-density 

measurements are made at a point farther away from 

the wind measurements than are the temperature 

measurements, potentially resulting in greater error in 

measuring vapor fluxes. Hence, the vapor-flux computations are subject to many more instrumentation uncertainties than are the sensible heat-flux determinations. 

2. The Leb estimates for the different saltcedar sites 

for May, September, and October (described below) are 

in close agreement, whereas the Lee estimates show 

substantial variance. Soil moisture storage was high at 

those times owing to recent rains, and it is plausible 

that the rates should be similar. 

3. A multiple regression of the energy budget 

(Rn -G) against sensible heat flux (H) and latent heat 

flux (Lee) for the data in table 3 yielded the following 

equation (forced through the origin): 

(Rn -G)=l.13H +1.61Lee (28) 

This relationship suggests statistical confirmation that 

most of the underestimation occurs in the latent heat 

determination. 

4. Estimates of evapotranspiration by the Leb 

method are in good agreement with those determined 

by some other studies, as described below, whereas estimates determined by the eddy-correlation technique 

are much lower than most other measurements. 

Because of the preponderance of data favoring the 

use of the Leb values, those values are emphasized. 

WATER USE BY SALTCEDAR 

Two sets of water-use estimates for saltcedar, 

derived by multiplying Lee and Leb in table 3 by a 

conversion of0.41 mm water per MJm-2d-1, are summarized in table 4 and in figures 7A and 7B. The conversion factor is based on a value for the latent heat of 

vaporization of water of2,450 Jg-1. These figures show 

the water-use measurements for each saltcedar thicket 

for the 3-yr period of measurements. Water use varies 

substantially among sites. Depth to water, plant density, and plant age also vary substantially among sites, 

but there is no apparent correlation between measured 

water use and these factors. For example, the oldgrowth site provides about SO-percent ground cover, 

although it was mapped as dense saltcedar by Mower in 

1958 (Mower and others, 1964). The saltcedar at the 

old-growth site was burned sometime prior to 197 4, but 

the growth seems to be again quite mature. The mowed 

saltcedar thicket, located about 1 km southwest of the 

old-growth site (fig. 1), currently provides about 50-

percent ground cover, although it was mapped as moderately dense (70-90-percent cover) by Mower in 1958. 

TABLE 4.-Summary of daily water-use estimates at the various sites 

Site Date 

Water use (Evapotranspiration) 

(millimeters per day) 

Saltcedar 

1980 

Old growth -------- June 25-26 

Oct. 22-23 

1981 

May 12 

June 24-25 

Sep. 14-15 

1982 

June 24-25 

Aug. 28-29 

1980 

Mowed ------ --- --- June 26 

Oct. 22-23 

1981 

Eddy 

correlation 

1.2 

1.6 

3.1 

0.7 

4.1 

1.2 

1.3 

1.8 

1.4 

May 13 2.2 

June 24-25 3.5 

Sep. 14-16 2.6 

1982 

Aug. 28-29 2.2 

Burned ------------June 26-27 1.9 

Aug. 27-28 2.8 

Wet old growth ----June 27-29 4.3 

Aug. 30-Sep. 1 2.2 

Replacement vegetation 

1980 

Grass and forbs ---- June 24 

Oct. 21 

1981 

May 11 

June 22-23 

Sep. 16 

1982 

June 22-26 

Aug. 24-28 

Grass----------,.--- June 27 

Aug. 30-Sep. 1 

Forbs-------------- Aug. 31-Sep. 1 

1.1 

1.0 

1.4 

0.5 

1.1 

1.4 

1.8 

0.6 

1.6 

1.2 

Energy budget 

residual 

1.9 

1.8 

4.8 

1.3 

4.7 

2.7 

2.8 

2.1 

4.8 

6.0 

4.4 

4.3 

4.1 

3.2 

5.4 

4.4 

1.5 

1.1 

2.9 

1.1 

2.2 

3.3 

2.7 

2.9 

3.1 

2.9 

The site was mowed in the fall or winter of 1977, and no 

shoots were visible in March 1978. This site has a substantial understory of alkali sacaton grass. Depth to 

water at both sites ranges from 3 to 3.5 m. 

The burn site is located about l/2 km south of U.S. 

Highway 83 and about 5 km south-southeast of the 

old-growth site. The site consists of dense saltcedar and 

is regrowth from a burn that occurred in 1974. The 

depth to water is about 2 m, and the vegetation is about 

4 m high. The site is south of the area mapped by 

Mower. 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G23 

10 

WET 

A BURNED OLD GROWTH MOWED OLD GROWTH 

1982 - ~ 8 

-

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AUG. 27-

SEPT. 1 

ITIIIIlJ 

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C'.i 

0 

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SEPT. 14-16 OCT. 22-23 

FIGURE 7.-Summary of estimates of daily evapotranspiration by saltcedar by (A) the energy-budgetresidual method, and (B) the eddy-correlation method. 

The wet old-growth site is a dense saltcedar thicket 

about 3 km north of the U.S. Highway 70 crossing. 

There are no signs that the saltcedar ever burned, and 

the site has a very shallow (less than 1 m) depth to 

water. The saltcedar is about 5 m tall. 

Water use by the different thickets showed great 

variation, particularly during June and August. For 

example, in June 1981 evapotranspiration from the 

mowed saltcedar was about five times that from the old 

growth, based on either method. Other supporting data 

suggest that these differences are qualitatively correct. 

Stomatal diffusion resistances of fronds of the old 

growth were about three to five times greater than 

those of fronds for the mowed saltcedar in June of each 
G24 STUDIES OF EVAPOTRANSPIRATION 

year. Also, in June 1982, the old-growth saltcedar had 

midafternoon frond temperatures, as measured by an 

infrared thermometer, of 43°-45°C, whereas the 

mowed saltcedar frond temperatures were 36°-38°C. 

