Soil-Moisture and 

Energy Relationships 

Associated With 

Riparian Vegetation Near 

San Carlos, Arizona

GEOLOGICAL SURVEY PROFESSIONAL PAPER 65S-E

Soil-Moisture and 

Energy Relationships 

Associated With 

Riparian Vegetation Near 

San Carlos, Arizona

By I. S. McQUEEN and R. F. MILLER

GILA RIVER PHREATOPHYTE PROJECT

GEOLOGICAL SURVEY PROFESSIONAL PAPER 655-E

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

First printing 1972 

Second printing 1979

For sale by the Branch of Distribution, U.S. Geological Survey 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Page

Abstract ___________________________..___ El

Introduction ___________________________ 1

Acknowledgments ____________________-__ 2

Theories of moisture retention and migration _ __ 2

Moisture retention ______________ __ 2

Capillary retention _________________ 2

Adsorbed-fllm retention _______________ 3

Osmotic potential _______________.___ 4

Pressure in the vapor phase ___________ 4

Moisture migration ____________________ 4

Migration as liquid, and the field-capacity

concept ________________________ 4

Migration as vapor _________________ 5

Procedures _____________________________ 6

Methods of measuring moisture-retention forces _.._ 6

Methods of measuring pressure in the liquid

phase ____________________ 7

Methods of measuring pressure in the vapor

phase _________________________ 8

Indirect methods of measurement _______ 8

Procedures Continued

Sampling program __________________

Field procedures _ .

Laboratory procedures ____ .

Results _________ _________________

Thermal data -

Soil-chemistry data ____ .

Moisture and moisture-stress data .

1962 saltcedar sites _ -

1962 mesquite sites _________________

1962 cottonwood sites _______________

1964 saltcedar sites _______________

1964 unshaded mixed-community site 

1964 willow site ____ __ - 

1964 cottonwood site _______________

1964 mesquite site _________________

Discussion of moisture use and dissipation ___ 

Phreatophytes and soil-moisture dissipation _ 

Conclusions -

Page

E8 

10

12

13

15

18

21

23

25

27

27

30

30

30

33

33

44

47

Bibliography ____ 49

ILLUSTRATIONS

Page

FIGURE 1. Index map of the area of confluence of the Gila and San Carlos Rivers, showing locations of sampling

sites ___________________________________________________________. E9 

2-6. Photographs showing sampling sites :

2. 62OT and 62UT ___________________________________________________ 10

3. 62OC and 62UC _______________________________________ - - 10

4. 62OM and 62UM ___________________________________________ - - 11

5. 64OS, 64UW, 64UC, and 64UM _____________________ _ - 11

6. 64OT and 64UT _____________________________________________________ 12 

7. Graph showing calibration relationships for wide-range gravimetric method of measuring soil-moisture

stress ___________________________________________________________- 13

8-10. Soil-temperature profiles :

8. Sites 62OT and 62UT, May; sites 64OT and 64TJT, July ___________ _ 15

9. Sites 64OS, 64UW, 64UC, and 64UM, May and July ______________________ 16 

10. Sites 64OT and 64UT, July and February__________________________________- 16 

11. Seasonal temperature fluctuations at selected depths at sites 64OT and 64UT __ _ 17 

12-20. Logarithmic profiles of the ratio of monovalent to divalent cations and the total soluble cations in soils:

12. Sites 62OT and 62UT, May _______________________________________________ 19

13. Sites 62OC and 62UC, May _____________________________________ 19

14. Sites 62OM and 62UM, May __________________________________________ _ 20

15. Site 64OT, May and July ________________________________________________ 21

16. Site 64UT, May and July _________.______________________________________ 22

III
IV CONTENTS

	Page

FIGURES 12-20. Logarithmic profiles Continued

17. Site 64OS, May and July _________-__ E22

18. Site 64UW, May and July ___________ - 23

19. Site 64UC, May and July ____________ 24

20. Site 64UM, May and July _______________ 24 

21-47. Profiles of soil-moisture-retention data:

21. MC, SMC, and pF profiles for site 62OT, May 25

22. MC, SMC, and pF profiles for site 62UT, May 26

23. ApF profiles for sites 62OT and 62UT, May 27

24. MC, SMC, and pF profiles for site 62OM, May ____________________ 28

25. MC, SMC, and pF profiles for site 62UM, May _____ _ - 29

26. ApF profiles for sites 62OM and 62UM, May ___________ __ ___ 30

27. MC, SMC, and pF profiles for site 62 OC, May 31

28. MC, SMC, and pF profiles for site 62UC, May 32

29. ApF profiles for sites 62OC and 62UC, May __________________ 33

30. MC, SMC, CMR, and pF profiles for site 64OT, May ________________ _ ____ 34

31. MC, SMC, and pF profiles for site 64OT, July _______ _ .. 35

32. ApF profiles for site 64OT, May and July _________________ _ 36

33. MC, SMC, CMR, and pF profiles for site 64UT, May _ 37

34. MC, SMC, and pF profiles for site 64UT, July _ _ ___ 38

35. ApF profiles for site 64UT, May and July __ _ __ 39

36. MC, SMC, CMR, and pF profiles for site 64OS, May ______ 40

37. MC, SMC, and pF profiles for site 64OS, July _______ ..___ 41

38. ApF profiles for site 64OS, May and July __ 42

39. MC, SMC, CMR, and pF profiles for site 64UW, May _____ _ 43

40. MC, SMC, and pF profiles for site 64UW, July ______ ________ ___ 43

41. ApF profiles for site 64UW, May and July _______ __________________ ____ 44

42. MC, SMC, CMR, and pF profiles for site 64UC, May ______________________________ 45

43. MC, SMC, and pF profiles for site 64UC, July _________________________________ 45

44. ApF profiles for site 64UC, May and July ____________________________ _ 46

45. MC, SMC, CMR, and pF profiles for site 64UM, May ____________________________ 47

46. MC, SMC, and pF profiles for site 64UM, July _______________________________ 48

47. ApF profiles for site 64UM, May and July _________________________________ _ 49

TABLES

Page

TABLE 1. Descriptions of sites sampled during this study ________________________________________ E14

2. Soil textures derived from field notes and laboratory tests _____________________________..___ 14

3. Comparison of depths at which MC equals CMR with depths at which stress equals pF 2.3 ______..___ 42
GILA RIVER PHREATOPHYTE PROJECT

SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED 

WITH RIPARIAN VEGETATION NEAR SAN CARLOS, ARIZONA

By I. S. McQuEEN and R. F. MILLER

ABSTRACT

Measurements of soil-moisture content and stress made under 

and near selected species of riparian trees in Arizona supply 

additional knowledge about the mechanism of moisture movement and utilization. Solar radiation provides energy to move 

soil moisture both upward and downward from the upper part 

of a soil profile. Profiles of chemical analyses show higher 

concentrations of salts at the surfaces of unshaded sites than 

at the surfaces of shaded sites, which indicates that evaporation loss is greater at unshaded sites than at shaded sites. 

This relation is confirmed by moisture-stress profiles. Profiles 

of soil-moisture stress under saltcedar and willow trees show 

no evidence of soil-moisture depletion by the trees. At one site, 

soil moisture increased during the season of maximum plant 

growth, even though a dense stand of annual grasses was 

growing on the site. The tree roots may have supplied moisture 

to the soils to maintain the soil moisture in hydraulic equilibrium with the water table. Cottonwood and mesquite trees, 

however, deplete soil moisture even when grourd water is 

available.

INTRODUCTION

The use and dissipation of water by riparian vegetation (phreatopliytes) in the arid regions of the Western United States has become a subject of intense study. 

A joint study of the Safford Valley in Arizona by the 

U.S. Geological Survey and the U.S. Army Corps of > 

Engineers in the early 1940's focused attention on the 

Gila Kiver and suggested the possibility of salvaging 

the water used by phreatopliytes. The Gila River 

Phreatophyte Project, initiated by the U.S. Geological 

Survey in May 1962, was designed to study the effects 

on the water supply of the basin caused by replacing a 

large stand of salt-cedar (Tartiarix pentandra. Pall.) 

with a beneficial vegetation (perennial grasses). Studies of soil-moisture and energy relationships were corducted by the authors as a supplement to the major 

study.

In the principal investigation, a 25.7-kilometer reach 

of the Gila River above its confluence with the San 

Carlos River was divided into three subreaches that 

were instrumented for water-budget determinations. 

The two upper subreaches have wide, naturally developed flood plains with at least one alluvial terrace, 

and the flood plains slope gently toward the channel. 

The lower subreach is a flat plain formed by sediment 

deposition in the San Carlos Reservoir. The upper 

subreaches contain 2,630 hectares of phreatopliytes, of 

which about half are saltcedar and half are mesquite. 

The lower subreach contains 810 hectares of saltcedar.

Stream flow is gaged at four stations on the Gila 

River and ait 62 stations on tributaries. Ground-water 

levels are measured in 72 wells, and soil moisture is 

estimated at 72 locations with neutron-scattering soilmoisture meters. Radiation, wind, humidity, and 

temperature are measured at Coolidge Dam for energybudget computations. Supplementary energy-budget 

measurements are made with a portable station.

Objectives of the Gila River Phreatophyte Project, 

as summarized by the project chief, R. C. Culler 

(1965), are

(1) to evaluate water conservation by phreatophyte control 

on a flood plain typical of many areas of existing and proposed applications;

(2) to describe the hydro logic and ecologic variables of th^ 

project area for the purpose of transposition of water 

conservation evaluations to other sites;

(3) to test and develop methods for evaluating hydrologi" 

variables on a large area.

El
E2 GILA RIVER PHREATOPHYTE PROJECT

In connection with the third objective of the Gila 

River Phreatophyte Project, the authors conducted 

the supplementary investigation reported here. This 

investigation had several interrelated objectives. (1) 

Measure the moisture and energy gradients in the soils 

under and adjacent to the dominant types of riparian 

vegetation in the Gila River Phreatophyte Project 

area. (2) Determine the effect of shading on soilmoisture migration and dissipation by comparing soilmoisture and energy data from adjacent shaded and 

unshaded sites. (3) Determine the moisture-use characteristics of dominant riparian species by comparing 

soil-moisture and energy data obtained before and 

after the annual maximum-growth season. (4) Interpret shading and moisture-use data in terms of probable effects of vegetation modification on the water 

supply. (5) Develop, calibrate, and test equipment and 

methods for measuring moisture and energy gradients 

in the soil. (6) Define relationships between moisture 

migration through soils and patterns of soil chemistry 

under and adjacent to dominant types of riparian vegetation in the Gila River Phreatophyte Project area.

This report presents a new look at soil moisture in a 

semiarid flood-plain environment and the role of 

phreatophytes in moisture utilization. Some of the 

methods used are new and have not yet received general acceptance. These methods produce data that have 

not been obtainable by commonly used methods and 

require new forms of data presentation and analysis. 

Therefore, there may be no background for parts of 

this report in bibliographies of previous investigations.

Traditional soil-physics terminology has been modified to conform more closely to dictionary definitions 

and to permit bringing together two theories of moisture retention. It is hoped that the data, methods, and 

interpretations reported herein will be useful to those 

studying moisture and energy relationships in soils.

Water use by phreatophytes has been studied by 

several agencies using various types of evapotranspirometers, including tanks, tents, weather instruments, 

and lysimeters. Many of these devices are discussed in 

other chapters of this professional paper series. For 

details of laboratory techniques used in measuring or 

controlling the moisture-retention forces in soils, the 

reader is referred to numerous reports, such as those by 

Richards (1949), Slater and Bryant (1946), and Eckardt (1960). The effects of temperature gradients on 

soil moisture have been reported by Gardner (1955), 

Rao and Ramacharlu (1955), Philip and DeVries 

(1957), Hutcheon (1958), Kulick (1967), and others. 

Basic theories of soil moisture and its energy have been 

discussed by Edlefsen and Anderson (1943) and summarized recently by Remson and Randolph (1962),

Stallman (1964), and Kuzmak and Sereda (19E8). In 

the present study, a new method (McQueen and Miller, 

1968) was used to measure the moisture-retention forces 

existing in soil samples obtained under vegetation and 

in adjacent clear areas; the measurements were then 

used to define the probable effect of tree removal on the 

thermal and moisture regimes.

ACKNOWLEDGMENTS

Special thanks are extended to L. M. Shown, who 

assisted in the field sampling, and to F. R. Steputis, 

who assisted in maintenance and reading of field instruments. Karl Ratzlaff and William Buller assisfed in 

laboratory analyses of soil samples. The authors gratefully acknowledge constructive criticism and review of 

the manuscript by L. M. Shown, M. L. Sorey, R. C. 

Culler, R. L. Hansen, J. W. Gary, R. L. Harlan, R. F. 

Hadley, C. C. McDonald, and D. A. Webster.

THEORIES OF MOISTURE RETENTION 

AND MIGRATION

An extensive review of the literature on soil-moisture 

retention and migration disclosed considerable confusion and some disagreement on basic theories. Published methods for estimating moisture moveirent in 

the unsaturated zone are applicable only to homogeneous isotropic sediments with known hydrologic 

properties, but one dominant characteristic of floodplain soils is heterogeneity. For these reasons and because in the present study the moisture stresses above 

a shallow water table were measured for the first time 

by a wide-range method, this part of the report is devoted to soil-moisture theories and their application 

to a flood-plain environment.

