Progress Report on Use of Water 

by Riparian Vegetation, 

Cottonwood Wash, Arizona

GEOLOGICAL SURVEY 

CIRCULAR 434

DDFPADFH IKI rrtrtDFDATITkkl WITW ARI70KIA CTATF IAK1H RFPABTMFNT AND SALT RIVER VALLEY WATER USERS' ASSOCIATION
Progress Report on Use of Water by 

Riparian Vegetation, Cottonwood Wash,

Arizona

By E. L. Hendricks, William Kam, and James E. Bowie

Prepared in cooperation with Arizona State Land Department and Salt River Valley Water Users' Association

Geological Survey Ckcular 434

Washington, D. C. 

1960
United States Department of the Interior

FRED A. SEATON, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

Free on application to the U. S. Geological Survey, Washington 25, D. C.
CONTENTS

Page Page

Abstract._ ________________ 1 Method of investigation 4 

Introduction .__.. _._______. 1 Results___.. 7 

Project plan ........................... 3 Summary and conclusions ............. 10

Geology of the project area ............ 3

ILLUSTRATIONS

Page 

Figure 1. Map of Cottonwood Wash project area, Mohave County, Ariz. 2

2. Computed weekly use of water by riparian vegetation in 4.1-mile reach of

Cottonwood Wash, Ariz., and weekly average air temperatures at Kingman

and at McKinney ranch in the study area..............--._-.._-------------- 7

3. Graphs of water-table and stream-discharge fluctuations on March 3,

March 19, April 16, and May 6, 1959 _________..... 9

III
Progress Report on Use of Water by Riparian Vegetation, 

Cottonwood Wash, Arizona

By E. L. Hendricks, William Kam, and James E. Bowie

ABSTRACT

Measurements of streamflow, ground-water levels, and 

meterological data obtained in a 4.1-mile reach of the flood 

plain of Cottonwood Wash, Mohave County, Ariz., define the 

use of water by riparian vegetation in that part of the stream 

valley. The computed evapotranspiration loss during the 

growing season of 1959 was 175 acre-feet, which represented 

about 33 percent of the water that entered the reach. The 

maximum rate of loss during the season was slightly more 

than 8 acre-feet per week, or about 60 percent of the inflow.

The project reach is divided into two parts: An upstream 

subreachof 2.6 miles and a downstream subreach of 1.5 miles. 

Seasonal losses in the upstream and downstream subreaches 

were 75 and 100 acre-feet respectively. Losses in the 

shorter downstream subreach were larger because of the 

greater plant population.

During the summer of 1960 the vegetation in the lower 

subreach will be chemically defoliated as a part of the experiment to determine the savings in water losses that can be 

effected by modifying riparian vegetation. Tests on chemical defoliants indicate that a single spraying eliminates the 

leaves on Cottonwood trees for 7 or 8 days and that no permanent damage results.

INTRODUCTION

Arizona has many thousands of miles of 

stream channels. Wherever channels carry 

a sufficient flow of water, their banks are 

lined with willow, cottonwood, saltcedar, and 

other water-loving plants. These plants, 

known as phreatophytes, pump water from the 

stream or from ground water tributary to the 

stream as effectively as if man had installed 

his mechanical pumps to divert water for his 

use. Thus the flow of many streams is greatly 

depleted as it travels from where it originates 

to the points where it can be used beneficially.

The transpiration of water by phreatophytes 

that line the banks of streams or draw from 

the water in the flood-plain sediments and the 

accompanying direct evaporation from water 

and soil surfaces are known as riparian 

losses. The losses involve two processes: 

(a) transpiration, which is the discharge of 

water vapor through the stomata of the plant 

leaves; and, (b) evaporation from the water and

soil surfaces. These two processes, together, 

are called evapotranspiration.

Inasmuch as riparian losses by evapotranspiration aggregate large volumes of water, it is natural to inquire whether some of 

the water dissipated in this manner can be 

conserved for beneficial use. The most logical direction of inquiry into this question is to 

investigate the possibility of effecting this 

conservation by reducing the transpiration of 

the riparian vegetation. The project on Cottonwood Wash, Ariz., currently in progress, 

is designed to determine whether a water savings for beneficial use can be accomplished by 

modifying the vegetation to reduce transpiration losses. The purpose of this report is to 

describe the progress to date on this project 

and to indicate the direction that the project 

will take in future phases of the investigation.

