Evapotranspiration by 

Woody Phreatophytes in 

The Humboldt River Valley 

Near Winnemucca, Nevada

GEOLOGICAL SURVEY PROFESSIONAL PAPER 491-D
Evapotranspiration by 

Woody Phreatophytes in 

The Humboldt River Valley 

Near Winnemucca, Nevada

By T. W. ROBINSON

fTith a section on SOIL-MOISTURE DETERMINATIONS

By A. O. WAANANEN

STUDIES OF EVAPOTRANSPIRATION

GEOLOGICAL SURVEY PROFESSIONAL PAPER 491-D

Quantitative studies of water use 

by greasewood, rabbitbrush^ 

willow, and wildrose

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1970
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

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

Washington, D.C. 20402 - Price 55 cents (paper cover)
CONTENTS

Page

Abstract. __________________________________________ Dl

Introduction.______________________________________ 1

Purpose and scope._____________________________ 2

Acknowledgments_ _ ___________________________ 2

Climate._______________________________________ 2

Species of woody phreatophytes__________________ 3

Winnemucca test site..__________________________ 3

Weather station._______________________________ 3

Evapotranspiration studies_________________________ 3

Definition of evapotranspiration__________________ 3

Evapotranspiration tanks.-______________________ 4

Construction of evapotranspiration tanks. ______ 4

Water-supply system..______________________ 5

Transplanting, survival, and growth of woody

phreatophytes. _______________________________ 7

Operation of the tanks. _ _________________________ 8

Water levels______________________________ 9

Sources of water for evapotranspiration__________ 9

Rainfall.-_-_-__-----____-_________________ 10

Water supplied to the tanks..________________ 10

Soil moisture.______________________________ 10

Computations of evapotranspiration.._____________ 11

Plant growth and development-__________________ 11

Foliage volume.____________________________ 12

Problems._________________________________________ 16

Damage by webbing insects._____________________ 16

Damage by rabbits.____________________________ 17

Page 

Problems Continued

Salt content of the soils.________________________ D17

Boron toxicity..______-__--______-----__-___ 17

Source and accumulation of boron in the top soil- 19 

Leaching as a corrective measure-_________________ 22

Factors influencing evapotranspiration rates__________ 24

Expression of evapotranspiration losses and extrapolation to growth areas ______________________ 26

Arealmethod._____________________________ 27

Volume-of-foliage method-___________________ 27

Results of evapotranspiration studies __________ 27

Greasewood__ _ _____________________________ 27

Rabbitbrush____.___----------------------- 29

Willow--_-_-_----------------------------_ 30

Wildrose.______________________________ 30

Bare soil________________-_____-_--_-___-- 31

Summary. ______________________________--------_-- 31

Soil-moisture determinations, by A. O. Waananen __ 32 

Soil-moisture observations at the Winnemucca test

site_________________________________ 32

Equipment and procedure__________________ 32

Water contents observed___-______--_-------- 34

Water-content variations in 1963____________ 35

Water-content changes in shallow flood-plain

deposits _____________-_-______--__--___-----_ 35

References. ______________________-___-__----------_ 41

ILLUSTRATIONS

Page 

FIGURE 1. Map showing location of Humboldt River Research Project test site._______________-___--_----------- D2

2. Plan sketch of Humboldt River Research Project evapotranspiration test site._______------_----------- 4

3. Photograph showing installation of a plastic membrane liner in a 30- by 30-foot evapotranspiration tank:

1X 12-inch boards on edge outline the sides of the tank__ _ __________________-_-----___-_----------- 5

4. Photograph of the bottom of the tank in figure 3 showing the water-distribution system in place_____----_ 5

5. Plan and section of evapotranspiration tank showing water-distribution system___________--_-_--------- 6

6-11. Photographs showing:

6. A 30- by 30-foot tank about half completed, with the corners of the membrane fastened at the land

surface. The photograph shows the method of backfilling_____________________----_-_---- 7

7. Installation of the water-distribution system in a 10- by 10-foot evapotranspiration tank________ 7

8. The 5-year-old greasewood plants in tank 2 and the two 350-gallon elevated reservoirs that supply

water to greasewood tanks 1 and 2______________________________-__----__-_-----------_- 7

9. Greasewood plants in tank 2 on September 30, 1960______________________-___-__-------_----- 8

10. Growth of plants in willow tanks on October 1, 1960, from cuttings planted on April 14, 1960______ 8

11. Scientist measuring cover density and plant height on a transect across greasewood tank 2________ 12

12. Graphs showing seasonal foliage volumes and operating water levels for the four species of phreatophytes

grown in evapotranspiration tanks-__-__--_____________-_-_-___--_-_-_----------------------- 14

13. Graph showing soluble boron content of the soil in greasewood tanks 1 and 2 before leaching and in tank 1

after leaching.___ ___________________________________________________________________________ 19

14. Plan sketches showing location of points and lines used in sampling undisturbed soil, in relation to clumps of

greasewood-_______________________________________________________________________________ 20

ni
IV CONTENTS

Page 

FIGURES 15-19. Graphs showing:

15. Results of boron, sodium, and electrical-conductance determinations of soil samples collected from

sampling points shown in figure 14... ________-_----_---__---__-_-.__________________.._____ D21

16. Decrease in boron and specific conductance of the effluent from greasewood tanks 1 and 2 during

leaching_ ________________________-___-_-__-------__--__-__--____________..__________ 23

17. Comparison of specific conductance and boron of the effluent from rabbitbrush tanks 2 and 3 for the

teachings of 1963 and 1966----_---_----_-------------------______________________ 23

18. Relation of monthly draft on ground water to temperature April through October 1966 for three

species of phreatophytes.____________-_-_-_-__--_--_-__-__---_-_____________..__________ 26

19. Evapotranspiration by four species of phreatophytes during the growing seasons 1963 through 1967

for indicated depth of water level..-____________________________________________________ 29

20. Photograph of neutron-meter sealer, probe, and shield, with soil augers and typical access tubes.________ 33

21. Generalized section of evapotranspiration tank showing neutron-meter depth probe in access tube for soilmoisture observations, and typical soil-moisture profile- __-__---___--_-_--________________________ 33

22. Photograph of neutron meter set up for use in greasewood tank 1_____________________________________ 34

23. Photograph showing rabbitbrush tanks and access tubes installed for soil-moisture observations-.-______ 34

24. Soil-moisture profiles at the beginning and end of the growing seasons 1962-67 in evapotranspiration tanks

and the flood plain at the Winnemucca test site._______________________________________________ 38

25. Graphs showing variations in water content of soils in evapotranspiration tanks at the Winnemucca test site

during the 1963 season.__________________-_-_-_--_---__---_---_----________________________ 40

26. Graph showing water content at end of each season and maximum content observed in June 1962, in inches,

as observed at three sites in Humboldt River flood plain near Winnemucca, Nev_ _____________________ 41

TABLES

Page 

TABLE 1. Growth and development of woody phreatophytes grown in evapotranspiration tanks at the Winnemucca

test site___-_-__-_________________________-__--_---_-----_-_-.._--------_----_-__-___-_-_-__ D13

2. Comparative data on greasewood growth in the field under different growth conditions and the average

in the two evapotranspiration tanks-__________________________________________________________ 15

3. Chemical analyses of water from the saturated soil in the evapotranspiration tanks of the Winnemucca test

site.__-_-__________________________-_____-_--__-_____-_-__--_-----__-__----_____-__--_____ 18

4. Results of analysis of samples of undisturbed soil profile.___________________________________________ 20

5. Results of analysis of surface soil sampled to 3-inch depth at 1-foot intervals from central stem of a greasewood plant-- _-__________________________----_------------_---__----------------_----__---__ 21

6. Quantity, rate of inflow to tank, and chemical analyses of samples of the effluent from greasewood tank 1 and

rabbitbrush tank 1, October 1962..______________________________________________________________ 24

7. Comparative warmth for the period, April 1 to October 31, 1962-67, by months, at the Winnemucca test

site, in degree days above a base of 32° F __--___-------_---__---__----_-_--__---_---_-_--__--_ 25

8. Climatological data for the April 1-October 31 period, 1962-67, at the Winnemucca test site______________ 26

9. Evapotranspiration by four species of phreatophytes grown in tanks at the Winnemucca test site and

evaporation from bare soil during the growing seasons 1963-67________________________________ 28

10. Soil-moisture changes in evapotranspiration tanks at the Winnemucca test site during summer and winter

periods 1962-67_____________________-_____-________-___--_-__-_----_--_---------_----_----- 36

11. Water content of soils in evapotranspiration tanks at the Winnemucca test site, as observed during the 1963

season._ ___________________________________________________________________________________ 40
STUDIES OF EVAPOTRANSPIRATION

EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE 

HUMBOLDT RIVER VALLEY, NEAR WINNEMUCCA, NEVADA

By T. W. ROBINSON

ABSTRACT

This report presents the results of cooperative studies of 

evapotranspiration by phreatophytes in the Winnemucca reach 

of the Humboldt River valley. Water that is wasted by evapotranspiration from areas of low beneficial phreatophytes is one 

of the largest unknowns in the water budget of the reach. 

In order to obtain information with which to evaluate the 

consumptive waste, studies of the water use of four woody 

phreatophytes greasewood, rabbitbrush, willow, and wildrose were undertaken in evapotranspiration tanks at the Winnemucca test site.

Twelve tanks ranging in size from 30 feet square and 10.5 feet 

deep to 10 feet square and 7 feet deep were constructed. Seedlings of greasewood were planted in 2 tanks and rabbitbrush in 

3 tanks; cuttings of willow were planted in 3 tanks and wildrose in 3 tanks. The twelfth was left bare. The tanks were 

constructed in place by lining excavated pits with watertight 

plastic membranes, providing a water-distribution system on the 

bottom and backfilling with the excavated material. The tanks 

were operated during the growing season April 1 to October 20 

from 1961-67, inclusive. Water metered into elevated reservoirs 

was supplied by gravity to the evapotranspiration tanks.

Evapotranspiration by the plants was computed as the sum of 

rainfall, soil-moisture depletion and water supplied to the tanks 

during the growing season. Plant growth and development were 

recorded by photographs and transects. Foliage volumes were 

computed from the transect data as the product of the average 

height, the cover density of the plants, and the area of the tank.

Foliage volumes and water use were affected by difference in 

depths to the water level, damage to the plants by rabbits and by 

insects, and, for the greasewood and rabbitbrush, by accumulation of boron in toxic concentrations in the root zone. The damages were alleviated by removing rabbits from the test site, by 

spraying the insect-infested plants, and by reducing the boron 

content in the root zone by backwash leaching.

Boron content of undisturbed soil, adjacent to the greasewood tanks, ranged from 13 to 32 milligrams per kilogram in the 

top 1.5 feet, and from 5 to 0.9 milligrams per kilogram between 

5 and 9 feet. Green greasewood leaves had a boron content that 

ranged from 196 to 233 milligrams per kilogram.

Evapotranspiration by a given species of phreatophytes is 

affected by climatic conditions, of which temperature is the 

most important. Water use by greasewood and rabbitbrush in 

April was only about 2 percent of the yearly total, whereas during the months of peak use it was 28 percent. More than twothirds of the annual use occurred during June, July, and August. 

Evapotranspiration, expressed on an areal basis as depth over 

a unit area, gives no indication of growth conditions for which 

the information was obtained and may result in serious error 

when transposed to areas of dissimilar growth conditions. Some 

of the difficulties and uncertainties of the areal method may be 

avoided by expressing evapotranspiration on a volume-of-foliage 

basis, as a quantity of water per unit of foliage volume. The 

method presumes that transpiration by a species is proportional 

to the total transpiring leaf area, and so proportional to the 

foliage volume. In the results of the studies, evapotranspiration 

is expressed in both quantities. The annual use of water ranged 

rather widely over the study period, as the plants responded to 

the effect of plant damage, boron toxicity, depth to the water 

level, and warmth and length of the growing seasons. Draft from 

the water table, equivalent to the water supplied to the tanks, 

varied with the rainfall. It was greatest when rainfall was scant, 

and least when the rams were copious.

INTRODUCTION

The Humboldt River Research Project is a FederalState cooperative project concerned with developing 

data and techniques by which to evaluate the water 

resources of the Winnemucca reach of the Humboldt 

River. The agencies cooperating in the study were the 

U.S. Geological Survey, the U.S. Bureau of Reclamation, and the Department of Conservation and Natural 

Resources of the State of Nevada.

The Winnemucca reach extends from the Comus gaging station downstream to the Rose Creek gaging station. The Comus gage is about 23 miles east, and the 

Rose Creek gage is about 15 miles southwest of the city 

of Winnemucca. The distance between the stations along 

the meanders of the river in the flood plain is about 92 

miles, whereas the distance along the meandering flood 

plain is about 45 miles. The flood plain ranges in width 

from 0.2 to 5 miles; the average width is 0.8 mile. The 

altitude at the Comus gage is slightly less than 4,400 feet 

above mean sea level, and that at the Rose Creek gage is 

about 4,200 feet (fig. 1).

Dl
D2 STUDIES OF EVAPOTRANSPIRATION

FIGURE 1. Location of Humboldt River Research Project test

site.

One of the largest unknowns in the water budget for 

this reach of the Humboldt River is the evapotranspiration loss. This loss includes the losses from water and 

bare-soil surfaces by evaporation and losses from soil 

moisture and ground water through transpiration by 

phreatophytes. Phreatophytes are plants that habitually send their roots to the water table, and obtain their 

water supply primarily from ground water. Several 

species of woody phreatophytes thrive along the 

streambanks, in the flood plain, and on the lower parts 

of the adjacent alluvial fans. Water use by such plants 

is by far the largest of the several evapotranspiration 

losses.

The purpose of this study, one of several in the interagency Humboldt River Research Project, was to determine the water use by woody phreatophytes, notably 

several species of low beneficial use. The water used by 

these plants constitutes a consumptive waste of water, 

for it is discharged into the atmosphere with but little 

benefit to man. This is one of nature's preemptive taxes, 

and results in depletion of the water resources of a 

region and reduction in the quantity of water available 

to man. With proper management, man can reduce this 

draft and benefit from the reduced draft to the extent 

of the salvageable portion. Possible modifications might

be to replace the phreatophytes of low economic value 

with plants of higher economic value or to salvage the 

water otherwise -consumptively wasted and use it beneficially. To assess the economic feasibility of such operations, quantitative evaluations of the consumptive waste 

are needed.

PURPOSE AND SCOPE

The purpose of the evapotranspiration studies was to 

obtain data for evaluation of the consumptive use of 

water by four woody phreatophytes widespread in the 

Winnemucca reach. These shrubs occur generally 

throughout the Humboldt River basin and in other 

areas in Nevada. The studies involved growing the 

plants in large evapotranspiration tanks and determining their seasonal water use for different depths to the 

water level and for different cover densities. The first 

tanks were constructed at the Winnemucca test site (see 

page D4) in the fall of 1959 and were planted in the 

spring of 1960. Later plantings were made in 1961 and 

1962 as additional tanks were constructed .

ACKNOWLEDGMENTS

Funds for construction of the tanks and for part of 

the operation and maintenance costs were provided by 

the U.S. Bureau of Reclamation. The evapotranspiration studies were under the technical direction of the 

Water Resources Division, U.S. Geological Survey.

The cooperation and assistance provided by personnel 

of the Nevada Department of Conservation and Natural 

Resources are acknowledged. The author is especially 

grateful to George Hardman, Assistant Director, for 

his unflagging interest, his counsel and helpful suggestions. Vernon Laca, assisted at times by James Starr, 

provided 7-day-a-week operation of the tanks during 

the growing season. The wholehearted cooperation of 

Mr. and Mrs. H. T. Harrer, on whose property the Winnemucca test site and access road are located, is 

appreciated and acknowledged.

CLIMATE

The Winnemucca reach of the Humboldt River lies 

largely in the 5- to 8-inch rainfalj. zone. The climate 

is characterized by few cloudy days and moderate wind 

movement and may be classified as arid to semiarid. 

About two thirds of the annual precipitation falls as 

rain or snow during the winter period, December to 

May. During the growing season, April to October, 

the precipitation falls largely as scattered summer 

showers that occasionally exceed one-half inch. On rare 

occasions as much as 1 inch of rain has been recorded 

during a single storm. The prevailing winds are westerlies, and these lose much of their moisture as they pass 

over the Sierra Nevada about 150 miles to the west.
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D3

The summers are marked by warm days and cool 

nights. Temperatures tend to rise sharply with the 

sunrise and remain comparatively high during the daylight hours, then drop rapidly about sundown. A daily 

temperature variation of 50°F is not uncommon. 

According to the U.S. Weather Bureau, the average 

monthly temperature during the growing season for 

92 years of record at and near Winnemucca has ranged 

from 47.1 °F in April to 7l.7°F in July. Extremes of 

temperature have ranged from 36°F belowT zero in January to a high of 108 °F in July. Temperatures rise to 

100°F or more on the average of 2 days a year. The 

growing season during the 92-year period of record 

has varied considerably, the shortest on record being 

only 63 days and the longest 184 days; the average 

growing season is about 140 days.

Measurements of pan evaporation for the months of 

April through October were made at the Winnemucca 

test site from 1962 through 1967. Evaporation during 

this 6-year period ranged from 52.7 inches in 1965 to 

66.8 inches in 1966 and averaged 58.0 inches.

SPECIES OF WOODY PHREATOPHYTES

The common woody phreatophytes that are native 

to the Winnemucca reach of the Humboldt River 

include grease wood (Sarcobatus vermiculatus] , rabbitbrush (Chtvysothamnus}, and willow (Salix). Wildrose 

(Rosa] and buffaloberry (Shepherdia) occupy less extensive areas. Saltcedar (Tamarix] , an exotic plant from 

the Mediterranean area introduced into this country 

about the turn of the century, is invading the lower 

part of the Winnemucca reach. Saltcedar is also grown 

as an ornamental shrub in several places in the city of 

Winnemucca. In addition, hydrophytes, such as cattails 

and bullrushes, grow in several small areas adjacent to 

the river.

Greasewood is the most extensive phreatophyte in the 

area, and willow and rabbitbrush are the next most 

common species. The open spaces on the bottom lands 

of the flood plain are covered with a variety of beneficial phreatophytic grasses. The two common species 

are blue joint or creeping wildrye (Elymus triticoides] 

and saltgrass (DistichUs stricta). Great Basin wildrye 

(Elymus cinereous) was once common in the flood plain, 

but is now found only in areas protected from heavy 

livestock grazing.

WINNEMUCCA TEST SITE

The Winnemucca test site is a parcel of land 300 by 

600 feet on the Harrer ranch, in the NEi/4 sec, 2, T.35, 

R.37 E., Mount Diablo base line and meridian, on the 

south side of the Humboldt Kiver. The site is 314 

miles southwest of Winnemucca, and three-fourths mile

west of U.S. Highway 40. About half the site lies on the 

present flood plain of the Humboldt River, and the remainder on a terrace about 4 feet higher. Both parts are 

nearly flat, with some slope northward toward the river. 

The terrace is at an altitude of about 4,260 feet.

To prevent inundation of the part of the test site on 

the flood plain during periods of high water in the 

Humboldt River or when the adjacent flood plain is 

ponded for flood irrigation, an earthen dike, approximately 21/^-3 feet high, was constructed. The site also 

was fenced with a combination of barbed wire and wire 

mesh, which was ostensibly rabbit tight and stock proof.

The layout of the test site, showing the evapotranspiration tanks, water-supply reservoirs, pipelines, supply well, and weather station, is shown in figure 2.

WEATHER STATION

A class "A" weather station, installed at the test site 

in March 1962, was operated each season during the period April 1 to November 1. Instrumentation consisted 

of a 4-foot evaporation pan, a totalizing anemometer, 

maximum and minimum thermometers, hygrothermograph, and an 8-inch rain gage. The temperature and humidity instruments were housed in a cotton-region type 

shelter. A pyrheliograph for measuring incoming radiation was installed in the low-lying meadow a short distance from the weather station.

In addition, a thermograph was installed at greasewood tank 2 to measure the surface temperature of the 

soil in the tank. The sensing element was covered with 

about one-fourth inch of soil, enough for shielding from 

the direct rays of the sun. All the nonrecording instruments were read daily, and the charts on the recording 

instruments were changed weekly. The station is about 

30 feet lower in altitude than the Weather Bureau 

station at the Winnemucca Airport, 3 miles to the south.