MOISTURE RETENTION

Two of many possible forces for retaining water in 

soil seem to be dominant and amenable to study in 

the range of temperatures and of pressures normally 

found in natural soils: the capillary retention force, 

and the adsorbed-film retention force. Water of crystallization, or combined water, and water trapped 

within impermeable rock units are not usually considered important. Osmotic potentials influence pressures 

and moisture movement in the vapor phase, so they 

must be considered in all measurements .of moistureenergy relationships.

CAPILLARY RETENTION

The primary characteristic of a capillary-force system is a curved air-liquid interface, usually with at-
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E3

mospheric pressure on the concave or vapor side of the 

interface and with negative pressure, equal to the 

capillary stress, on the convex or liquid side. The radius 

of curvature of the interface is related to the moisture 

stress by the equation for the height of rise in a capillary tube:

h = 2r/rpg 

where

h = stress, or height of rise,

T = surface tension,

r = radius of curvature of the meniscus,

p = density of fluid, and

g = gravitational constant.

This equation usually includes the cosine of the angle 

of wetting. However, for water and soils the capillary 

meniscus is anchored to a multimolecular film adsorbed 

to the solid particles, and the angle of wetting must be 

nearly zero; therefore, its cosine is 1 (Remson and 

Randolph, 1962, p. 6; Stallman, 1964, p. 12, 13). For 

porous media with complex curved interfaces the 

radius of curvature is usually expressed as an equivalent radius.

The curved air-water interface of a capillary meniscus is the expression of a pressure difference between 

the air and the water. Before the capillary formula is 

applied indiscriminately, pressure, temperature, and 

phase relationships of water must be studied. Basic 

physics states that whenever the saturation vapor pressure of a liquid exceeds the confining pressure on the 

liquid, bubbles of vapor form within the liquid as the 

liquid changes phase, or starts to boil (Worthing and 

Halliday, 1948, p. 302). This condition can be induced 

either by heating the liquid to a higher saturation 

vapor pressure or by reducing the confining pressure to 

the saturation vapor pressure.

When water is at 20 °C the saturation vapor pressure is 17.5 mm (millimeters) of mercury or 23.77 cm 

(centimeters) of water. Standard atmospheric pressure 

at sea level is 1,032.27 cm of water. Therefore, in a 

capillary system at 20°C with standard atmospheric 

pressure on the vapor side, whenever the tension or 

pressure reduction within the water approaches 1,008.5 

cm, which is equivalent to 23.77 cm absolute pressure, 

the water must change phase or boil, and the capillary 

menisci can no longer exist. This suggests a limit on 

the capillary moisture stress and on the height of capillary rise. Dissolved gases may start to form bubbles 

within the liquid at tensions below this limit. Bolt 

and Miller (1958) discussed the possible existence of 

tensions in soil water and suggested that soil water 

should have tension limitations similar to those in 

other hydraulic systems. They stated on page 928:

"We suspect that the fraction of water which is actually in tension is never large except perhaps at very 

high water contents and small tensions." They concluded: "We regard apparently large values of soil 

moisture tension or soil suction to be artifacts accounted for by the influence of the electrical double 

layer" [adsorption to surfaces].

ADSORBED-FILM RETENTION

Moisture is attracted to and adsorbed on particle 

surfaces by molecular forces. The quantity of moisture 

adsorbed is a function of the attractive forces between 

molecules, but it may be influenced by electrical charges 

and chemical valence forces. Widtsoe (1914) discussed 

soil moisture from the standpoint of film thickness, 

indicating several stages in water movement over soil 

grains. Meinzer (1923b, p. 18-22) discussed the molecular forces of water in rocks and included several 

diagrams illustrating basic principles. He indicated 

(p. 20) that "the force of molecular attraction is believed to extend through several times the space occupied by one molecule * * * ." The thermodyramics 

of adsorbed films of moisture was discussed extensively by Edlefsen and Anderson (1943, p. 195-243).

Langmuir (1916 and 1917) proposed that mc ;sture 

was adsorbed to soil-particle surfaces by chemical 

valence forces only and, therefore, could only H one 

molecular layer thick. Smith (1936) and other proponents of the universal application of the capillary 

theory still accept this monolayer limitation on adsorption in spite of evidence for multimolecular adsorption 

presented by workers in physical chemistry (Young 

and Crowell, 1962).

Capillary water bodies and adsorbed multimolecular 

films are both involved in soil-moisture retentior. The 

problem is defining the range of applicability of each 

theory. Data obtained with the method used in this 

study to measure moisture tensions indicate tl at at 

stresses above pF 2.30 (0.2 bar) 1 moisture is retained 

primarily as films adsorbed on the particle surfaces. A 

capillary meniscus (at sea level with water at 20°C) 

can exist at stresses below a theoretical upper limit of 

pF 3 (1 bar). Therefore, the two theories overlap in 

the range from pF 2.3 to 3.0 (0.2 to 1 bar). This is a 

small part of the total range of stresses, but it is important. Russell (1959) said: "50 percent or more of 

the water used by plants is held at tensions less than 1 

atmosphere."

1 pF Is the common logarithm of the moisture tension m?asured 

in centimeters of water (Schofleld, 1935).
GILA RIVER PHREATOPHYTE PROJECT

OSMOTIC POTENTIAL

Osmotic pressure as applied to soils is defined as:

That pressure to which a pool of water, identical in composition to the soil water, must be subjected in order to be 

in equilibrium, through a semipermeable membrane, with a 

pool of pure water (semipermeable means permeable only to 

water). [Osmotic pressure] may be identified with osmotic 

potential * * *. (Soil Science Society of America Committee 

on Terminology, 1965, p. 348.)

To this definition should be added the concept that 

where two solutions are separated by a semipermeable 

membrane, the pressure difference is proportional to 

the difference in concentration (osmotic potential) of 

the two solutions.

In order for a pressure to exist because of minerals 

dissolved in water, there must be a difference in the 

dissolved-mineral concentrations of two solutions separated by a semipermeable membrane. Plant tissues 

may act as semipermeable membranes and, therefore, 

may develop osmotic gradients that aid or resist absorption of water from the soil. Salt concentrations in 

plant fluids will usually be slightly higher than those 

in the soil moisture, so the osmotic pressure gradients 

will usually aid moisture flow into the plant. Osmotic 

gradients will usually be small because concentration 

differences are usually small.

An air-water interface is a semipermeable membrane 

with zero osmotic potential on the vapor side and the 

full solution potential on the liquid side. The osmotic 

gradient in this case will oppose evaporation and may 

induce condensation if the vapor pressure of the air 

approaches saturation.

Certain pseudomembranes associated with differences 

in salt concentrations at liquid-solid interfaces have 

been described (Day and others, 1967; Gary and Taylor, 1967). However, the presence and function of the 

pseudomembranes in transporting moisture have not 

been adequately defined by controlled experiments. 

Kemper (1961) pointed out that osmotic gradients are 

probably not very effective in transporting water 

where moisture stress is less than 1 bar (pF 3.0). No 

data were obtained during this study to further define 

moisture movement in response to osmotic pressure 

gradients.

PRESSURE IN THE VAPOR PHASE

The vapor pressure of the soil atmosphere is a function of the forces retaining moisture in the soil. This is 

true if the moisture is present only as adsorbed films, if 

capillary moisture is present, or if osmotic forces 

are present.

The relationship between vapor pressure and 

moisture stress has been presented by numerous authors, including Edlefsen and Anderson (1943), Smith 

(1936), Remson and Randolph (1962), Stallman 

(1964), and Croney and others (1952) :

h = -(RT/Mg}\HP/P0 , 

where

h = moisture stress in centimeters of water, 

R = gas constant (8.315 X107 ergs mole"1 °C'1 ), 

T = absolute temperature of soil moisture (°K), 

M = molecular weight of water (18.0 g (grams)

mole"1 ),

g = gravitational constant (981 ergs g"1 cm"1 ), 

In = natural or Naperian logarithm (base e), 

P = partial pressure of water vapor in soil

atmosphere (millibars), and 

P0 = vapor pressure over pure distilled water.

At 20°C, P0 = 23.347 and pF = 6.502 +log 

(1.368 log P), where log = common logarithm (base 

10).

MOISTURE MIGRATION

Moisture migrates through soils as a liquid or as a 

vapor in response to energy gradients. Liquid and vapor migration can be in concert or in opposition. In 

order to predict or define moisture migration, the 

gravitational, moisture-stress, and thermal -energy 

gradients, as well as the liquid and vapor conductivities of the. soils, must be defined.

MIGRATION AS LIQUID, AND THE FIELD-CAPACITY 

CONCEPT

In any hydraulic system, flow occurs whenever a 

pressure difference is not-compensated by a positional 

(elevation) difference. Moisture-stress measuremer ts 

are usually referred to an arbitrary datum of a flat 

free-water surface. In a system containing a water 

table or a phreatic surface, this surface can be used as 

the datum. When the moisture retention force, either 

capillary or aclsorptive, is equal to the height above the 

water table, then the moisture-stress in the sample is 

said to be in equilibrium with the gravity potential. 

Departures from equilibrium (differences between the 

stress in the sample and its height above a water table) 

represent either excess moisture that is on its way to 

the water table or deficiencies in soil moisture induced 

by evapotranspiration processes. Moisture-stress mersurements must be adjusted for height above (or belov) 

the datum to define energy gradients.

The term "field capacity" has been defined as uthe 

amount of water held in the soil after the excess gravi-
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E5

tational water has drained away and after the rate of 

downward movement of water has materially decreased, usually 2 or 3 days after irrigation." (Veihmeyer and Hendrickson, 1931, p. 181). This definition 

implies that at a certain moisture content in a soil 

there is an abrupt change in moisture flow and probably a change in the moisture stress-moisture content 

relationship. Two values are given for moisture stress 

at field capacity. One-third atmosphere (pF 2.52) 

is the accepted value for normal, or fine-grained, soils, 

and one-tenth atmosphere (pF 2.00) is the value for 

sandy soils. Data obtained in Geological Survey laboratories (Prill and Johnson, 1959; McQueen, 1963) 

point out the reason for a range of values. The 

standard centrifuge moisture-equivalent test, used to 

establish relationships between moisture stress and field 

capacity or specific retention, permits heating which 

dehydrates sand samples. Also, clay samples may not 

reach equilibrium in the time allowed for a standard 

centrifuge test (Croney and others, 1952). When 

temperature and humidity in the centrifuge are controlled and samples are run for 6 hours instead of 

1 hour, there is little difference in stress between coarse 

and fine samples.

Meinzer (1923a), in discussing subsurface water, 

divided the zone of aeration into three belts: the belt 

of soil water, the intermediate belt, and the capillary 

fringe.

The belt of soil water is that part of the soil subjected to moisture depletion by evaporation and transpiration. Moisture content and retention forces vary 

with the seasons and with the utilization characteristics 

of the plants. The depth of this belt is limited to that 

part of the zone of aeration occupied by plant roots or, 

if there are no roots, to that part of the profile influenced by diurnal temperature fluctuations.

The capillary fringe is that part of the zone of aeration immediately above the water table where the 

moisture content exceeds field capacity.

The intermediate belt is a moisture transmission zone 

between the belt of soil water and the capillary fringe. 

Moisture-stress values are thought to be relatively constant, and moisture content is assumed to be a function 

of soil characteristics. Meinzer (1923a, p. 26) referred 

to the intermediate belt as "the residual part of the 

zone of aeration." He described the belt of soil water 

and the capillary fringe as having finite thicknesses 

and the intermediate belt as the part between the two, 

if any exists. Water is supposed to move through the 

intermediate belt under gravity gradients and then only 

when the moisture content exceeds field capacity.

The wide-range method for measuring the moisture 

stress existing in soil samples, combined with standard

methods for estimating field capacity, permits a new 

look at moisture relationships in an arid flcod-plain 

environment and enables an evaluation of the fieldcapacity concept.

Data obtained in this study indicate a change in 

energy status of soil moisture at a stress of 0.21 bar 

(pF 2.32) (McQueen and Miller, 1968). TTe have 

chosen this value (pF 2.32) as the best estimate of 

the stress at field capacity.

The intermediate belt, if it exists, should appear on 

the pF profile as an appreciable depth increment at a 

pF of about 2.3, with increasing stress abov^ it and 

decreasing stress below it. The capillary fringe may 

extend as much as 2 meters above the water table. The 

belt of soil water is often less than 2 meters d«-ep, but 

it may extend as deep as 20 to 25 meters if there are 

deep-rooted plants present (Meinzer, 1927).

MIGRATION AS VAPOR

Numerous investigators have shown that thermal 

gradients are capable of moving moisture through soils. 