Riparian vegetation, may be modified in two 

ways; by eradication and by chemical defoliation. Eradication of the vegetation is an extreme measure, for costs are necessarily 

high and the values of the vegetation would be 

destroyed. Thus, for these and other reasons 

any proposal to remove riparian vegetation in 

large-scale operations would likely meet with 

strenuous objections from many people. On 

the other hand, chemical defoliation of vegetation removes the leaves for a very limited 

time. However, the water savings, if any, 

would be effective for only a short time. Both 

methods of vegetation modification may be 

used in this project.

It is important that the role of the Cottonwood Wash project in relation to the basic 

question of water conservation by riparianvegetation modification be seen in proper 

perspective. For example, the quantitative 

answers obtained from this project will apply 

strictly to the reach of Cottonwood Wash selected for study. Also, the present investigation is a pilot study in that it will furnish
113°32' 

R. 18 W. 17 16 15 14 13 12 II 10

INDEX MAP

Figure 1. Map of Cottonwood Wash project area, Mohave County, Ariz.
GEOLOGY OF THE PROJECT AREA

an answer for one type of environment and in 

that it is being used as a means to explore 

the techniques for conducting such studies. 

Experience is lacking on the kinds of data 

needed and the techniques to be used for analysis of the data. Similar studies in other 

environments will be needed before generalizations can be made.

The Cottonwood Wash project is sponsored 

by the Arizona Water Resources Committee. 

Installation of the gaging stations and groundwater wells was made possible by financial 

support of Arizona State Land Department and 

the Salt River Valley Water Users' Association. The project has subsequently been carried on in cooperation with the Arizona State 

Land Department.

PROJECT PLAN

A 4.1-mile reach of Cottonwood Wash, beginning at a point about 10 miles upstream 

from the Big Sandy River, is being used as 

the study area (fig. 1). For experimental 

purposes the reach is divided into two subreaches; an upstream subreach 2.6 miles 

long and a downstream subreach 1.5 miles 

long. A stream-gaging station at each end of 

the study reach and one at the dividing point 

between the upper and lower subreaches, a 

total of three gaging stations, permit analysis 

of the changes in streamflow that occur in 

each of the two subreaches.

The general plan of the experiment is to 

maintain the upper subreach unchanged 

throughout the study. Vegetation modifications will be made in the lower subreach to 

test for water savings. In the first phase of 

the study, which is now complete, the natural 

losses in the two subreaches were measured.

In the second phase of the study, which will 

be started in the spring of 1960, the vegetation on the flood plain of the lower subreach 

will be chemically defoliated and the duration 

of the defoliation will be controlled. The water savings, if any, resulting from the reduction in transpiration will be determined by 

analysis of the streamflow, ground-water, and 

meteorological records collected during the 

test period.

If it is shown in the second phase that sensible quantities of streamflow can be conserved by reducing transpiration losses, a

third phase of the study will be started later 

in the summer of 1960. The vegetation in the 

lower subreach will be permitted to return to 

its original state and.then a large grove of 

cottonwood and willow trees close above the 

upstream end of the study reach will be defoliated to determine whether the gain in 

streamflow obtained therefrom persists unabated through the three measuring points or 

is partly or completely lost through increased 

evapotranspiration as the water is being 

transported downstream.

In a final phase of the study, probably during the winter of 1960-61, it is planned that 

the flood-plain vegetation in the lower subreach will be removed by cutting. Records 

will be continued long enough, at least through 

the following summer and thereafter, to evaluate the effects of the regrowth of vegetation 

on the streamflow. It cannot, of course, be 

said at this time precisely how the study will 

proceed. Decision to conduct any one of the 

phases of work as outlined will depend on the 

results of the antecedent work.

GEOLOGY OF THE PROJECT AREA

The geologic environment is a major factor 

to be considered in the study of the surfacewater and ground-water relation in the Cottonwood Wash area. A geologic investigation 

will be made, therefore, of the immediate 

area of the pilot study to determine, primarily, whether significant gains or losses in 

streamflow or ground-water storage are attributable to effluent or influent seepage. Seepage of ground, water from tributary valleys 

or terrace deposits along the canyon walls 

may contribute to streamflow, or seepage of 

streamflow into fault zones or joint systems 

may cause losses from the stream. The following paragraphs give a very brief, general 

description of the geologic environment of the 

pilot-study area.