These frond temperatures suggest that much less evaporative cooling was occurring from the old growth than 

from the mowed saltcedar. Finally, the old-growth 

saltcedar showed visible signs of moisture stress and 

had not flowered, whereas the mowed saltcedar was 

vigorous and in full bloom. Measurements made during 

each June followed a long sequence of hot, dry, sunny 

days, when soil moisture was substantially depleted. 

Finally, examination of hand-auger samples obtained 

at the old-growth site indicated that live roots were 

prevalent in the top 1.8 m of material above the clay 

layers (fig. 8) but were sparse in and beneath the clay 

layers. At the mowed site, on the other hand, live roots 

were prevalent throughout the section down to the 

water table. These observations suggest that the oldgrowth saltcedar derives most of its water from soil 

moisture, while the mowed saltcedar relies heavily on 

ground water during periods of extreme moisture 

stress. 

Because of the alkali sacaton in the understory, 

water use at the mowed site represents a combination 

of water use by the saltcedar and the grass. It is assumed that the saltcedar accounted for most of the 

evapotranspiration. 

Evapotranspiration determined as the energybudget residual (Eeb) for wet old growth (4.4 mm/d) 

slightly exceeded that for the mowed site ( 4.3 mm/d) in 

August 1982. No measurements of evapotranspiration 

from the mowed saltcedar were made in June 1982, but 

the June 1981 evapotranspiration from the mowed site 

(6.0 mm/d) exceeded the June 1982 evapotranspiration 

from the wet old-growth site (5.4 mm/d) by about 

10 percent. Both measurements of Eeb from the burn 

site are intermediate between the old growth and the 

mowed site. 

WATER USE BY REPLACEMENT VEGETATION 

Estimates of water use by the replacement vegetation are listed in table 4 and shown in figures 9A and 

9B. Water use, as measured by the residual-energybudget method, ranged from as low as 1.1 mm/d to as 

much as 3.3 mm/d and typically averaged about 3 mm/ 

d. The value of 2.2 mm/d measured on September 16, 

1981, was measured on a cloudy day and may be atypically low. 

The forbs at the grass-and-forbs site varied greatly 

between measurements, although alkali sacaton and 

saltbush (Atriplex patula) are perennials and were always present. In June 1980, desert seepweed (Suaeda 

suffractescens) was the dominant species, with some 

Russian thistle present. Large bare patches were 

present, and young kochia had died. By the ill-fated 

visit in August 1980, when rain caused instrumentation failures, kochia was the dominant annual and 

desert seepweed had nearly disappeared. Kochia remained dominant in October 1980. 

In May and June 1981, desert seepweed, horsenettle 

(Solanum elaeagnifolium), and an unidentified weed 

were prevalent. By August and September, kochia, a 

mustard plant (Brassica nigra), and Russian thistle 

were again common. In June 1982, kochia, the mustard, and desert seep weed were prevalent (the first 

June in which kochia was dominant). The kochia and 

mustard were still prevalent in August 1982. 

Vegetative cover at the forbs site is dominantly 

kochia. The dominant cover at the grass site is alkali 

sacaton, a grass that apparently was one of the common 

types of cover before the saltcedar infestation began in 

the Pecos valley. 

COMPARISON WITH 

POTENTIAL EVAPOTRANSPIRATION 

Potential evapotranspiration is defined as the evapotranspiration rate from a well-watered reference crop 

that completely shades the ground, such as short grass 

(Penman, 1956) or alfalfa (Jensen, 1973). Potential 

evapotranspiration is dependent on climatic factors, including solar radiation, air temperature, vapor pressure or vapor density, and windspeed, and on crop properties, including the aerodynamic roughness of the 

vegetation, reflectance of the canopy surface to shortwave radiation, the canopy temperature, and the resistance to vapor loss imposed by the leaf stomates. The 

vegetative properties of short grass or alfalfa can be 

estimated, so that potential evapotranspiration can be 

defined by climate alone. The concept is widely used to 

estimate evapotranspiration from phreatophytic vegetation. Hence, a comparison of values measured in this 

study with potential evapotranspiration is useful to 

evaluate the suitability of the potential evapotranspiration concept for such estimates. 

Many different equations exist for computing potential evapotranspiration; the selection depends on the 

climatic data available and on usually site-specific calibration constants (Jensen, 1973). The Jensen-Raise 

equation (Jensen, 1973, p. 73, 74) was used to compute 

potential evapotranspiration for this study. The equation is 

where 

EP =potential evapotranspiration, in mm/d; 

Ta =mean monthly air temperature, in oc; 
en 

a: 

w 

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WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. 

0--..-=----

1 

3 

4 

5 

.... ' 

..... · 

:: :. :_ :.-.: 

··:: .. · ........ 

. ·.·:: .. . ... · ..... . ... · . . . . . . 

Sand, fine to medium 

Silt, sandy 

Sand, silty 

Clay, silty 

Sand, silty, clayey 

Clay, silty, with 

gypsum crystals 

Sand, clayey, silty 

Clay, silty, with gypsum 

crystals above 3.5 m. 

Water table 

Sand, clayey 

A 

Sand, fine to medium 

Clay 

Sand, clayey, to 

clayey sand 

Clay, silty 

c 

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:x::: .. :.:.:.:::. 

Silt, sandy, with 

gypsum crystals 

Clay, silty, with 

gypsum crystals 

Silt, sandy to clayey 

:·.:::x:·:::: Sand, silty to clayey, 

·.::~::x: with gypsum crystals . ·.x·:. ·.::=.:... ·--·- ·::·:?<:::::: 

:.::?. .. -:-: 3-~·:·~ Clay, sandy, with 

· · ·-·(t: caliche nodules 

~--~:-: 

--····- --··-

··-····- ····-- --····· ······-· ··-··-

··········· ·······- 1-:::::..:..:..:.::. 