Kates of moisture movement are often much greater 

than those predicted by theory (Philip and DeVries, 

1957; Hutcheon, 1958). There are indications that 

most of the moisture moved by thermal gradients migrates in the vapor phase (Kuzmak and Sereda, 1958) 

and that temperature has little influence on cr.pillaryheld water. Hutcheon (1958, p. 130) concluded :

A temperature gradient imposed on a soil column, iritially of 

uniform moisture content and temperature, will create a relatively large gradient of vapor pressure from warm to cold 

regions, but only a very slight gradient of moisture tension 

within the liquid phase in the same direction.

An examination of two basic formulas shows why this 

must be true. In the capillary formula h = Qr/rpg, only 

the surface tension (T) and the density (p) are influenced by temperature. Dorsey (1940, p. 516) gave 

the rate of change of surface tension with temperature 

at 20°C as 0.151 dyne per centimeter per degree 

Celsius ( 0.00015 gram per centimeter per degree 

Celsius). The density of water does not change at a 

uniform rate, but at 20°C its rate of change is given as 

 0.00002 gram per cubic centimeter per degree Celsius. 

The effect of a small temperature change on th^ capillary tension is obviously small.

In the equation for the relationship between 

total stress and vapor pressure of the soil air 

(h = RT In PV/PQ), a small change in temperature 

may cause a relatively large change in stress, because 

the variables are all temperature dependent. Tv<?, effect 

of temperature on moisture films adsorbed on particle 

surfaces is not known because adsorption relationships 

are not well defined.
E6 GIL A RIVER PHREATOPHYTE PROJECT

Although numerous studies have been made of 

moisture movement under thermal gradients, the exact 

mechanisms involved are not well defined or understood. One thing common in these studies is that the 

moisture moved by a thermal gradient exceeds that 

allowed by theories of vapor diffusion.

Hutcheon (1958, p. 131) said: "For moisture conditions above the permanent wilting percentage, the observed coefficients of vapor diffusion were from 6 to 

8 times greater than those calculated from known relationships for isothermal conditions."

Philip and DeVries (1957) tabulated data from several previous reports and computed ratios of the observed movement to the value predicted by simple 

theory. They then proposed a theory of combined liquid 

and vapor movement that included condensation into 

one side of a capillary "contact" water body, flow 

through the water body, and evaporation on the other 

side of the water body. Their, theory would have been 

more plausible if they had allowed flow of multimolecular adsorbed films instead of trying to restrict liquid 

movement to flow through capillary water bodies that 

probably do not exist at stresses of pF 3.0 (1 bar) and 

certainly do not exist at the higher stress they cited.

Thermal energy is supplied to the soil surface by 

solar radiation and conduction from the air in a complex cyclic function involving annual and diurnal 

primary frequencies modified by an irregular stormfrequency pattern and possibly by longer climaticcycle patterns. Some energy is available from below the 

soil as indicated by the geothermal gradient. Energy 

is removed from the soil by evaporative cooling and 

by radiation. Attenuation of the cyclic patterns of 

thermal-energy that move moisture in soils is influenced 

by the amount of moisture available for movement and 

by the frequency of the energy fluctuations. The diurnal cycle, having the higher frequency, is attenuated 

at shallower depths than the annual cycle and, therefore, influences less of the soil mantle. The effectiveness of cyclic temperature fluctuations in transporting 

soil moisture may depend upon the moisture content 

and the depth of the water table, as well as the frequency of the fluctuations. The direction of movement 

will usually be in the direction of decreasing temperature, but this may be modified by stress gradients.

Whenever moisture is moved through a porous medium there is a concurrent transfer of energy. When 

the movement includes capillary flow or film flow, there 

is a very small change of hydraulic energy to heat 

because of frictional resistance. If the movement involves evaporation and condensation, the heat of vaporization and condensation must be included in the 

energy transferred. When temperature and energy

measurements can be made with sufficient precision, 

flow of soil moisture may be defined by thermoine^ric 

measurements. Unfortunately, the precision required 

was beyond the limitations of this study.

PROCEDURES

Procedures used in this study were developed in the 

Soil and Moisture Conservation Program of the IT.S. 

Geological Survey for use on arid lands. Many of the 

procedures are based on popular or standard argicultural methods, but some were developed to cover the 

broader scope of conditions that exist on range lands.

Soils on river flood plains are often extremely variable. Cobbles and clays may be found adjacent to e^ch 

other. The sampling procedures and laboratory 

methods were chosen to define this variability and to 

define energy relationships without interference from 

the variability. Sampling depth increments were kept 

small enough to define trends and abrupt changes. 

Each increment was handled as an individual sample.

The usual methods involve defining the moistureretention characteristics of representative samples, 

measuring the moisture content, and then determining the stress from the characteristic curve. THse 

methods were considered inadequate. The wide-range 

method used in this study permits measuring both the 

moisture content and the moisture tension of each 

sample.

As part of a continuing program of development 

and testing of soil-moisture instrumentation and 

methods, four experimental plastic columns, each containing 42 electrical-resistance moisture-stress transducers and 42 thermistors, were installed on May 10 

and 11, 1964, in the sampling pits for sites 64OT and 

64UT 2 in section 23 of the Gila River Phreatopl yte 

Project area. These columns were later destroyed by 

submergence in rising waters of the San Carlos Reservoir. Before their destruction they yielded valuable 

information oil temperature gradients, and they enabled a qualitative evaluation of electrical-resistrnce 

moisture instruments.

METHODS OF MEASURING MOISTURE-RETENTION T 

FORCES

A graph showing the relationship between the 

moisture content of a soil and the magnitude of the 

forces retaining the moisture in the soil, commonly 

called a moisture-retention curve or a moists re2 Site number code consists of sampling year (62 = 1962), shading 

on site (O, unshaded; U, shaded), and trees associated with site 

(T, saltcedar; C, cottonwood; M, mesquite; W, willow; and S, 

mixed community).
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E7

characteristic curve, is essential in estimating moisture 

movement in soils. Many laboratory procedures have 

been developed to define this relationship. A few 

methods have been developed for measuring existing 

moisture-retention forces in the field. The principles 

involved in these measurements are discussed here because they aid understanding of moisture movement, 

and they help to explain the writers' choice of methods. 

The forces of moisture retention can be measured 

directly as the pressure in the liquid phase or as the 

pressure in the vapor phase, or they can be measured 

indirectly as some moisture-dependent property of an 

inert porous material in contact with the soil moisture. 

Some examples of available methods and their characteristics follow.

METHODS OF MEASURING PRESSURE IN THE 

LIQUID PHASE

The capillary tensiometer, which was developed at 

Utah State Agricultural College under the direction 

of J. A. Widtsoe, measures negative pressure within 

the liquid phase. A porous bulb in contact with the 

soil is connected to a manometer or aneroid vacuum 

gage. The bulb and connecting lines are filled with 

distilled water, and the system is sealed. Water flows 

from the bulb into the soil until the negative pressure 

in the instrument equals the capillary tension in the 

soil water. The tensiometer is limited to measuring a 

stress of less than about pF 2.9 (0.8 bar) because 

vapor bubbles form in the instrument when the absolute pressure in the water approaches the saturation 

vapor pressure of the water. The porous bulb of a 

tensiometer tends to act as an imperfect osmotic membrane; therefore, the instrument may respond to the 

osmotic component of the soil moisture until diffusion 

of salts through the cup equalizes the solution 

concentrations.

A tension table is a porous surface used in laboratories to impose a negative pressure on the water in 

soil samples. The samples are placed on the porous 

surface and a negative pressure is maintained beneath 

the porous surface by means of a hanging water column or a controlled vacuum pump. The tension table 

is subject to the same stress limitation as the tensiometer. However, it is normally operated well below 

its limit. (60 cm of water, or pF 1.8, has become a 

standard stress in many laboratories).

Pressure methods, suggested by Woodruff (1940) 

and developed by Kichards (1941, 1947), are similar to 

the tension-table method in that they involve the application of pressure differentials to soil samples to 

establish points on the moisture-characteristic curves 

of the soils. There is one important difference between

the two methods. In the tension-table method, decreased pressure is applied to the liquid phase, and 

the sample and its vapor phase are allowed to remain 

at atmospheric pressure; in the pressure methods, 

increased pressure is applied to the sample and its 

vapor phase, and the liquid phase is allowed to remain 

at atmospheric pressure. Allowing the liquid phase/ to 

remain at atmospheric pressure bypasses the capillarylimit problem, but it introduces an error into the results that has been noted and discussed by Chahal r.nd 

Yong (1965). Within the pressure device, the capillary 

water, retained as rings around contact points between 

particles, develops radii of curvature that are smaller 

than those permitted by the capillary limit. When the 

samples are returned to atmospheric pressure, vapor 

bubbles form in the capillary-retained water, and the 

rings are disrupted so that the water contained in 

them flows into the adsorbed films. The films are tHn 

thicker than they should be for the pressure difference 

applied, and the actual stress is lower than the intended stress.

The centrifuge has been used in soils laboratories 

since the early 1900's to determine the moistureholding power of soils. Two methods are in use. On e, 

called the moisture-equivalent test, was proposed by 

Briggs and McLane (1907); the other employs modeling techniques in which the soil sample becomes p^rt 

of a model of a soil column with a controlled water 

table. The moisture-equivalent test determines one 

point on the retention-relationship curve, whereas the 

modeling techniques are capable of defining the relationship from pF 2.3 to about pF 4.5. Higher values 

are possible, but the time and energy requirements become prohibitive.

In the moisture-equivalent test, capillary moisture 

is removed readily from soils by centrifugal force, 

whereas adsorbed films are removed slowly if at all. 

By properly choosing the amount and duration of 

energy applied, the capillary moisture can be removed 

and the adsorbed films left. The moisture content retained against centrifugal force is called the moisture 

equivalent and is accepted as <a laboratory estimate of 

the field capacity, or the moisture a soil will retain 

against gravity drainage.

In the centrifuge modeling techniques, soil samples 

are supported on porous blocks or sand beds in special 

centrifuge cups. A drain is provided so that water 

remains in the outer ends of the cups, and a controlled 

water table or zero stress datum is simulated. The 

centrifugal force applied to the model distorts the 

equilibrium stress profile so that the stress at a given 

distance from the zero datum is equal to the distance 

multiplied by the relative centrifugal force. A sample
E8 GILA RIVER PHREATOPHYTE PROJECT

supported 5 cm from the cup drain when spun at a 

relative centrifugal force of 1,000 times the force of 

gravity would have an equilibrium stress of 5,000 cm, 

or pF 3.7.

Results of centrifuge moisture measurements are influenced by temperature and humidity in the centrifuge; speed and duration of centrifuging; dimensions 

of the sample, sample container, and centrifuge head; 

and the time allowed between stopping the centrifuge 

and weighing the sample.

Freezing-point methods, such as the dilatometer 

technique described by Bouyoucos (1917), measure 

pressure in the liquid phase. The freezing temperature 

of water decreases as the pressure on the water increases, so a lowered freezing point would be found in 

water that is retained under positive pressure. Moisture adsorbed to particle surfaces is under positive 

pressure, but capillary absorbed moisture is under 

negative (gage) pressure or tension; therefore, a lowered freezing point could not be a result of capillary 

absorbed moisture. Dissolved salts also lower the 

freezing point of water, so the osmotic component 

would be included in freezing-point methods. The 

relationship between the freezing point and the moisture stress is given (Croney and others, 1952) as

where 

h

h = L(t0 -t}/Tg,

= moisture stress in centimeters of

water, 

L = latent heat of fusion of pure water

(3.336X109 ergs g-1 ),

(to £) = freezing-point depression (°C or °K), 

T = absolute temperature of freezing

point of pure water (273°K), and 

g = gravitational constant (981 ergs g"1

cm"1 ).

On the pF scale

pF = 4.095 + Iog10 (£0 t).

METHODS OF MEASURING PRESSURE IN THE 

VAPOR PHASE

Measurements of pressure in the vapor phase are 

possible with several types of instruments. However, 

in order to measure moisture stress, extreme accuracy 

must be possible in the range of P/P0 from 0.98 to 

1.00 (98 to 100 percent relative humidity). An instrument that has been successful in laboratory use is the 

thermoelectric psychrometer (Monteith and Owen, 

1958; Richards and Ogata, 1958; Spanner, 1951). Accuracy requirements have so far prevented development 

of a successful field instrument. Laboratory measurements of stresses below pF 3 should be considered of 

questionable accuracy because they are near the limits 

of accuracy of the best laboratory equipment.3

INDIRECT METHODS OF MEASUREMENT

Moisture-retention forces can be measured by indirect 

methods employing materials such as electrical resistance blocks (gypsum, fiberglass, and nylon, for example), gravimetric blocks, air permeability cones 

(Kemper and Amemiya, 1958), and, as in this study, 

filter paper (McQueen and Miller, 1968). All these 

methods are dependent upon the stability and accuracy 

of the calibrations of the instruments. Usually, F, direct 

determination of the quantity of water absorbed by an 

inert absorbent will be more accurate than a secondary 

calibration, such as electrical conductivity or air permeability. Characteristics of indirect methods are varied. In general they are less accurate than direct 

methods, but they cover a wider range of stresses

The wide-range gravimetric method, which WTS used 

in this study, uses filter paper as sensors and measures 

stress from pF 0 to pF 6.2 (0 to 1,500 bars). Data 

obtained with this method, including the data presented in this report, show that its accuracy is good 

and that the variability between samples is small.