The Cottonwood Wash canyon traverses the 

Cottonwood Cliffs in southeastern Mohave 

County, Ariz. That part of- the canyon with 

which this investigation is concerned is located in sees. 25, and 26, T. 21 N., R. 12 W., 

and sees. 29 and 30, T. 21 N., R. 11 W.

The canyon of Cottonwood Wash is a youthful valley in which the. stream has excavated 

a narrow and relatively steep-walled trench 

in crystalline rocks. The bedrock floor of the
USE OF WATER BY RIPARIAN VEGETATION, COTTONWOOD WASH. ARIZ.

canyon is irregular, resulting in rapids and 

falls along the stream course, and is covered 

discontinuously with gravel, sand, and silt. 

The walls of the canyon, within the area, rise 

to maximum altitude of about 5,000 feet above 

sea level and the maximum relief is about 

1,000 feet. The average gradient of the 

stream within the area of study is about 125 

feet per mile.

The rocks in the area are principally granite and gneiss of Precambrian age in which 

the Cottonwood Wash canyon has been cut. 

Extrusive and intrusive rocks of probable 

Tertiary age are present in the eastern part 

of the area. These consist primarily of andesite, tuff, and basalt. Unconsolidated to 

poorly consolidated sediments of Quaternary 

age form terrace remnants along the steep 

slopes of the canyon walls and in small basins, and thin lenticular mantles along the 

course of the stream. The terrace lies about 

20 feet above the bed of the stream in the 

lower subreach and indicates the upper level 

of an older phase of aggradation in the canyon. The terrace deposits, consisting of silt, 

sand, and gravel, have been cut through in 

most places, to the crystalline-rock floor of 

the channel. It is, apparent, therefore that a 

recent change in the local base level has occurred in the area. The valley fill that has 

accumulated in the present channel of Cottonwood Wash consists of unconsolidated lenticular deposits of boulders, sand, and clay. 

These sediments lap up against the base of 

the terrace deposits except where they abut 

the crystalline rocks in the more deeply excavated parts of the channel.

METHOD OF INVESTIGATION

The basic method to be used for determining the riparian water losses in each subreach, and in the total study reach, is the 

water-budget method. This method requires 

that the outflow from the reach be subtracted 

from the inflow to the reach and that the difference be adjusted for changes in groundwater storage and soil-moisture storage in 

the valley fill. Either the outflow or inflow 

may occur by surface or subsurface means. 

The study reach of Cottonwood Wash was specifically chosen because a reconnaissance indicated that the subsurface inflow and outflow 

would be quite small if, indeed, it exists at 

all during the critical water-budget periods.

The three stream-gaging stations (fig. 1) 

are located at valley sections where the 

stream is flowing over bedrock. Concrete 

cutoff walls resting on the bedrock were constructed at each station to force all downvalley flow through the measuring flumes. If 

storm periods are omitted, the streamflow 

stations measure the principal inflow and 

outflow in each subreach under consideration. 

Thus, a first approximation of the water used 

by evaporation and transpiration in the reach 

is determined by subtracting the flow at the 

downstream gaging station from the flow of 

the upstream gaging station. The difference 

in streamflow thus computed should be adjusted for changes in ground-water storage 

and in soil-moisture storage. For example, 

if water levels fall during a period of observation, the water that came out of groundwater storage should be added to the loss in 

streamflow to obtain a corrected estimate of 

the evapotranspiration in a subreach. Preliminary estimates of the possible magnitude 

of the ground-water storage item were based 

on estimates of the volume of saturated valleyfill sediments, the probable rates of watertable recession, and the coefficient of drainage (storage coefficient). These estimates 

indicated that, in comparison to the volumes 

of water consumed by transpiration in the 

study reach, the quantities of water drawn 

from ground-water storage are small.

Although three wells are located in the valley fill in the vicinity of McKinney's ranch 

house to indicate changes in ground-water 

storage and for use as transpiration wells 

(see discussion that follows), it has been decided that it would not be feasible to install 

enough observation wells to define changes in 

water levels in all segments of the valley fill 

within the study reach. Too, the heterogeneity 

of the sediments wouldmake.it a difficult task 

to determine an average coefficient of drainage.