EVAPOTRANSPIRATION STUDIES 

DEFINITION OF EVAPOTRANSPIRATION

The first use of the term "evapo-transpiration" was 

by Sondregger (1929) in May 1929; he used it as a 

synonym of evaporation-transpiration losses, a term 

coined by Lee (in Lee and others, 1926) to describe 

water lost to the atmosphere. Later in the 1930's as the 

term came into common usage the hyphen was omitted. 

The term is generally considered to be synonymous with 

the term "consumptive use" and is defined in Manual 

No. 43, "Nomenclature for Hydraulics," of the American Society of Civil Engineers (1962, p. 156) as "water 

withdrawn from soil by evaporation and plant 

transpiration."
STUDIES OF EVAPOTRANSPIRATION

600 FEET

Elevated water reservoirs

VX?

Greasewood 

tanks

Rabbitbrush tanks

FIGURE 2. Humboldt River Research Project evapotranspiration test site.

EVAPOTRANSPIRATION TANKS

Twelve evapotranspiration tanks were installed at the 

Winnemucca test site. Eleven tanks were used to measure the evapotranspiration from four species of woody 

phreatophytes greasewood, rabbitbrush, willow, and 

wildrose and the twelfth was used to measure the 

evaporation from bare soil. The plant species, tank 

sizes, and planting dates are given in the following 

tabulation:

Evapotranspiration tanks

Greasewood-- _ Willow________

Rabbitbrush- _

Bare soil___

Number

2

3

3

3

1

Tanks

Size (ft)

30X30X10. 5

10X10X 7. 5

inv 1 nv 7 n

20X20X10. 0

10X10X 7. 0

Apr. 13, 1960.

Ar»r 14. 1 QfiO

May 20, 1961.

Apr. 10 and 11, 1962.

The greasewood and rabbitbrush tanks are on the 

terrace at the test site, and the willow, wildrose, and 

bare-soil tanks are on the low-lying flood plain. The 

terrace, according to Cohen (1965, p. 28, pi. 1), consists of deposits of Quaternary age, whereas the flood 

plain consists of deposits of Holocene (Recent) age. 

The soils and vegetation in the two parts are quite different. The terrace deposits consist of poorly sorted 

gravel, sand, silt, and clay, whereas the flood-plain deposits consist of clay loam with considerable organic 

matter and occasional balls of volcanic ash. The terrace 

deposits support a vigorous and well-established

growth of greasewood that averages 2.5 feet in height; 

individual plants are us much as 5 feet high. Saltgrass 

grows in open spaces between the clumps of greasewood. About three fourths of the flood-plain part of 

the test-site meadowland is covered with several kinds 

of grasses, and about one fourth, in the northwest corner of the site, has a dense growth of willows and wildrose. The willow and wildrose tanks are in this northwest corner. The bare-soil tank is in the meadow-grass 

area, at the edge of the willow growth. Saltgrass occurs 

among the willows and also in the meadow. The predominant meadow grass, however, is blue joint or creeping wildrye. As the result of the fencing of the site for 

protection from grazing, a few clumps of Great Basin 

wildrye have made a healthy growth in the meadow.

CONSTRUCTION OF EVAPOTRANSPIRATION TANKS

The tanks were constructed in place by installing 

watertight plastic-membrane linings in pits excavated 

into the terrace and flood-plain deposits. The membranes 

for the 30- by 30-foot tanks and the 10- by 10-foot tanks 

were fabricated in sheets 57 and 30 feet square, respectively, from black poly vinyl plastic sheets 20 mils (0.02 

inch) thick. The plastic sheets for the 20- by 20-foot 

tanks were 45 feet square and were fabricated from aqua 

poly vinyl material 22 mils thick.

The pits for the tanks were excavated to the water 

table and were 2-3 feet larger than the finished size. 

Excavation was done by a dragline equipped with a 1- 

yard bucket. When the excavation was completed, the 

bottom of the pit was leveled manually, raked clean of
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D5

stones and sharp objects, and shaped to size. Next, the 

dimensions of the finished tank were outlined on the 

bottom of the pit by using 1X 12-inch boards placed on 

edge, and the plastic membrane was positioned in the 

bottom of the pit as shown in figure 3. Marks on the 

center of the membrane served as guides to fix the liner 

position in the pit. After the membrane was positioned, 

the part forming the bottom of the tank as outlined by 

the 1X 12-inch boards was covered with a 4 5-inch layer 

of pit-run medium to coarse sand. A water-distribution 

system was then placed on the sand layer as shown in 

figure 4. The system consisted of a horizontal E-shaped 

component made of 2-inch rigid plastic pipe perforated 

with %6 -inch holes on 14-inch centers on the underside, 

and a 4-inch riser pipe for adding or withdrawing water 

from the system. The riser pipe extended 0.5 foot above 

the ground surface and was also used to monitor the 

water level in the tank. A plan and section view of the 

tank, including the water-distribution system, is shown 

in figure 5. The 2-inch pipes were covered with sand 

throughout their perforated sections in order to facilitate movement of water into the sand layer which has 

been described.

With the water-distribution system in place, the pit 

was backfilled with the material previously excavated, 

a clamshell bucket being substituted in place of the dragline bucket for this purpose. The excavated material was 

replaced only approximately in the reverse of the order 

in which it had been removed. The positioning of the 

membrane for the sides of the tank was accomplished 

by alternately draping the membrane on the outside of 

the 1X 12-inch boards and placing fill 011 the inside, and 

then draping on the inside and placing fill on the outside. As the level of the fill on both sides became even 

with the top of the boards, the boards were raised 8-10 

inches and the backfilling process repeated. Thus, as the 

backfilling progressed, the membrane that was origiFIGURE 3. Installation of a plastic membrane liner in a 30- by 30-foot 

evapotranspiration tank: 1 X 12-inch boards on edge outline the 

sides of the tank.

nally draped over the 1X 12-inch boards in the bottom of 

the pit became the sides of the tank. Experience demonstrated the advantages of keeping the backfill higher 

along the sides than in the center of the tank, and the 

desirability of fastening the membrane corners on the 

land surface at the corners of the pit as shown in figure 6. 

The 30- by 30-foot and the 20- by 20-foot tanks were 

constructed in the fall of the year. Upon completion, the 

tanks were tested for watertightness by being filled with 

water to within about 4 feet of the surface and allowed 

to stand in this condition during the winter months. No 

leaks developed. A 30- by 30-foot tank was constructed 

in about 4 days; the excavation required about iy2 days; 

the installation of the membrane, water-distribution 

system, and the backfilling about 2^2 days. The smaller 

20- by 20-foot tanks were constructed in 2i/£ days each, 

with the excavation usually completed in about 7 hours; 

the 10- by 10-foot tanks required l-l1/^ days each.

The method of construction and the design of the 

water-distribution system in the shallower 10- by 10- 

foot tanks was different from that for the larger tanks. 

For the smaller tanks, the pit was excavated to the 

finished size, and the membrane suspended loosely from 

the top edges of the pit. During backfilling, care was 

taken to keep the membrane on the sides of the pit loose 

and free from tension. The distribution system resting 

on a 4- to 5-inch layer of sand, as shown in figure 7, was 

an 8-foot circle of 2-inch flexible plastic pipe that was 

cross connected on one diameter, perforated on the bottom, and connected to a 6-inch riser pipe.

WATER-SUPPLY SYSTEM

Water for use in the test site was obtained from a 

drilled well, 25 feet deep, located inside and near the 

entrance to the site (fig. 2). The well was equipped with 

an electrically operated jet pump that delivered water 

under a working pressure of 30 pounds per square inch

«»

..-J!^^^^

FIGURE 4.- -The bottom of the tank in figure 3 showing the waterdistribution system in place.

372-487 O 7C
D6 STUDIES OF EVAPOTRANSPIRATION

 30'0" 

-10'-

o o ^ u>

00

(U J=

a 5 o

4-inch water-supply 

,and dewatering pipe

PLAN

Land surface

Fine sandy 

silt

Sand and \oo»' 

small gravel

Excavation line

Backfilled with excavated 

material

,,4-inch water-supply 

and dewatering pipe R surface

4-5-inch layer 

of pit-run sand\ 

.....7?.--- - - ~V -

SECTION 

FIGURE 5. Plan and section of evapotranspiration tank showing water-distribution system.

to a 500-gallon buried pressure tank. Water from the 

pressure tank was delivered through a pipeline to elevated metal reservoirs adjacent to the evapotranspiration tanks (fig. 8). The pipeline consisted of about 500 

feet of %-inch iron pipe and 400 feet of 1-inch plastic 

tubing.

The volume of water delivered to the reservoir was 

measured by industrial cold-water meters that have 

horizontal nonsetback-reading registers and a 10-gallon

dial circle calibrated to one-fourth gallon. Each meter 

was tested for accuracy before installation; for flow 

rates greater than 3 gallons a minute, the measurement 

error was found to be less than 1 percent.

Water from the reservoir was delivered by gravity 

through a %-inch pipeline to a float-controlled valve at 

the 4-inch supply pipe (fig. 5) of the evapotranspiration tank. Water levels in the evapotranspiration tanks 

were controlled by means of a 3^-inch float in the
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D7

FIGURB 6. A 30- by 30-foot tank about half completed, with the corners 

of the membrane fastened at the land surface. The photograph shows 

the method of backfilling.

177'

FIGURE 7. Installation of the water-distribution system in a 10- by 

10-foot evapotranspiration tank.

FIGURE 8. The 5-year-old greasewood plants in tank 2 and the two 

350-gallon elevated reservoirs that supply water to greasewood tanks 

1 and 2.

supply pipe that was directly connected by a %6-inch 

rod to the float-valve mechanism. The float valve was 

activated by the float; as the water level in the evapotranspiration tank fell, the valve opened to admit water; 

when the water level rose, the valve closed. The interval between the high and low position of the water levels 

was less than 0.1 foot; so a nearly constant water level 

was maintained in the evapotranspiration tank.

TRANSPLANTING, SURVIVAL, AND GROWTH OF 

WOODY PHREATOPHYTES

Transplanting of greasewood and rabbitbrush to the 

tanks was undertaken with some trepidation, for it was 

the first time transplanting on a large scale for experimental purpose had been attempted. Seedling plants 

were used for both species. Some consideration was 

given to transplanting mature plants, but it was feared 

that damage to their deep-root systems would be so 

great that many of the plants would not survive. During 

excavation for the tanks, greasewood roots were observed and photographed at depths of 7 and 8 feet; 

judging from the root size at these depths, the roots 

probably extended to a depth of at least 10 feet.

The seedling greasewood plants were obtained in the 

general vicinity of the tanks. In the transplanting procedure, the seedlings were dug from the ground with 

care taken to keep as much soil as possible 'around the 

roots, placed in a wheelbarrow, covered with wet burlap, and transported to the tank for planting. Each 

seedling was examined carefully before planting, and 

seedlings with obvious root damage were discarded. 

Prior to planting, the average spacing of the plants in 

the area was ascertained by random transects. Because 

some loss of the transplanted shrubs was anticipated, the 

seedling spacing was closer in the tanks than in the surrounding growth, for which the average spacing was 3- 

3y2 feet- Eighty-five plants were planted in greasewood 

tank 1 and 105 in tank 2 in mid-April 1960. At the end of 

September, 71 plants were growing in tank 1 and 89 in 

tank 2, a survival rate of 84 and 85 percent, respectively, 

for the two tanks. Figure 9 shows the growth in greasewood tank 2, 5i/^ months after planting. Some additional transplanting was done in the spring of 1961 to 

fill the spaces where two or more adjacent plants had 

died.

Eabbitbrush plants were not available in the immediate vicinity of the test site; however, an extensive growth about one-half mile distant provided an 

ample supply of seedlings. The young plants were
D8 STUDIES OF EVAPOTRANSPIRATION

FIGUEE 9. Oreasewood plants in tank 2 on September 30, 1960. Of 105 

seedling's planted on April 13, 1960, 89 rooted and thrived.

easily dug from sandy loam with a minimum of damage to the roots. The seedlings were planted in irregular 

rows, and at a closer spacing than for the greasewood, 

to simulate observed field conditions. The plantings on 

April 10 and 11,1962, were as follows: 75 plants in tank 

1, 86 in tank 2, and 90 in tank 3. A count of the living 

plants on July 9, 1962, showed 63 surviving in tank 1, 

75 in tank 2, and 86 in tank 3. An item of interest was 

the location of the dead plants. Plant mortality in each 

of the three tanks was greatest in the center of the tank 

and least along the edges. Replanting in the spring of 

1963 did little to alter this pattern.

The willow tanks were planted on April 14, 1960, 

using cuttings from the thicket in which the willow 

tanks were located. The cuttings, about 10 inches long, 

were stuck into the wet soil surface of the tanks to a 

depth of about 6 inches at approximately 1-foot intervals. An inspection on September 30 showed that about 

99 percent of the cuttings had taken root and were 

growing. The growth by October 1, as shown in figure 

10, ranged in height from ll/£ to 3y2 feet and averaged 

about 2y2 feet.

The wildrose tanks were also planted with cuttings 

from the surrounding area. The plantings on May 20 

took root, thrived, and were well established by the end 

of the 1960 growing season. Survival rate was high, 

averaging about 95 percent.

OPERATION OF THE TANKS

With the exception of the year of planting, studies 

of evapotranspiration in the tanks were started each 

year on April 1 and ended on October 20. The emphasis 

following planting was on the establishment of a healthy 

and vigorous growth. The period April 1 to October 

20 was selected because it spanned the dates of earliest 

and latest beginning and end of the growing season. 

Generally water could not be supplied to the tanks for

the entire period each season. Water systems were disrupted in some years near the beginning of the season 

and in other years near the end of the season when below 

freezing temperatures at night caused water lines and 

water meters to freeze and burst.

Samples of the soil and of the water in the tanks 

were collected once or twice a year, following the first 

full growing season, for chemical analysis. The analyses 

provided data on the concentration of salts in the soil 

and water, and warned of any salt buildup that could 

be detrimental to the plants.

Once the operating water level for the growing 

season was established and the float mechanism for 

controlling the water level adjusted, the day to day 

operation was fairly simple. At the start of the season, 

the reservoirs were filled to their operating capacity, 

indicated by a reference mark on a manometer tube 

attached to the side. The reservoirs supplying the greasewood and rabbitbrush tanks had a capacity of 350 gallons, and those for the willow, wildrose, and bare soil 

tanks 100 gallons. With the outlet valve of the reservoirs 

open, water delivered to the evapotranspiration tanks 

was controlled at the tanks by the float-valve mechanisms. The quantity of water supplied to the evapotranspiration tank in any period replaced water used 

by the plants. It is a measure of the draft from the 

ground-water reservoir.

The quantities of water used were determined by 

refilling the supply tanks to the reference marks at the 

end of each period with water measured through the 

water meter. It \vas not feasible to meter water directly 

into the evapotranspiration tanks because of the low 

demand-rate of flow. The rate varied from a trickle to 

about 1 gallon a minute during the course of a day. 

These rates were too low for proper actuation and operation of the meter.

The interval between refills varied, depending on the

FIGURE 10. Growth of plants in willow tanks on October 1, 1960, from 

cuttings planted on April 14, 1960. Average height, 2% feet, density 

comparable to that in surrounding thicket.
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D9

date within the growing season. At the beginning and 

end of the season, when water use by plants was low,- 

the reservoirs were refilled at weekly or 10-day intervals. During the height of the season, in July and 

August, it was not uncommon to fill them every day 

or every second day. All reservoirs, regardless of the 

need, were refilled on the first day of each month. In 

this way, the water use by months was obtained. Refilling was usually done in the morning, generally 

between 9 and 10 o'clock. At each filling, the operator 

compared the water use of the current period with that 

of the previous period, and thus was alert to any 

unusual condition. Some difficulty was experienced with 

sediment clogging the float-valve outlets, reducing and 

even stopping delivery of water to the evapotranspiration tanks. On a few occasions, the submergence of a 

float that developed a leak kept the float valve open and 

thereby allowed excess water to enter the tank. On 

other occasions, the horizontal-distribution system in 

the bottom of the greasewood tanks became partly 

clogged and restricted inflow. Unusual conditions such 

as these were quickly noted and easily corrected.

WATER LEVELS

With the exception of the interval following planting, while the plants were getting established, the water 

levels in the evapotranspiration tanks were controlled 

at predetermined depths below the surface of the tank. 

During and following planting of the seedlings or cuttings, the depth to water was maintained from 1 to 2 

feet below the surface so that the soil at the root level 

might be kept moist. As the plants became established 

and the roots developed, the water level was allowed to 

decline slowly, until the approximate operating level 

was reached.

All the evapotranspiration tanks were operated for 

the first 3 of 4 years with a water level 5 feet below the 

surface of the tank. When adequate data for that depth 

had been obtained, the water level was raised or lowered, 

and water-use data was obtained for different depths to 

water. The operating water levels for the different species and the different tanks, for the period of study, are 

given in the following tabulation:

Operating water levels for the evapotranspiration tanks

Growing season

Depth to 

Tank water level 

(ft)

Remarks

Greasewood

1961 _ _ 

1962-63.

1964.. ..

1965.. .. 

1965-.-.

1966.

1966.

1967.

1,2 

1,2 

1,2

1

2

1

2 

1,2

5.0 

5.0 

6.0 

7.5 

6.0

7.6 

6.2

7.8

Aug. 1 to Oct. 21.

Apr. 1 to Aug. 4. Membrane 

accidentally perforated on 

Aug. 4.

Willow

1961.......

1962-63-64. 

1965.......

1965.....--

1966.......

1966....__.

1967.......

1967.......

1,2,3 5.0 July 13 to Oct. 21.

1,2,3 5.0

1 5.7 Tank 3 discontinued.

2 3.5

1 5.8

2 4.2

1 5.4

2 4.1

Rabbitbrush

1963-64-65.

1966-..._..

1967---..-

1,2,3 

1,2,3 

1,2,3

5.0 

5.3 

6.2

Wildrose

1963-64. 

1965....

1965....

1966....

1966....

1967.... 

1967....

1,2,3

1

2

1

2

1

2

5.0 

5.0 

4.2 

5.9 

4.2 

6.1 

4.4

Tank 3 discontinued.

Bare-soil tank

1962.

1963.

1964.

1965.

1966.

4.0 

2.2 

1.9 

2.3 

4.0

SOURCES OF WATER FOR EVAPOTRANSPIRATION

Evapotranspiration rates, as determined from the 

tank studies, are for the period of the growing season, 

April 1 to October 20. The data are indicative but may 

not be applicable to the full year, as precipitation was 

not measured during the nongrowing season. In other 

similar tank studies in the southwestern United States 

where the winters are mild, evapotranspiration rates are 

determined for the full year.
D10 STUDIES OF EVAPOTRANSPIRATION

The water that made up the evapotranspiration discharge was supplied from three sources, namely, (1) 

rainfall, (2) water supplied to the tanks, and (3) soil 

moisture.

RAINFALL

As noted earlier, rainfall was measured by a standard 

8-inch rain gage at the weather station. The catch of the 

rain gage was considered the water supplied by rain. 

During the period of study, rainfall ranged widely not 

only in amount for the growing season but also in the 

intensity and frequency of individual storm periods. 

Rainfall for storm periods ranged from a low of 0.01 

inch in July and September 1966 to a high of 1.96 inches 

during a 4-day period in June 1964. During July 1962 

and 1963 and October 1964 and 1966, there was no measurable rainfall.

Rainfall, in inches, from April 1 to October 20, is 

given in the following tabulation:

Rainfall, in inches, from April 1962 to October 1967

Year

1962... ..........

1963..--. ........

1964-...........

1965... ......... .

1966...- ..

1967--.. ......

Apr.

 0.23

... 1.20

.87

.81

.38

.98

May

0.35

1.65

.97

.77

.33

.21

June

0.25

2.67

2.10

1.06

.48

1.51

July

0.00

.00

.35

.34

.01

.36

Aug.

0.13

.25

.08

.42

.02

.19

Sept.

0.05

.25

.46

.09

.42

.22

Oct.