SAMPLING PROGRAM

Records of precipitation for southern Arizona show 

that a dry period usually exists from May to tl °> middle of July. Trees along the Gila River start to leaf 

out in April and May and reach their full growth by 

July. Consequently, all sampling for this study was 

conducted during the months of May and July.

Initial sampling on May 26-30, 1962, was of shaded 

and unshaded sites associated with saltcedar (Tamarix 

pentandra Pall.; fig. 2), cottonwood (Populus fremontii Wats.; fig. 3), and honey mesquite (Frosopis 

juliflora (Swartz) DC; fig. 4). The two saltcedar sites 

were near section 11 of the Gila River Phreatophyte 

Project area (Culler, 1965). The cottonwood sites were 

near where U.S. Highway 70 crosses the San Carlos 

River at Peridot. The mesquite sites were soutt of the 

Southern Pacific Railroad bridge on the San Carlos 

River. Site locations are shown on the map in figure 1.

In 1964, four sites near the 1962 cottonwood sites 

were chosen so that one was unshaded, a second was 

shaded by a willow tree (Salix gooddingii Ball), a third

3 At pF 3.0, P/Po 0.99927, wet bulb depression is less than 

0.01 °C, and the emf produced by the thermojunctions is about 0.6 

microvolts.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E9

be

c

1a
E10 GILA RIVER PHREATOPHYTE PROJECT

FIGURE 2. Sampling sites associated with salteedar trees near 

section 11 of the Gila River Phreatophyte Project area. 

Sampled May 30, 1962. a, unshaded site 62OT; ft, shaded 

site 62UT.

was shaded by a cottonwood tree, and a fourth was 

shaded by a mesquite tree (fig. 5). These sites were 

sampled May 13-14, when the trees were leafing out, 

and again July 18-20, when moisture use was beginning to decline.

Two sites were selected near section 23 of the Gila 

River Phreatophyte Project area, one of them shaded 

by a salteedar tree and the other one in the open 

(fig. 6). They were sampled May 10-11 and July 

16-17,1964.

FIELD PROCEDURES

At each of 18 sites sampled during this study, soilmoisture samples were obtained from a column of soil 

extending from the surface to the water table. Tinned 

sample boxes were forced into the undisturbed soil 

and then excavated to provide relatively undisturbed 

samples. A disk of treated filter paper was placed in 

each sample, and the sample boxes were sealed with 

plastic tape to prevent moisture loss during transport 

and storage. Additional soil to be used for chemical 

and other laboratory analyses was obtained at each 

sampling depth. Soil temperatures were measured in 

place with either a dial thermometer or a thermister 

probe. Root concentrations, soil horizons, and soil

B

FIGURE 3. Sampling sites associated with cottonwood trees 

on the San Carlos River flood plain. Sampled May 28-29, 

1962. A, unshaded site 62OC; B, shaded site 62UG.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION Ell

FIGURE 4. Sampling sites associated with mesquite trees on 

the San Carlos River flood plain. Sampled May 26-27, 1962. 

A, unshaded site 62OM; B, shaded site 62UM.

B

FIGURE o. Sampling sites associated with willow, cottonwood, 

and mesquite trees on the San Carlos River flood plain. 

Sampled May 13-14 and July 18-20, 1964. A: a, unshaded 

site 64OS; &, shaded site 64UW under willow trees; d, shaded 

site 64UM under mesquite trees. B: a, unshaded site 64OS; 

c, shaded site 64UC under cottonwood trees.
E12 GILA RIVER PHREATOPHYTE PROJECT

FIGURE 6. Sampling sites and experimental moisture-stress 

and temperature measuring units associated with saltcedar 

trees near section 23 of the Gila River Phreatophyte Project 

area. Sampled May 10-11 and July 16-17, 1964. a, unshaded 

site 64OT; 6, shaded site 64UT.

structure were determined by visual observation of 

the profile. Soil textural classification was determined 

by standard field methods (U.S. Dept. Agriculture, 

1951; Shaw, 1928).

LABORATORY PROCEDURES

Tests to determine the chemistry of the soils were 

similar to those described in U.S. Department of Agriculture Handbook 60 (Richards and others, 1954) except as noted. Saturated soil paste was prepared from 

samples obtained for chemical analysis (method 2). 

Paste conductivity (method 5), paste pH (method 

21a), and saturation moisture content (method 27a) 

were determined. For the conductivity measurements, 

a two-electrode probe was used in place of the Bureau 

of Soils electrode cup. Saturation extracts were obtained using a pressure extractor. Soluble calcium plus 

magnesium was determined by titration with versenate, 

and soluble sodium was measured with a Beckman 

Model DU flame photometer.

Centrifuge moisture retention was determined for 

the May 1964 samples using a moisture-equivalent 

centrifuge wT ith a drum head and square soil boxes 

that held samples weighing 40 to 50 grams. This permitted larger grain sizes to be included in the samples. 

The air in the centrifuge was maintained at 20 °C 

and 100 percent relative humidity, and a relative 

centrifugal force of 1,000 times the force of gravity 

was applied for 6 hours (McQueen, 1963).

Moisture contents and moisture tensions were determined using the wide-range gravimetric method 

described by McQueen and Miller (1968). Moisture 

samples were stored in a constant-temperature chamber 

at 20°C (± 2°C) for at least 2 weeks to allow equalization of moisture-retention forces between the soil 

sample and the filter-paper sensor disk. After equilibration, the disks were separated from the samples 

and placed in small weighing boxes. Moisture contents 

of the samples and of the sensor disks were determined 

gravimetrically. Moisture stresses were computed from 

the moisture contents of the filter papers using the calibration relationships:

pF = 6.24617 7.23 M

for disks with moisture-weight ratios (M) less 

than 0.54,

and

pF = 2.8948 1.025 M

for disks with moisture-weight ratios (M) greater 

than 0.54.

These relationships are shown in figure 7 along with 

confidence limits determined from the data by statistical analyses.

Any given measurement consists of the true value 

plus or minus each of the inaccuracies involved in the 

measurements. In soil samples obtained from a continuous column, the differences between measurements 

on contiguous samples include the difference between 

true values plus or minus the inaccuracies. As the 

depths of sampling increments are reduced by increasing the number of samples taken, the differences between true values approach zero, and the differences 

in measured values approach the total variability of 

the measuring methods. The standard deviation of the 

differences between the measured stresses in contiguous 

samples becomes an indication of the variability of 

the method used to measure stress.

Differences in moisture contents of the sensor disks 

from contiguous samples were computed for the 18 

soil columns sampled during this study. These differences were grouped according to the moisture content 

of the disk from the deeper sample. The class intervals 

in the range from 0 to 55 percent moisture content 

were 5 percent, and from 55 to 205 percent they were 

10 percent. The mean and the standard deviation were
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E13

CALIBRATION RELATIONSHIPS 

WITH CONFIDENCE LIMITS

1.0 1-5 

MOISTURE-WEIGHT RATIO (M)

FIGURE 7. Calibration relationships for wide-range gravimetric method of measuring soil-moisture stress. ConfHence-limit 

envelope (. shaded) represents one standard deviation of total variability on both sides of calibration line.

computed for each class interval. These standard deviations were plotted on both sides of the calibration 

curve (fig. 7) to indicate a confidence-limit envelope 

for the wide-range gravimetric method used in this 

study for measuring moisture stress.

The differences between the true values for the 

samples cannot be distinguished from the measured 

differences because of practical limitations on sampling. 

So, in this analysis, the indicated variability exceeds 

the actual variability. Because the indicated variability 

is generally less than that for other available methods, we consider the confidence limits shown to be 

acceptable.

Moisture- stress measurements made for this study 

are reported as the log of the stress, in centimeters of 

water, or pF as proposed by Schofield (1935). If the 

log of the height above a water table is subtracted 

from the log of the stress, the difference is the log of the 

ratio of the measured stress to the equilibrium stress:

S'

log $ log H = log -jj = ApF tL

In order to simplify chart headings, we have labeled 

this ratio ApF. Where ApF is negative, the stress is 

less than equilibrium and there is an excess of moisture. Where ApF is positive, the stress is higher than 

equilibrium and there is a moisture deficiency. Measurements showing equilibrium with, or departures from 

equilibrium with, a water table have not been made 

successfully in the past. The wide-range filter-paper 

method used in this study for measuring moisture 

stress makes these measurements possible.

RESULTS

Descriptive data are given in table 1 for each site 

sampled during this study. Locations are given to the 

nearest minute of latitude and longitude. Section, 

township, and range are not given because the standard land grid has not been established for the Indian 

reservation lands upon which this study was conducted. 

Elevations were estimated from topographic maps. 

Relative saturation data were computed from moisture 

contents and saturation moisture capacities as a dimensionless ratio.
E14 GILA RIVER PHREATOPHYTE PROJECT 

TABLE 1. Descriptions of sites sampled during this study

Location

Site 

No.

62OT 

62UT 

62OM 

62UM 

62OC 

62UC 

640T

64UT 

64OS 

64UW 

64UC 

64UM

Lat (N.)

33°12' 

33°16' 

33°17'30" 

33°13' 

33° 13' 

33°17'30" 

33°17'30" 

33°17'30" 

33°17'30"

Long

(W.)

110°27' 

110°27' 

110°22' 

110°22' 

110°27' 

110°27' 

110°27' 

110°27'

Elevation 

(ft)

2,520 

2,540 

2,500 

2,460 

2,460 

2,500 

2,500 

2,500 

2,500

Date 

sampled

May 30, 

do

May 27, 

May 26, 

May 28, 

May 29, 

May 10, 

July 16, 

May 11, 

July 17, 

May 13, 

July 19, 

May 13, 

July 18, 

May 14, 

July 20. 

May 14, 

July 20,

1962

1962 

1962 

1962 

1962 

1964 

1964 

1964 

1964 

1964 

1964 

1964 

1964 

1964 

1964 

1964 

1964

Trees

Saltcedar _ 

do

do

Cottonwood _ 

-_ do

__ do __ 

__ do __ 

__ do __ _ Mixed _

__ do __

___ do ____ 

Cottonwood _ 

__ do __ 

Mesquite ___ 

___ do __

Shading 1

O 

U 

0

u 

o 

u

0

o 

u 

u 

o

0

u

o 

u 

u 

u 

u

Ground cover

Shrubs and annual weeds __

do _

do

dododo _ . _

do

_ do _

do

Depth 

to

water 

(cm)

159 

160 

400 

420 

175 

174 

198 

290 

190 

290 

140 

180 

130 

180 

140 

165 

130 

175

Depth 

sampled 

(cm)

180 

180 

393 

440 

180 

190 

200 

210 

210 

210 

140 

180 

125 

175 

135 

165 

125 

175

Relative 

saturation (dinnnsionl<?ss)

0.734 

.573 

.373 

.473 

.232 

.401 

.417

.514

!320 

.5f7 

.471 

.5?9

6i4

1 0 = unshaded; U = shaded.

TABLE 2. Roil textures derived from field 

laboratory tests

Site Date 

No. sampled

62OT May 30, 1962

62UT May 30, 1962 

R2OM May 27, 1962

R2UM May 26, 1962

R20C May 28, 1962 

62DC May 29, 1962

Depth 

increment 

(cm)

0-7 

7-80 

80-90 

90 105

105-140 

140-180

0-13 

13-24 

24-120 

120-180

0 30

30 75

75-95 

95-125 

125-145 

145-175 

175-200 

200-265 

265-400

0 105

105 120 

120-135 

135-155 

155-175 

175-195 

195-235 

235-440

0-30 

30-60 

60-180

0-17 

17 30

30-45 45 SO

80 105

105-175 

1 75 1 90

Soil 

texture

Fine sand

Silt loam ____

Clay

Sand and gravel

Silt loam

Sandy loam _ 

Clay __ _

Silt loam

Clay _ _ _____ 

Silt loam ___

Sandy loam ___ 

Clay __ - -

Sandy loam ___ 

Sand

Silt loam __

Sandy loam _ _

Silt loam

Loam _ _

Sandy loam ___ 

Sand and gravel

Sandy silt loam 

Loam to sand _ 

Sand and gravel

Sandy silt loam 

Silt loam

Sandy loam ___

Sand

Pnnrsf> sand

notes

Average 

saturation moisture 

capacity 

(g/g)

0.48 

.32 

.37 

.24 

.60 

.20

.51 

.40 

.33 

.60

.37 

.50 

.41 

.52 

.40 

.33 

.54 

.30 

.20

.42 

.26 

.35 

.19 

.39 

.49 

.34 

.19

.36 

.40 

.22

.37 

.43 

.36 

.44 

.37 

.27 

23

and

Particle 

surface 

area 

(mVg)

146 

72 

96 

23 

190

< 10

157 

111 

76 

190

96 

154 

114 

162 

111 

76 

169 

60

< 10

120 

37

87

< 10

106 

152

82

< 10

91 

111

< 10

97 

125 

92 

129 

97 

42 

^ 1O

TABLE 2. Soil texture derived from field 

laboratory tests Continued

Depth 

Site Date increment 

No. sampled (cm)