In this study water budgets for weekly periods will be computed. Weeks in which rainfall resulted in either storm runoff or recharge in the valley fill will be eliminated 

from consideration. Thus only weeks during 

which the water table was in continuous recession will be used. In this way, the necessity for evaluating quantitatively the changes 

in ground-water storage is minimized. Observed changes in ground-water levels during 

a water-budget period are used in this study
METHOD OF INVESTIGATION

only for qualitative evaluation of the computed evapotranspiration losses.

The evaluation of changes in soil-moisture 

storage is not being attempted in this study. 

As in the case of ground-water storage 

changes, it is estimated that changes in soilmoisture storage will be relatively small for 

the periods used in the study and that adjustments of the computed evapotranspiration for 

these changes would not appreciably improve 

the results.

Theoretically, of course, changes in storage in the stream channel should be considered in the water-budget method of measuring 

the evapotranspiration in the study reach. 

However, the water volumes involved in these 

changes are clearly inconsequential.

A second method for determining the evapotranspiration losses from the flood plain is 

known as the transpiration-well method. Although extensive work would be required to 

estimate the losses from all segments of the 

flood-plain vegetation by this method, a limited effort may yield estimates of evapotranspiration losses useful for comparison 

with those obtained by the water-budget 

method.

Records of the daily fluctuation of the water table in the vegetation covered parts of 

the flood plain are needed for computations 

by the transpiration-well method. For this 

purpose three wells were constructed in the 

downstream sub reach in August 1958. The 

wells were dug by hand to about l^feet below 

the water table. Samples of material in two 

wells were collected from within a foot above 

the water table. The physical properties of 

the samples from the zone of fluctuations of 

the water table were determined by laboratory procedures. A water-stage recorder 

was installed over each well.

The transpiration-well method involves 

obtaining continuous records of the daily 

water-table fluctuations and determining the 

coefficient of drainage of the material in 

which the daily fluctuations of the water table 

occur. From these data, transpiration volumes are computed and related to volume 

density of vegetation at the individual wells. 

The results are then applied to the larger 

area. No computation of losses by the 

transpiration-well method have been made 

for this progress report; however, before a

final project report is prepared, this method 

will be explored further.

A limited ' observation-well network was 

established in the area in order to measure 

changes in storage in the ground-water reservoir and to determine the direction of 

movement of ground water. Two observation 

wells were dug and four were drilled in the 

terrace deposits, which occur generally above 

the level of the modern flood plain. One well 

was dug in the gravel and sand of the flood 

plain. Three of the wells intersected the water table, one in the flood plain and two in the 

terrace deposits; the others were dry. The 

altitude of the measuring point of each of the 

wells will be determined by instrument leveling. Thus, for any well it will be possible 

to compute the elevation of the water table 

above sea level and determine any gradients 

that are of interest. The location of both the 

transpiration and the observation wells is 

shown on figure 1.

Rates of evapotranspiration are related to 

the climatic factors that control the physical 

and physiological processes and to the density and types of vegetation. This project 

has been planned, therefore, to provide data 

on these factors in addition to the volumetric 

measurements of the quantities of water used 

in the evapotranspiration process in the study 

reach.

A partial weather station (fig. 1) is operated at McKinney's ranch. The group of instruments operated at this site include a 

recording rain gage, a hygrothermograph for 

measuring air temperature and relative humidity, a pyrheliograph for measuring radiation, and an anemometer to measure wind 

movement. Two other automatic rain gages 

are operated in the study area (fig. 1). The 

rain gages are used primarily to indicate the 

periods in which local recharge to the valley 

fill occurred and to indicate whether storm 

runoff measured at the gaging stations originated within the study area or above it.

A study of the flood-plain vegetation in the 

study reach is planned for sometime during 

the summer of 1960. In this part of the project the types and density of plants drawing 

water from the stream or from ground water 

in the valley fill will be described. The 

climatologic data and the quantitative vegetative descriptions being collected in the 

study area should provide the link 'between
Measuring site

or 

instrument

Stream gage: 

1________ 

2________

USE OF WATER BY RIPARIAN VEGETATION, COTTONWOOD WASH, ARIZ.

Location

Transpiration wells 

Dug well 1 _______

At upstream end of 4.1-mile study reach.__ _____._

2.6 miles downstream from stream gage 1, at junction

of upstream and downstream subreaches. 

4.1 miles downstream from stream gage 1, at downstream end of study reach. 

In valley fill opposite McKinney ranch house _________

On flood plain about 0.10 mile southeast of McKinney 

ranch house.