0.35

.49

.00

.14

.00

.05

Period

1.36

6.51

4.83

3.63

1.64

3.52

WATER SUPPLIED TO THE TANKS 

The water supplied to the evapotranspiration tanks 

was the largest of the three sources of water for evapotranspiration. This water represented the draft on the 

ground water by the plants. Draft on the ground water 

will vary from season to season depending on the rainfall, the climatic conditions, and the portion of the 

winter rainfall available to the plants. The quantities, 

expressed as acre-feet per acre, supplied to each tank 

during the growing seasons from 1962 thru 1967 are 

given in the following tabulation:

Water, in acre-feet per acre, supplied to the evapotranspiration 

tanks during the growing seasons from 1962 to 1967

Tank 1962 1963 1964 1965 1966 1967

Willow---..-..--..-...

Wildrose.. .............

..... 1

2

..... 1 ..

2 ..

3 ..

.... . 1

2 

3

..... 1 ..

2 ..

3 ..

1.32

.98

1 on

1.55 

1.60

.17

0.90

.88 

1 39

1.42

1.76

2.89

2.97 

3.82

1.04

69

.58

.30

0.56

.51 

1.10

.98

1.20

1.76

1.80 

1.87

.86

89

.76

.63

0.42

0) 

.58

.53

.75

1 da

2.13

.83

1.26

.23

0.83

.98

1 1ft

.85

1.16

2 on

3.19

1

0°.

1 71

1ft

0.56

.62 

1 39

.72

91

1 55

1.85

1 08

1 dfi

/3\

1 Membrane perforated August 1965.

2 Discontinued in 1965.

' Unmeasured water entered tank 1967.

NOTE. 1 foot of depth of water over the tanks is equivalent to: 6,733 gallons for 

greasewood 1; 6.695 gallons for greasewood 2; 2,992 gallons for rabbitbrush 1, 2, and 3; 

748 gallons for willow 1, 2, and 3, wildrose 1, 2, and 3, and bare soil.

SOIL MOISTURE

The plants in the evapotranspiration tanks obtained 

part of their seasonal water supply from soil moisture 

in the unsaturated zone above the water table. The 

differences in the water content of the soils in the tanks 

at the beginning and end of the growing season provided a measure of the quantity of water obtained by 

the plants from this source.

Determinations of the volumes of water in the soils 

in the evapotranspiration tanks and of seasonal variations in the water content were started in 1962. Neutronmeter observations of soil moisture were made in access 

tubes installed in each of the tanks at the beginning, 

middle, and end of the growing season except in 1962 

and 1963 when additional observations were made. The 

tubes were of sufficient length to permit sampling to 

nearly the full depth of the tanks, and the bottoms were 

sealed in order to permit observations at depths below 

the water levels in the tanks.

The quantities of water, in acre-feet per acre, obtained by the plants from soil moisture during the 

growing seasons 1962 through 1967, and the observed 

changes in water content of the soil in the bare-soil 

tank are shown in the following tabulation: These data 

represent net changes in water content for the full profiles sampled, although the principal depletions of soil 

moisture occurred in the unsaturated zone.

Soil-moisture depletion, in acre-feet per acre, in evapotranspiration 

tanks at the Winnemucca test site during the growing season

[Parentheses indicate gain in soil moisture, generally as result of adjustment of water 

level in tank after start of seasonal operation]

Tank 1962 1963 1964 1965 1966 1967

Greasewood. 0.37 

.38

0.44 

.53

0.26 

.39

0.42 0.24 

.25

Rabbitbrush. 1 .09 .22 .19 .21 .65

2 .09 .22 .09 .14 .28

3 .10 .23 .12 .09 .36

Willow. 1 .34 .27 .10

2 .51 .28 .23

3 .38 .25 .26

.29 

.15

.28 

.22

0.43 

.64

.40 

.37

.52

.25 

.24

Wildrose. (.01) 

.03 

.15

.14 

.21

.27

.10 .10 

.14 .18 

.20 ........

.19 

.24

Bare soil. .14 .03 (.03) (.01) .04 .08

The water provided to the plants from soil moistur* 

each season constituted a significant part of the seasons' 

supply for each species. This source supplied 27 percerf 

of the total use in the greasewood tanks during the stud v 

period, 17 percent in the rabbitbrush, 10 percent in the 

wildrose, and 8 percent in the willow tanks. The contributions varied widely from year to year owing to differences in plant growth, water level, and the availability of soil moisture. The changes in water content in 

the bare-soil tank, however, were very small, and the 

water lost by evaporation was derived in approximately
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY Dll

equal shares from precipitation and water supplied to 

the tank.

Infiltration from winter precipitation was the principal source for replenishment of moisture in most of the 

soil above the capillary fringe. The water available to 

the plants from soil moisture varied annually as a result 

of variations in winter precipitation and opportunity 

for infiltration. The changes in soil-moisture volumes 

between the end of one growing season and the begining of the next were a measure of the portion of the 

winter precipitation that was stored durirg the nongrowing season. The total precipitation during this period actually was greater than the measured quantity, 

as part was lost by evaporation from the soil and plant 

surfaces, perhaps some by evapotranspiration, and some 

by sublimation of snow and ice.

A description of the instruments and procedure used 

in the measurement of soil moisture in the evapotranspiration tanks, illustrations of composite soil-moisture 

profiles in the tanks for the species studied, and some 

selected results are given in the section "Soil-moisture 

determinations" in this report.

The section also includes a brief discussion of seasonal 

soil-moisture changes in shallow^ flood-plain deposits 

as observed in the vicinity of the Winnemucca test site. 

These latter findings demonstrate (1) the characteristics 

of soil-moisture availability to phreatophytes growing 

on the flood plain and (2) the relative volumes of water 

stored in the unsaturated flood-plain sediments and carried over after wet seasons to the following season or 

discharged by evapotranspiration or by ground-water 

outflow in dry seasons. The volumes of water in storage 

in these sediments also may be indicative of the capacities available for inflow and storage of floodflows and 

the corresponding effects on streamflow downstream. 

Observations during a flood period in June 1962, for 

example, indicated the water content of the sediments to 

be more than 1 acre-foot per acre greater than in September 1961, which was near the end of a 3-year dry 

period. By October 1962, the \vater content had declined 

only about a half of this amount, with resultant carryover of about one-half acre-foot per acre to the following season. The water content at the end of the following seasons, 1963-65, increased slightly, but by the end 

of the 1966 season, which was dry, the water content 

dropped to the September 1961 level. The large changes 

in storage in 1962, 1966, and 1967 thus would be representative of significant differences between the annual 

precipitation, streamflow, and wrater available to plants 

on the flood plain.

COMPUTATIONS OF EVAPOTRANSPIRATION

Evapotranspiration was computed as the sum of the 

water supplied to the tank, precipitation during the 

growing season, and the difference in soil moisture at the 

beginning and end of the growing season. Because the 

water from the three sources was measured in different 

units, each was converted separately into equivalent 

depth of water in feet for the area of the tank; that is, 

acre-feet per acre. Water supplied to the tank during 

the season wras considered to represent the draft on 

ground water under the conditions of growth in the tank.

Water that may have been added or withdrawn prior 

to the beginning of the growing season in order to establish the desired operating water level for the season was 

not included in the computations of evapotranspiration.

PLANT GROWTH AND DEVELOPMENT

The growth and development of the plants were observed and recorded by photographs and measured by 

means of transects across the tanks. Photographs in color 

(35 mm transparencies) and in black and white were 

taken at 4-6-week intervals during the growing season. 

The color transparencies provided a chronological record of changes in the color of the foliage. Subtle changes 

in color, indicative of distress in the plants resulting 

from an unfavorable environment, could be detected by 

comparing the transparencies writh transparencies taken 

at a prior time or with the actual growth in the tanks. 

For example, during the summer of 1962 there was a 

slow progressive change in the color of the foliage of the 

greasewood plants in both tanks from dark green to yellowish green, the result of a deleterious salt buildup in 

the soil.

After the plants had become well established, measurements of the vegetative growth were made, beginning 

with the second full growing season. From a series of 

measurements made in July, August, and September 

of 1963 and 1964, it was found that the foliage was at a 

maximum and that most of the plant growth for the 

season had occurred by about the first of August. When 

the early part of the growing season was warm, maximum plant development occurred in late July; when the 

season was cool, the maximum occurred in early August. 

Some variation in foliage development among the four 

species was observed also. Willows were the earliest to 

reach their peak in growth and foliage development, and 

rabbitbrush was the latest. In a given season the difference in time was not great, generally only 2 to 3 weeks.
D12

FOLIAGE VOLUME

Foliage volume, computed from measurements of the 

height and crown intercept of the plants, is the product 

of cover density (expressed in percent), vertical thickness of the foliage, and surface area of the tank. In general, the foliage of the four species extended from the 

crown of the plants to the surface of the tanks, and the 

height of the plants thus was equal to the thickness of 

foliage. The cover density and thickness of foilage were 

obtained by transects across the tanks. The number of 

transects for a tank depended on the size of the tank. 

Four transects having a total length of about 140 feet 

were measured across each greasewood tank two on the 

diagonal and two at the midsections. Diagonal transects 

were used for the smaller rabbitbrush tanks in which 

the two transects measured totaled 57 feet, and the willow and wildrose tanks in which the two transects 

totaled 28.5 feet.

Cover density is considered synonymous with crown 

cover and is defined by Horton, Kobinson, and McDonald (1964, p. 9, 36) as "the amount of ground covered 

or shaded by the vegetation foliage." Cover density is 

measured by vertically projecting the transect intercept 

of the crown of the plant onto a tape stretched horizontally across the tank and by noting the length of the 

intercept. (See fig. 11.) The summation of the vertical 

projections of the crown intercepts, expressed in percentage of the transect length, is the measure of the 

crown cover for the transect.

FIGURE 11. Scientist measuring cover density and plant height on a 

transect across greasewood tank 2.

The results of one diagonal transect across greasewood tank 2, the computations for obtaining the average weighted height or thickness of foliage, and the 

cover density together with the method for computing

the volume of foliage are shown in the following 

example:

Transect across greasewood tank 2, southeast to northwest corners, 

Aug. 4, 1966

Interval 

(ft)

0.0 to 1.0 _ __ .

1.0 to 2.0_______.

2.0 to 5.5________

5.5 to 8.2 _ _ _.

8.2 to 9.5__ ___ -

9.5 to 12.2 _______

12.2 to 16.8 __ ____

16.8 to 21.7 ____ .

21.7 to 26.0____ _ .

26.0 to 31.0 ____ .

31.0 to 35.9 ____ _.

35.9 to 38.0 ____ _.

38.0 to 38.5 _ __

38.5 to 39.0 ____ _

39.0 to 41.1 _ _____

41.1 to 41.9 __ _ _

41.9 to 42.9 __--_..

Total. __ ___

Species '

______ B

______ G

_____ B

_______ G

_____ B

.__.___ G

______ G

______ B

_______ G

.__.___ G

____ B

_______ G

______ G

______ B

_______ G

____ B

G

Vegetation I

Height Intercept in 

(ft) (ft) i

1. 1

1. 1

2.0 

3.2

3. 0 

2. 5

1.3 

(Dead

1.7

. 8

2

1. 0

2.7

2.7 

4. 6

4.3 

5. 0

2. 1

plant)

2. 1

1.0

!5. 5

leight 

times 

.tercept 

(sqjt)

1. 10

2.97

5.40 

14. 72

12.90 

12. 50

2.73

3.57

.80

56.69

1 B, bare; G, greasewood.

NOTE. Length of transect 42.9 ft. Other computations are as follows: 

Computations for transect:

cc an

Average weighted height= '-=2.22 ft.

Cover density=25.5X100 

42.9

25.5 

=59.4 percent.

Computations for tank, based on four transects:

Average weighted height....____------ .--- 2.05 ft.

Average cover density..____ 49.55 percent.

Areaof tank-... . -- . - -- SOOsqft. 

Foliage volume: 2.05X900X0.4955=915 cu ft.

Variation occurred in the height of the plants as well 

as in the height of different parts of individual plants, 

for example, between the center and perimeter of the 

crown. These variations necessitated refinement of the 

method of height measurement so that an average plant 

height for each transect could be determined. For individual plants, the results of several measurements of the 

height of the crown along the transect were averaged. 

For plants of different heights, the transects were segmented into intercepts across a plant or group of plants 

of approximately equal height.

An example of the variation in height is afforded by 

the willow tanks. During the winter of 1963-64, rabbits present despite the ostensibly rabbit-proof fence 

gnawed the 'bark of the stems girdling and killing some 

plants and severely damaging others. Regrowth from 

the crown was rapid, and by August 1964 the new 

growth was about one half as tall as the undamaged 

growth. This combination of new and old .growth made 

it quite difficult to obtain a measure of the average
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D13

height of the plants. In computing the foliage volume, 

a weighted average plant height was used. The weighted 

average plant height for each tank was obtained by 

dividing the sum of the products of crown intercept and 

height of the plant for the transects by the total length 

of the transects. 

The results of the transect measurements for each 

season for the various tanks and the foliage volume computed from them, together with the depth to the operating water levels for that season, are given in table 1.

TABLE 1. Growth and development of woody phreatophytes grown 

in evapotranspiration tanks at the Winnemucca test site

Cover Plant Foliage Depth to 

Year density height volume water level Remarks 

(percent) (ft) (cu ft) below surface 

of tank (ft)

Grease wood tanks

1961 

Sept. 14............ 25.5 1.21 276 5.0 Tank 1.

29.5 1.35 358 5.0 Tank 2. 

1962

39.1 1.43 503 5.0 Tank 2. 

Aug. 8..... ....... 55.4 1.58 784 5.0 Tank 1.

38.9 1.43 501 5.0 Tank 2. 

1963 

July 17... ......... 61.9 1.97 1,091 5.0 Tank 1.

47.9 1.91 823 5.0 Tank 2. 

Sept. 6........... 56.4 2.01 1,015 5.0 Tank 1.

46.4 1.75 730 5.0 Tank 2. 

1964 

Aug. 4............ 62.4 2.14 1,195 6.0 Tank 1.

8........... 47.3 1.97 839 6.0 Tank 2. 

^965 

Aug. 3... ......... 59.2 2.19 1,161 7.5 Tank 1.

4............ 50.4 2.06 935 (») Tank 2.

1966 

Aug. 4............ 51.7 2.26 1,046 7.6 Tank 1.

6............ 49.6 2.05 915 6.2 Tank 2.

1967 

July 26.. .......... 52.1 2.24 1,045 7.8 Tank 1.

49.3 2.12 940 7.8 Tank 2.

Rabbitbrush tanks

1963 

July 17,18..--..... 37.8 1.29 197 5.0 Averageof 3 tanks.

Sept. 6-.-..-...-... 46.2 1.45 272 5.0 Do.

1964

1965 

Aug. 4,6 ......... 54.1 1.90 411 5.0 Do.

1966

1967 

July 26.-..--...-... 58.8 2.35 553 6.2 Do.

Willow tanks

1961 

Sept. 14. .____...... 81.6 3.26 266 5.0 Tank 1.

77.7 2.82 219 5.0 Tank 2. 

78. 8 2. 94 232 5. 0 Average of 3 tanks.

1962 

June 14-.-.,.....--- 84.8 3.21 272 5.0 Tank 1.

Aug. 30__________.__ 96.8 4.24 411 5.0 Do.

86.9 4.38 381 5.0 Tank 2. 

92.8 4.32 400 5.0 Average of 3 tanks. 

1963 

Sept. 6---........_- 88.3 4.40 389 5.0 Tank 1.

87.2 5.06 441 5.0 Tank 2. 

89.5 4.68 419 5.0 Average of 3 tanks.

1964

290.5 M.97 2450 5.0 Tank 2. 

2 87. 7 24.51 2 396 5.0 Average of 3 tanks

See footnotes at end of table.

TABLE 1. Growth and development of woody phreatophytes grown 

in evapotranspiration tanks at the Winnemucca test site Con.

Cover Plant Foliage Depth to 

Year density height volume water level Remarks 

(percent) (ft) (cu ft) below surface 

of tank (ft)

Willow tanks Continued

1965 

Aug. 6............ 83.7 4.34 363 5.7 Tank 1.

4.-..--. . 94.3 5.60 528 3.5 Tank 2.

(3)

1966 

Aug. 5.........,._ 58.1 3.74 217 5.8 Tank 1.

74.8 5.23 391 4.2 Tank 2. 

1967 

July 27..-......-.- 56.7 3.74 212 5.4 Tank 1.

75.2 5.72 430 4.1 Tank 2.

Wildrose tanks

1962 

Aug. 8........... 63.5 1.73 110 5.0 Tankl.

24. 1 1. 20 29 5. 0 Tank 2. 

50. 5 1. 54 78 5. 0 Average of 3 tanks. 

1963

Sept. 4......----.- 77.7 2.11 164 5.0 Tankl. 

60. 6 1. 67 101 5. 0 Tank 2.

1964 

Aug. 4_..-..--.... 79.8 2.34 187 5.0 Tankl.

73.6 2.57 189 5.0 Tank 2. 

80.6 2.64 214 5.0 Average of 3 tanks. 

1965 

Aug. 4 -.....--.. 75.5 2.13 161 5.0 Tankl.

72.7 2.65 193 4.2 Tank 2.

_.-......._.-.-.-......----_. «

1966 Aug. 5-..-..-.-.. 64.0 1.73 111 5.9 Tankl.

60. 0 2. 42 145 4. 2 Tank 2. 

1967 

July 27... ......... 76.6 1.77 136 6.1 Tankl.

73. 0 2. 81 205 4. 4 Tank 2.

1 Variable water level in tank 2, resulting from leak in plastic membrane. 

2 Plants damaged by rabbits gnawing bark; some stems died. 

3 Tank 3 discontinued on account of leaks in membrane.

The variations in growth and development of the 

plants are reflected in the changes in foliage volumes 

from one season to the next, as shown in figure 12. Only 

those tanks are shown for which the record of foliage 

volume and operating water level are continuous for 

the period of record. Curves for willow tank 3 and wildrose tank 3 are not shown, as they were discontinued in 

1965. For the most part, the foliage volumes showed a 

rather uniform increase during the early part of the 

study when the plants were becoming established and 

the operating water levels in all the tanks were maintained at a depth of 5 feet. 

In order to observe the effects of differences in depth 

to water on the growth rates after the plants had become established, the operating water levels were 

changed from the 5-foot depth. The relation of foliage 

volume to those changes in water level is shown in 

the curves of the foliage volumes of greasewood, willow, 

and wildrose. For the rabbitbrush tanks, in which the 

5- foot water level was maintained after the plants had 

become established, the foliage volume increased at 

nearly the same rate over the 3-year period 1963 through 

1965; practically the same rate continued in 1966 and

372-487 O 7C
D14 STUDIES OF EVAPOTRANSPIRATION

1200

RABBITBRUSH

Averag 5 of 3 tanks

£* 

z5

Uluj -"O

- 

NJ U.

XuJ t m UJ

OUU

^500

UJ

i\GE VOLUME, IN CUBIC F

£w rv>

O Q 

O0 

0

100 

0

WILLO

-

-

~'X

W

f* ^<

Tanks' 

1 and 2

Tank 2 ,

-Tank 1 

V--^-

xx\

\

A

\

Ta"nk2~ 

VTank 1

- 

* -»

1961 1962 1963 1964 1965 1966 1967

FIGURE 12. Seasonal foliage volumes and operating water levels

WILDF

s

X

OSE

Tanks 

1 and 2

Tank 1

^ X 

X

'Tank 2

*7^

Tank 1

^"^v

Tank 2

Tank 1

L^X 

^v^

-

_s 

^-

W X VI O1l £. f\ FEET DPTH WATER LEVEL, IN TO

BELOW SURFACE OFTANK

1962 1963 1964 1965 1966 1967

tanks.

1967, when the water level was lowered from 5.0 feet to 

5.3 feet in 1966 and from 5.3 to 6.2 feet in 1967.

There was a moderate to small increase in foliage 

volumes for the greasewood tanks in 1964 when the 

water level was lowered from 5 feet to 6 feet below the 

surface of the tank; however, the amount of increase 

was less than with the 5-foot water level from 1962 to 

1963. The record for tank 2 was interrupted during the 

1965 season because a leak in early August resulted in a 

variable water level for the remainder of the season. 