64OT May 10, 1964 0-5 

5-20 

20-60 

60 70

70-100 

100-120 

120-190 

190-205

July 16, 1964 0-10 

10-20 

20-110 

110-140 

140-170 

170-195 

195-205 

205-210

64UT May 11, 1964 0-5 

5-15 

15-25 

25-75 

75-80 

80-130 

130-180 

180-210

July 17, 1964 0-5 

5-15 

15-100 

100-135 

135-165

165-180 

180-210

640S May 13, 1964 0-65 

65-140

July 19, 1964 0-20 

20-60 

60-85 

85 175

Soil 

texture

Clay _

Silt loam _

Sandy loam ___

Silt loam

Sand and gravel

Sandy silt loam 

Fine sand __ __ 

Sandy loam ___ 

Coarse sand _ 

Sand to silt ___ 

Sandy loam _ 

Sand

Clay (organic)-

Sandy loam _ _ Sand __ _ __

__ do ______ 

Sandy loam _ 

Sand

Silt loam __

Clay (organic)- Silt loam _ _

Sandy loam _ _ 

Sand to loam _ 

Loam to silt

Fine sand _

Fine sand __ _

Silt loam _

Sandy loam ___ 

Sand

notes and

Average 

saturation moisture 

capacity 

(g/g)

0.61 

.40 

.32 

.19 

.32 

.40 

.20 

.40

.52 

.35 

.27 

.36 

.23 

.36 

.35 

.26

.85 

.39 

.28 

.21 

.33 

.36 

.22 

.37

.59 

.40 

.30 

.41

.44 

.30 

.42

.40 

.30

.54 

.45 

.34 

.26

Particle 

surface 

arm

(mVg)

192 

111 

72

< 10

72 

111

< 10

111

162 

87 

43 

92

< 10

92

87 

37

290 

106 

49

<r 10

77 

92

< 10

96

186 

111 

60 

116

129 

60 

120

111 

60

169 

134

82 

36
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E15

TABLE 2. Soil textures derived from field notes and, 

laboratory tests Continued

Average 

saturation moisture Particle 

Depth capac- surface 

Site Date increment Soil ity area 

No. sampled (cm) texture (g/g) (m2/g)

64UW May 13, 1964 0-40 Loam 0.50 154

40-75 Clay _ _ .63 200

75-125 Sandy loam __ .31 65 

July 18, 1964 0-45 Loam _ .50 154

45-80 Clay _ _ .63 200

80^140 Sandy loam __ .34 82 

140-175 Sand ___ __ .25 23

64UC May 14, 1964 0-15 Clay loam ___ .53 166 

15-30 Silt loam _ _ .44 129

30-45 Clay .58 183

45-95 Fine sand ___ .27 43 

95-105 Silt loam _ .39 106

105-115 Sandy loam __ .31 65 

115-135 Sand __ _ .23 10

July 20, 1964 0-15 Clay loam __ .57 180

30-85 Clay ___ - .65 208

85-165 Sand __ __ .25 23

64UM May 14, 1964 0-35 Loam __ _ __ .52 162

35-80 Sand __ __ _ .24 24

80-85 Silt loam _ _ .38 102

85-125 Sand __ ___ .26 36

July 20, 1964 0-35 Loam ___ - .52 162

35-45 Silt loam ___ .45 134

45-80 Sand ____ .27 43

80-90 Sandy silt loam .35 87 

90-155 Sandy loam __ .29 54 

155-175 Sand ___ _ __ .25 23

Flood-plain soils are extremely heterogeneous because they form on the deposits of aggradation mixed 

with the residues of violent flood events. The soil textures in each soil column are summarized in table 2. 

The textural designations were determined from field 

observations and saturation moisture capacities. The 

particle surface areas were estimated from the saturation moisture capacities using a tentative statistical 

relationship. 

Laboratory and field data obtained from soil 

samples during this study are presented graphically 

as function profiles. Individual data points are plotted 

at the midpoints of the sampling depth increments, 

or the data are given in bar graphs where the bar 

width represents the sampling depth increment. Occasionally an error in a laboratory measurement or in 

a computation created an apparently erroneous point 

on a profile. Where no source of error could be found 

in the data, the values were retained. 

Three general types of data are presented: thermal, 

chemical, and moisture.

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tuat 

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umn 

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THERMAL DATA

oil temperatures are a dynamic variable that flir 

es with the energy available and the changing 

mal properties of the soil. Records of soil tempera 

s are significant only if they illustrate principle 

Ived in energy transmission. For this reason, a temperature measurements have been selected to shov 

cal patterns of energy transfer within the soil, 

ril temperatures were obtained during sampling a 

ites. Data from selected sites are presented in fig 

8 and 9. 

our experimental moisture-stress measuring col 

s installed at sites 64OT and 64UT near sectioi 

i May 1964 included individual thermisters at 5-cn

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0 20 30 4 

TEMPERATURE, IN DEGREES CELSIUS

E 8. Soil-temperature profiles for shaded and unshaded 

s associated with salteedar trees near sections 11 and 23 

the Gila River Phreatophyte Project area. Sites 62OT 

62UT sampled May 30, 1962; sites 64OT and 64TJT 

ipled July 16-17, 1964.
E16 GILA RIVER PHREATOPHYTE PROJECT

depth increments. Selected data from these units are 

presented in figures 10 and 11.

Conduction of heat in a solid or a porous mass is not 

instantaneous but progresses at a finite rate; so if a 

variable source of heat is applied to a surface, a temperature profile within the mass will be a record of the 

fluctuations in heat applied at the surface. Because 

some heat is absorbed or used in the solid, the temperature record is attenuated so that at some depth in the 

mass no changes in temperature can be detected. The 

soil temperature profiles in figures 8, 9, and 10 show 

diurnal temperature cycles as sharply attenuated sine 

waves superimposed on the longer wave length annual 

temperature cycles. The sampling depth was too small 

to include more than a small fraction of the annual 

cycle.

Evaporation of soil moisture is inhanced by cyclic 

fluctuations in soil temperature, so the depth required 

to attenuate the diurnal temperature cycle may be the 

effective depth of evaporative moisture depletion. Complete attenuation of the diurnal fluctuations may be assumed where their wave forms cannot be distinguished 

from the annual fluctuation wave form. These depths, 

as indicated in figure 9 for the mixed community, 

were about 40 cm at the shaded sites and about 60 cm 

at the unshaded site. These depths are corroborated by 

data from other projects where continuous temperature 

recordings have been made, by depths of diurnal influence frequently reported in the literature, and by the 

depths of evaporative loss inferred from the stress 

data of this report.

The seasonal pattern of temperature fluctuations for 

five depths shown in figure 11 indicates a reversal of 

gradients in September or October. The other reversal 

occurs in April or May, although it was not shown in 

the data obtained. The temperature profiles obtained 

in May 1962 and 1964 (figs. 8 and 9) show almost uniform temperature with depth, and this event would 

occur at the time of gradient reversal.

Moisture moves in the direction of decreas; ng temperature. In the top 40 to 60 cm, movement vould be

u

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t i i i 1 i i i i i i i i i 1 i i i i i i i i i 1 i i i i i i i i i

10 20 30 4(

TEMPERATURE, IN DEGREES CELSIUS

FIGURE 10. Soil-temperature profiles obtained wit]

10 20 30 

TEMPERATURE, IN DEGREES CELSIUS

40

FIGURE 9. Soil-temperature profiles for shaded and unshaded 

sites associated with willow, cottonwood, and. mesquite 

trees on the San Carlos River flood plain. Sites 64OS, 64FW, 

64FC, and 64UM sampled May 13-14 and July 1S-20, 1964.

thermisters in experimental moisture-stress me?suring 

columns installed at sites 64OT and 64UT near section 

23 of the Gila River Phreatophyte Project area. A 

temperature profile obtained by manual sampling 

methods at the unshaded site 64OT is included for 

comparison of methods. Measurements were obtained 

on July 16, 1964, and February 19, 1965.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION

40

E1T

10

MAY JUNE JAN FEB 

1965

FIGURE 11. Seasonal temperature fluctuations at selected .depths for the period May 15, 1964, to February 

25, 1965, at sites 64OT and 64UT near section 23 of the Gila River Phreatophyte Project area. Measurements were obtained with thermisters in experimental moisture-stress measuring columns.
E18 GILA RIVER PHREATOPHYTE PROJECT

downward during the day and upward during the 

night. Below 40 to 60 cm, movement in response to 

thermal gradients would be downward during the 

summer and upward during the winter. The quantity 

of moisture moved during a single diurnal cycle is not 

measurable with any method presently available. Also, 

the quantity of moisture moved by thermal gradients 

annually is not measurable because it is usually masked 

by recharge events and moisture utilization by plants. 

Elimination of shading would result in increased 

downward movement of moisture in the vapor phase 

in response to increased thermal gradients. However, 

this downward movement in response to thermal gradients may be offset by increased capillary flow upward 

where the water table is less than 2 meters below the 

surface.

SOIL-CHEMISTRY DATA

Studies of the ratio between monovalent and divalent soluble cations (Na+1/Ca+2 + Mg+2 ) and the TSC 

(total soluble cations) measured with depth in soils 

indicate that a chemical imprint is produced in the 

soil by migrating moisture. Some basic principles, 

which apply to the processes involved, have been reported by Miller and Ratzlaff (1965) :

1. The chemical pattern left in soils by moving moisture is influenced more by drying processes than 

by wetting processes.

2. Ion-exchange phenomena cause an increase in monovalent ions and a concomitant decrease in 

multivalent ions in the direction of moisture 

movement.

3. If water that contains an excess of monovalent ions 

is being supplied, principle 2 may be reversed, 

and monovalent ions may decrease in the direction of water movement (Brooks and others, 

1958).

4. Where water movement occurs as film flow at tensions greater than 1 bar, a process of "salt sieving" (Kemper, 1960; Miller and Ratzlaff, 1965) 

may cause a decrease in TSC in the direction of 

water movement.

5. Where moisture movement is dominated by movement in the vapor phase, salts are left behind because their concentration soon exceeds solubility 

levels and they precipitate out. The result is an 

increase in salts in the direction of water movement. Such a concentration of salts is often found 

near the soil surface, where evaporation occurs.

Interpretations of water-movement patterns have 

been made from profiles of soluble-sodium percentage, sodium-adsorption ratio, cation-valence ratio, 

and pH compared with TSC. The cation-valence 

ratio (Na + 1 /Ca+2 + Mg +2 ) and the TSC 

(Na+1 + Ca+2 + Mg+2 ) are plotted as logarithms 

against depth, in meters, in figures 12 through 20. In 

each figure, data for two related profiles are plotted 

for easy comparison.

In a soil column with a shallow water table (less 

than 2 meters), little drying may occur beyond a stress 

of 0.2 bar except at the surface, where evaporation 

withdraws moisture and stress exceeds 0.2 bar; so 

principle 1 may apply only near the surface. The salt 

sieving of principle 4 may only operate near the surface, where tensions exceed 1 bar.

Principles 2 and 3 may yield opposite interpretations, depending upon the cation concentration of the 

water supplied. The ground water in the Gilp. and 

San Carlos River flood plains is variable chemically, 

so interpretations of chemistry profiles become extremely complex. Because of these and other cons; derations, detailed analyses of chemical data by the methods of Miller and Ratzlaff (1965) were considered 

impractical at this time. However, the data arc presented to make them available for further study.

Soluble salt concentrations may be greater at or 

near the soil surface than at depth owing to either of 

two processes: capillary supplied evaporation, or guttation (exudation). Where the water table is ner.r the 

soil surface, evaporation of water supplied from the 

saturated zone by capillary rise will leave a deposit of 

salts near the surface. This should produce the highest concentration of salts at unshaded sites at the 

depths where evaporation occurs at the most rapid 

rate. Some species of trees are known to guttate or 

exude salt solutions, whereas other species transpire 

only pure water vapor. Salts will be deposited on the 

surface of soils under those trees that guttate. The 

profiles of total soluble cations presented in figures 12 

through 20 show the results of these processes.

Capillary rise and evaporation have increased the 

salts near the surface of unshaded sites on the, San 

Carlos River. See figure 13 and compare figure 17 with 

figures 18, 19, and 20. Capillary rise is not effective 

where the water table is deeper than 2 meters below 

the surface. See figures 14, 15, and 16.

Salts are deposited on the soil surface under saltcedar trees. See figure 12 and compare figures 15 and 

16. Cottonwood trees also appear to deposit salts on 

the soil surface by guttation. See figure 13 and compare figure 19 with figures 18 and 20. There is no evidence of salts being deposited on the soil surface 

under willow and mesquite trees. See figure 14 and 

compare figures 18 and 20 with figures 15, 16, and 17.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E19

^ A*1 0i 0i v< ' i s ' *A ' r

A A AA A * A A A

~i i i i r~i r

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A A

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A 00$ A > A A 

*V A ^ V EXPLANATION

v A A 

A A *

A 0 A A Ratio: Na+VCa+2+Mg+2, shaded site

A 0 AA 0

A A Ratio:Na+i/Ca+2+Mg+ 2 , unshaded site

I * VA . AA * A

0v . Total soluble cations, shaded site

	A 

> OA A A A

* A A Totah soluble cations, unshaded site

0v A A

A A

0v A A

v A

i - i i i i i i i I________i i i i i i L_l_l I I l l I I l I

0.1 1 10 100 1000 

RATIO Na+1/Ca+2+Mg+2 AND TOTAL SOLUBLE CATIONS, IN MILLIEQUIVALENTS PER LITER

FIGURE 12. Logarithmic profiles of the ratio of monovalent to divalent cations and the TSC for shaded and unshaded sites 

62UT and 62OT associated with saltcedar trees near section 11 of the Gila River Pheatophyte Project area. Sampled 

May 30, 1962.