~Y::::· 

-··-

8 

Sand, fine, silty 

Clay, sandy, sltly plastic 

to sand, clayey 

Sand, fine, with 

silt and clay 

D 

FIGURE 8.-Sample logs showing the nature of the materials from land surface to the water table for each saltcedar 

measurement site: (A) old-growth site, (B) regrowth site, (C) burn site, and (D) wet old-growth site. 

G25 
G26 STUDIES OF EVAPOTRANSPIRATION 

8r-----------------------------------------------------------~ 

>- 6 <( 

c 

a: 

w 

a. 

en 

a: 

w 

1-

w 

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A GRASS AND 

GRASS FORBS FORBS 

1982 • [ill]] ~ 

1981 D 

1980 Em 

.... 

.... 

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~ 4r-----------------------------------------------------------~ 

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::::l 

a: 

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1-

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~ 2 

8 

.... 

.... q 

.... 

MAY 11 JUN. 22-27 AUG. 24 - SEPT. 1 SEPT. 16 OCT. 21 

FIGURE 9.-Summary of estimates of daily evapotranspiration by replacement vegetation by (A) the 

energy-budget-residual method, and (B) the eddy-correlation method. 

Tx =-2.5oC-1.4(e2 -e1)°C/kPa -(altitude in m/ 

550 m)°C; 

R8 =incoming solar radiation, in MJm-2d- 1; 

C1 =38°C+( -2°Cxaltitude in m/305 m); 

C2=7.6oC; 

CH=5.0 kPa/(e2 -e1); and 

eh e2=saturation vapor pressures, in kPa, at the 

maximum and minimum average temperatures for the warmest month of the year. 

Solar radiation records from El Paso, Tex., and mean 

monthly temperatures at Roswell, N. Mex., as provided 

by the National Weather Service, were used for the 

computations. El Paso is located about 205 km southwest of Artesia, and Roswell is located about 60 km 

north of Artesia. The climates of the two cities are 

relatively similar to the climate of Artesia, although 

solar radiation at El Paso might be 10 percent or so 

greater than at Artesia on an annual basis. Results of 

these computations are shown by the potential evaporation curves in figures 10 and 11. Annual potential 

evapotranspiration was computed to be 1. 7 5 m. 

The energy-budget-residual estimates of water use 

by saltcedar ranged from 13 percent of potential evapotranspiration for the lowest value measured at the oldgrowth site to about 65-70 percent for the largest values measured during each month. 

The fact that the estimated maximum water use by 

saltcedar is less than Jensen-Haise potential evapotranspiration is consistent with observations made 

during the study. The empirical constants used in the 

Jensen-Haise method were developed to provide estimates that agree with the measured water use by al-
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G27 

falfa. The stomatal resistance to vapor loss from alfalfa 

is very low, so that the resistance to vapor transport 

from the canopy is comparable to the resistance to heat 

transport. Hence, the alfalfa behaves as a freely transpiring surface and is usually cooler during daytime in 

summer in 'the Artesia area than the air, owing to 

evaporative cooling. All of our observations indicated 

that saltcedar is less freely transpiring, and that the 

resistance to vapor transport is greater than the resistance to heat transport. This is evidenced by the fact 

that saltcedar surface temperatures during sunny summer days, as measured by an infrared thermometer, 

were always higher than the air temperature, both at 

our eddy-correlation measurement sites and at various 

other saltcedar thickets in the floodplain, as spotmeasured in June and August 1982. Thus, the JensenRaise equation will overestimate water use by 

saltcedar, at least in the Roswell basin area, unless a 

crop coefficient is used io adjust the values downward. 

A similar conclusion regarding water use by saltcedar 

in the Gila River floodplain was reached by Culler and 

others (1982, p. P39) on the basis of their water-budget 

measurements of saltcedar evapotranspiration. 

ANNUAL WATER USE 

The potential for salvage of ground water by 

saltcedar eradication is dependent on annual water use 

by both the saltcedar and the replacement vegetation. 

However, only spot measurements were made, and 

these were made only during the growing season. 

Hence, it was necessary to interpolate between measJAN. 1 

A 

ANNUAL WATER USE (m) 

(From areas under curves) 

MAR. 1 MAY 1 JULY 1 SEPT. 1 NOV.1 DEC. 31 

urements, and to extrapolate the measurements 

through the nongrowing season. 

For the growing season, maximum, median, and 

minimum estimates of water use were made by drawing straight-line segments through selected data. 

(More formal interpolation schemes were attempted 

initially, but these schemes inferred low water use 

after the start of the wetter "monsoon" season in July. 

Hence they were deemed less realistic than the use of 

straight-line segments.) For the energy-budgetresidual estimates, minimum water use by saltcedar 

was obtained by drawing line segments through the 

measured values for the old-growth site, including the 

average of only the two lower June measurements. The 

median estimates are based on average values for all 

available saltcedar measurements for each month. The 

maximum water use was determined by the data from 

the mowed site, which gave the largest water-use values in midsummer. Evapotranspiration during the 

nongrowing season (approximately November 15 to 

Aprill), both for saltcedar and for replacement vegetation, was arbitrarily assumed to be 40 percent of potential evapotranspiration. Saltcedar in the Roswell area 

drops its fronds after a killing frost and leafs out again 

in mid-April. Hence, it is reasonable to assume that 

evapotranspiration for both cover types, consisting 

mainly of evaporation from the soil surface, is about 

equal during the nongrowing season. 

The estimation procedure is illustrated in figure lOA. The daily water-use rates for saltcedar measured by the energy-budget-residual method are plotted 

ANNUAL WATER USE 

8 (From area under curve) 

~ 0.6 m 0 

a: 

~ 

ffi 6 1 .... 

~ ...1 

...1 

:E 

;e; 4 

u.i 

gs 

a: 

w 

.... 