When the leak was repaired in the spring of 1966, the 

tank again became operational. The foliage volume (as 

shown) was measured in 1965, but the water level is not 

shown because of its variation. Foliage volume for tank 

1 decreased in 1965 and 1966, when the water level in 

tank 1 was lowered from 6.0 to 7.5 feet, and in tank 2

in 1966, when the level was lowered to 6.2 feet, following repair of the ruptured membrane. In 1967, however, 

with the water level in both tanks at 7.8 feet, there was 

no further decrease in the foliage volume of tank 1, 

and there was a small increase in tank 2. It is inferred 

from this fact that the plants had become established 

and adjusted to the deeper water levels. If such is the 

case, it points up an important principle of phreatophyte growth; namely, that a lowered water level affects 

the plant only temporarily pending readjustment of 

the root system to the new environment, provided the 

lowered water level is not beyond the reach of the roots. 

The effect of the damage to the willow plants by 

rabbits during the winter of 1963-64 resulted in a decrease in the foliage volume for tank 1 and a reduction in the rate of increase in foliage volume in tank 2
EVAPOTRANSPIRATTON BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D15

during the 1964 growing season. The foliage volume in 

tank 1 increased during the 1965 season despite a water 

level lower than that in 1964. The increase in foliage 

volume for willow 2 was much greater in 1965 than 

in 1964. Part of the increase was attributable to the 

higher water level, raised from a 5-foot depth to a 3.5- 

foot depth, and part to a continued recovery from the 

damage by rabbits.

On the basis of the differences in the increase of foliage volume in the two willow tanks having different 

water levels, it is inferred that the plants did not recover 

completely in 1965 from the damage in the winter of 

1963-64. During the second growing season, the plants 

evidently were still recovering from the damage. The 

decrease in foliage volume in tank 2 in the 1966 growing season is believed to reflect the effect of the greater 

depth to water level in 1966 when the water level was 

lowered 0.7 foot from 3.5 to 4.2 feet below the surface 

of the tank. The cause for the sharp decrease in foliage 

volume in tank 1 in 1966 is uncertain. Several plants 

died in 1966, and this is reflected in a decrease in cover 

density from 83.7 percent in 1965 to 58.1 percent and 37 

percent in 1966. The cause of death was not apparent. 

There was only minor damage by rabbits and none by 

insects. In 1967 the water level was raised 0.4 foot in 

tank 1, from 5.8 feet to 5.4 feet below the surface of the 

tank, while in tank 2 the raise was only 0.1 foot from 

4.2 to 4.1 feet. The foliage volume of tank 1 was almost 

unchanged from 1966, but in tank 2 there was a substantial increase. The halt in the downward trend in 

tank 1 and the increase in tank 2 are believed due in part 

to the higher water level and in part to the adjustment 

of the roots to the changed water environment as described for the greasewood tanks.

The plants in the wildrose tanks had in general a 

similar pattern in that large foliage volumes were produced when the water levels were shallow, and small 

volumes when the water levels were deep, except in 

1965. In that year, with no change in water level, the 

foliage volume in tank 1 decreased about 14 percent. The 

reduction resulted from the death of stems of several 

plants in July.

Many variables affect plant growth and development 

such as climate, soils, salts in the soil, and water supply. 

However, none of these are responsible for the change in 

foliage volume of the tanks from year to year, as conditions were the same for each pair of tanks for each 

species. The differences in the paired tanks, shown in 

figure 12, can be due only to changes in the depth to 

water level during the growing season.

In order to have some basis for comparing greasewood 

growth in the tanks to growth in the field outside the 

tanks, several 100-foot transects were measured in the 

field. The transects, measured during the 1964,1965, and

1966 growing seasons, sampled three localities having 

different growth conditions. The first locality was within 

the test-site enclosure near the north side; there the 

growth had not been disturbed by livestock grazing since 

the site was fenced in 1960. Although greasewood is not 

very palatable, cattle will browse on it at certain times 

of the year when hungry. At the same time, the plants 

may be damaged through trampling or by breaking of 

plant stems.

The second locality was outside the test-site enclosure 

and about 30 feet away from the first locality. There 

browse conditions were the same as those within the 

enclosure at the time it was fenced. This locality is in a 

pasture that is grazed moderately during the winter 

months.

The third locality, also outside the test site, was in a 

pasture that was heavily grazed and had frequently held 

small herds of cattle for periods of as much as 1 month. 

Damage to the plants from grazing and trampling was 

quite apparent. The results of the transect measurements, together with comparative data on average 

foliage volume of the two greasewood tanks, are shown 

in table 2.

TABLE 2. Comparative data on greasewood growth in the field 

under different growth conditions and the average in the two 

evapotranspiration tanks

Year

Plant Cover Foliage

height density volume

(ft) (percent) (cu-ft per acre)

Inside Winnemucca test-site enclosure

1964.

1965.

1966.

2.37

2. 39

2.30

51. 3

57. 3

57.8

52, 960

59, 650

57, 910

Outside test-site enclosure (grazed moderately)

1964. 2.35 41.3 42, 280

Outside test site (grazed heavily)

1964.

1965.

1966.

2. 08

2. 04

2. 04

28.0

33. 1

36.8

25, 370

29, 410

32, 700

Average of two evapotranspiration tanks

1964.

1965.

1966.

2. 06

2. 12

2. 16

54. 8

54. 8

50.6

49. 170

50. 620

47, 590

Comparisons of plant growth on the basis of foliage 

volumes show that the undisturbed growth in the enclosure is about 15 percent greater than the average of 

the growth in the two tanks. This difference is not surprising in view of the fact that these plants were well 

established at the time the tanks were planted and have
D16 STUDIES OF EVAPOTRANSPIRATION

been able to grow undisturbed since that time. The 

growth outside the enclosure, in the moderately grazed 

pasture, was about 10 percent less than the average of 

the two tanks. The difference in growth between the two 

localities, for which the plant growth was about the 

same in 1960, show that there was a substantial improvement in growth when the plants were protected from 

livestock grazing.

The effect of heavy grazing is reflected in the foliage 

volume of the transects in the heavily grazed pasture. 

Plant growth there was less than half that in the enclosure, about 40 percent of that in the moderately 

grazed pasture, and about 65 percent of that in the 

tanks.

These data indicate that man's activities in the management of livestock operations adversely affect greasewood growth. Other woody phreatophytes may be affected to a greater extent, as the palatability of some, 

such as willow, is higher than that of greasewood.

PROBLEMS

In the planning of the evapotranspiration studies, 

some problems were anticipated in the operation of the 

tanks, in transplanting, and in establishing growths 

representative of those outside the tanks. Some problems 

arose as the result of nature's handiwork, whereas others 

resulted from the disturbance of nature's balance. An 

important objective of the project construction and 

development was the maintenance of the natural environment insofar as possible. In the tank construction, 

disturbance of the soils could not be avoided, and the 

soils were mixed to some extent and could not be replaced in the same layered sequence in which they occurred naturally.

The problems caused by nature included damage by 

insect infestation during the growing season and by 

rabbits during the winter months; they were not related to construction or operation of the tanks. Insect 

infestation was not restricted to the plants grown in the 

tanks but was widespread over the countryside. That 

condition recurs at intervals of several years. Damage 

by rabbits gnawing the bark of willow plants may be 

local or widespread depending on the severity of the 

winter and the scarcity of food available to the rabbit 

population.

The accumulation of deleterious salts in the root zone 

in several of the tanks was one of the problems resulting 

from the disturbance of nature's balance.

DAMAGE BY WEBBING INSECTS

In early July 1960, a webbing insect spun webs and 

laid eggs on the greasewood plants in the tanks and 

on about 90 percent of the area of greasewood growth

in the Humboldt Eiver valley, including the Winnemucca reach. The insect was identified as belonging to 

the family Pyrolidae, genus Eumysia sp. Infestations 

of the insect appear periodically throughout greasewood areas in Nevada and Idaho, and doubtless other 

western States. However, their only appearance during 

the period of study was in July 1960. Eumysia sp. was 

the night-flying insect that was discovered in 1950 by 

George Zappettini, State Forester for Nevada, and reported on in his dissertation for a master's degree at 

the University of Idaho. The larvae has a snout mouth, 

is from 10 to 16 millimeters long, is 3 millimeters in 

diameter, and has a voracious appetite. The adult has 

a wing span of about 23 millimeters and a body length 

of about 10 millimeters.

Webbing was confined almost entirely to greasewood 

and associated species such as rabbitbrush, shadscale, 

and occasionally a sagebrush plant. No webbing from 

this insect was observed in willow growth. The webs 

had a white or silver luster and were so numerous that 

in some areas when the sun was low on the horizon, 

as in early morning or late afternoon, they gave a soft 

silver sheen to vast areas of greasewood growth.

Damage due to the larvae feeding on the leaves was 

extensive; many of the leaves on the plants were 

shriveled and brown and presented a dead appearance. 

Serious damage to the greasewood plants in the tanks 

was avoided by spraying with the insecticide DDT on 

July 16. The plants in the tanks were only about 3 

months old, and it was evident that unless the ravages 

by the insects were checked, the plants would be 

severely damaged and perhaps die. Although damage 

was extensive, the plants recovered rapidly following 

the spraying; by early August the plants presented a 

healthy green appearance and extensive new growth 

was apparent.

A different webbing insect, the so-called "tent 

caterpillar", spun webs, laid eggs, and produced larvae 

that damaged the willow plants. This insect, although 

also widespread, did not blanket the willow growth, 

but occurred randomly in groups or clusters. With the 

exception of the 1966 growing seasons, webs of the 

insect were found on one or more of the plants in the 

willow tanks beginning with the 1962 season. Serious 

damage again was avoided by spraying with DDT.

It could be argued that preventive measures should 

not be taken to guard against damage by insects, that 

they are a part of nature's checks and balances, and 

hence that the studies of evapotranspiration that use 

such measures do not simulate natural conditions. 

However, the newly transplanted greasewood shrubs 

were in grave danger of high mortality, and unchecked 

damage would have delayed the studies a full year.
EVAPOTRANSPIRATTON BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D17

Furthermore, the occurrence of the infestations at intervals of severals years, rather than yearly, reduces their 

impact on the average annual evapotranspiration loss. 

As the tent caterpillar does not damage the willow 

growth everywhere, but only in spots, evapotranspiration losses from undamaged plants would be a closer 

representation than that from damaged plants.

DAMAGE BY RABBITS

The fence enclosing the test site was constructed to 

be rabbit proof, but rabbits did manage to enter. The 

rabbits probably came through the gate at times when 

it was open and workmen were in a remote part of the 

site. During the severe winter of 1963-64, rabbits damaged the willow plants in the tanks so badly that a 

number of the stems died. The rabbits gnawed the bark 

of the stems just above the surface of the tank and those 

that died were girdled or nearly so. Although vigorous 

regrowth occurred from the root crown during the 

1964 growing season, a comparison of foliage volumes 

of the 1963 and 1964 seasons indicated that it was not 

enough to compensate for the loss of foliage from the 

dead stems. Damage to the plants by rabbits was noted 

also during the winters of 1964-65 and 1965-66, but 

it was not extensive and only a few stems died.

During the 1963-64 winter, there was damage also 

to the willows growing in the site outside the tanks, 

but it did not appear to be as severe as that in the tanks. 

Possibly the bark from the younger trees in the tanks 

was more palatable than that from the older trees.

In an effort to prevent or at least reduce further 

damage, a program to remove the rabbits from the test 

site was begun in the fall of 1964. Using a humane trap, 

in which the rabbits were captured alive, several rabbits 

were caught and then released outside the fenced site. 

The trapping program is believed responsible for the 

lesser damage in the following years.

SALT CONTENT OF THE SOILS

The soils of the terrace deposits, used in the greasewood and rabbitbrush tanks, were suspected to have an 

appreciable content of alkaline salts. However, because 

there was a luxuriant growth of greasewood on these 

soils, no difficulty was expected in growing greasewood 

plants in tanks containing the same soil, particularly 

as greasewood is known to have a high tolerance for 

alkaline salts.

The soils of the flood-plain deposits, used in the willow and wildrose tanks, were considered to have a low 

salt content. These deposits are subject to repeated 

leaching by overflow of the Humboldt River during 

spring runoff. The flood plain supported a dense and

luxuriant growth of willow, wildrose, and meadow 

grasses.

The higher salt content of the terrace deposits was 

confirmed by the appearance of an incrustation of salts 

on the surface of the greasewood and rabbitbrush tanks 

as the result of maintaining a high water level while 

the plants were becoming established, whereas there was 

no incrustation on the willow and wildrose tanks. A 

water-soluble portion of the incrustation samples collected in August 1960 was made by treating 1.015 grams 

of the ovendried sample with a liter of water. The 

sample gave the following concentration in the extract:

mg/l

Ga ____________ 2.4 

Mg ________ _ .7 

Na ____________ 115

Dissolved solids____ 350

PH

(CO 

SO 

Gl _

mg/l 

10.1 

93 

60 

28

In April 1962, samples of water from the saturated 

material below the water level were collected, for chemical analysis, from all but rabbitbrush tanks 1 and 3 and 

the bare-soil tank. The samples were obtained by withdrawing water from the water-distribution system of 

the tank by pumping. In order to have a representative 

sample from the 10- by 10-foot tank, at least 50 gallons 

was pumped to waste before sampling; greater volumes 

were pumped from the larger tanks. A sample of water 

was also collected from the well supplying water for the 

evapotranspiration tanks. Later, in August 1962, additional samples of water were collected from greasewood 

tank 2. The results of the chemical analyses, given in 

table 3, show clearly that the mineral content of the 

soils of the terrace deposits is considerably greater than 

that of the flood-plain deposits.

BORON TOXICITY

A wholly unexpected and serious effect on the growth 

of, and water use by, the greasewood plants occurred 

during the 1962 growing season. The first signs of distress, noticed in July 196-2, were tip burn of the leaves 

and a change in color of the foliage. The color change 

from a normal dark green to a yellowish green became 

more pronounced as the season advanced. In addition to 

those symptoms of distress, there was progressive defoliation during August and September, and by early 

October some plants had lost more than half their 

leaves. Chemical analysis of leaf samples and of the soil 

in the tanks indicated that the difficulty was probably 

due to toxic concentrations of boron in the root zone.

The first indication that boron was responsible for 

the difficulty was the chemical analysis of a second sample of the incrustation on the soil surface of greasewood 

tank 1 collected in the fall of 1962. The results of the 

second analysis were startling, for it showed a boron
D18 STUDIES OF EVAPOTRANSPIRATION

TABLE 3. Chemical analyses of water from the saturated soil in the evapotranspiration tanks of the Winnemucca test site

[Constituents given in milligrams per liter]

Gallons Specific 

pumped Calcium Magnesi- Sodium Potassi- Lithium Bicar- Carbon- Sulfate Chloride Phos- Boron Dissolved conductDate collected 1962 before (Ca) um (Mg) (Na) um (K) (Li) bonate ate (SOO (01) phate (B) solids ance pH 

collecting (HCO8) (CO3) (PO4) (micromhos 

sample at 25°C)

Project supply well

Apr. 12........ ..... 1,000+ 48 14 103 11 0. 12 320 0 78 51 0. 16 0. 44 507 785 7.81

Grease wood tank 1 1

9.. .......

11 .

..... 100

..... 770

..... 1,200

153

79 

56

43

28 

19

1,260 

1,030

942

31 0.29 

25 .24 

23 .21

2,750 

2,020 

1,840

0 

50 

39

518 

388 

326

384 

312 

274

5.5 13 3,800 

6.4 9.2 2,970 

7. 0 8. 8 2, 650

5,190 

4,190 

3,800

8.14 

8.42 

8.40

Greasewood tank 2 '

Apr. 7 

Aug. 8.........

8 .

g

9.........

9 ......

500

.... 50

960

..... 1,470

.... . 2,430

..... 2,650

99 

51 

56 

53 

51 

51

30 

15 

15 

15 

14 

14

768 

119 

298 

350 

433 

449

31 0. 19

19

IS

1Q

25 ..........

23 ..........

1,680 

360 

700 

798 

959 

1,020

0 

0 

0 

0 

0 

0

338 

88 

156 

171

176

179

262 

57 ..

106 ..

116 ..

130 ..

133 ..

6.7 7.1 2,420 

........ .47 ..........

........ 2.1 -.--......

........ 3.3 ..........

........ 4.8 ..........

........ 4,5 ..........

3,460 

877 

1,600 

1,780 

2,050 

2,150

8.19 

7.65 

8.13 

7.80 

7.70 

8.13

Rabbitbrush tank 2 >

Apr. 12... ........... 100 197 55 768 29 0.24 1,260 0 672 492 0.57 2.1 2,910 4,170 7.95

Willow tank 1 *

~~- 50 107 27 143 13 0.09 668 0 73 59 0.09 0.47 799 1,220 7.82

Willow tank 2 2

Apr. 7.... ...... - 50 115 31 143 13 0.09 632 0 73 97 0. 05 0.40 836 1,320 7.42

Willow tank 3 2

Apr. 12......... .. 50 117 34 140 13 0.09 668 0 73 89 0. 10 0.43 850 1,340 7.55

Wildrose tank 1 2

 .. 50 79 46 162 13 0.14 749 0 53 61 0. 02 0.51 835 1,310 7.60

Wildrose tank 2 2

 - 50 99 36 183 14 0.13 815 0 49 67 0. 06 0.61 887 1,380 7.87

Wildrose tank 3 *

Apr. 8... __ ... . - 50 120 49 140 11 0.10 792 0 57 80 0.03 0.38 896 1.410 7.44

1 Terrace soil. 

1 Flood-plain soil.

content of 416 milligrams per kilogram. To assess more 

exactly the amount of boron in the plant and in the soil, 

samples of leaves and of soil were collected in October 

1962. The soil was sampled at 1-foot intervals to a depth 

of 8 feet in greasewood tank 1 and to 6 feet in tank 2 

and to a depth of 4 feet in rabbitbrush tank 1. Samples 

were collected with a 3-inch earth auger. In augering for 

the samples in greasewood tank 2, a considerable quantity of root material was encountered between depths 

of 1 and 6 feet. The small roots and rootlets were carefully separated from the several soil samples and preserved for separate analysis of their boron content. 

Samples of the leaves shed were collected from both 

tanks and from plants growing outside the tanks, and 

samples of green leaves were plucked from plants 

growing in the two tanks. 

The results of the analyses for boron content of air

dried portions of the leaf and root samples are given 

below:

[Leaves were collected on Oct. 18, 1962]

Greasewood tank 1

mg/kg

Shed leaves from six plants_ _ _ 120 

Green leaves, from the tips of branches 233

Greasewood tank 2

Shed leaves from two plants -_ - 125

Green leaves' from tips of branches 196

Root material between the depth of 1-6 feet 54

Greasewood plants growing outside of tanks

Shed leaves from several plants 107

The boron content of the shed leaves was about 20 

percent higher for the plants in the tanks than for 

those on the outside, and the green leaves had nearly 

twice the content of boron that the shed leaves had. A
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D19

sample of green leaves was collected from plants growing outside the tanks, but unfortunately it was lost.

The soil samples were analyzed for their water soluble 

boron content by the Nevada Soil Testing Laboratory 

at the University of Nevada.1 The greatest concentration in both greasewood tanks occurred between the 

depths of 3 and 4 feet, as shown in figure 13. In the 

graphs of figure 13, the boron content is plotted as the 

midpoint of the sample interval, 0.5, 1.5 feet, and so 

forth. The four samples from the rabbitbrush tank all 

had the same boron content, 5 mg/kg.

SOURCE AND ACCUMULATION OF BORON IN THE 

TOP SOIL

The terrace deposits are rich in boron, as was established by the chemical analysis of soil samples. The 

mineral form of occurrence is not known, but the results 

of soil analysis indicate that some of the boron is readily 

soluble in water and that a larger amount is in a relatively insoluble or only slightly soluble form. Boron 

in the water soluble form is presumed to be readily 

available to the plants and was given the most 

attention.

fcz

LJ

Z

Z

IIL.

o

LJ 

Q

cc

(0

1

\

\

Greasewood 2

EXPLANATION 

Oct. 22, 1962, before leaching 

April 15, 1963, after leaching

10 20 

BORON, IN MILLIGRAMS PER KILOGRAM

30

FIGURE 13. Soluble boron content of the soil In greasewood tanks 

1 and 2 before leaching and in tank 1 after leaching.