~~A yi i i i i i i n i i i i i i i n -f i r^ i i i r

*A ^ S AA A "

A * A 0 A A A A

A 

A

A A

EXPLANATION v 0 A A

v A 0 A A

Ratio: Na+1/Ca+2 + Mg+ 2 , shaded site A 0 A A

A v v A A

Ratio: Na+1/Ca+2 + Mg+2, unshaded site A 0 A A

t

1 A

Total soluble cations, shaded site 0 A

v A A

A

A v

Total soluble cations, unshaded site

0v AA 

AA

I I I I I I I I________I____i i i i i i i I________|_____i i i i i

0.01 0.1 1 10 100 

RATIO Na+i/Ca+ 2+Mg + 2 AND TOTAL SOLUBLE CATIONS, IN MILLIEQUIVALENTS PER LITER

FIGURE 13. Logarithmic profiles of the ratio of monovalent to divalent cations and the TSC for shaded and unshaded sites 

62Trr and 62OC associated with cottomvood trees on the San Carlos River flood plain. Sampled May 28-29, 1962.
E20 GILA RIVER PHREATOPHYTE PROJECT

The chemical quality of the ground water appears 

to control the chemical imprint in the soils for an appreciable height above the water table. The quality of 

the ground water is reported to be extremely variable, 

and this variability makes interpretations difficult. 

Gatewood, Robinson, Colby, Hem, and Halpenny

(1950, p. 61) said: "An outstanding characteristic 

of the Gila Eiver in Safford Valley is the rapidity 

with which the concentration of dissolved solids in 

the water may change * * *." Hem (1950, p. 49) in 

discussing the chemical quality of water in the shallow alluvium of the Gila Eiver, said:

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EXPLANATION A<> A A

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Unshaded 0

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3 

0.01 0.1 1 10 100 

RATIO Na + l/Ca+2+Mg+2 AND TOTAL SOLUBLE CATIONS, IN MILLIEQUIVALENTS PER LITER

1000

FIGURE 14. Logarithmic profiles of the ratio of monovalent to divalent cations and the TSC for shaded and unshaded sites 

62UM and 62OM associated with mesquite trees on the San Carlos River flood plain. Sampled May 26-27, 1962.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E21

The shallow ground waters from various parts of the valley, 

however, differ greatly in chemical character and concentration, and in a few instances water from a single well has been 

known to change 50 percent or more in concentration over a 

period of a few months.

A comparison between shallow ground water in the 

San Carlos River and that in the Gila River is difficult because of this variability. However, Hem (1950) 

did indicate that the clissolved-solids concentrations in 

the San Carlos waters may have a higher sodium 

ratio. These characteristics are evident in the chemical 

data obtained in this study.

MOISTURE AND MOISTURE-STRESS DATA

Moisture-tension profiles have been obtained in the 

past only from tensiometer installations. These profiles have been limited in detail and have included 

tensions only up to the practical limit of the instruments, which is about 0.5 bar. Attempts have been 

made by previous workers to estimate moisture 

stress from moisture-content measurements by using

moisture-retention curves for "representative" samples, 

but natural variability and hysteresis factors have 

made results unsatisfactory. The wide-range gravimetric method used in this study for measuring 

moisture stress provides accurate detailed stress data 

over the total range of naturally occurring stresses.

Two useful measures of the moisture-retaining capacity of a soil are the CMR (centrifuge moisture retention) and the SMC (saturation moisture capacity). 

These properties are statistically related, and er°h is 

valuable for analysing the hydrology of soils. Both 

properties were determined for the samples obtained 

in May 1964. For all other samples, the CMR wrs not 

determined. The MC (moisture content) and the 

pF (moisture stress) existing in the samples at the 

time of sampling are presented in this section along 

with the CMR and the SMC. The CMR and SMC are 

both measures of the soil texture and are included to 

help explain the seeming disparity between MC and 

pF. This disparity exists because of variability in soil 

texture.

2 -

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EXPLANATION V v v V

V ' V

v v

0 V V 

Ratio: Na+VCa+2+Mg+2 May V ^ v

v V 

0 v v 

v v 

Ratio: Na+VCa+2+Mg+2, July V v

v v

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Total soluble cations, May v ^7

v V v v 

Total soluble cations, July V

^ V 

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

10 100 10

RATIO Na+1 /Ca+2+Mg+2 AND TOTAL SOLUBLE CATIONS, IN MILLIEQUIVALENTS PER LITER

FIGURE 15. Logarithmic profiles of the ratio of monovalent to divalent cations and the TSC for unshaded site 64OT associated with saltcedar trees near section 23 of the Gila River Phreatophyte Project area. Sampled May 10 and July 16, 1964.
E22 GILA RIVER PHREATOPHYTE PROJECT

A A T I

* A * T * V

A 

A. * 

A TV

VT

A A i V T

, » *« « V T*

. A * A % V V

 * « TV

 * VT 4 * v T

A ^ EXPLANATION v T

A V T

A A -u

A A Ratio: Na+i/Ca+2 + Mg+2, May ^ T

f A V

A A A V T

4 AA V

Y A* Ratio: Na+VCa+ 2 +Mg+2, July ^V T

f . A v T

A * T TV

A . A V w

" V A Total soluble cations, May TV

» J '. ' T-" '

^ A Total soluble cations, July

V T

T

V T 

V T

1 10 100 

RATIO Na+VCa+2 + Mg+"' AND TOTAL SOLUBLE CATIONS, IN MILLIEQUIVALENTS PER LITER

FIGURE 16. Logarithmic profiles of the ratio of monovalent to divalent cations and the TSC for shaded site 64UT under saltcedar trees near section 23 of the Gila River Phreatophyte Project area. Sampled May 11 and July 17, 1964.

EXPLANATION

0 

Ratio: Na+ VCa+ 2 +Mg+2 , May

0 

Ratio:Na+i/Ca+2+Mg + 2, July

1 10 100 

RATIO' Na+l/Ca+2+Mg+2 AND TOTAL SOLUBLE CATIONS, IN MILLIEQUIVALENTS PER LITER
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E23

Stress data obtained during this study were compared with stress data for equilibrium conditions. For 

hydraulic equilibrium, the measured moisture stress 

for a sample should equal the height of the sample 

above the water table. If the log of the height (in 

centimeters) of a sample above the water table is subtracted from the pF, the remainder is the log of the 

ratio of measured stress to equilibrium stress. We have 

labeled this ratio ApF to signify departures from 

equilibrium stress. When Schofield (1935, p. 46) introduced the concept of pF, he said, "In all cases we 

can apply the principle that water will not move except 

where there is a pF gradient (due allowance being 

made for gravity)." The ApF computation is one 

way of making allowance for gravity.

Because ApF is a log-ratio function, variability 

would appear greatest near the water table, where

effects of errors or fluctuations in water-table elevation 

would be greatest.

Several principles of moisture movement and utilization are illustrated by the moisture data of this 

study. A detailed examination of the moisture, and 

stress-departure profiles should help to define these 

principles.

1962 SALTCEDAR SITES

Saltcedar sites sampled in 1962 (figs. 21, 22, and 

23) show the influence of abrupt changes in soil texture where the water table may be changing and vhere 

a recharge event has occurred relatively recently (in 

terms of days). Moisture in clay layers at depths from 

0.8 to 0.9 meter and 1.05 to 1.4 meters at the unshaded 

site, and from 1.2 to 1.8 meters at the shaded site, 

was not at equilibrium with the water table. During

I I I 1 I I I I I I

EXPLANATION

Ratio: Na+i/Ca+2 + Mg+2, May

Ratio: Na+i/Ca+2+Mg+2, j u | y

V 

Total soluble cations, May

V 

Total soluble cations, July

I T V 

T V 

V

77 T

T v

TV 

TV

TV

V i 

V

VT 

W

V 

V 

V 

V

V T 

VT

V

T

VT 

V

I I I I I I I I I

T 

T 

T

T 

V

I I I I I I

0.1 1 10 

Ratio Na+VCa+2+Mg+2 AND TOTAL SOLUBLE CATIONS, IN MILLIEQUIVALENTS PER LITER

100

FIGURE 18. Logarithmic profiles of the ratio of monovalent to divalent cations and the TSC for shaded site 64TJW rnder 

willow trees on the San Carlos River flood plain. Sampled May 13 and July 18, 1964.

FIGURE 17. Logarithmic profiles of the ratio of monovalent to divalent cations and the TSC for unshaded site 64OS associated with willow, cottonwood, and mesquite trees on the San Carlos River flood plain. Sampled May 13 and July 19, 1964.
E24 GILA RIVER PHREATOPHYTE PROJECT

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DEPTH

2

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EXPLANATION -

V 4"

Ratio: Na+VCa+2 + Mg+2, May

?

Ratio: Na+VCa+2 + Mg+2, July ~

T

Total soluble cations, May

~

v

Total soluble cations, July

-

ii i l IIIIM

100 1COO

RATIO Na+l/Ca+2+Mg+2 AND TOTAL SOLUBLE CATIONS, IN MILLIEQUIVALENTS PER LITER

FIGURE 19. Logarithmic profiles of the ratio of monovalent to divalent cations and

cottonwood trees on the San Carlos River flood plain. Sampled May 

o 

C/3

CC

U

UJ

2 1

i"

t

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f. Illlll 1 1 1 1 1 1 1 1 1 1 1 ^1 1

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1 1 1 l i 1 1 l l i 1 l l i i i i ilil

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the TSC for shaded site 64UC under

14 and July 20, 1964.

 i i i y i i i i i iii

_

_

EXPLANATION

4

Ratio: Na+VCa+2+Mg+2, May

*,

Ratio: Na+VCa+2+Mg+2 , July ~~

T

Total soluble cations, May

V

Total soluble cations, July

-

iiil i i 111111

100 1000

RATIO Na+i/Ca+2+Mg+2 AND TOTAL SOLUBLE CATIONS, IN MILLIEQUIVALENTS PER LITER

FIGXTRE 20. Logarithmic profiles of the ratio of monovalent to divalent cations and the TSC for shaded site 64UM under mesquite trees on the San Carlos River flood plain. Sampled May 14 and July 20, 1964.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E25

the sampling, water rose in the auger hole when the 

coarse material below the clay layers was penetrated. 

These clay zones also contained fewer roots than the 

more permeable parts of the material at the sites. The 

moisture-content profiles show no indication of moisture deficiencies in the clay zones, but the pF and 

ApF profiles show higher stress in these zones.

Moisture from 0.05 to 0.3 meter at the shaded site 

was depleted and had a stress level of pF 3.5 to 4.2, 

indicating moisture loss by transpiration. The dense 

stand of annual grasses visible in figure 2 is probably 

responsible for this depletion. The zone of depletion is 

defined on the stress-departure profiles (fig. 23).

1962 MESQUITE SITES

No evidence of flooding or recent recharge was 

found at the 1962 mesquite sites, and the moisture

data show a moisture deficiency (figs. 24, 25, and 2").

Moisture stresses were higher at the unshaded rfte 

than at the shaded site. The moisture stress at depths 

from 0.60 meter to 2 meters at the unshaded site v^as 

nearly constant at pF 4.4 (25 bars). This value indicates depletion by a plant with high moisture-stress 

competence. At the shaded site the stress was nearly 

constant at pF 3.9 (8 bars) from 0.50 meter to 1.5 meters and at pF 3.5 (3 bars) from 1.7 to 3.2 meters. Th°.se 

values tend to indicate a lower stress competence for 

the plant roots at the shaded site. This situation might 

be explained by the field observation that the roots at 

the shaded site were mature primary roots and those 

at the unshaded site were active root hairs. Moisture 

had been depleted to a greater depth at the shaded rite 

than it had at the unshaded site, as shown in the 

stress-departure profiles.

0.2 0.3 0.4 0.5 

MOISTURE-WEIGHT RATIO

234 

pF (MOISTURE STRESS)

FIGURE 21. Profiles of MC, SMC, and pF at unshaded site 62OT associated with saltcedar' trees near section 11 of the.

Gila River Phreatophyte Project area. Sampled May 30, 1962.
E26 GILA RIVER PHREATOPHYTE PROJECT

-Water table

o.i 0.2 0.3 0.4 0.5 

MOISTURE-WEIGHT RATIO

234 

pF (MOISTURE STRESS)

FIGURE 22. Profiles of MC, SMC, and pF at shaded site 62UT under saltcedar trees near section 11 of the Gila River

Phreatophyte Project area. Sampled May 30, 1962.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E2T

Z 1

t

-WET DRY

EXPLANATION

 

Site 62UT, shaded

o 

Site 62OT, unshaded

O

o

-10 1 2345 

DEPARTURE FROM EQUILIBRIUM (ApF)

FIGUEE 23. Profiles of ApF for shaded and unshaded sites 

62UT and 62OT associated with saltcedar trees near section 

11 of the Gila River Phreatophyte Project area. Sampled 

May 30, 1962.