~ 

8 

MAR.1 MAY 1 JULY 1 SEPT. 1 NOV. 1 DEC. 31 

FIGURE 10.-Estimated annual water use by saltcedar in the Pecos valley, New Mexico, as determined by (A) the energy-budget-residual 

method, and (B) the eddy-correlation method. 
G28 STUDIES OF EVAPOTRANSPIRATION 

as bar graphs. Line graphs through the selected values 

or means show the estimated minimum, median, and 

maximum rates. The areas under these line graphs 

were integrated to determine the annual water-use 

rates given in table 5. This procedure indicates that 

water use by saltcedar at the old-growth site (the minimum value) may be about 0.77 m/yr, whereas that for 

the mowed site (the maximum value) is about 1.07 m1 

yr. The average for the four sites tested is about 0.9 

m/yr. 

Estimates of annual water use by saltcedar based on 

the eddy-correlation measurements (fig. lOB) were 

made only for the mean of the values for all sites for 

each month. These measurements indicated that water 

use at the mowed site in May and September was less 

than that at the old-growth site, although the mowedsite measurements were greater in June and August 

(table 4). Hence, the approach described above for analyzing the energy-budget-residual measurements 

would have resulted in maximum, mean, and minimum values that are all of about the same magnitude. 

The analysis of the eddy-correlation measurements indicates that the minimum value for average water use 

by saltcedar is 0.6 m/yr. 

Estimates of annual water use by the replacement 

vegetation (fig. 11) were obtained in a similar manner. 

For the energy-budget-residual measurements, minimum water use was determined as that for the grassand forbs-site, using only the seasonally low values for 

June 1980 and June 1981, the minimum value measured in August 1982, and measured values for May, 

September, and October (table 4). The median value 

was obtained using the averages of all the monthly 

measurements. The maximum estimate is based on the 

highest measurement each month, and, in addition, it 

was assumed that the September measurement, made 

on a cloudy day, was equal to the average of the maximum August measurement and the September measurement. These data indicate that water use by replacement vegetation ranged from 0.57 to 0.67 m/yr 

(table 5). 

TABLE 5.-Estimates of the range in annual water use by saltcedar 

and by replacement vegetation in the Pecos River floodplain and of 

projected water salvage due to saltcedar eradication 

Maximum 

Median 

Minimum 

Median 

[In meters per year] 

Saltcedar 

Replacement 

vegetation 

Based on the energy-budget residual 

1.07 

.90 

.77 

0.67 

.62 

.57 

Based on eddy correlation 

0.60 0.40 

Salvage 

0.40 

.28 

.20 

0.20 

ANNUAL WATER USE (m) 

>- (From areas under curves) 

C§ Maximum 0.67 

a: Median 0.62 

~ Minimum 0.57 

C/) 

ffi 6 1-

w 

::!: 

::i 

~ 

JAN. 1 MAR.1 MAY 1 JULY 1 SEPT. 1 NOV. 1 DEC.31 

FIGURE 11.-Estimated annual water use by replacement vegetation 

in the Pecos valley, New Mexico, as determined by the energybudget- residual method. 

Estimates of annual water use based on the eddycorrelation measurements of evapotranspiration (not 

shown in fig. 11) were made in a manner similar to 

those for saltcedar. This analysis resulted in a wateruse estimate for the replacement vegetation of 0.4 ml 

yr, as listed in table 5. 

COMPARISON WITH PREVIOUS RESULTS 

In this study, the best estimates indicate that summertime daily water use by saltcedar is as little as 

1.0 mm/d or as much as 6 mm/d, but estimated rates in 

the range 4 to 5 mm/d are common (table 4). Annual 

water use is estimated to range from 0. 77 to 1.07 m. 

These values are at the low end of the range of those 

measured by other investigators. For example, the 

minimum annual value coincides with the estimate of 

consumptive use by floodplain vegetation determined 

by Gatewood and others (1950) using the inflowoutflow technique. However, only about half the area 

for which they determined consumptive use contained 

saltcedar or other brush. Moreover, their analysis was 

based on 1 yr of record, and changes in soil-moisture 

and ground-water storage were not measured. Hence, 

their estimates could be in error, being either too large 

or too small. 

The median and maximum estimates of annual 

evapotranspiration compare closely with those Culler 

and others (1982) obtained using a water-budget technique. They measured annual water use, including precipitation, of about 1.0 m as an average for three 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G29 

J: 

1-

z 

0 

::E 

0: 

w 

a. 

en 

0: 

w 

1-

w 

::E 

i= 

z 

w 

u 

~ 

z 

0 

i= 

<C 

fa: 4 

(3 

w 

0: 

a. 

0--------0 NORMAL 

)(-- ~ 1980 

&--·----8 1981 

8-----B 1982 

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC 

FIGURE 12.-Monthly precipitation at Roswell, N.Mex., during the period 1980-82. 

reaches that include some mesquite and bare ground. 

The climate of the Gila River floodplain in the vicinity 

of Safford, Ariz., where both of the above studies were 

conducted, is similar to that of the Pecos River floodplain near Artesia. The latitudes are the same (33° N.), 

and rainfall is similar (292 mm at the Gila site versus 

about 320 mm at Roswell, N.Mex.). The altitude of the 

Gila site (about 770 m) is somewhat lower than that for 

the Artesia, N.Mex., site (about 1,000 m). On the basis 

of these comparisons, evapotranspiration rates might 

be anticipated to be slightly lower in our study area 

than in the Gila River study site. 

Our estimates of annual evapotranspiration may be 

somewhat low because most of the measurements were 

made during dry weather, partly by design and partly 

because equipment malfunctions induced by rain prevented measurements immediately afterward. Evaporation rates from wet canopies and wet soil are quite 

high but may be about equal for the saltcedar and the 

replacement vegetation. If the periods following rain 

had been sampled more representatively, our estimates 

of annual evapotranspiration would be at least a little 

higher. Monthly rainfall at Roswell, N. Mex., for the 

study period is shown in figure 12. 

Measurements of daily evapotranspiration using the 

Bowen-ratio method made by Gay and Fritschen (1979) 

and by Leppanen (1981) resulted in typical evapotranspiration rates of 7 mm/d. These values are about 50 

percent larger than those measured during our study. 