1 In addition to boron, the sample was found to have a lithium content of 20 mg/kg.

In order to obtain information about the total concentration of boron in the deposits, four soil samples 

were analyzed. The samples were collected from greasewood tank 1 with a Veihmeyer soil-sampling tube. The 

sampling on October 23, 1963, was from the surface 

to a depth of 5.5 feet, at i/^-foot intervals. Determinations were made of the soluble boron in all the samples 

and of total boron in four samples at depths from 0.5- 

2.5 feet. Total and soluble boron content of the four 

samples are given in the following tabulation:

Depth interval (ft)

0.5 to 1.0. 

1.0 to 1.5. 

1.5 to 2.0. 

2.0 to 2.5.

Total boron 

(mglkg)

700

875

1, 140

280

Water solvMe 

boron (mg/kg)

11.7

18. 1

7.34

11.7

The soluble boron content probably would have been 

higher had not the soil in the tank been leached the 

previous October. The analyses indicated that a large 

supply of boron is stored in the soil in some form not 

readily soluble; however, this material may be slowly 

decomposed by chemical reactions with water and solutes 

in the tank, and accumulations of the decomposition 

products in the tanks could lead to toxic quantities of 

soluble boron. The analyses for insoluble boron content 

illustrate the magnitude of the total boron supply in the 

tanks. The depths at which the samples were collected 

have no relation to natural soil profiles, as the soil in 

the tank had been disturbed.

In addition to the sampling in the tank, three sets 

of samples of the soil profile and two sets of surfacesoil samples were collected for soluble boron determination from the undisturbed soil outside the tanks. The 

sampling points were in the midst of a thicket of about 

15 greasewood plants. The starting point for the lines 

of lateral samples was the central stem of one of the 

large plants. Figure 14 shows the location of the sampling points, the lateral lines of sampling, and the surrounding greasewood growth. The dates, depths, and 

sampling interval are given in the following tabulation:

Vertical sampling

Sampling point Date

A____ _ __ Oct. 22, 1962___

B____ _____ Apr. 9, 1963 __

C_ __ __ Oct. 23, 1963 __

Depth S 

sampled ; 

(ft)

0-9

0-1. 5

0-5.5

ampling 

interval 

(ft)

1. 0

0. 25

0.5

Lateral sampling

Distance from Sampling

Sampling line Date central stem interval

of greasewood (ft) 

plant (ft)

D-E_______ Apr. 9, 1963__._ 1-8 1

D-F_______ Oct. 25, 1963..__ 0-8 1
D20 STUDIES OF EVAPOTRANSPIRATTON

Surface soil sampling lines, 3-inch depth

r, T, , . __» __-- D-E. 8 samples at D-F. 9 samples at^p ?" ^<^ i.foot intervals 

1-foot intervals / y

Enlarged view of a part of A 

B 1 in. = 10 ft

Soil sampling points A, B, and C (of soil profile)

C. Sampled 0-5.5 feet , 

at 6-inch intervals 4r

A. Sampled 0-9 feet 

at 1-foot intervalsB. Sampled 0-1.5 feet 

at 3-inch intervals

1 in. = 20 ft

FIGURE 14. Location of points and lines used In sampling undisturbed 

soil, in relation to clumps of greasewood.

The nine samples collected from sampling point A 

by means of a soil auger were analyzed by the Nevada 

Soil Testing Laboratory at the University of Nevada. 

All the other samples were analyzed by the Water 

Resources Division Laboratory of the Geological Survey 

at Menlo Park, Calif. The samples collected from point 

B were taken from the side of a 1.5-foot dug pit, and 

those collected from point C were taken with a Veihmeyer soil-sampling tube. All the samples were analyzed 

for water-soluble boron; in addition, the pH and specific 

conductance were determined for the samples for sampling point A, and sodium for the samples from line 

D-F. The results of these analyses are given in tables 

4 and 5 and are shown graphically in figure 15.

These analyses demonstrate that there is an accumulation of soluble boron in the surface and near-surface 

soil. The concentration in the surface soil is greatest in 

the vicinity of the greasewood plants, and that in the 

near-surface soil is greatest between the depths of 1 and 

2 feet. The data suggest that evapotranspiration by the 

greasewood is largely responsible for the accumulation 

and concentration of boron in the soil. The chemical 

analyses of the samples of the soil, roots, and leaves show 

that there is uptake of solu'ble boron by the roots, translocation of the solute through the roots and stems to the 

leaves, temporary storage in the leaves, and release to 

the soil. The process of release may include guttation, 

leaching by washing of the green leaves by rainfall, 

decay and leaching of the leaves after being shed, or a 

combination of these processes. Experiments with barley

TABLE 4. Results of analysis of samples of 

undisturbed soil profile

[The nine samples collected from sampling point A were 

analyzed by the Nevada Soil Testing Laboratory, University of Nevada, Reno, Nev. All other samples were 

analyzed by the Water Resources Division Laboratory, 

U.S. Geological Survey, Menlo Park, Calif.]

Depth interval (ft) PH

Specific 

conductance 

(micromhos

at 25° C)

Boron

Sampling point A

[Sample Oct. 22,1962]

0 to !______...__ 9.6 29,000 24

1 to 2__..._._.__ 9.6 24,000 10

2to3_--....._._ 9.6 7,500 8

3 to 4___..__..._ 9.6 3,500 5

4 to 5_.__.______ 9.5 1,500 4

5 to 6___-_-_--._ 9.2 1,100 6

6 to 7___._______ 9.0 700 5

7 to 8_________ 9.1 800 5

8 to 9__ _______ 9.0 700 5

Sampling point B Dug pit

[Sampled Apr. 9,1963]

0.0 to 0.25-_-_-_--_--------_--_-- 13

0.25 to 0.5_____ ___ _ 18

0.5 to 0.75----------------------- 29

0.75 to 1.0-__-_--_---____-_-_-___ 26

1.0 to 1.25__________-___ 32

1.25 to 1.5 _ __ 25

Sampling point C

[Sampled Oct. 23,1963]

0.0 to 0.5_________-____-___ 10. 5

0.5 to 1.0 20. 7

1.0 to 1.5__________________ 32. 0

1.5 to 2.0______________________ 20. 3

2.0 to 2.5.--.-------------------- 13. 5

2.5 to 3.0___----_-_-______-_ 4. 3

3.0 to 3.5 1. 8

3.5 to 4.0 1. 4

4.0 to 4.5 1. 0

4.5 to 5.0______-_____--_-__--_-_- 1. 0

5.0 to 5.5____-___-_-________ . 9

plants grown in a boron solution show that boron may 

be lost from the leaves by both guttation and rainfall 

washing the leaves (Oertli, 1964). Following release 

from the leaves, boron seemingly is moved downward in 

the soil by infiltration of rainfall or melted snow water. 

As about two thirds of the annual precipitation falls 

during the winter months when evapotranspiration is 

low, the downward movement probably occurs at that 

time. During the spring and summer months, evapotranspiration dissipates the winter accumulation of 

water, leaving behind the boron and mineral salts.

The boron and specific conductance curves for sampling point A, shown in figure 15, illustrate this condition. The concentration of boron in the top 1.5 feet of 

the soil column is shown in greater detail by the curve 

for sampling point B, where the samples were collected 

at 3-inch depth intervals. The distribution of boron in
 =3

&§ B »

P3 H

B p. 

E «

&p

H, po 

2 t?

B g <-»- o 0"

as B

P" 2 O <*

^ &

Sff

B 7

I-1 O3 CO M rf^H-'

po <x> o p-1 01 jf^ po ri^ jji. 

O3

s!
D22 STUDIES OF EVAPOTRANSPIRATION

6-month periods preceding the sampling dates shows 

3.37 inches for line D-E and 2.82 inches for line D-F, 

a difference of about half an inch. The mode of precipitation was also different. The 3.37 inches preceding the 

April sampling of line D-E occurred largely in the form 

of snow that accumulated during each storm and then 

melted slowly at temperatures above 32°F. Such a condition provides for a maximum infiltration opportunity 

that allows wetting of the soil to considerable depth and 

downward movement of boron. The 2.82 inches of precipitation preceding the October sampling of line D-F, 

fell as rain, in widely separated showers. With the exception of 1 day in July when 0.48 inches fell, the 

showers were light, none of more than 0.3 inch and most 

of them less than 0.25 inch. During the summer months, 

when the rainwater is dissipated rapidly by evapotranspiration, the soil is wetted only superficially, and so 

there is less opportunity for boron to migrate downward 

into the soil column than during the winter months.

LEACHING AS A CORRECTIVE MEASURE

The results of the chemical analyses of the soil and leaf 

samples indicated that the soil in the tanks was rich in 

boron, and that there was uptake of boron by the greasewood plants in quantities that damaged the plants. To 

assure continued growth and survival of the plants, a 

reduction of the boron content in the root zone was essential. The measure considered to be the simplest and most 

effective was to leach by backwashing. In this procedure, 

water was introduced into the bottom of the tank 

through the distribution system until the entire soil mass 

was saturated and water overflowed the tank. As the 

water moved upward through the soil, boron and other 

mineral salts were taken into solution and carried away 

in the effluent.

As the soil in both the greasewood and rabbitbrush 

tanks was similar in texture, and presumably similar in 

boron and mineral salts contents, both sets of tanks were 

leached, even though the rabbitbrush plants had not 

shown any signs of distress. Backwashing of greasewood tank 1 and rabbitbrush tank 1 began in late October 1962, and was nearly completed on November 5 when 

freezing temperatures effectively halted further operation for the year. Backwashing of the three remaining 

tanks, greasewood tank 2 and rabbitbrush tanks 2 and 3, 

was completed in April 1963.

During the summer of 1965, the rabbitbrush plants in 

the tanks began to show symptoms of distress. Leaftip 

burn was quite noticeable, and toward the end of the 

growing season, there was some defoliation. The plants 

in the greasewood tanks, however, did not show any

symptoms indicating boron toxicity. The backwashing 

evidently had not been as effective in the rabbitbrush 

tanks as in the greasewood tanks, or the greasewood 

plants had a greater tolerance to boron than rabbitbrush. The latter explanation seems more appropriate. 

These symptoms of boron toxicity indicated that the 

rabbitbrush tanks needed to be backwashed a second 

time. Backwashing of all three tanks was completed 

in early April 1966.

Sampling of the soil in the tanks to determine the 

reduction of its boron content as leaching progressed 

was not practicable. Collection of soil samples at depth 

would have been exceedingly difficult and facilities for 

analytical determinations were not available at the site. 

In lieu thereof, the specific conductance (in micromhos 

at 25°C) of the effluent water was used as a measure of 

the effectiveness of the treatment.

The volume of water used for leaching and the rate 

at which it was added to each tank was measured with 

water meters. As soon as the soil mass in each tank 

was saturated, a 5-foot extension was attached to the 

supply pipe of the tank to provide a head for the 

upward movement of water through the soil. The 

rate of flow through the soil was not the same for all 

tanks of equal surface area; however, an approximately 

uniform flow through each tank was achieved by a rate 

of inflow adjusted to maintain a head of about 5 feet 

above the surface of the tank. The rate of inflow to 

greasewood tank 1 and rabbitbrush tank 1 in October 

1962 was less uniform than the rates for greasewood 

tank 2 and rabbitbrush tanks 1, 2, and 3 in April 1963 

and 1966. The difficulty was due largely to the low 

temperatures during the night causing ice to form on 

the surface of the tanks and in the surface soil.

Samples of the effluent, for analysis, were collected at 

the overflow point of each tank. Field determinations of 

the conductivity of the samples were made as they were 

collected. The results of the conductivity determination were used as a guide in selecting the samples of 

the effluent for chemical analysis. As shown in figures 

16 and 17, the conductivities were high initially and 

decreased as the leaching progressed. Leaching was 

continued until the conductivity of the effluent had 

decreased to about 2,000 micromhos. The value of 2,000 

micromhos was arbitrarily selected on the assumption 

that at that conductivity, the boron content of the soil 

in the root zone had been reduced to a level that was 

not harmful to the plants.

The results of chemical analysis of the effluent from 

greasewood tank 1 and rabbitbrush tank 1 are given in 

table 6. These analyses show a general decrease in concentrations of all the constituents as the leaching progressed. The results for rabbitbrush tank 1 are somewhat
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D23

16,000

14,000

§ 12,000

I

10,000

8000

6000

4000

o

a! 2000

CO

tank 2. Boron

Greasewood tank 1 

Conductivity Greasewood tank 2. 

Conductivity

40

35

a: 

30 tf

a: LU 25°- 

co

<

a:

200

15 Z

Z

o 

o

CO

12 16 20 

EFFLUENT, IN THOUSANDS OF GALLONS

24 28 32

FIGURE 16. Decrease in boron and specific conductance of the effluent from greasewood tanks 1 and 2 during leaching.

12

10

CM

$ 6

CO

O

I 4

a:

o

u. 2 

O 

CO 

Q

1°

O

I 

H 12

o

D 

O 8

6

60

50

40

30

RABBITBRUSH TANK 2

EXPLANATION

 

Specific conductance

 

Boron

 1966 1966

20

10

60 i

50 g 

oc 

O

00

40

30

20

10

RABBITBRUSH TANK 3

EXPLANATION

1966 1963

6 8 10 

EFFLUENT, IN THOUSANDS OF GALLONS

12 14 16

FIGURE 17. Comparison of specific conductance and boron of the effluent from rabbitbrush tanks 2 and 3 for the leachings of 1963

and 1966.
D24 STUDIES OF EVAPOTRANSPIRATION

erratic, because of different rates of circulation. Owing 

to the below-freezing temperature at night during leaching, regulation of the low rate of flow of the leach water 

was difficult. Regulation of the higher rate of flow into 

greasewood tank 1 was not a problem until near the 

last 48 hours of the operation. During the latter period, 

the rate decreased from about 79 gallons an hour to 

about 46 and then to about 32 gallons an hour. The effect was to increase the concentration of most of the 

constituents in the effluent, as shown in table 6. The increase in boron and conductivity is shown graphically 

in figure 11. Boron increased from 8.7 to 16 mg/1 and 

conductivity from 1,720 to 2,450 micromhos with the 

decrease in rate of inflow from 79 to 46 gallons an hour. 

The increase was not unexpected, for with slower movement through the soil, the leach water was in contact 

with the soil grains for a longer period of time; thus, 

greater opportunity was afforded for the constituents to 

be taken into solution.

Less difficulty was experienced in maintaining a uniform flow through the tanks leached in April 1963. In 

greasewood tank 2, the rate averaged 224 gallons per 

hour, varying only about 2 gallons per hour during the 

period of leaching. In rabbitbrush tank 2, the rate 

averaged 70 gallons an hour, and for rabbitbrush tank 

3, it was 99 gallons an hour.

Comparison of the specific conductance curves of the 

effluent for the teachings of April 1963 and April 1966, 

shown in figure 17, indicate that the effluent in 1966 was 

less concentrated. Boron determinations were made only 

for rabbitbrush tank 3 in 1963 and for tank 2 in 1966; 

so a direct comparison of the difference in the boron

content of the effluent from the two teachings is not 

available for either tank. It seems reasonable, however, 

to assume that, like the conductivity, the boron content 

of the effluent was less in 1966 than 1963.

The rates of flow through the tanks were smaller in 

1966: 54 gallons an hour for rabbitbrush tank 2 and 81 

gallons an hour for tank 3, a decrease of 16 and 18 gallons an hour, respectively. As a result, the leach water 

had a greater opportunity to take the soluble salts into 

solution. The decrease in concentration of the effluent 

in 1966, even though the opportunity to take more salts 

into solution was greater, indicates that the leaching of 

1963 was effective in removing most salts; however, apparently the 1963 leaching did not reduce the boron content to a concentration that was not damaging to the 

plants. Because the plants in the greasewood tanks that 

were leached in 1962 and 1963 did not show any evidence 

of boron toxicity in 1965 or in 1966, the greasewood 

evidently has a higher tolerance to boron than does 

rabbitbrush.

FACTORS INFLUENCING 

EVAPOTRANSPIRATION RATES

Evapotranspiration was defined earlier as water 

withdrawn from soil by evaporation and plant transpiration. Evaporation from the soil surface generally is 

the smaller fraction of evapotranspiration. The results 

from the bare-soil tank studies at the Winnemucca test 

site indicated that the evaporation from soil was less 

than half the evapotranspiration from the vegetated 

tanks.

Transpiration by plants and evaporation from the soil

TABLE 6. Quantity, rate of inflow to tank, and chemical analyses of samples of the effluent from greasewood tank 1 and rabbitbrush tank 1,

October 1962

[Constituents in milligrams per liter]

Effluent

(gal)

Elapsed

time

(hr)

Rate of

inflow to

tank (gph)

Conductivity Calcium Magnesium Sodium Potassium Bicarbonate

(micromhos

at 25°C)

(Ca) (Mg) (Na) (K) (HCOs)

Carbonate

(C0 3)

Sulfate

(804)

Chloride

(Cl)

Phosphate

(P04)

Boron pH

(B)

Greasewood tank 1

0

2,150

3,780

6,140

8,440

10,700

12, 570

14,570

16,460

17, 570

18,330

19,080

0

20.5

37.0

60.0

85.0

109.0

133.0

157.0

181.0

205.0

228.5

253.0

_

105

99

103

92

94

86

83

79

46

32

31

11,300

9,400

8,170

6,180

4,750

3,740

3,020

2,320

1,720

2,450

2,490

2,370

68

50

30

15

11

11

28

11

7.2

6.5

25

21

16

8.5

4.8

3.9

9.8

3.1

1.9

2.3

2,830

2,500

2,180

1,630

1,190

740

373

587

597

567

326

243

191

137

101

60

32

42

41

41

4,060

4,720

4,210

3,090

2,240

1 420

818

1,300

1,340

1,160

59

143

245

280

290

172

59

54

44

103

1,680

854

606

358

225

135

102

116

117

104

1,270

653

475

285

182

96

66

68

66

67

24

44

52

48

39

27

13

23

23

23

20 8. 32

33 8.42

37 «. 62

38 8.80

33 8.91

 

19 8.90

 

8. 7 8. 63

16 8.40

16 8.40

15 8. 78

Rabbitbrush tank 1

0

550

1,820

3,060

3,710

4,320

4,530

5,160

_

23.5

51.5

nao

163.5

211.5

235.5

284.0

_

23

45

19

14

13

9

13

6,550

6,430

6,620

3,460

4,150

1,890

5,210

3,840

13

18

24

47

43

54

43

44

18

11

10

16

15

16

11

17

1,630

1,710

1,720

766

972

366

1,250

864

90

79

82

37

42

22

56

40

2,320

3,330

3,070

1,330

1,600

750

1,870

1,410

202

108

103

0

0

0

34

7

793

507

591

333

464

198

645

443

508

356

488

296

384

152

528

3SO

9.0

12

15

6.5

8.5

2.9

12

7.9

12 8.83

18 8.48

25 8.48

8. 6 8. 10

11 8.04

3. 3 7. 99

13 8. 33

8. 0 8. 23
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D25

occur in response to the same energy sources as evaporation from a water surface. The response, however, is 

modified by the physical characteristics of the soil and 

the physiological characteristics of the plants; the effects of the modifications are not fully understood. For 

a given climatic condition, with water nonlimiting, as 

for phreatophytes, the rate of transpiration depends on 

the species, cover density and plant size, stage of maturity, and tolerance to mineral salts in the soil and water. 

For a given plant species, the annual rate is affected 

by climatic conditions such as temperature, wind movement, humidity, solar radiation, rainfall, and length of 

growing season. Of these, temperature is the most important for it determines the warmth of the growing 

season and controls the length of it.

The growing season has been defined by the Phreatophyte Subcommittee of the Pacific Southwest InterAgency Committee (1966) as "the season that is warm 

enough for plants to grow." Generally it is considered 

as the period between the last killing frost in the spring 

and the first killing frost in autumn. The minimum 

temperature that constitutes a killing frost for one 

species may have little or no effect on more hardy species.