Date

of 

establishmen t

September 1958 

Do.

Do.

August 1958 

June 1958

Drilled well: 

2_____. 

3______.

Rain gage:

1__________ 

2___________. 

3___________

Anemometer ______

Hygrothermograph. 

Py rheliograph _____

Water-temperature

recorder:

1...............

3__________.

On terrace southeast of McKinney ranch house ___ . 

Bank of tributary wash and road junction northeast of 

McKinney ranch house.

November 

Do.

1959

2,200 feet southeast of stream gage 1. 

3,600 feet northwest of stream gage 1 

At McKinney ranch house _________

______________do_»______________ 

_____ _______do __________________

______________do __________________

Attached to stream gage 1. 

Attached to stream gage 3.

June 1959

Do.

Do.

October 1959 

June 1959

Do.

Do. 

Do.

results obtained from this area and those 

from other areas when these studies have 

been extended. Also, it is probable that the 

climatic and vegetative factors may provide 

the basis for generalizing the answers obtained at several study sites for application 

to unmeasured areas.

In a pilot study such as this the possibility 

that the water quality may be altered owing 

to the evapotranspiration withdrawals as flow 

moves down the channel will not be overlooked. Water samples for chemical analysis 

were collected at each stream-gaging site in 

January 1960 during a base-flow period to 

provide an indication of the water quality unaffected by evapotranspiration. In the next 

phase of the study, on the effects of defoliating vegetation in the downstream subreach, 

sampling for water quality will be intensified 

and will include the periods before, during, 

and after treatment in order to evaluate possible changes in water quality. Inasmuch as 

the water transpired and evaporated in pure 

water, it is possible that during periods of 

high evapotranspiration losses the chemical 

quality of the streamflow may progressively 

deteriorate as the water that enters the project area proceeds downstream. Thus, reducing the evapotranspiration losses should

improve the water quality, whether it will 

improve measurably remains to be determined.

In the defoliation phase of the study, the 

removal of the leaves from vegetation in the 

downstream subreach will be accomplished 

by the application of magnesium chlorate. 

This defoliant will be applied by crop-dusting 

methods. Magnesium chlorate was determined to be most feasible by preliminary 

field testing conducted on a one-quarter acre 

grove of cottonwood trees in a canal wasteway on the Salt River Indian Reservation in 

the vicinity of Scottsdale, Ariz. Magnesium 

chlorate was tested after consultation with 

several chemical companies1 and the State 

universities. The recommended dosage used 

was 20 pounds of magnesium chlorate in 200 

gallons of water per acre, which is the usual 

dosage for cotton. One quart of a wetting 

agent per acre was added t© the mixture. 

This concentration of the defoliant is about 

one-tenth that commonly used to defoliate 

cotton but as indicated, the dosage used was 

twice that used for cotton. Magnesium chlorate is nontoxic to humans or animals. The

iChemicals contributed by White Chemical Co. and Arizona 

Pest Control Co.
RESULTS

defoliant as made up is estimated to cost about $5.00 per acre. Application costs have 

not been estimated.

A summary of the instruments and datacollection sites being used in this study on 

the preceding page.

RESULTS

The losses of water in the 4.1-mile study 

reach and in the two subreaches were computed by subtracting the streamflow at the 

downstream end of the reach from that at the 

upstream end of the reach. No adjustments 

to the loss figures thus computed have been 

made. Error from this cause should be 

slight, however, for the reasons previously 

described. The water-budget period used in

the computation is a week. Weeks in which 

there was significant storm runoff or groundwater recharge in the valley fill were, eliminated from the computations.

The computed weekly loss rates are shown 

on figure 2. Although a smoothed line has 

been drawn on figure 2 to represent the 

changes in loss rate during the growing season, it is probable that the actual changes 

from week to week were somewhat more irregular than the smoothed line indicates. The 

air-temperature records show irregular 

weekly variations. Other climatological data 

collected to date have not been processed but 

when combined with the temperature record 

may provide the basis for a stricter interpretation of the week-to-week variations in 

evapotranspiration than has been made for 

this progress report.

ACRE-FEET, PER LOSS, IN WEEKBETWEEN TEMPERATURE, IN 

DEGREES STREAM GAGES3FAHRENHEIl 1 AND

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

Figure 2. Computed weekly use of water by riparian vegetation in 4.1-nrile reach of Cottonwood Wash, Ariz., and weekly average 

air temperatures at Kingman and at McKinney ranch In the study area.
8 USE OF WATER BY RIPARIAN VEGETATION. COTTONWOCD WASH, ARIZ.