Leppanen's measurements were made over rapidly 

growing young saltcedar, which may not be comparable to mature stands. On the other hand, there is no 

obvious explanation for the difference between our results and those of Gay and Fritschen. 

The Bowen-ratio measurements by Gay (1980) and 

the eddy-correlation measurements by Fritschen and 

others (1980) both indicate evapotranspiration rates of 

about 3 mm/d. However, their measurements were 

made on a cloudy day, and the results are not fully 

comparable to most of those in this study. The portion 

of the energy budget used in evapotranspiration during 

their studies was about the same as that which would 

result in an evapotranspiration rate of about 5 mm/d on 

a sunny day. Estimates of evapotranspiration based on 
G30 STUDIES OF EVAPOTRANSPIRATION 

the temperature and humidity profiles, net radiation, 

and soil-heat-flux data presented by van Hylckama 

(1980) for a saltcedar thicket near Gila Bend indicate 

that daily evapotranspiration rates at that site are on 

the order of 8 mm/d, which is in better agreement with 

the results of the other Bowen-ratio studies than with 

our results. On the other hand, the analyses of van 

Hylckama's data are uncertain enough to preclude 

definitive comparisons. 

The estimates of daily evapotranspiration from 

saltcedar made during this study are one-fourth or onefifth those measured in tanks in Arizona by Gatewood 

and others (1950) or by van.Hylckama (1980), but are 

nearly the same as those measured in tanks at 

Bernardo, N.Mex. (U.S. Bureau of Reclamation, 1973). 

The discrepancies with the Arizona tank studies probably arise from oasis effects on those tanks, as described 

in the section "Discussion of Previous Results." 

Energy-budget estimates of water use by replacement vegetation (fig. 11) are in the general range of 

those determined by other workers. Culler and others 

(1982, fig. 10) found that replacement vegetation in the 

Gila floodplain generally was 1-1.5 mm/d in April and 

May and about 2-2.5 mm/d during June through August. Their values are slightly lower than those found 

in this study. However, their vegetative cover was 

mainly poorly established bermuda grass, annual 

weeds, and bare ground. Also, many of their measurements were made immediately after clearing and 

before the replacement vegetation could become well 

established. Hence, the differences are easily accepted. 

Measurements by Fritschen and others (1980) and by 

Gay (1980) at the forbs site indicated that evapotranspiration was about 3 mm/d in July 1979, which agrees 

with our results. 

Although several evapotranspirometer-tank studies 

of evaporation from bare soil and evapotranspiration 

from saltgrass, sacaton, and Russian thistle have been 

made, the depths to water are not comparable to those 

in this study and no comparisons are warranted. 

In summary, the energy-budget evapotranspiration 

measurements made during this study agree exceptionally well with the results of the water-budget study 

conducted by Culler and others (1982), and with 

Bowen-ratio and eddy-correlation measurements made 

in the Pecos River floodplain by Gay (1980) and by 

Fritschen and others (1980). Agreement is less good 

with energy-budget studies conducted in other areas, 

and agreement with tank-measured evapotranspiration is very poor. The extensive water-budget measurements of Culler and others (1982) probably are the 

most representative of the entire floodplain environment, and the good agreement of our results with those 

of that study give substantial credence to our results. 

ESTIMATES OF EVAPOTRANSPIRATION SALVAGE 

Evapotranspiration salvage can be estimated as the 

difference in water use between saltcedar and thereplacement vegetation. Such estimates are, of course, 

fraught with uncertainty, because of the limited number of measurements (17 for saltcedar and 10 for replacement vegetation), the potential error in the measurements, and the difficulty in extrapolating the 

measurements in time and space. Nonetheless, the 

high, low, and median estimates for evapotranspiration provide probable bounds to the magnitude of 

evapotranspiration salvage. One set of salvage estimates can be obtained by comparing high, median, and 

low estimates for each cover type (table 5). These estimates suggest that the reduction in evapotranspiration 

might be as much as 0.4 m/yr or as little as 0.2 m/yr, 

with a median estimate for the energy-budget method 

of about 0.3 m/yr. These represent estimates of the 

maximum anticipated annual salvage. 

An estimate of the minimum anticipated annual 

water salvage can be obtained by subtracting annual 

water use by the replacement vegetation, as determined by the eddy-correlation method, from the eddycorrelation-based estimate of annual water use by 

saltcedar. This analysis indicates that annual water 

salvage by saltcedar eradication is about 0.2 m. 

The above analyses indicate that annual water salvage by saltcedar eradication probably is more than 

0.2 m but less than 0.4 m. Because the energy-budget 

residual or maximum values are considered more reliable than the eddy-correlation values, a salvage estimate of about 0.3 m/yr is the best available. 

Alternate estimates of salvage can be obtained from 

the energy-budget values in table 5 by subtracting the 

minimum evapotranspiration for replacement vegetation (0.57 m/yr) from the maximum water use estimate 

for saltcedar (1.07 m/yr) to determine an upper limit for 

water salvage of0.5 m/yr. Similarly, the minimum estimate can be obtained by subtracting maximum water 

use by replacement vegetation (0.67 m/yr) from the 

minimum for saltcedar (0.77 m/yr). This minimum is 

0.1 m/yr and tends to confirm studies of base flow gain. 

However, the validity of both these high and low estimates is suspect because the measured low saltcedar 

water use generally was coincident with measured low 

replacement-vegetation use. Hence, these alternate estimates lack credence. 