The four species of phreatophytes studied were hardy 

plants, native to the Humboldt River basin, and acclimated to the basin. Data on the minimum temperature 

conditions that would constitute a killing frost for these 

species are uncertain. A killing frost for them may be 

interpreted as that which damages the foliage severely 

enough to cause defoliation, thus restricting the rate 

of water use. It is not a killing frost in the sense that the 

plants are killed, for the shrubs are perennials that 

persist from year to year and survive below-zero temperatures during winter months. However, some generalizations may be made. Observations in the area indicate that the four transplanted species withstood, 

without apparent damage, temperatures of 32 °F or 

slightly lower that severely damaged or killed less hardy 

plants such as alfalfa, garden and flowering ornamental plants. Threshold temperatures of 32°F, 28°F, 

24°F and 16°F are reported regularly by the Weather 

Bureau as an aid for those people concerned with damage to plants of different degrees of hardiness. The 

threshold temperature that appears to stop growth in 

the four species studied is 28°F. This interpretation is 

based on observations of the plants and on a decrease in 

water use following minimums of 28°F.

The yearly periods between minimums of 32 °F are 

shorter than those between minimums of 28 °F. For 

comparisons of the lengths of the growing seasons controlled by minimums of 32°F and of 28°F the earliest 

and latest dates of these temperatures and the lengths

of the periods between them are shown in the following tabulation:

Threshold temperatures

32°F 28°F

Year Latest Earliest Days Latest Earliest Days

1962.

1963.

1964.

1965.

1966.

1967.

. June 5 Aug. 28

. May 12 Sept. 24

.May 21 Aug. 30

May 15 Sept. 9

June 3 Sept. 2

. May 14 Sept. 14

84 Apr. 30 Sept. 19

135 Apr. 20 Oct. 24

101 May 7 Sept. 19

117 May 6 Sept. 17

91 June 3 Oct. 4

123 May 14 Sept. 14

142

187

137

134

123

123

Average. 108 141

The warmth of a growing season, of a month, or of 

any period of time may be described and compared on 

the basis of degree days a degree day being 1° of the 

average daily temperature above a base of 32°F for 1 

day. Thus an average temperature of 42°F for 1 day is 

equivalent to 10 degree days. The degree days by months 

and for the period April through October at the test 

site are given in table 7. The data in the table show that 

the warmth in the early and late months varies widely. 

For example, April 1967 was cooler by 354 degree days 

than April 1962 (equivalent to an average daily temperature difference of 11.8°F; October 1962 was warmer 

by 456 degree days than October 1966 (equivalent to an 

average daily difference of 14.7°F). In the midsummer 

months of June, July, and August, however, the range 

of difference was much less, being about 125 degree days 

for each month.

The relation of monthly draft on ground water (water 

added) to temperature during the period April through 

October 1966 for three of the species grown in the 

evapotranspiration tanks is shown in figure 18; the 

values are expressed in percentage of the seasonal total. 

The appearance of some new leaves and buds on the 

rabbitbrush and greasewood shrubs in April indicated 

plant activity and water use. Growth activity by the 

willows, however, was barely discernible.

During April, the draft by the rabbitbrush and 

greasewood plants was a little more than 2 percent of the 

total use for the season, while that for the willow plants 

was less than 1 percent. Evapotranspiration during

TABLE 7. Comparative warmth for the period April 1 to October 31, 1962-67, by months, at the Winnemucca test site, in 

degree days above a base of 32°F

Month 1962 1963 1964 1965 1966 1967 Average

April...-.--------.-. 591 363

May. .---... -.. 657 831

June --.__-..-.__- 942 819

July___ 1,147 1,088

August.----- .... 1,029 1,073

September..____...... 885 945

October.... - -- 958 701

645

834

1,215

1,060

723

507

589

891

1,184

1,051

660

605

495

871

903

1,119

1,162

882

502

237

694

867

1,223

1,268

960

558

426

714

876

1,163

1,107

842

Total degree days .. 6,209 5,820 5,525 5,487 5.934 5,807 5.797
STUDIES OF EVAPOTRANSPIRATION

Apr. May June July Aug. Sept. Oct

1966

FIGURE 18. Relation of monthly draft on ground water to temperature April through October 1966 for three species of 

phreataphytes. Monthly values of evapotranspiration and 

warmth in degree days are expressed in percentage of the 

seasonal totals.

May, June, and July increased at a rather uniform rate 

for the three species. The peak rate of ground-water 

use for rabbitbrush and willow occurred in July, but 

that for greasewood was not reached until August. The 

decrease in rate of use following the peaks was rapid 

for rabbitbrush and greasewood, but much slower for 

willow. The ground-water use by the rabbitbrush and 

greasewood during the peak month was 28 percent of 

the seasonal total; the ground-water use by the willows 

was about 27 percent. During the period June through 

August, the ground-water use by the rabbitbrush and 

willow was about 67 percent of the seasonal total and 

that by the greasewood about 72 percent, whereas the 

corresponding warmth was only 54 percent of the total 

for the season.

The graphic representation of evapotranspiration in 

figure 18 demonstrates for the three species of phreatophytes the rates of ground water use, the differences in 

rates at the beginning and end of the growing season, 

and the months of peak use. These differences emphasize 

the need for studies of evapotranspiration by species, 

for proper assessment of evapotranspiration discharge 

and, more importantly, the period and rate of draft on 

the ground-water reservoir.

Wind movement affects evapotranspiration by removing the humidity-laden air adjacent to the transpiring leaf and replacing it with air of lower humidity.

Wind also affects evaporation from soil and water surfaces in much the same manner.

Kainfall, as described earlier, is scant and generally 

occurs as showers of less than 0.5 inch. During rain periods, there is an increase in the humidity of the air that 

results in a reduction of the evapotranspiration rate. 

The principal effect of rainfall in the Humboldt River 

valley is its influence on the draft from the ground water 

by phreatophytes. During seasons of high rainfall, the 

draft from the ground water is reduced by the quantity of rain that enters the soil and becomes available 

to the plants as soil moisture, as well as by the lesser 

evapotranspiration rate resulting from increased humidity. Conversely, during periods of low rainfall, the 

draft on the ground water is greater, as there is little 

opportunity for replenishment of the soil moisture.

As evaporation from a water surface occurs in response to the same energy source as evapotranspiration, 

the seasonal evaporation from a standard evaporation 

pan may be considered as an index to the relative evapotranspiration for that growing season. Thus, when 

pan evaporation is high, evapotranspiration may be expected to be correspondingly high.

The values for the elements of climate (wind, rain, 

and pan evaporation) as observed at the Winnemucca 

test site during the April through October periods from 

1962 through 1967 are given in table 8.

EXPRESSION OF EVAPOTRANSPIRATION LOSSES AND 

EXTRAPOLATION TO GROWTH AREAS

The transposition of evapotranspiration losses by 

phreatophytes from the place of measurement such as 

evapotranspiration tanks to natural growth areas involves many variables. The two most important are differences in climate and in plant growth. Other variables 

include differences in soil texture and fertility, kind 

and amount of salts in the soil, depth to the water table, 

and quality of the ground water. Consideration must 

also be given to the relative stage of plant development 

at the two locations. Studies at the Winnemucca test 

site of the relationship of water use to plant development of the four species of phreatophytes indicate that 

evapotranspiration was generally greater after the 

plants had become established and had entered a PCTABLE 8. Climatological data for the April 1-October 31 period, 

1962-67, at the Winnemucca test site

Year Wind move- 

(April through October) ment (miles)

1962____________

1963____________

1964____________

1965______-_____

1966__. _________

1967____________

__________ 11,383

-_____.___ 10,803

__________ 9,555

-_--______ 11,487

__________ 9,120

__-_-__-._ 9.487

Rainfall Pan evapora- 

(in.) tion (in.)

1.36 

6. 66 

5.21 

3.63 

1. 64 

3. 60

62. 04 

53.94 

56. 60 

52. 72 

66. 79 

56. 10
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D27

riod of vigorous growth than after the plants had 

reached maturity. The extrapolation of evapotranspiration losses from immature phreatophytes to areas of mature growth would result in estimates that would be too 

large.

Realistic transposition of values for evapotranspiration losses from the place of measurement to natural stands of phreatophytes is as important as determination of the losses. The two methods commonly used 

are based on (1) area and (2) volume of foliage.

AREAL METHOD

Until about 1950, the usual method of expressing 

evapotranspiration was on an areal basis, that is, depth 

over a unit area, as acre-inches or acre-feet per acre. 

Evapotranspiration expressed on an areal basis describes the water use of a vegetated area for the existing 

growth condition without any indication as to the 

growth condition. Thus extrapolation on an areal basis 

is valid only when the conditions of growth, climate, 

soil, water supply, and water quality are similar. Generally, when the experimental studies are situated within or adjacent to the growth area under consideration, 

the only variables that need to be considered are the 

variations in plant growth and depth to water. Variations in plant growth are not uncommon in stands of 

phreatophytes. The plants may range in size from seedlings to mature plants, and growth density may range 

from a few percent to 100 percent. As a result, an appreciable error may be introduced into the computation of 

the water use by a natural stand when the conditions 

of growth for the measurement-area values are not 

known. In the past, adjustments for marked differences in growth conditions were made arbitrarily by 

assuming a linear relationship between evapotranspiration and variation in cover density. The validity of this 

assumption is doubtful, as available fragmentary data 

suggest a greater use when cover density is in the 80 to 

90 percent range than that indicated by a direct proportion. This may be due, in part, to the "oasis effect." 

More information is needed on the relation of evapotranspiration rates to variations in cover density so that 

the extrapolation may be made more realistically.

VOLUME-OF-FOLIAGE METHOD

To avoid some of the difficulties and uncertainties inherent in the areal method, the volume-of-foliage method was developed. In this method the evapotranspiration loss or water use is expressed as a unit quantity 

per unit of foliage volume. The method presumes that 

transpiration, by a plant species, is proportional to the 

total transpiring leaf area and thus is proportional to 

the foliage volume. Transpiration rates vary for different plant species, in proportion to the leaf area and to 

the rate per unit of leaf area. The transpiration rate 

per unit of leaf area has been found to differ markedly 

among species (Tomanek and Ziegler, 1962).

The volume-of-foliage method requires detailed 

measurements of cover density and thickness or canopy 

depth for the computation of foliage volume. The measurements for volume determination of the growth in 

the tanks were made by the techniques and to the standards outlined in the manual by Horton, Robinson, 

and McDonald (1964) for surveying phreatophyte 

vegetation.

Evapotranspiration on a volume-of-foliage basis may 

be expressed as acre-feet of water per acre-foot of foliage or in cubic-foot units. This method obviates the 

corrections for differences in plant growth, except possibly for growth areas of low density. In these latter 

areas, the method may have limitations due to the 

"oasis" effect, and this facet needs further study.

The evapotranspiration-tank studies were designed 

and the required data were collected so that the evapotranspiration rates could be calculated by both methods.

RESULTS OF EVAPOTRANSPIRATION STUDIES

The evapotranspiration data for the four species of 

phreatophytes and the bare-soil tank are tabulated in 

table 9 for the two methods and for the different water 

levels. The two sets of graphs in figure 19 depict evapotranspiration computed by the areal and the volumeof-foliage methods during the five seasons 1963 

through 1967, together with the operating water levels. 

Graphs are shown only for those tanks for which the 

records of water use were unbroken over the 5 years; 

those for greasewood tank 2, willow tank 3, and wildrose tank 3 are not shown. In the graph of evapotranspiration based on the areal method, the amounts of the 

three sources of water that make up the total evapotranspiration loss ground water, soil moisture, and 

rainfall are shown for each tank. The graph of the 

volume-of-foliage method shows only the total evapotranspiration loss. The evapotranspiration rates for the 

different tanks and the factors that influence them will 

be discussed separately by species.

GREASEWOOD

The decrease in evapotranspiration from 1963 to 1964 

is believed to have resulted from the 1-foot lower water 

level in 1964 and the shorter and cooler growing season. 

Both tend to reduce the water use. The period between 

the minimum temperature of 28 °F was 50 days less 

in 1964 than in 1963, and the period April through 

October was 295 degree days cooler. The slight decrease 

from 1964 to 1965 seems to be due largely to the lower-
D28 STUDIES OF EVAPOTRANSPIRATION

TABLE 9. Evapotranspiration by four species of phreaiophytes grown in tanks at the Winnemucca test site and evaporation from bare soil during the growing seasons 1963-67

Year water level 

in feet

Sources of water in acre-feet per acre

Rainfall

Soil Water 

moisture added

Kvapotranspiration

Acre-feet Acre-feet of 

per acre water per acrefoot of foliage

Remarks

Grease wood

1963 '__________.

1963 i..__. _._._.

1964___________.

1966

1966

1967

1967__ _____ _ .

_____ 5.0

_____ 5.0

_____ 6.0

_____ 6.0

.____ 7.5

_____ 7.6

_____ 6.2

_____ 7.8

_____ 7.8

0. 44 

.44 

.40 

.40 

.30 

. 14 

. 14 

.30 

.30

0.48 

.44 

.32 

.26 

.42 

. 24 

.25 

.43 

. 64

0.89 

. 90 

.53 

. 56 

.42 

.83 

.98 

.56 

.62

1.81 

1.78 

1. 25 

1.22 

1. 14 

1.21 

1.37 

1.29 

1.56

1. 70 

1.45 

1.06 

. 91 

.88 

1.04 

1.35 

1. 10 

1. 49

Average 

Tank 1. 

Average 

Tank 1. 

Do. 

Do. 

Tank 2. 

Tank 1. 

Tank 2.

of 2 tanks. 

of 2 tanks.

Rabbitbrush

1963 !__________.

1965

1966

1967

.____ 5.0

_____ 5.0

.____ 5.0

._.__ 5.3

.____ 6.2

0.44 

.40 

.30 

. 14 

.30

0.23 

. 13 

. 15 

.43 

.43

1.52 

1.09 

.62 

1. 06 

1. 01

2. 19 

1.62 

3 1. 07 

1. 63 

1.74

3.22 

1.94 

3 1. 04 

1.42 

1.25

Average 

Do. 

Do. 

Do. 

Do.

of 3 tanks.

Willow

1963 !_________..

1963

1964

1964- __________

1965 «__ _________

1966

1967

1967_ __________

._--- 5.0

_____ 5.0

.__._ 5.0

.____ 5.0

._.__ 5.0

._.__ 5.0

.____ 5.7

.____ 3.5

.____ 5.8

.____ 4.2

._-__ 5.4

4 1

0.44 

.44 

. 44 

.40 

.40 

.40 

.30 

.30 

. 14 

. 14 

.30 

.30

0.27 

.27 

.28 

.20 

. 10 

.23 

.29 

. 15 

.28 

.22 

.25 

. 24

3. 23 

2.89 

2. 97 

1.81 

1.76 

1.80 

1.48 

2.13 

2. 30 

3. 19 

1. 55 

1. 85

3.94 

3.60 

3.69 

4 2.41 

4 2. 26 

4 2. 43 

2.07 

2. 58 

2.72 

3.55 

2. 10 

2.39

0. 94 

. 93 

. 84 

4 .61 

4 . 66 

4 . 54 

.57 

.49 

1. 26 

.91 

. 99 

.56

Average 

Tank 1. 

Tank 2. 

Average 

Tank 1. 

Tank 2. 

Tank 1. 

Tank 2. 

Tank 1. 

Tank 2. 

Tank 1. 

Tank 2.

of 3 tanks. 

of 3 tanks.

Wildrose

1963 ». .__.

1963-__-__._____

1963----_--__.__

1964 _ -_-_-.___

1964_.__________

1964___. ________

1965_-__--_---__

1965 5__ _________

1966_-_-_-_-.___

1966_-_-________

1967-_--___--___

1967--__-.______

--_- 5.

--__ 5.

__.- 5.

-._- 5.

-.-- 5.

-__. 5.

_._- 5.

__ 4.

____ 5.

_ -_ 4.

_ 6.

4.

0

0

0

0

0

0

0

2

9

2

1

4

0.44

. 44

.44

.40

.40

.40

.30

.30

. 14

. 14

.30

.30

0.20

. 14

.21

. 15

. 10

.14

. 10

. 18

. 19

.24

.21

. 16

0.

1.

1.

1.

1.

1.

1.

77

04

69

84

86

89

83

26

23

75

08

46

1.

1.

1.

1.

1.

1.

1.

1.

1.

2.

1.

1.

41

64

34

39

36

43

23

74

56

13

59

92

0. 92

1. 00

1.33

.65

.73

.76

.76

.90

1.40

1.47

1.17

. 94

Average

Tank 1.

Tank 2.

Average

Tank 1.

Tank 2.

Tank 1.

Tank 2.

Tank 1.

Tj-nk 2.

Tank 1.

Tank 2.

of 3 tanks.

of 3 tanks.

Bare soil

1963 > _ --_-__._

1964____________

1965-_--________

1966 «___________

____ 2.2

1 q

_.__ 2.3

.___ 4.0

0.44

.40

.30

. 14

0. 03

(Y_

-. 01

.04

0.30

.63

.23

. 18

0.77 ____-_-__-_-

1.00 _-_----.-_--

. 52 ____________

.36 ____________

1 May l to October 20.

2 Membrane in tank 2 perforated in August 1965.

3 The decrease in water use in 1965 may reflect slower growth caused by 

an accumulation of salts in the root zone. Their presence was indicated 

by tip burn of the leaves, and by a change in color of the foliage during 

the growing season.

4 Plants damaged, and some stems dead, as a result of rabbits gnawing 

the bark of the plants, 

s Tank 3 discontinued in 1965. 

« Unmeasured water entered tank 1967.
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D29

Wild rose 1

EVAPOTRANSPIRATION PER UNIT OF AREA AND SOURCE OF THE WATER Source of water 

Rainfall 

Soil Moisture 

Ground water

EVAPOTRANSPIRATION PER UNIT OF FOLIAGE

FIGURE 19. Bvapotranspiration by four species of phreatophytes during the growing seasons 1963 through 1967 for indicated depth of

water level.

ing of the water level from 6.0 to 7.5 feet below the 

surface of the tank, as the length and warmth of the 

two growing seasons were comparable. The differences 

in the period between minimums of 28 °F was only 3 

days and in growing season warmth was 38 degree days. 

Evapotranspiration increased a small amount in 1966 

and again in 1967. During these 2 years, the water level 

was only slightly deeper than in 1965; the periods between 28°F averaged 11 days less than in 1965, but the 

growing season was warmer by 447 degree days in 1966 

and 320 degree days in 1967. During at least a part of 

1965 and in 1966 and 1967, the root system of the plants 

in all probability had become adjusted to the water 

environment of the 7.5- to 7.8-foot water levels. Thus it 

seems that the length and warmth of the growing season 

exert a greater effect on evapotranspiration of greasewood than does a lowering of water level in the 5.0- to 

7.8-foot range.

The evapotranspiration values for greasewood are 

believed to be representative for plants in the Humboldt River basin having approximately similar growth 

and water-level conditions. The average value for tank 1 

for the four seasons 1964 through 1967 for water levels 

from 6.0 to 7.8 feet was 1.21 acre-feet per acre and 0.98 

acre-foot per acre-foot of foliage. Ground water supplied 50 percent of this water, rainfall 23 percent, and 

soil moisture 27 percent. For tank 2, which had about 

100 cubic feet less foliage, the evapotranspiration averaged 1.46 acre-feet per acre and 1.42 acre-feet per acrefoot of foliage during the 1966 and 1967 seasons, for 

water levels in the depth range of 6.2 and 7.8 feet. 

Ground water supplied 55 percent of the water, rainfall 15 percent, and soil moisture 30 percent.

RABBITBRUSH

The decrease in the evapotranspiration rate from 1963 

through 1965 probably resulted largely from the adverse effect of boron that still remained in the root zone 

in toxic amounts after leaching. Climatic effects such 

as the shorter and cooler growing seasons were doubtless 

contributing factors, but the impact - " , masked by the 

deleterious effect of the boron. The in^iease in water use 

in 1966 and 1967 is largely the result of leaching of the 

tanks and removal of the boron in the spring of 1966. 

The warmer growing seasons of 1966 and 1967 were an 

important factor also.