Additional scatter of the observed data 

above or below the smoothed loss curve is 

probably caused in large part by slight inaccuracies in the observed streamflow data. 

The loss figures computed by subtracting 

streamflow records contain all the error inherent in both records. Thus small errors 

in the streamflow records may cause much 

larger percentage errors in the computed 

losses. For example, although the departures 

of the observed data from the smoothed curve 

on figure 2 appear large, corrections to each 

of the streamflow records of as little as 3 

percent in the right direction would account 

for all but about half a dozen of the departures. Failure to adjust the loss figures for 

changes in ground-water and soil-moisture 

storage may account for a small part of the 

scatter of the data plotted on figure 2.

The data plotted on figure 2 are for the 

loss in the 4.1-mile study reach, which includes the two subreaches. Similar diagrams 

have been prepared for each of the subreaches but are not reproduced in this report.

The computed evapotranspiration loss in 

the study reach for the growing season, 

March through October, is 175 acre-feet. 

This loss is 33 percent of the water that entered the reach, as measured at streamgaging station 1. In the peak months of July 

and August the maximum weekly loss rate 

was 8.3 acre-feet, equivalent to slightly more 

than 60 percent of the water that entered the 

reach.

The computed season losses in the upstream (2.6 miles) and downstream (1.5 

miles) subreaches were 75 and 100 acre-feet, 

respectively. It is noteworthy that, although 

the lower subreach is shorter, the losses are 

larger than in the upper subreach. The difference is due largely to a higher total population of phreatophytes in the lower reach.

The actual evapotranspiration losses may 

be somewhat greater than computed because 

of unmeasured inflow to the study reach. The 

data plotted on figure 2 show that there is a 

small gain in flow in the reach during the 

winter months. The gain in flow appears to 

be appreciably greater in the upstream subreach. Inasmuch as data for weeks having 

storm runoff have not been used, the conclusion is that the inflow occurs by subsurface 

routes. This is not totally unexpected, for 

several of the small tributary washes along

the reach discharge onto or through the terrace remnants and thus there is opportunity 

for a limited amount of ground-water storage 

from storm runoff to provide the inflow. Evidence of this inflow from one tributary was 

detected during a reconnaissance in December prior to the start of the current project. 

Although inflow was not observed during the 

summer period it cannot be stated positively 

that none existed. There is^ a possibility, 

then, that both the total inflow to the reach 

(streamflow plus subsurface inflow) and the 

evapotranspiration losses are higher than 

have been computed. However, it is believed 

that the sources of subsurface inflow are 

largely depleted early in the summer and that 

error from this source is not large during the 

growing season. The subsurface-inflow factor will receive increased emphasis in the 

subsequent phases of the investigation.

The weekly changes in water levels in the 

three transpiration wells were computed. 

With few exceptions, the water table in the 

valley fill was in recession during the weeks 

for which evapotranspiration losses were 

computed. The weekly recession was usually 

less than 0.05 foot, confirming that the corrections for ground-water storage which 

could be applied to the computed losses were 

small.

Duringthe spring of 1959 a series of weekly 

observations on the stage of leafage of the 

riparian vegetation was made. The graphs in 

figure 3 show the water-level fluctuations in 

the transpiration wells and the streamflow 

fluctuations.

Figure 3d, shows that as early as March 3 

a pronounced pattern of diurnal fluctuations 

in streamflow rates had been established even 

though the vegetation was only starting to leaf 

out. Inasmuch as the water-table levels were 

showing only slight diurnal changes on this 

date (fig. 34), there is indication that evaporation.from the valley-fill surface preceded 

the onset of transpiration losses. Figure 3C 

H demonstrates the increasing range of diurnal fluctuations as the vegetation leafted out 

and spring passed into summer.

The graphs of diurnal fluctuations in watertable levels and streamflow shown in figure 3 

are typical and demonstrate the effects of 

variations in evapotranspiration rates during 

a day. The lowest water levels and streamflow rates occur during the afternoon when
RESULTS

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Figure 3. Graphs of water-table and stream-discharge fluctuations on March 3, March 19, April 16, and May 6, 1959. Numbers refer to

transpiration wells and stream gages, respectively.
10 USE OF WATER BY RIPARIAN VEGETATION. COTTONWOOD WASH. ARE.

the temperature and radiation are highest and 

the humidity is lowest. The highest water 

levels and streamflow rates occur during the 

night or early morning as solar radiation is 

reduced, temperature drops, and humidity 

rises.