A major assumption in the foregoing analyses of 

water salvage is that saltcedar at the measurement 

sites uses water at a rate typical of that formerly used 

by saltcedar in the now-cleared areas. Selection of sites 

for measurement of water use by saltcedar was limited, 

as most of the remaining saltcedar thickets in the floodplain in the Acme-Artesia reach of the Pecos River 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G31 

were too small to provide adequate fetch for micrometeorological measurements. Hence, it was necessary to 

select sites upstream and downstream from the cleared 

reach to obtain additional sample sites. Of all the sites, 

only the burn site had a depth to water typical of much 

of the cleared area, whereas all three replacementvegetation sites had typical depths to water. However, 

the lack of any apparent relationship between depth to 

water and measured water use by saltcedar, coupled 

with the fact that the water-use rate at the burn site 

was intermediate among rates measured at the other 

sites, suggests that water use may have been adequately sampled. 

Use of water-salvage values to estimate the baseflow gain in the Pecos River between the Acme and 

Artesia gaging stations also is difficult. Although the 

USBR has cleared about 8, 700 ha (21,500 acres) of 

saltcedar in the reach (Leon Marcell, written commun., 

1983), Mower and others (1964, table 7) indicate that in 

1958, 7,160 ha (17,900 acres) had an areal saltcedar 

density of greater than 10 percent and 5,280 ha (13,000 

acres) had an areal saltcedar density of greater than 

25 percent. Neither this study nor that of van 

Hylckama (1980) confirmed a linear relationship between water use and area or volume density. However, 

it seems likely that water use by very sparse stands of 

saltcedar is substantially less than that from moderate 

to dense stands. The lack of transpiring material and 

the sparseness of the thickets suggest that the plants 

are not able to obtain large quantities of ground water. 

Hence, it may be reasonable to assume that water use 

by saltcedar of greater than 10-percent cover is equal to 

that measured, while that at less than 10-percent cover 

might be substantially smaller. However, the expected 

low use by the less than 10-percent cover might be 

compensated for by a large-scale oasis effect on the 

evapotranspiration from the remaining 15-m-wide 

strips of saltcedar along either side of the river. Also, 

the 30-m-wide strip of saltcedar along the 132-km-long 

river reach contains about 400 ha (1,000 acres) of 

saltcedar, all or most of which probably constituted 

dense stands at the time Mower and others (1964) prepared vegetation-density maps. Thus, about 6,800 ha 

(17 ,000 acres) of saltcedar of greater than 10 percent 

density may have been cleared. Such a clearing operation would have resulted in roughly 1.2x 107 to 2.5x 107 

m3 (10,000-20,000 acre-ft) of salvage annually, based 

on the measurements described in this report. This 

value is less than the 3.5 x 107 m3 (28,000 acre-ft) of 

salvage predicted by Mower and others (1964, table 14), 

but it is still sizable. 

The indicated 10,000--20,000 acre-ft of salvage is 

large enough that it should appear as a base-flow gain 

in the reach. The reason it has not been detected to date 

may be that it is masked by large variations in the 

annual water budget. For example, saltcedar mowing 

appears to have been an ineffective means of control 

that had little impact on water use. The root plowing in 

1974 coincided with an unusually wet year, resulting 

in large base-flow gains that year and some residual 

gains the next. Variations in ground-water pumpage 

and precipitation may have masked the instantaneous 

impact, and the overall base-flow gains that would be 

expected to occur for stable conditions actually may be 

compensated for by continuing declines resulting from 

ground-water pumpage in the shallow aquifer in the 

Roswell basin. 

SUMMARY AND CONCLUSIONS 

The eradication of saltcedar from 8,700 ha of the 

Pecos River floodplain between Acme and Artesia, 

N.Mex., did not result in base-flow gains in the reach 

that could be reliably related to saltcedar clearing. This 

suggests that consumptive use by the replacement vegetation, mainly annual weeds and alkali sacaton, was 

about equal to that by saltcedar. To investigate that 

possibility, water use by selected saltcedar thickets and 

selected plots of replacement vegetation was periodically measured using an eddy-correlation method. 

Measurements were made at approximately monthly 

intervals at two saltcedar sites and one replacementvegetation site during the growing season in 1980 

and 1981, and at four saltcedar sites and three 

replacement-vegetation sites in 1982. 

The eddy-correlation approach provides direct measures of the sensible heat flux and the vapor flux, which 

may be converted to latent heat flux, from the thickets 

or plots. The sum of these fluxes should equal the energy budget; however, the measurements frequently 

were only 60 to 70 percent of the energy budget on a 

daily basis. A second estimate of latent heat flux and 

evapotranspiration was obtained by subtracting sensible heat flux from the energy budget. These estimates 

provided minimum and maximum values of water use 

from the two types of vegetative cover. Extrapolation of 

the measurements indicates that water use by the 

saltcedar thickets is between 0.6 and 1.1 m/yr, and by 

the replacement vegetation, 0.4 to 0.7 m/yr. The estimated probable range of water salvage by saltcedar 

eradication is 0.2 to 0.4 m/yr. Use of these estimates to 

compute the possible salvage for the entire reach is 

difficult because of the areal variability in hydrologic 

and soil conditions and in the density and vigor of the 

saltcedar prior to their eradication. However, salvage 

of 1.2-2.5x 107 m3/yr (10,000 to 20,000 acre-ft/yr) may 

have occurred, but is not recognized as base-flow gain 

because it is masked by declines in base flow related to 

ground-water development. 
G32 STUDIES OF EVAPOTRANSPIRATION 

Measurements of evapotranspiration by saltcedar 

made during this study are in agreement with those of 

some other studies but are in wide disagreement with 

still others. In summary, measurements of water use 

by saltcedar for this study are in good agreement with 

those determined by the massive water-budget study 

by Culler and others (1982) and with measurements in 

the U.S. Bureau of Reclamation evapotranspirometers 

at Bernardo, N. Mex. (U.S. Bureau of Reclamation, 

1973). Results of our study and that of Culler and others (1982, p. P39) indicate that evapotranspiration by 

saltcedar is substantially less than potential evapotranspiration. Also, Hughes (1972) showed that the 

saltcedar evapotranspiration rates measured by the 

U.S. Bureau of Reclamation (1973) could be simulated 

by the Penman (1956) equation for potential evapotranspiration only if a stomatal-resistance term was 

added. 