1962 COTTONWOOD SITES

Cottonwood sites sampled in 1962 (figs. 27, 28, r,nd 

29) show depletion of soil moisture by grasses and annual weeds and loss by evaporation. The unshaded site 

had a nearly constant stress of pF 4.2 (20 bars) at 

depths from 0.2 to 0.55 meter. The shaded site showed 

two stress levels: pF 4.0 (10 bars) from 0.15 to 0.4 

meter and pF 3.5 (3 bars) from 0.45 to 1.0 meter. The 

top 0.1 meter of the unshaded site appears to have 

been influenced by a light rainstorm.

1964 SALTCEDAR SITES

Moisture at the 1964 saltcedar sites (figs. 30, 31, 32, 

33, 34, and 35) remained at equilibrium with the 

water table except near the surface, where diurnal 

temperature fluctuations probably increased evaporation loss. The unshaded site showed a decrease in 

soil moisture in the top 0.60 meter during the growth 

period, from May to July. This loss appears to hr.ve 

been due to increased evaporation. The shaded site 

showed an increase in soil moisture in the top 0.4 

meter during the growth period. There is no record 

of significant recharge of soil moisture either from 

precipitation on the sites or from flooding during the 

period May 15 to July 15, 1964. A light rainstorm on 

the night of July 17-18, 1964, influenced the soil moisture near the surface of the unshaded site. The shaded 

site was sampled prior to the rain, so it was rot 

influenced.

A high water flow of 1,000 cubic feet per second in 

the Gila River occurred about 12 hours before the 

July 16 sampling of the unshaded site. Although 

neither the unshaded nor the shaded site was inundated by the flooding, some excess moisture, perhaps 

due to the flooding, was recorded from 1.2 to 1.3 

meters at the unshaded site on July 16 and at 1.1 meters and below 1.8 meters at the shaded site on July 17
E28 GILA RIVER PHREATOPHYTE PROJECT

MC 

SMC

Water table

0.2 0.3 0.4 

MOISTURE-WEIGHT RATIO

0.6 0 234 

pF (MOISTURE STRESS)

FIGURE 24. Profiles of MC, SMC, and pF at unshaded site 62OM associated with mesquite trees on the San Carlos

River flood plain. Sampled May 27, 1962.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E29

MC 

SMC

 Water table 

o.i 0.2 0.3 0.4 

MOISTURE-WEIGHT RATIO

0.6 0 234 

pF (MOISTURE STRESS)

FIGURE 25. Profiles of MC, SMC, and pF at shaded site 62UM under mesquite trees on the San Carlos River flood p'ain.

Sampled May 26, 1962.
E30 GILA RIVER PHREATOPHYTE PROJECT

-WET

Z 2

O

o*

DRY-

 O

 O

V

o 

o

o

o

o 

o 

o 

o 

o 

o

o

o

o

o

EXPLANATION

Site 62UM, shaded

O Site 62OM, unshaded _

 0

.

o 

 

o

-101234 

DEPARTURE FROM EQUILIBRIUM '(ApF)

(figs. 32 and 35). Stress departures were computed 

from ground-water levels prior to the flooding.

1964 UNSHADED MIXED-COMMUNITY SITE

The stress-departure profile for the May 1964 sampling at the unshaded mixed-community site shows 

the effect of the spring runoff in the San Carlos River 

and of the falling water table at the time of sampling 

(figs. 36, 37, and 38). Excess moisture was present 

from 0.25 meter to 1.2 meters. Evaporation that caused 

depletion influenced only the top 0.05 meter.

The July sampling showed that evaporation had 

depleted moisture to a depth of 0.4 meter. Recharge 

from a light rainstorm on the night of July 17-18 

influenced the top 0.2 meter at this site. Excess moisture present at the time of the May sampling had 

drained by the time of the July sampling, and equilibrium conditions had been established below 0.4 meter.

1964 WILLOW SITE

Soil at the 1964 willow sites (figs. 39, 40, and 41) 

contained excess moisture below a depth of 0.8 meter 

in May and July. No evidence was found of soilmoisture depletion by transpiration during the growth 

period. Evaporation had removed moisture to a depth 

of 0.20 meter in May and 0.3 meter in July ^Recharge 

from a light rainstorm on the night of July 17-18 

influenced the top 0.2 meter.

1964 COTTON WOOD SITE

Soil at the 1964 cottonwood site (figs. 42, 43, and 44) 

had excess moisture below a depth of 0.65 meter in 

May. By July the soil moisture had been depleted to a 

depth of 1.15 meters; no evidence was found that soil 

moisture was in equilibrium with the water table or 

that field capacity had been established. Recharge from 

a light rainstorm on the night of July 17-18 influenced 

the top 0.1 meter. Soil-moisture utilization during the 

growth period was extensive. A nearly constant pF of 

3.75 (5.6 bars) from 0.4 to 0.6 meter in July may indicate the stress capabilities of cottonwood trees; the 

1962 cottonwood profiles show pF levels of 3.5 to 4 

(3 to 10 bars).

FIGURE 26. Profiles of ApF for shaded and unshaded 

sites 62UM and 62OM associated with mesquite 

trees on the San Carlos River flood plain. Sampled 

May 26-27, 1962.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION

MC 

SMC

-Water table 

0.2 0.3 0.4 

MOISTURE-WEIGHT RATIO

234 

pF (MOISTURE STRESS)

FIGURE 27. Profiles of MC, SMC, and pF at unshaded site 62OC associated with cottonwood trees on the San Carlos

River flood plain. Sampled May 28, 1962.
E32 GILA RIVER PHREATOPHYTE PROJECT

c;|

T

MC 

SMC

0.2 0.3 0.4 

MOISTURE-WEIGHT RATIO

--Water table0.5 0.6 0 234 

pF (MOISTURE STRESS)

FIGURE 28. Profiles of MC, SMC, and pF at shaded site 62UC under cottonwood trees on the San Carlos River flood

plain. Sampled May 29, 1962.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E33

-WET

2 

DRYO 

O

o

O

EXPLANATION

Site 62UC, shaded

O 

Site 62OC, unshaded

-101234 

DEPARTURE FROM EQUILIBRIUM (ApF)

FIGURE 29. Profiles of ApF for shaded and unshaded sites 

62UC and 62OC associated with cottonwood trees on the 

San Carlos River flood plain. Sampled May 28-29, 1962.

1964 MESQUITE SITE

Excess moisture was present below a depth cf 0.4 

meter in May at the 1964 mesquite site (figs. 45, 46, 

and 47). Moisture was near equilibrium belov7 0.7 

meter in July. Moisture dissipation during the growth 

season was evident from the surface to 1.2 meters. Recharge from the light rain on the night of July 17-18 

did not affect the soil-moisture record, but thif site 

was protected from direct rainfall by intercepting 

vegetation.

DISCUSSION OF MOISTURE USE AND 

DISSIPATION

Some new ideas on movement and utilization of 

soil moisture are discussed here without rigorous proof 

because proof is not within the scope of this project. 

The authors hope that these ideas and the new methods 

used in this study will contribute to the development of 

the science of soil moisture.

The depths where soil moisture is depleted by 

evaporation and (or) transpiration can be inferred 

from the ApF profiles. A positive ApF with a high 

value at the surface that approaches the zero, or equilibrium, line at a uniform high rate usually indicates 

evaporation loss. A positive ApF value that remains 

nearly constant or shows slight decrease with depth 

may indicate transpiration. In any given profile, both 

forms of moisture dissipation may be evident.

Transpiration can remove moisture from any depth 

where the soil is occupied by live roots. Annual grasses, weeds, and other ground-cover plants deplete soil 

moisture to a depth of about 0.6 meter (fig. 27). Depletion was evident under a mesquite tree to a depth of 

3.4 meters (figs. 25 and 26).

In many profiles, depletions due to evaporation may 

be indistinguishable from those due to transpiration.
E34 GILA RIVER PHREATOPHYTE PROJECT

0.1

MC 

CMR

SMC

Water table0.2 0.3 0.4 0.5 

MOISTURE-WEIGHT RATIO

0.6 0.7 0 2345 

pF (MOISTURE STRESS)

FIGURE 30. Profiles of MC, SMC, CMR, and pF at unshaded site 64OT associated with salteedar trees near section 23

of the Gila River Phreatophyte Project area. Sampled May 10, 1964.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E35

NIC 

SMC

r- 

|

Zl

Water table at 2.90 meters

0.2 0.3 0.4 

MOISTURE-WEIGHT RATIO

0.6 0 234 

pF (MOISTURE STRESS)

FIGURE 31. Profiles of MC, SMC, and ^>F at unshaded site 64OT associated with saltcedar trees near section 23 of

the Gila River Phreatophyte Project area. Sampled July 16, 1964.
E36 GILA RIVER PHREATOPHYTE PROJECT

WET

#

°i*

 Jfr

DRY

i&

« o

EXPLANATION

May 10, 1964

July 16, 1964

-10 1 2345 

DEPARTURE FROM EQUILIBRIUM (ApF)

FIGURE 32. Profiles of ApF for unshaded site 64OT associated with saltcedar trees near section 23 of the Gila River 

Phreatophyte Project area. Sampled May 10 and July 16, 

1964.

The combined term "evapotranspiration" was coined 

because the two types of depletion are difficult to separate. The moisture stress data for the 1962 saltcedar 

sites (figs. 21 and 22) show decreasing stress with 

depth that is typical for evaporation depletion, but 

the depth of depletion and the stresses are greater 

for the shaded site than for the unshaded site. The 

ApF data for these sites (fig. 23) offers an explanation 

for this anomaly because transpiration is indicated as 

possible for the depth interval 0.05 to 0.3 meter at the 

shaded site. Field data show a heavy stand of g~ass at 

the shaded site (fig. 2). A stress of pF 3.5 to 4.2 (3 to 

16 bars), as shown for the 0.05 to 0.3 meter depth interval (fig. 22), would be about right for an area of annual grass in a rapid growth stage.

Relatively impermeable clay layers in a soil may 

have ApF values that are high or low depending upon 

the direction of movement of the water table. Clay 

-layers at the 1962 saltcedar sites at depths cf 1.05 

to 1.40 meters at the unshaded and 1.20 to 1.80 meters 

at the shaded site have high ApF values which are 

probably due to slow response to a rising water table 

(figs. 21, 22, and .23}.

Evaporation of soil moisture appears to be proportional to the amplitude of the soil temperature fluctuations. Attenuation of the diurnal cycle of temperature 

depends both upon the amount of radiation received at 

the soil surface (degree of shading) and upon the 

moisture content. These relationships are indicated in 

the stress profiles. Moisture stresses decrease rapidly 

to the depth where diurnal fluctuations are attenuated. 

Below this depth, the stress values form an equilibrium 

curve related to the water table or to a transpiration 

stress level imposed by plants. The depth which corresponds to the transition from the evaporation segment of the curve to the equilibrium or transpiration 

segment is greater in July than it is in May, and it is 

greater at the unshaded sites than it is at the shaded 

sites. There is one exception to the first generalization. 

The depth corresponding to the transition at the
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION

il

MC 

CMR 

SMC

Water table

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 

MOISTURE-WEIGHT RATIO

123456 

pF (MOISTURE STRESS)

FIGURE 33. Profiles of MC, SMC, CMR, and pF at shaded site 64UT under saltcedar trees near section 23 of the

Gila River Phreatophyte Project area. Sampled May 11, 1964.
E38 GILA RIVER PHREATOPHYTE PROJECT

MC 

SMC

0.2 0-3 0.4 

MOISTURE-WEIGHT RATIO

n

Water table at 2.90 meters

0.6 0 234 

pF (MOISTURE STRESS)

FIGURE 34. Profiles of MC, SMC, and pF at shaded site 64TJT under saltcedar trees near section 23 of the Gila

River Phreatophyte Project area. Sampled July 17, 1964.
SOIL-MOISTURE AND EXERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E39

WET

* * 

*

* *1

** 

«*

 *

k

 «

 

^ 

^

* 

ji

#

# 1 

* 

EXPLANATION

May 11, 1964

July 17, 1964

-10 1 234 5 

DEPARTURE FROM EQUILIBRIUM (ApF)

FIGURE 35. Profiles of ApF for shaded site 64TJT under saltcedar trees near section 23 of the Gila River Phreatophyte 

Project area. Sampled May 11 and July 17, 1964.

shaded saltcedar site is greater in May than in July. 

The reason for this exception is discussed in a following section of this report.

Trees and other perennials may deplete soil moisture around the perimeter of their root system sooner 

than in the center, near the older, established roots. 

Stress measurements made under and near mesKuite 

trees in May 1962 showed lower stress under the trees 

than in the open, where there was little or no vegetation (fig. 2). Both sampling sites were permeate! by 

roots, but the roots at the unshaded site were smaller 

and more numerous than those at the shaded site. 