During the season of highest evapotranspiration rates the maximum range in water 

levels in the transpiration wells during a day 

is about 0.15 foot. Typical of the diurnal 

fluctuations in streamflow is a variation in 

flow rate from 0.6 cfs to 0.85 cfs at stream 

gage 1 and a variation from about 0 to 0.5 cfs 

at stream gage 3.

In the defoliation experiment the defoliant 

tested is estimated to have been about 90 percent effective where reasonable spray coverage was attained. However, the period when 

transpiration was effectively eliminated was 

probably no longer than about 7 or 8 days, as 

shown by the chronology of observations as 

follows:

September 17 Cottonwood grove sprayed with

defoliant.

18 No effect observed. 

21 Leaves falling rapidly. 

24 All affected leaves either down

or dry. Apparent that spray

coverage not sufficient in

densest parts of grove. 

28 New buds formed and leafing

started. 

October 2 Leafing well advanced. Some

leaves completely open. 

6 Foliage nearly as dense as

before spraying. 

12" Foliage back to normal density

The defoliant was not damaging to the vegetation, for foliage waa observed to be back to 

normal density in about 3 weeks.

When the vegetation in the downstream subreach is defoliated in June as planned, it is 

expected that a first spraying will be followed 

by a second about 3 days later to insure complete defoliation. Supplementary hand spraying of theunderstory vegetation will probably 

be needed also. After the initial sprayings, 

other dosings at about weekly intervals may 

be necessary to effect a leaf-free period of 

about 3 weeks in which to observe the watersaving effects of the treatment.

SUMMARY AND CONCLUSIONS

1. Cottonwood Wash flows in a relatively 

steep-walled trench in massive crystalline 

rocks* Subsurface inflow or outflow from 

these rocks appears to be negligible; however, 

inflow from terrace deposits along parts of 

the canyon walls may contribute measurable 

quantities of seepage to the stream after periods of high rainfall.

2. Evapotranspiration losses along the 

stream valley computed by subtracting 

stream outflow from stream inflow are subject to only small errors from failure to adjust for ground-water and soil-moisture 

storage.

3. Results to date show that a measurable 

quantity of ground-water seepage enters the 

study reach during the winter periods. This 

seepage is believed to be very small, however, during the summer periods when water 

budgeting for computation of evapotranspiration losses is critical.

4. Computed evapotranspiration loss in the 

4.1-mile project reach during the growing 

season, March through October, 1959, was 

175 acre-feet. This loss is about 33 percent 

of the water that entered the reach. Maximum 

rate of .loss during the season was slightly 

more than 8 acre-feet per week, or about 60 

percent of the inflow.

5. Seasonal losses in the upstream and 

downstream subreaches were 75 and 100 

acre-feet, respectively. Losses in the shorter 

downstream subreachwere larger than in the 

upper sub reach because of the greater plant 

population.

6. Net weekly changes in ground-water 

level measured in three transpiration wells 

was generally 0.05 foot or less during the 

water-budget weeks. Maximum diurnal range 

in stage in these wells was about 0.15 foot.

7. Typical maximum diurnal fluctuations 

in streamflow were as follows:

Stream gage 1 

Stream gage 3

0.6 to 0.85 cfs 

Nearly 0 to 0.6 cfs

8. During the spring of 1959 a pronounced 

diurnal pattern of streamflow was exhibited
SUMMARY AND CONCLUSIONS

prior to the leafing out of the vegetation. This 9. A test to determine an effective defolisuggeststhat direct evaporation of soilmois- ant to be used in the defoliating phase of the

ture from the valley-fill surface is an appre- project indicates that magnesium chlorate is

ciable part of the losses observed in the proj- highly effective on cottonwood trees. Howect reach and leads to the speculation -that ever, a single spraying eliminates the leaves

the water savings that will be accomplished for an effective period of about 7 or 8 days,

by modifying the riparian vegetation may not In project use, a multiple-spraying technique

be as great as hoped. will be used to prolong the leaf-free period.

INT.DUP.,D.C.60- »OB15