Results of this study indicate that evapotranspiration from different saltcedar thickets varied greatly, 

but no relationship among volume density of the plant 

cover, depth to water, and water use was apparent. The 

lack of d.ependency of water use on depth to water also 

was noted for the data from the evapotranspirometer 

tanks at Bernardo, N. Mex. (Hughes, 1972; U.S. Bureau of Reclamation, 1973). The lack of a linear relationship between volume density and water use also 

was noted by Hughes (1972) and by van Hylckama 

(1974). 

RECOMMENDATIONS FOR FUTURE STUDIES 

OF EVAPOTRANSPIRATION 

The main purpose of this study was to determine 

water use by saltcedar and by the vegetation that replaced it in areas of the Pecos River floodplain from 

which it had been eradicated. A strong second motive 

among all of the authors was to test the eddycorrelation method for measuring evapotranspiration 

on the basis of newly developed equipment for making 

and recording the measurements. The method has 

many desirable attributes, including portability of the 

instruments, ability to make measurements at a single 

height above the canopy, and ability to check the fluxes 

by comparing them with the energy budget. In addition, the conventional wisdom for many years has been 

that the eddy-correlation method would provide reliable estimates of evapotranspiration once the instrumentation problems were solved (Tanner, 1967, p. 545; 

van Hylckama, 1980, p. F13). 

Perhaps as a consequence of high expectations, our 

inability to close the energy budget, or even to improve 

closure in light of equipment modifications made during the study, has been very frustrating. Although we 

are confident that the eddy-correlation and energybudget estimates of evapotranspiration flux do bracket 

actual water use, the range between the estimates is 

disappointingly large. Consequently, in future studies, 

estimates of evapotranspiration should also be 

obtained by the Bowen-ratio method. Battery-operated 

data loggers that provide user-programmable instrument control, such as the Campbell Scientific CR-21, 

CR-21X, or CR-7 data loggers, can be used to control 

a battery-operated device that switches the thermometer and psychrometer positions periodically (Simpson 

and Duell, 1984). Such a device is sufficiently portable 

to be used for the measurement of evapotranspiration 

at fairly remote wild land sites. Estimates of evapotranspiration based on the Bowen-ratio technique 

would provide a useful adjunct to eddy-correlationbased estimates, or vice versa. 

A disadvantage exists in attempting to determine 

possible evapotranspiration salvage by saltcedar eradication from a previously cleared area because of uncertainty about whether the remaining or offsite thickets 

are representative of those already cleared. Such uncertainty would be reduced by making measurements 

before and after clearing. However, climatic variability 

is large, even in the arid Southwest, so that it is desirable to obtain measurements for a few years before 

clearing and for a few years after the replacement vegetation is established. As it may take a few years to 

fully establish the replacement vegetation, a sequential study might require several years to complete. 

Consequently, it is difficult to judge, even in hindsight, 

whether a sequential or a parallel approach is more 

desirable. 

Extrapolating measurements in time and transferring them in space, even to nearby areas, requires that 

assumptions of uncertain reliability be made. The 

plant-soil-atmosphere modeling study now in progress, 

as preliminarily described by Weaver (1984), should 

provide insights into evaluating the effects of soil type, 

depth to water, ground-water and soil-water salinity, 

and plant vigor on evapotranspiration by saltcedar and 

replacement vegetation. These results in turn might be 

used to estimate evapotranspiration at other sites. 

REFERENCES 

Blaney, H.F., 1933, Water losses under natural conditions in wet 

areas in southern California, Part I, Consumptive use by native 

plants growing in moist areas in southern California: California 

Department of Public Works, Water Resources Division, Bulletin 44, p. 19-139. 

--1961, Consumptive use and water waste by phreatophytes: 

American Society of Civil Engineers Proceedings, v. 87, no. 183, 

pt. 1, p. 37-46. 
WATER USE BY SALTCEDAR AND BY REPLACEMENT VEGETATION, N.MEX. G33 

Blaney, H.F., and Hanson, E.G., 1965, Consumptive use and water 

requirements in New Mexico: New Mexico State Engineer Technical Report 32, 82 p. 

Blaney, H.F., Taylor, C.A., and Young, A.A., 1930, Rainfall penetration and consumptive use of water, Part II, Evaporation and 

transpiration losses from moist areas: California Department of 

Public Works, Water Resources Division, Bulletin 33, p. 107-

146. 

Blaney, H.F., and others, 1942, Consumptive water use and requirements, in [U.S.] National Resources Planning Board, Pecos 

River Joint Investigation-Reports of the participating agencies: Washington, U.S. Government Printing Office, p. 170-230. 

Brutsaert, Wilfried, 1975, On a derivable formula for long-wave radiation from clear skies: Water Resources Research, v. 11, p. 742-

744. 

Buck, Arden, 1976, The variable path Lyman-alpha hygrometer and 

its operating characteristics: Bulletin of the American Meteorological Society, v. 57, p. 1113-1118. 

Campbell, G.S., 1977, An introduction to environmental biophysics: 

Heidelberg, Springer-Verlag, 159 p. 

Campbell, G.S., and Unsworth, M.H., 1979, An inexpensive sonic 

anemometer for eddy correlation: Journal of Applied Meteorology,v. 18,p. 1072-1077. 

Culler, R.C., Hanson, R.L., Myrick, R.M., Turner, R.M., and Kipple, 

F.P., 1982, Evapotranspiration before and after clearing 

phreatophytes, Gila River flood plain, Graham County, Arizona: 

U.S. Geological Survey Professional Paper 655-P, 67 p. 

Eakin, M.E., and Brown, C.B., 1939 (rev. ed.), Silting of reservoirs: 

U.S. Department of Agriculture Technical Bulletin 524, p. 11-

18. 

Fritschen, L.J., 1965, Miniature net radiometer improvements: Journal of Applied Meteorology, v. 2, p. 165-172. 