The evapotranspiration values for rabbitbrush, like 

greasewood, are believed to be representative for that 

plant in the Humboldt River basin under similar conditions. The average value, for the three tanks for the 

1964, 1966, and 1967 seasons for water levels of 5.0-6.2 

feet is 1.66 acre-feet per acre and 1.54 acre-feet per acrefoot of foliage. The 1963 and 1965 seasons were not 

included in the average because the plants were not 

mature in 1963, and the adverse effect of boron was ap-
D30 STUDIES OF EVAPOTRANSPIRATION

parent in 1965. Of the 1.66 acre-feet per acre-foot, 64 

percent was supplied from ground water, 17 percent 

from rainfall, and 20 percent from soil moisture.

WILLOW

The large decrease in evapotranspiration in the two 

willow tanks from 1963 to 1964 was caused largely by 

damage to the plants by rabbits during the winter of 

1963-64. Evapotranspiration continued to decrease in 

tank 1 in 1965 while it increased in tank 2. The differences are ascribed to the changes in water levels. In tank 

1, the water level was lowered from 5.0 to 5.7 feet, while 

the water level in tank 2 was raised from 5.0 to 3.5 feet 

below the surface of the tank. As other conditions were 

the same, the difference in evapotranspiration can be 

accounted for only by the lower and higher water levels.

Evapotranspiration in 1966 was appreciably higher 

in both tanks than in either 1964 or 1965. The water 

level in tank 1 was virtually unchanged from 1965 and 

in tank 2 was lower by 0.7 foot. The increased evapotranspiration was due to the warmer growing season, 

especially during the months May through September, 

the period of active willow growth. This period was 

warmer by 562 degree days in 1966 than in 1965.

Evapotranspiration in both tanks was markedly less 

in 1967 than in 1966. As the differences in water levels 

and in the warmth of the May through September 

period were relatively small, some other explanation 

must be sought to explain the decrease. As shown in 

table 1, the differences in cover density and foliage 

volumes between 1966 and 1967 were small. Maturation 

of the plants or concentration of deleterious salts in the 

root zone in toxic amounts provide the best explanations 

of the decrease. The decrease is attributed, however, to 

the adverse effect of the salts because the plants, which 

had been planted in 1960, were considered to have 

reached maturity by 1963 and certainly had by 1964. 

The water supplied to the tanks shown in table 4, although low in dissolved solids, is highest in sodium. It 

is conceivable that the alkali salts in the supply water 

may have accumulated in the root zone and reached a 

concentration in 1967 that affected the use of water by 

the plants. Willows have a low tolerance for alkali salts, 

and the threshold tolerance may have been exceeded in 

1967. Unfortunately data are not available to indicate 

the threshold tolerance of willows to alkaline conditions.

The striking feature of evapotranspiration by willow 

shown in figure 19 is the heavy draft on ground water, 

which was the highest for the four species studied. For 

the 2 years when evapotranspiration was highest, ground 

water supplied to the tanks averaged 83 percent of the 

total water use; in the 3 years of lower evapotranspiration 1964, 1965, and 1967 ground water supplied to 

the tanks averaged 76 percent of the total water use. 

These data indicate that draft on the ground water by 

willow is relatively independent of rainfall. Thus, in 

1963, the year of highest seasonal precipitation, rain accounted for 12 percent of the total water use, and in 

1966, the year of lowest seasonal precipitation, rain 

accounted for only 5 percent of the water use. In the 

other 3 years, seasonal rainfall averaged 15 percent of 

the water use.

Excluding the 2 years 1964 and 1967, when water use 

seems to have been adversely affected by damage by 

rabbits and an alkaline condition in the root zone, the 

average evapotranspiration for the two tanks was 3.03 

acre-feet per acre and 0.83 acre-foot per acre-foot of 

foliage. Ground water supplied 82 percent, rainfall 10 

percent, and soil moisture 8 percent of the total evapotranspiration during these 3 years.

WILDROSE

The causes of the differences in evapotranspiration 

in the wildrose tanks are not as apparent as for the 

other three species. The decrease in tank 1 from the 

1963 to the 1965 season may be the result of the cool 

seasons of 1964 and 1965. The depth to the water level 

remained unchanged at 5.0 feet during this period. In 

tank 2 there was a slight increase in evapotranspiration 

from 1963 to 1964, while the depth to the water level 

remained unchanged. The slight increase, in contrast to 

the decrease in tank 1, may have been due to the 90 

cubic feet increase in foliage volume. In 1964 the water 

level in tank 2 was raised 0.8 foot, from 5.0 to 4.2 feet 

below the surface of the tank, and remained at that 

level through 1966. The increase in evapotranspiration 

in these 2 years is believed due to the higher water level 

in 1965 and to the warmer growing season in 1966. In 

tank 1 the water level in 1966 was lowered 0.9 foot from 

5.0 to 5.9 feet. Normally this would have caused a decrease in evapotranspiration; however, the effect of the 

warm growing season seems to have more than compensated for the effect of the greater depth to water. In 

1967, with slightly lower water levels in both tanks, 

evapotranspiration increased slightly in tank 1 and 

decreased slightly in tank 2.

The average evapotranspiration for the 5 years of 

record was 1.48 acre-feet per acre and 1.01 acre-feet 

per acre-foot of foliage for tank 1, which had the deeper 

operating water levels, and 1.71 acre-feet per acre and 

1.08 acre-feet per acre-foot of foliage for tank 2, which 

had the higher operating water levels. Over the 5-year 

period, ground water supplied 70 percent, rainfall 20 

percent, and soil moisture 10 percent of the total evapotranspiration. Wildrose was second to willow in its use
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D31

of ground water and utilized less soil moisture than 

any of the other three species.

BARE SOIL

The values of evaporation from bare soil given in 

table 9 show that evaporation from a bare-soil surface 

is less than half the evapotraiispiratioii losses shown for 

the phreatophytic vegetation. During the 4 years of 

record, with the depth to the water level ranging from 

1.9 foot to 4.0 feet below the surface of the tank, draft 

from the ground water, based on the water added to 

the tank, was-51 percent of the total loss, while rainfall 

supplied 48 percent. Evaporation for the 4 years ranged 

from 0.36 to 1.00 acre-foot per acre and averaged 0.66 

acre-foot per acre. As expected, the loss was least at the 

deepest water level, 0.36 acre-foot per acre from a 4.0- 

foot water level, and 1.00 acre-foot per acre with a 

1.9-foot water level. Loss from the ground water ranged 

from 0.18 to 0.63 acre-foot per acre and averaged 0.34 

acre-foot per acre. During June of 1963,1964, and 1965, 

some recharge to the ground water in the tank resulted 

from showers of 0.50-0.75 inch, as indicated by small 

rises of wT ater level in the tank following showers of onehalf inch or more. No large showers occurred in 1966, 

and water from the light summer rains seemingly did 

not percolate to the ground water in the tank.

SUMMARY

One of the largest unknowns in the water budget of 

the Winnemucca reach of the Humboldt River is the 

consumptively wasted water from areas of phreatophytes of low-beneficial usefulness. Studies were begun 

in 1959 to evaluate the unit annual consumptive waste 

of four of the common woody phreatophytes greasewood, rabbitbrush, willow, and wildrose growing in 

the reach. The water use by these shrubs was determined 

by growing the plants under controlled conditions in 11 

evapotranspiration tanks that ranged in size from 10 

feet square and 7 feet deep to 30 feet square and 10.5 

feet deep. In addition, evaporation was determined 

from unplanted bare soil in a tank 10 feet square.

Evapotranspiration was computed as the total quantity of water added to the tanks, the rainfall on the 

tanks during the growing season, and the reduction in 

soil moisture between the beginning and end of the 

growing season. Plant growth, development, and water 

use were adversely affected during some years by damage to the plants and by boron toxicity. Damage resulted from rabbits gnawing the bark of willows and 

from insects feeding on the leaves of greasewood and 

willows. Boron toxicity resulted from concentrations of 

soluble boron in the soils at the root zones of greasewood

and rabbitbrush. The causes of plant damage were corrected by catching and removing rabbits from the test 

site enclosure and by spraying the insect infestation 

with insecticide. Boron toxicity was corrected by reducing the concentrations of boron in the root zone 

through backwash leaching.

Foliage volumes, a measure of plant growth and 

development, were computed from transects across the 

tanks. They provided a basis for comparison of growth 

by species from year to year and seasonally between individual tanks of the same species. Foliage volumes also 

provided a basis for expressing water use in terms of 

foliage.

Climatic conditions and the lengths of the growing 

seasons affected the annual evapotranspiration rates. 

The most important climatic element was temperature. 

The highest water use by the plants occurred in June, 

July, and August when more than two thirds of the 

seasonal use took place. The least water use occurred 

in April and October when the use each month was approximately 2 percent of the seasonal use.

Evapotranspiration rates were computed by two 

methods and expressed in two different units on an 

areal basis (in depth over a unit area of land) and on a 

volume of foliage basis (in volume of water per unit 

volume of foliage). Evapotranspiration expressed 

areally gives no indication of the growth conditions for 

which the information was obtained. When expressed 

by volume of foliage, however, the growth conditions, 

which are represented by the product of the cover density and thickness of foliage for a unit area, are inherent 

in the expression, because the evapotranspiration is 

presumed to be proportional to the transpiring leaf 

area, and thus is proportional to the foliage volume. 

In the extrapolation of experimental data to field areas 

of dissimilar growth, the volume-of-foliage method is 

preferable as less uncertainties are involved than in the 

areal method.

The results of the studies ranged rather widely between species. For the same species the seasonal results 

varied, differences being due to the operating water 

level, the warmth of the growing season, and plant response to the effects of damage or alleviation of damage 

by rabbits, insects, and boron toxicity. Draft from the 

water table (equivalent to the water supplied to the 

tanks) varied with the seasonal rainfall, being greatest 

when the rainfall was scant and least when it was 

copious.

The data obtained in the evapotranspiration tank 

studies at the Winnemucca test site indicate that during 

1963-67 average water use by greasewood ranged from 

1.21 to 1.45 acre-feet per acre in tanks 1 and 2, of which
D32 STUDIES OF EVAPOTRANSPIRATION

50 to 55 percent was supplied by ground water. The 

average evapotranspiration by rabbitbrush for 3 years, 

1964, 1966, and 1967, was 1.66 acre-feet per acre, of 

which 64 percent was supplied by ground water. Evapotranspiration by willow was the highest of the four 

species, amounting to 3.03 acre-feet per acre for the two 

tanks during the 1963, 1965, 1966 seasons. It was also 

the highest user of ground water, obtaining 82 percent 

of its water from that source. Wildrose was the second 

highest user of ground water and the smallest user of 

soil moisture. Evapotranspiration by wildrose averaged 

1.48 acre-feet per acre in tank 1 with operating levels 

ranging from 5.0 to 6.1 feet below the surface of the tank, 

and 1.71 acre-feet per acre in tank 2 with operating 

water levels ranging from 4.2 to 5.0 feet below the surface of the tank. On the average, ground water supplied 

70 percent of the total use, and soil moisture only 10 

percent.

SOIL-MOISTURE DETERMINATIONS

By A. O. WAANANEN

The woody phreatophytes under study in the evapotranspiration tanks at the Winnemucca test site of the 

Humboldt River Research Project receive part of their 

seasonal water supply from soil moisture in the unsaturated zone. Precipitation and water added to the tanks 

during the growing season constitute the principal part 

of the water supply, but water from winter precipitation 

stored as soil moisture above the water table may represent a significant part of the water budget. Evaluations 

of evapotranspiration water use thus would be incomplete without information on the quantities of water 

provided from this soil moisture.

Initial observations of the water content of the soils 

in the evapotranspiration tanks were made in September 1961 using a neutron-scattering soil-moisture meter 

in access tubes installed in each tank for this purpose. 

A regular program of observations was started in April 

1962.

The purpose of the soil-moisture observations was to 

determine the water content of the soils at the beginning 

and end of the growing season and at selected intervening times to define seasonal variations. The change in 

water content during the growing season thus provides a 

measure of the volumes of water provided to the plants 

from this source. Data were obtained also at several 

sites on the Humboldt River flood plain near Winnemucca, including one tube installed within the Winnemucca test site, to explore the range of variations in 

water content in the zone of fluctuation of the water 

table adjacent to the river and in the unsaturated soils 

near the land surface.

SOIL-MOISTURE OBSERVATIONS AT THE 

WINNEMUCCA TEST SITE

EQUIPMENT AND PROCEDURE

The neutron meter provides a convenient means for 

determining changes in the moisture content of soils. 

After installation of suitable access tubes, rapid and 

repetitive observations can be taken in the tubes at any 

time as the soils are not subjected to further disturbance. Differences in the quantity and distribution of 

moisture in the soils as shown by subsequent observations 

represent a measure of the changes in the water content.

The neutron-scattering soil-moisture meter used consists of a depth probe equipped with a 28-milligram 

actinium-beryllium neutron source, detector tube, and 

preamplifier connected by cable to a portable counting 

device (sealer). The probe is stored and transported in 

a shield that serves also as a standard for relating 

meter counts to water content and for checking meter 

operation. In normal use, the probe is lowered in an 

access tube to desired depths. Fast neutrons emitted by 

the source enter the surrounding soil materials and are 

moderated by hydrogen ions present principally in the 

moisture in the soil. Thermal (slowed) neutrons are 

detected and counted. The observed count varies 

directly with the number of hydrogen ions in the soil, 

contained principally in the water. The effective diameter of the sphere of influence of the neutron source 

varies inversely with the water content of the soil. 

Appropriate calibration relations permit conversion of 

the observed counts to moisture content in percent by 

volume or to weight of water per unit of volume.

Figure 20 presents a view of the neutron-meter sealer, 

probe, and shield as well as soil augers and types of 

access tubes used in the soil-moisture studies. A typical 

application of the neutron meter in the tank studies is 

illustrated in the generalized section of an evapotranspiration tank shown in figure 21.

Initially only one access tube was installed in each 

of the tanks planted to woody phreatophytes and in the 

bare-soil tank. These were placed near the center of 

the tanks but not over any conduit of the water distribution systems. Additional tubes were installed later in 

the two 30-foot tanks planted to greasewood and in one 

of the 20-foot tanks planted to rabbitbrush to provide 

information also on the lateral distribution of moisture 

in the tanks. Four tubes were installed in greasewood 

tank 1, of which two were placed directly over units of 

the water system. Tubes of aluminum (2-inch outside 

diameter, 0.065-inch wall) or alloy steel (1.75-inch outside diameter, 0.035-inch wall) were used, and these 

were sealed at the bottom so that observations could be 

taken at depths below the water level in the tanks. The 

tubes in the larger tanks were of sufficient length to
EVAPOTRANSPIRATTON BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D33

FIGURE 20. Neutron-meter sealer, probe, and shield, with soil augers 

and typical access tubes.

permit sampling nearly the full depth of the tanks, 

but those in the shallower 10-foot tanks limited sampling to 60-inch profiles.

Figure 22 presents a view of the neutron meter set 

up for use in greasewood tank 1 in July 1962. The 

access tubes installed in the rabbitbrush tanks are visible in the photograph (fig. 23) taken July 1963. These 

photographs also give some indication of the relative 

plant growths at the respective times.

Soil-moisture observations were taken in the access 

tubes at half-foot depth intervals below the land surface, and moisture contents were computed in successive 

6-inch-thick zones. The observed values of water content, representing the integrated result for spherical 

volumes of soil ranging from 12 to 30 inches in diameter depending on the moisture present, were used as the 

mean for each zone. Near the land surface, the sphere 

of influence of the source intersects the air-soil interface, and the inclusion of some air in the observed 

volume results in a reduction in the neutron count. 

Therefore, moisture in the top increment was computed 

for a 9-inch depth using the moisture value observed

1 1

c

1

1 1

1

^

b portable sealer

-Shield

Neutron probe

Sphere of influence 

of neutron source

Plastic 

membrane 

tank liner

> Capillary 

fringe

xWater level

V

Sealed access tube

\ 0 Water content

SECTION OF EVAPOTRANSPIRATION TANK SOIL-MOISTURE 

PROFILE

FIGURE 21. Generalized section of evapotranspiration tank showing neutron-meter depth probe in access tube for soilmoisture observations, and typical soil-moisture profile.
D34 STUDIES OF EVAPOTRANSPIRATION

at the 6-inch depth. Tests had indicated that this procedure yielded reasonably reliable values as the reduced 

count was offset by the moisture differences at the 6-inch 

and the 4^-inch mid-increment depths. This procedure 

obviated the need for the precise determination of moisture at the surface.

Calibration relations for the specific soils at the Winnemucca test site were reviewed. The heavy flood-plain 

soils in the meadow area are well leached, and the 

normal calibrations for the tubes used were deemed 

applicable. The terrace sands and gravels, in place, 

contain alkali salts and some boron, as discussed in the 

main report, that vary in concentration with depth and 

that have the greatest concentrations in the upper 2rA 

feet of the soil profile (fig. 13). Boron is a neutron 

moderator, and its presence in appreciable concentrations may cause sufficient absorption to reduce the meter 

count for given water content. Some other salts present 

in the soils also may have some moderating influences. 

The materials in the greasewood and rabbitbrush tanks 

(fig. 2), however, were mixed during the construction 

of the tanks, and the pattern of the concentrations of 

salts is uncertain. Gravimetric sampling indicated a

FIGURE 22. Neutron meter set up for use in greasewood 

tank 1.

small effect on the calibration relations, but definition 

was inconclusive owing to the mixing of the soils and 

the variations with time resulting from concentration 

of boron in the root zone by evapotranspiration and 

subsequent leaching of the tanks to reduce the concentrations of boron and other salts. Accordingly, the normal calibration curves were used without adjustment, 

but with recognition that the resulting values might 

be small. Determination of changes in water content, 

as shown by differences in moisture rather than the

FIGURE 23,. Rabbitbrush tanks and access tubes installed for soilmoisture observations.

total content, was the principal objective. The effect of 

small errors on the seasonal water budget is minor.

WATER CONTENTS OBSERVED

Soil-moisture observations were made at the beginning, middle, and end of each of the growing seasons 

from 1962-67. Two additional sets of observations were 

made during the 1962 season and three in 1963. Postseason observations were made also in December 1963. 

Water-use evaluations for the first two seasons indicated that about one-half of the seasonal water use in 

the tanks had occurred by the end of July or early 

August. The summer observations therefore were scheduled usually for that time to provide a convenient midseason check on tank operations and an index of 

midsummer moisture conditions.

The results of the spring and autumn observations 

during the 1962-67 seasons, in the individual tubes in 

the tanks and in the tube in the meadow area at the test 

site, are listed in table 10. The changes in water content 

occurring during the summer and winter periods are 

also indicated. The losses in the summer season indicate 

the part of the soil moisture that was discharged by 

evapotranspiration. These data and the tabulations in 

table 9 in the main report indicate that soil moisture 

supplied 26 percent of the total water used in the greasewood tanks during the study period, 15 percent in the 

rabbitbrush, 12 percent in the wildrose, and 8 percent 

in the willow tanks. The contributions varied widely 

from year to year owing to differences in plant growth, 

water level, and the availability of soil moisture. The 

changes in water content in the bare-soil tank were very 

small, and the soil-moisture contributions to the seasonal 

evaporation were minor.

The data listed in table 10 represent net changes in 

water content for the full profile sampled, although the 

principal depletions of soil moisture occurred in the 

unsaturated zone. Expression of the moisture contents
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D35

in terms of depth per unit of area permits ready comparison among tubes and tanks or with precipitation. 

The results are expressed in inches, but the seasonal 

water-use values were reported in equivalents of acrefeet per acre in the section on soil moisture and in table 

9 in the main report.

Profiles of soil moisture in the tariks and the meadow 

at the beginning and end of each season are shown in 

figure 24. The water content is expressed in percent of 

volume, and the differences between the spring and 

autumn profiles delineate the zones of moisture depletion. The profiles for each of the four species of plants 

are composites of the data from several tanks and tubes 

for a particular species. The depths of water in the tanks 

shown also are average levels at the time of sampling; 

owing to variations in evapotranspirative draft and 

early season water-level adjustments, these depths may 

differ from the operating levels for the season. In 1964, 

the water content observed in the first foot of the capillary fringe above the water table in the tanks with plants 

was about 1 percent by volume greater in October than 

in April. This condition might have resulted from an 

increase in soil-moisture tension as available moisture 

higher in the capillary fringe was reduced by transpiration. The additional profiles shown for the meadow area 

represent the maximum and minimum conditions observed during the period September 1961-October 1967. 

The maximum occurred June 11,1962, when flooding of 

the meadow caused essentially full saturation of the 

soils.

Winter precipitation was the principal source of water 

for replenishment of moisture in the soil above the 

capillary fringe. The annual replenishment varies 

widely as a result of variations in the type and rate of 

precipitation and opportunity for infiltration. The 

winter changes in water content shown in table 10 are 

a measure of the part of the winter precipitation that 

was stored during the nongrowing season. The remainder of the precipitation was lost by evaporation, some 

evapotranspiration, and sublimation of snow and ice.

WATER-CONTENT VARIATIONS IN 1963

Variations in the water content of soils in the evapotranspiration tanks during a typical season are shown 

by the seven sets of observations made in 1963. The water 

contents for the depths sampled in the tanks, expressed 

in inches depth per unit area, and the depths to water 

at the observation times are listed in table 11; the average contents are shown in figure 25. The September 4 

observations showed the least content for the season, 

except that in the bare-soil tank. The increased content 

in October and December may be both the result of increase in capillary water above the water table on the

reduction or cessation of evapotranspirative draft and 

the result of small additions from precipitation. Capillary rise above the shallow water table in the bare-soil 

tank kept the surface soils moist. Evaporation of the 

small quantity of free water in these heavy soils caused 

a 1-foot lowering of the water table and a small reduction in total water content between October 20 and 

December 16.

WATER-CONTENT CHANGES IN SHALLOW 

FLOOD-PLAIN DEPOSITS

The flood-plain deposits in the Humboldt River valley provide storage space for large volumes of water in 

the ground-water reservoir and in the unsaturated soils 

near the land surface during floodflows. Water stored 

in these deposits during periods of rising river stage 

and released to the stream when the river stage falls 

is one of the principal sources of water that sustains 

low flows in the Humboldt River. The water may be 

stored as soil moisture in the unsaturated zone, including the capillary fringe, and as ground water in the 

saturated zone.

The discharge of water from the flood-plain deposits 

in areas where the water table is at shallow depth may 

occur by evaporation from the land surface, by transpiration from riparian and flood-plain vegetation 

commonly woody phreatophytes and by underflow to 

stream channels. All the water going into storage in a 

given season may not be released in the subsequent 

low-water season, or for several seasons of copious precipitation or heavy streamflow. The quantity of water 

stored or released seasonally, or carried over, may be 

substantial.

Data on changes in the water content of the flood-plain 

deposits have been obtained during the period 1962- 

67 at three sites in the Humboldt River valley; one is at 

the test site 4 miles southwest of Winnemucca, a second 

at Winnemucca, and a third at the Kearns Ranch 6% 

miles northeast of Winnemucca. The access tubes at 

these sites permit sampling the soil profiles for depths 

of 81, 100, and 90 inches, respectively, which are adequate to cover the full range of the zone of soil-moisture 

change. Observations were made at the same intervals 

of time and depth as those in the evapotranspiration 

tanks.

Water-content profiles at these sites, based on five 

sets of observations made in 1962 as reported by 

Waananen (1965), showed seasonal changes in a year 

of high flow in the Humboldt River. Data obtained 

June 11, 1962, after the meadow had been flooded, represent the largest content observed during the study and 

indicate the water content of the soils under nearly full 

saturation. The initial measurement in September 1961
D36 STUDIES OF EVAPOTRANSPIRATION

TABLE 10. Soil-moisture changes in evapotranspiration tanks at the

Tank and tube

Depth of 

profile 

observed , 

in.

Water content and seasonal gain (+) or loss ( ), in inches depth

Apr. 8-10, 

1962

Change Oct. 18-20, 

1962

Change May 1-2, 

1963

Change Oct. 20-21, 

1963

Change Apr. 13-14, 

1964

Change

Grease wood

1-A

1-B

1-C

2-B

81

99

93

93

Qiy

75

99

87

22. 16 

24. 69

17. 20 

26. 81

-4.97 

-3.91

-4. 44 .

-3. 65

-5. 38

-4. 52 .

17. 19 

20. 78 

16. 62 

19.90

13.55 

21. 43

+ 4.71 

+ 6. 54 

+ 4. 68 

+ 4. 11

+ 5. 01 .

+ 8.04 

+ 6.93

+ 7.48 .

21.90

27.32 

21. 30 

24. 01

21.59 

28. 36

-5. 33 

-5.47 

-5. 45 

-4.71

-5. 24 _

-6. 44 

-6. 21

-6. 32 _

16.57

21.85 

15.85 

19. 30

15. 15 

22. 15

+ 1. 78

+ 1.91

+ .83 

+ . 60

+ 1. 28 .

+ 3. 35 

+ 3. 16

+ 3. 26 .

18. 35 

23. 76 

16. 68 

19.90

18. 50 

25.31

-3. 31 

-4. 73 

-2. 22 

-2. 02

-3.07

-4. 63 

-4. 79

-4. 71

Babbitbrush

1-A

1-B

2 _

3

93

87

93 

93

93

92

26. 02

26.94 

27. 06

-1. 03

-1. 08 

 1. 25

-1. 12 .

24. 99 

22. 70

25. 86 

25. 81

-2. 25 

-2.40

-2. 89 

-1.84

-2.35 .

22. 74 

20. 30

22. 97 

23. 97

-2. 37 

-3. 01

-2. 66 

-2. 78

-2. 71 _

20. 37 

17. 29

20. 31 

21. 19

+ 1. 06

+ 1. 59

+ 1.41 

-hi. >.<6

21.43

18.88

21.72 

22. 55

-1.99 

-2.65

 1. 12 

-1.40

-1. 61

Willow

1 __________________

2

60

60

60

60

22. 09 

22.47 

22. 15

-4. 12 

-6. 11 

-4. 57

17. 97 

16. 36

17.58

+ 2. 62 

+ 3. 20 

+ 3. 35

20. 59 

19. 56 

20. 93

-3. 25 

-3. 32 

-3. 05

-3. 21 _

17. S , 

16, ^4 

17. 81

-2. 18 

2. 79 

-2.47

19. 52 

19.03 

20.35

-1. 21 

-2. 76 

-3. 06

-2. 34

Wildrose

l

2

3

60

60

60

60

3 24. 26 

3 23. 48 

3 24. 65

+ 0. 09 

-. 31 

-1. 82

24. 35 

23. 17 

22. 83

-0. 89 

-.98 -.45

23.46 

22. 19 

22. 38

 1. 62 

-2. 55 

-3. 19

-2.4;, 

21. 84

19. 34 

19 19

+ 0. 70 

+ 1. 29 

+ 1.70

22.54 

20.93 

20. 89

-1. 26 

-1.70 

-2.44

 1. 80

Bare soil

60 3 23. 23 -1. 72 21. 51 +1. 59 23. 10

Meadow

81 3 26. 83 -5.59 21.24 +3.13 24.37 -I

22.71 +0.04 22.75 +0.37

21.05 +3.84 24.89 -2.52

1 Data not representative owing to puncture of tank membrane liner.

2 Change in depth of profile from 87 to 93 inches.
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY 

Winnemucca test site during summer and winter periods 1962-67

D37

Water content and seasonal gain (+) or loss ( ), in inches depth Continued

Oct. 19-21, 

1964

Change Apr. 20-21, 

1965

Change Oct. 25-26, 

1965

Change May 1-3, 

1966

Change Oct. 19-20, Change Apr. 19-20, 

1966 1967

Change Oct. 19-20, 

1967

Grease wood Continued

15.04 

19.03 

14.46

17.88

13. 87 

20. 52

+ 3.05

+ 3.68 

+ 2.07 

+ 2. 11

+ 2. 73 _

+ 3.61

+ 4. 14

+ 3.88 .

18.09 

22. 71 

16.53 

19.99

17.48 

24.66

-5. 14 

-6. 15 

-4. 13 

-4. 63

-5.01 .

1 -7. 72 

1 -9.40

1 -8. 56 _

12.95 

16. 56 

12.40 

15.36

1 9.76 

1 15. 26

+ 1.60 

+ 1. 84 

+.35

+. 99

+ 1. 20 .

1 +4. 49 

1 +5. 59

i +5. 04

14.55 

18.40 

12.75 

16.35

14. 25 

20. 85

-3. 32 

-3.39 

-2. 08 

-2.59

-2. 84 .

-3. 26 

-2.86

-3. 06 .

11. 23 +4. 91 

15. 01 +4. 51 

10. 67 +3. 22 

13. 76 +3. 09

._--_--. +3.93 .

10. 99 +5. 36 

17. 99 +4. 97

........ +5. 16 .

16. 14 

19. 52 

13. 89 

16.85

16.35 

22. 96

-5. 27 

-6.50 

-4.62 

-4.22

-5. 15 _

-7. 60 

-7.71

-7. 66 .

10.87 

13. 02 

9.27 

12. 63

8.75 

15.25

Rabbitbrush Continued

19.44 

16. 23

20. 60 

21. 15

+ 1.73

+ 2.47

+ 2. 00 

+ 2. 12

21. 17 

18. 70 

2 20. 43 

22. 60 

23. 27

-2.64

-2.98 

-1. 69 

-1. 04

-1.76 .

19. 03

17.45 

20.91 

22. 23

+ 4. 54

+ 5.84 

+ 3.05 

+ 2. 39

23.57

23. 29 

23. 96 

24. 62

-7. 19

-8.46 

-3.33 

-4.30

-5. 15 .

16.38 +1.67

14. 83 +3. 58 

20. 63 + 1. 82 

20. 32 +2. 03

18. 05

18. 41 

22. 45 

22.35

-4.06

-5.48 

-4.40 

-6.23

-5. 13 .

13.99

12.93 

18.05 

16. 12

Willow Continued

18. 31 

16.27 

17. 29

+ 1.85 

+ 3. 37 

+ 3.44

20. 16 

19. 64 

20. 73

-3.45 

-1.75

16.71 

17.89

+ 1.68

+ 2. 21

18.39 

20. 10 

21. 82

-3.30 

-2.65

15. 09 +3. 99 

17.45 +3.58 

12. 70 +3. 48

19. 08 

21.03 

16. 18

-3.05 

-2.90

16.03 

18. 13

Wildrose Continued

21. 28 

19. 23 

18.45

+ 1. 54 

+ 2.46

+ 2.78

22. 82 

21.69 

21. 23

-1. 17 

-2. 18

21.65 

19.51

+ 0. 65 

+ 3.39

22. 30 

22.90

22.58

-2. 27 

-2. 84

20. 03 +2. 12 

20.06 +2.55 

14. 11 +4. 82

22. 15 

22. 61 

18.93

-2.56 

-1.87

19.59 

20.74

Bare soil Continued

23.12 -0.66 22.46 +0.09 22.55 -0.80 21.75 0.42 21.33 +1.25 22.58 0.99 21.59

Meadow Continued

22.37 +4.56 26.93 -1.86 25.07 +3.63 28.7.0 -13.51 15.19 +4.65 19.84 +0.34 20.18 

' Apr. 4,1962.
GREASEWOOD

60

0 10 20 30 40 50

1962

10 20 30 40 50

1963

20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 5

1964 1965 1966 

WATER CONTENT, IN PERCENT BY VOLUME
WILDROSE

1 1 1 1 

^HHk

Water Level ll 

_! Oct. 18'

! I 1 1

30

CO

LU

I 60

Z BARE SOIL

30

60

T I

Oct. 22

Water 

_ Level

O MEADOW

LJ 

CD

30

60

90

1 T

May 2

1 I

J__L

Oct. 21

I I I I

1 T

Oct. 20

I I

1 [

April 22

I I

Oct. 25

J__L

T I

I I I

-May 2

Oct. 20 <

I I

J__I

0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 5

1962 1963 1964 1965 1966 

WATER CONTENT, IN PERCENT BY VOLUME

FIGURE 24. Soil-moisture profiles at the beginning and end of the growing seasons 1962-67 in evapotranspiration tanks and the flood plain at the 

areas on profiles in the tanks indicate seasonal changes in water content. Shaded areas for meadow profiles indicate difference between en

minimum content September 14, 1961. Meadow profile for June 11,1962, is the maximum observed and represents nearly full saturation of the 
D40 STUDIES OF EVAPOTRANSPIRATION

in the meadow area at the test site fortunately provided 

information on the water content at the end of a 3-year 

dry period, thus perhaps representing the minimum 

that might be expected.

The water content observed at the end of each season 

from 1961 to 1967 at the three sites is shown in figure 26, 

together with the maximum content as observed in June 

1962. These data demonstrate that the storage in the

24

22

20

18 

23

21

19 

24

W 

u 22

O

Z

z 20

Z

u

Z 18 

O O

u h- i«

i

-

_

Water 

^Content

<

I

1

GREASEWOOD 

Depth of profile, 90 inches 

Depth to water, 60 inches 

i i

-

_

i

i

i

i

RABBITBRUSH

Depth of profile, 92 inches 

Depth to water, 60 inches 

 1 1 1

-

(

, 

i

1 t

(

1 WILLOW 

Depth of profile, 60 inches 

Depth to water, 60 inches 

i

(

i

-

1 

24

22

20

18 

26

24

22

20 

Af

1

-

1 i

i

i

1 1 WILDROSE 

Depth of profile, 60 inches 

Depth to water, 60 inches 

{

i

i

-

-

i

Water 

^Content

i

BARE SC

' i

)IL 

Depth of p 

Depth to \A

i

rofile, 60 inche 

later, 60 inches

<

s

ril May June July July Sept. Oct. Dec.De 

LI 11 1 26 4 20 16 3

flood-plain deposits increased more than 1 acre-foot per 

acre between September 1961 and June 1962, but in October 1962 only about one-half acre-foot per acre of this 

increase still remained in storage. In subsequent seasons 

the storage increased slightly, but in the dry season of 

1966 the soil moisture was depleted to the 1961 levels ; 

in 1967 the storage increased again, but less than 

one-half acre-foot per acre.

The distribution of moisture in the soil profile in the 

meadow area at the test site is shown in figure 24. The 

profiles for September 1961 and June 1962 are repeated

FIGURE 25. Variations in water content of soils in evapotranspiration 

tanks at the Winnemucca test site during the 1963 season.

in each graph to indicate the relation of the water conTABLE 11. Water content of soils in evapotranspiration tanks at 

the Winnemucca test site, as observed during the 1968 season

Depth of Water content, in inches depth (upper number) and 

profile depth to water, inches below land surface (lower number)

(in.) May June July 

1-2 11 1

July

26-27

Sept. Oct. 

4-5 20-22

Dec. 

16-17

Greasewood

1 

2

.. _ ............ 91# 23.63 22.10 21.26

59 60 60 

................. 87 24.98 23.41 22.53

57 60 J60

Average 

water 

content..... 90 24.30 22.76 21.90

19.23 

60 

19.53 

60

19.38

18.00 

60 

18.26 

60

18.13

18.39 

60 

18.65 

60

18.52

17.53 

91 

19.52

72

18.52

Rabbitbrush

1 

2 

3

................. 90 21.52 20.83 19.92

61 60 60 

................. 93 22.97 21.91 21.38

59 60 60 

. .-.....----. 93 23.97 23.39 22.44

61 60 60

Average 

water 

content..... 92 22.82 22.08 21.25

18.48 

60 

19.57 

60 

21.17 

60

19.74

18.54 

60 

19.81 

60 

20.65 

60

19.67

18.83 

60 

20.31 

60 

21.19 

60

20.11

18.87

74 

19.87 

71 

21.07

72

19.94

Willow

1 

2 

3

1

2 

3

. _ ............. 60 20.59 20.17 ........

56 60 ........

................. 60 19.56 19.27 223.66

57 54 ........

................. 60 20.93 22.76 * 24.03

61 25 .... ....

Average 

water 

content..... 60 20.36 20.73 "23.84

Wildrose

................ 60 23.46 22.95 2 24. 00

59 60 ........

................. 60 22.19 22.75 223.96

56 30 ........

......... ....... 60 22.38 21.99 * 24.38

55 48 ........

Average 

water 

content..... 60 22.68 22.56 224.11

19.97 

60 

19.07 

60 

20.19 

62

19.74

23.05 

59 

21.96 

59

22.58 

56

22.53

16.51 

60 

15.52 

65 

17.43 

64

16.49

21.12 

61 

19.12 

62 

18.77 

62

19.67

17.34 

60 

16.24 

62

17,88 

61

17.15

21.84 

61 

19.64 

63 

19.19 

62

20.22

18.11 

16.94 

18.90

17.98

22.12 

76 

20.35 

77 

19.69 

79

20.72

Bare soil

60 23.10 22.82 22.72 

26 18 23

22.27

28

22.16

28

22.71 

28

22.00 

40

i Inlet tube dry. 

2 Tanks flooded; water content from 1962 data under full saturation, 

a Dry.
EVAPOTRANSPIRATION BY WOODY PHREATOPHYTES IN THE HUMBOLDT RIVER VALLEY D41

1961 1962 1963 1964 1965 1966 1967

FIGURE 26. Water content at end of each season and maximum content observed in June 1962, in inches, as observed at three sites in

Humboldt River flood plain near Winnemucca, Nev.

tent each spring and fall with the extremes observed. 

The shaded areas indicate the distribution in the profile 

of moisture in excess of the minimum storage. The October 1966 data differ slightly from those for 1961 because the initial observations were taken at different 

depths.

It might be inferred from the slight increase in the 

October water content from 1963 to 1965 that water 

storage in the flood-plain deposits had only a minor 

effect on the annual water budgets for the Humboldt 

River in those years. The large storage increase in 1962, 

sharp decrease in 1966, and lesser gain in 1967, however, 

when related to the full extent of the flood plain affected 

in the basin, may represent significant differences in 

the relation between annual precipitation and streamflow, as well as in the water available to plants on the 

flood plain. The depletion of soil moisture and ground 

water in 1966 may have resulted in a larger sustained 

summer flow in the Humboldt River than would have 

been produced by the annual precipitation alone, 

whereas the increased retention in 1962 and 1967 

reduced the streamflow.

The seasonal water-content determinations thus provide an index of storage capacity and a means for estimating the volumes of water that could be accepted by 

and stored in flood-plain deposits during subsequent 

floodflows, or that would be effective in maintaining 

streamflow during dry seasons.

REFERENCES

American Society of Civil Engineers, 1962, Nomenclature for 

hydraulics: Am. Soc. Civil Engineers Manual No. 43, 501 p.

Cohen, Philip, 1905, Water resources of the Humboldt River 

valley near Winnemucca, Nevada: U.S. Geol. Survey 

Water-Supply Paper 1795,143 p.

Horton, J. S., Robinson, T. W., and McDonald, H. E.3 1964, 

Guide for surveying phreatophyte vegetation: U.S. Dept. 

Agriculture, Forest Service, Agriculture Handbook 266, 

37 p.

Lee, C. H., and others, 1926, Evaporation on the United States 

Reclamation Projects: Am. Soc. Civil Engineers Proc., 

August 1926, p. 1196.

Oertli, J. J., 1964, Loss of boron from plants through guttation: 

Soil Science, July-December, v. 94, p. 214-219.

Pacific Southwest Inter-Agency Committee, Phreatophyte Subcommittee, 1966, Glossary of terms relating to the phreatophyte problem in Vegetation management on flood plains 

and riparian lands symposium presented by the Phreatophyte Subcommittee at 66-3 meeting of the Pacific Southwest Inter-Agency Committee, Albuquerque, New Mexico, 

Aug. 30,1966, p. 48-54.

Sondregger, A. L., 1929, Water supply from rainfall on valley 

floor: Am. Soc. Civil Engineers Proc., v. 55, no. 5, p. 

1139-1165.

Tomanek, G. W., and Ziegler, R. I., 1962, Ecological studies of 

saltcedar: Div. Biological Sciences, Fort Hays Kansas State 

College, Fort Hays, Kansas, 128 p.

Waananen, A. O., 1965, Water-content changes in shallow floodplain deposits at three sites in Water resources of the Humboldt River valley near Winnemucca, Nevada: U.S. Geol. 

Survey Water-Supply Paper 1795, p. 104-108.

U.S. GOVERNMENT PRINTING OFFICE: 1970 O 372-487