Fritschen, L.J., and Gay, L.W., 1979, Environmental instrumentation: New York, Springer-Verlag, 216 p. 

Fritschen, L.J., Simpson, J.R., and Smith, M.O., 1980, Eddycorrelation measurements of evaporation from bare soil and of 

evapotranspiration from saltcedar groves in the Pecos River 

flood plain, New Mexico: Final Report to U.S. Geological Survey, 

Water Resources Division, Denver, Colo., 45 p. 

Gatewood, J.S., Robinson, T.W., Colby, B.R., Hem, J.D., and 

Halpenny, L.C., 1950, Use of water by bottom-land vegetation in 

lower Safford Valley, Arizona: U.S. Geological Survey WaterSupply Paper 1103, 210 p. 

Gay, L.W., 1980, Energy budget measurements of evaporation from 

bare ground and evapotranspiration from saltcedar groves in the 

Pecos River flood plain, New Mexico: Final Report to U.S. Geological Survey, Water Resources Division, Denver, Colo., 18 p. 

Gay, L.W., and Fritschen, L.J., 1979, An energy balance budget analysis of water use by saltcedar: Water Resources Research, v. 15, 

p. 1589-1592. 

·Hughes, W.C., 1972, Simulation of saltcedar evapotranspiration: 

American Society of Civil Engineers Journal, v. 98, no. 1R4, 

p. 533-542. 

Jensen, M.E., 1973, Consumptive use of water and irrigation water 

requirements: New York, American Society of Civil Engineers, 

Irrigation and Drainage Division, 215 p. 

Leppanen, O.E., 1981, Evaporation from rapidly growing young 

saltcedar in the Gila River Valley of Arizona: U.S. Geological 

Su~ey Open-File Report 81-485, 31 p. 

McDonald, C.C., and Hughes, G.H., 1968, Studies of consumptive use 

of water by phreatophytes and hydrophytes near Yuma, Arizona: 

U.S. Geological Survey Professional Paper 486-F, 24 p. 

trU.S. G.P.O. 1987-181-407:60025 

Montieth, J .L., 1973, Principles of environmental physics: New York, 

American Elsevier, 236 p. 

Mower, R.W., Hood, J.W., Cushman, R.L., Borton, R.L., and Galloway, S.E., 1964, An appraisal of potential ground-water salvage along the Pecos River between Acme and Artesia, New 

Mexico: U.S. Geological Survey Water-Supply Paper 1659,98 p. 

Penman, H.L., 1956, Evaporation: An introductory survey: Netherlands Journal of Agricultural Sciences, v. 4, p. 9-29. 

Priestly, C.H.B., 1959, Turbulent transfer in the lower atmosphere: 

Chicago, University of Chicago Press, 130 p. 

Redford, T.G., Jr., Verma, S.B., and Rosenberg, N.J., 1980, Humidity 

fluctuations over a vegetated surface measured with a Lymanalpha hygrometer and a fine wire thermocouple psychrometer: 

Journal of Applied Meteorology, v. 19, p. 860-867. 

Robinson, T.W., 1965, Introduction, spread and areal extent of 

saltcedar (Tamarix) in the Western States: U.S. Geological Survey Professional Paper 491-A, 12 p. 

--1970, Evapotranspiration by woody phreatophytes in the 

Humboldt River valley near Winnemucca, Nevada: U.S. Geological Survey Professional Paper 491-D, 41 p. 

Simpson, M.R., and Duell, L.F.W., Jr., 1984, Design and implementation of evapotranspiration measuring equipment for Owens valley, California: Ground-Water Monitoring Review, v. 4, no. 4, 

p. 155-163. 

Suomi, V.E., and Tanner, C.B., 1958, Evapotranspiration estimates 

from heat-budget measurements over a field crop: American 

Geophysical Union Transactions, v. 39, no. 2, p. 298-304. 

Tanner, C.B., 1963, Basic instrumentation and measurements for 

plant environment and micrometeorology: Madison, University 

of Wisconsin College of Agriculture, Department of Soil Science, 

Soils Bulletin 6, 216 p. 

--1967, Measurement of evapotranspiration, in Hagan, R.M., 

ed., Irrigation of agricultural lands: American Society Agronomy Monograph 11, p. 534-574. 

Thomas, H.E., 1963, Causes of depletion of the Pecos River in New 

Mexico: U.S. Geological Survey Water-Supply Paper 1619-G, 

14p. 

U.S. Bureau of Reclamation, 1973, Progress report, phreatophyte 

investigations, Bernardo evapotranspirometers: U.S. Bureau of 

Reclamation, Middle Rio Grande Project office, v. 2, 50 p. 

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

the water budget method: U.S. Geological Survey Professional 

Paper 491-E, 30 p. 

--1980, Weather and evapotranspiration studies in a saltcedar 

thicket, Arizona: U.S. Geological Survey Professional Paper 

491-F, 78 p. 

Veihmeyer, F.J., and Brooks, F.A., 1954, Measurement of cumulative evaporation from bare soil: American Geophysical Union 

Transactions, v. 35, no. 4, p. 601-607. 

Weaver, H.A., 1984, A mechanistic model of evapotranspiration from 

saltcedar (Tamarix chinensis ): Unpublished doctoral dissertation, Washington State University, Pullman, Wash. 

Webb, E.K., Pearman, G.I., and Leuning, R., 1980, Correction of flux 

measurements for density effects due to heat and water vapor 

transfer: Quarterly Journal of the Royal Meteorological Society, 

v. 106, p. 85-100. 

Welder, G.E., 1983, Geohydrologic framework of the Roswell groundwater basin, Chavez and Eddy Counties, New Mexico: New Mexico State Engineer Technical Report 42, 28 p., 28 figs. 

Young, A.A., and Blaney, H.F., 1942, Use of water by native vegetation: California Department of Public Works, Division of Water 

Resources Bulletin 50, 160 p.