The moisture-stress profile for the unshaded site (fig. 

24) shows a nearly uniform stress of pF 4.4 (25 bars) 

from 0.6 to 2.0 meters. The stress in June or July, 

after an active growth season, would probably be much 

higher, indicating that the mesquite tree is capable of 

competing for soil moisture with many plants, that 

are classed as xerophytes. The shaded site had a stress 

of about pF 3.9 from 0.5 to 1.5 meters. Moisture contents at the two sites were similar, but slightly more 

moisture occurred under the tree.

The two sites sampled under and near cottomrood 

trees in May 1962 were influenced by smaller plants 

growing on the sites. The unshaded site (fig. 3) was far 

enough from the cottonwood trees to be free of tree 

roots. The smaller plants growing on and near the site 

induced a stress of pF 4.3 (20 bars) at depths from 

0.2 to 0.55 meter. Moisture at the site was near equilibrium from O.Y meter to the water table at 1.75 meters 

(fig. 29). The moisture and stress profiles of the site 

shaded by cottonwood trees indicate evaporation loss 

to only 0.15 meter, transpiration loss to 1.15 meters, 

and equilibrium from 1.15 meters to the water table 

at 1.64 meters (figs. 28 and 29).

Moisture at the sites associated with saltcedar trees 

sampled in May and July 1964 appeared to be maintained at equilibrium with the water table except in the 

top 60 cm, where evaporation was high because of 

diumal temperature fluctuations.
E40 GILA RIVER PHREATOPHYTE PROJECT

i.o

1.5

MC 

CMR

SMC -Water table

0.1 0.2 0.3 0.4 

MOISTURE-WEIGHT RATIO

0.6 0 2 3 

pF (MOISTURE STRESS)

FIGURE 36. Profiles of MC, SMC, CMR, and pF at unshaded site 64OS associated with willow, cottonwood, and rresquite trees on the San Carlos River flood plain. Sampled May 13, 1964.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITR RIPARIAN VEGETATION E41

MC 

SMC

-Water table 

0.2 0.3 0.4 

MOISTURE-WEIGHT RATIO

0.5 0.6 0 234 

pF (MOISTURE STRESS)

FIGURE 37. Profiles of MC, SMC, and pF at unshaded site 64OS associated with willow, cottonwood, and mesquite tree?5

on the San Carlos River flood plain. Sampled July 19, 1964.
E42 GILA RIVER PHREATOPHYTE PROJECT

The saltcedar trees did not deplete soil moisture 

from May 11 to July 17, 1964. At the site under the 

trees, an increase in soil moisture occurred during this 

period, especially in the top 0.4 meter. Part of this 

increase may be attributed to physical differences in 

the soils of the two sampled columns. However, the 

soil-moisture stress is independent of the soil texture, 

as indicated by the moisture-stress and saturationo 

o 

o 

o

o 

o 

o

o 

o 

o 

o

ft

ft ft

il

ft ft

#

ft

te

o * 

o

ft

-ft

oft

EXPLANATION

o 

May 13, 1964

July 19, 1964

ft

ft

ft

WET DRY-

-101 2345 

DEPARTURE FROM EQUILIBRIUM (ApF)

FIGURE 38. Profiles of ApF for unshaded site 64OS associated 

with willow, cottonwood, and mesqxiite trees on the San 

Carlos River flood plain. Sampled May 13 and July 19, 1964.

capacity profiles, and the pattern of equilibrium with 

the water table is so well defined in the six profiles for 

sites associated with saltcedar trees that some mechanism of moisture translocation through the root systems 

must be considered to explain the data. This irechanism will be discussed in a following section of this 

report.

The field-capacity concept implies that for a given 

soil there is a moisture-retention force and a corresponding moisture content at which there is an abrupt 

change in the rate of moisture movement induced by 

gravity gradients. The moisture retained by F. soil 

sample in a centrifuge is assumed to be approxiir ately 

equal to the field capacity. The retention force at field 

capacity is in the range pF 2.0 to 2.5 (0.1 to 0.3 bars).

As a qualitative test of the methods for determining 

field capacity, it is assumed that a column of soil 

containing moisture in equilibrium with the water 

table should have a depth at which the moisture content (MC) is equal to the centrifuge moisture retention (CMR). Furthermore, this depth should coincide 

with the depth at which the moisture stress is equal to 

the stress at field capacity, for which an average value 

of pF 2.3 may be assumed. Also, the most frequently 

occurring stress in a group of soil columns should be 

the field-capacity stress (pF 2.3). An examination of 

the moisture data shows that these conditions exist 

within reasonable -limits. The depth correspondence 

data for those sites for which centrifuge data are 

available are given in table 3.

The moisture data do not indicate the existence of an 

intermediate belt as described by Meinzer, because at 

every site the belt of soil water and the capillary fringe 

merged so that there was no room for an intermediate 

belt.

TABLE 3. Comparison of depths at which MC equals CMP with 

depths at which stress equals pF 2.3

Figure 

No.

30 ____

33 _ _

36 ______

39 _ __

42__

45__ _

Site

64OT

64UT

64OS

64UW

64UC

64UM

Depth at which 

MC=CMR 

(cm)

40-55

25, 45-50

15

30-50

55

20

Depth at which 

pF = 2.3 

(cm)

40-55

40-55

10

20-50

55

20

FIGURE 39 (upper right). Profiles of MC, SMC, CMR, and pF at shaded site 64TJW under willow trees on the San Crrlos

River flood plain. Sampled May 13, 1964.

FIGURE 40 (lower right). Profiles of MC, SMC, and pF at shaded site 64UW under willow trees on the San Carlos F'ver

flood plain. Sampled July 18, 1964.
^OIL-MOISTTJRE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E43

0.2 0.3 0.4 0.5 0.6 

MOISTURE-WEIGHT RATIO

- -Water table--

0.7 0.8 0 12345 

pF (MOISTURE STRESS)

0.2 0.3 0.4 0.5 

MOISTURE-WEIGHT RATIO

0.6 0.7 0 1234 

pF (MOISTURE STRESS)
E44 GILA RIVER PHREATOPHYTE PROJECT

PHREATOPHYTES AND SOIL-MOISTURE 

DISSIPATION

Vegetation established in a soil may withdraw moisture from those parts of the soil that are penetrated 

by live roots. Few measurements have been made to 

define depths of moisture depletion by plants in their 

natural habitat or to define the energy relationships 

involved. Moisture utilization is different for different 

species. Shallow-rooted species, such as annual grasses, 

will not deplete moisture in the same depth zone as a

deep-rooted mesquite tree. A phreatophyte may obtain 

all its moisture from below the water table and, therefore, not deplete soil moisture.

Meinzer (1923a, p. 55) defined a phreatophyte as "a 

plant that habitually obtains its water supply from 

the zone of saturation, either directly or through the 

capillary fringe." Obviously, riparian, or stre^.mbank, 

vegetation would include many phreatophyte?, but it 

may also include some species that use only soil moisture and depend upon occasional flooding to replenish 

their moisture supply. A knowledge of where a plant 

species or community obtains its water is essential to 

proper management of water resources by vegetation 

modification.

Soil-moisture depletion during a maximum growth 

season should define part of the moisture-use characteristics of a plant. Relative depletion of soil moisture 

can be indicated by changes in moisture stress, or it 

can be determined from changes in moisture content 

of the soil.

The texture and the moisture-retention properties of 

flood-plain soils are extremely variable, so direct 

comparisons of moisture contents at each depth at two 

adjacent sampling sites may not be valid. However, a 

comparison between the totals or averages for two sites 

may be significant.

The ratio of the measured MC of a sample to the 

measured SMC (saturation capacity) of the sample 

is the RS (relative saturation). The average PS for a 

site can be compared with the average RS for other 

sites provided the total sampling depths are similar. 

The average RS for each site is reported in the site 

descriptions (table 1).

A comparison between the average RS before and 

after a growth season should define depletion of moisture by transpiration in a plant community plus depletion by evaporation. The difference between th°. initial 

average RS and the final average RS divided by the 

initial average RS can be defined as the soil-moisturedepletion index. Six sites were sampled in May and 

July 1964. The soil-moisture-depletion indices for these 

sites, expressed as percentages, are:

Unshaded saltcedar (Gila) ____________ 13.2 

Shaded saltcedar (Gila) ______________ 3.0 

Unshaded mixed community (San Carlos) ____ 39.1 

Shaded willow (San Carlos) ___________ 19.8 

Shaded cottonwood (San Carlos) _ __-___ 26.5 

Shaded mesquite (San Carlos) __________ 40.6

FIGURE 42 (upper right). Profiles of MC, SMC, CMR, and pF at shaded site 64UC under cottonwood trees on the San Carlos

River flood plain. Sampled May 14, 1964.

FIGURE 43 (lower right). Profiles of MC, SMC, and pF at shaded site 64UC under cottonwood trees on the San Carlos River

flood plain. Sampled July 20, 1964.

u

/IETERS

^

Z 1

DEPTH,

2

-

~ 

*

.**

9

 * 

 *

*

*

*

*

-

 « WET

1 m 1 1

* 

 "#"

 *

 *

 #"

*

* 

 # 

 "$" 

*.

c

-

^ EXPLANATION

^-

May 13, 1964 

July 18, 1964

*

DRY >

1 1 1

-101 234 5

DEPARTURE FROM EQUILIBRIUM (ApF)

FIGURE 41. Profiles of ApF for shaded site 64UW under willow

trees on San Carlos River flood plain. Sampled May 13 and

July 18, 1964.
DEPTH, IN METERS DEPTH, IN METERS

I I

^ '#£

1 I I T

(/> O 

O SJ O

J.

I I I I

~i r
E46 GILA RIVER PHREATOPHYTE PROJECT

Although the depletions indicated here represent evaporation and transpiration, the differences between communities probably are significant indicators of soil 

moisture utilization by phreatophytes.

The reasons for different soil-moisture utilization by 

different plant communities are not well understood. 

Meinzer (1927) discussed the value of various species 

of natural vegetation for indicating the presence of 

shallow ground water. He recognized that different 

species responded differently to water-table depths. He 

considered willow a positive indicator of shallow 

ground water, but he knew cottonwood could survive 

without a shallow water table. He said (p. 42-43) of 

mesquite:

z i

* 

*

*

#

*

-WET

 * *

' * 

*

*

*

*

*

*

*

EXPLANATION

 

May 14, 1964

* 

July 20, 1964

DRY-

-1012345 

DEPARTURE FROM EQUILIBRIUM (ApF)

FIGURE 44. Profiles of ApF for shaded site 64TJO under cottonwood trees on the San Carlos River flood plain. Sampled 

May 14 and July 20, 1964.

Studies make it clear that mesquite is essentially a deeprooting plant which under favorable conditions has a capacity 

for tapping the ground-water supply, even where the water 

table is as much as 50 feet below the surface; also, tha t it has 

excellent adaptations for obtaining water from other sources 

and for resisting drought.

He did not discuss saltcedar, probably because- large 

stands had not developed when his observations were 

made.

Meinzer's observations on the moisture-use characteristics of willow, cottonwood, and mesquite trees are 

supported and explained by the ApF profiles of this 

study.

Willow trees (figs. 39, 40, and 41) use very little 

soil moisture. The ApF profiles (fig. 41) shov7 some 

depletion near the surface that may be due to evaporation.

Saltcedar trees, as indicated by figures 23, f 2, and 

35, do not use soil moisture when ground wter is 

available, and they may even transfer moisture from 

the water table to zones of moisture depletion. The 

dense stand of annual grasses on the site shaded by 

saltcedar trees should have depleted the soil moisture 

 to a depth of 0.5 or 0.6 m and increased the pF to 

3.5 or 4.5. Instead, a decrease in stress with a concomitant increase in soil-moisture content occurred 

during the period from May to July in the depth interval from 0.2 to 0.5 meter (figs. 33 and 34).

Laboratory investigations have demonstratei that 

some plants will transport moisture through their root 

systems from moist soil to dry soil or from a saturated 

atmosphere to dry soil (Breazeale, 1930; Breazeale and 

Crider, 1934; Volk, 1947; Breazeale and others, 1950; 

and Bormann, 1957). Eapid transfer of moisture from 

one branch of a tree to another branch in response to 

wind and (or) vapor-pressure differences has been 

measured by Daum (1967). The term "translocation" 

has been applied to these processes, and the following 

characteristics have been noted:

1. Moisture movement is always toward moisture-stress 

equilibrium. That is, flow is from a part of the 

plant in a low-stress environment to a part of the 

plant in a higher stress environment.

2. Moisture may be exuded from the roots into the 

soil if the" difference in stress is sufficient.

3. Movement from a saturated atmosphere irto the 

leaves, through the plant, and out of the roots 

seems to be more efficient than movement into the 

plant through the roots (Breazeale and others, 

1950).

4. Moisture may transfer from one plant to an adjacent plant if the roots are intertwined (Bormann, 

1957).
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E47

-Water-table 

0.2 0.3 0.4 

MOISTURE-WEIGHT RATIO

0.6 0 234 

pF (MOISTURE STRESS)

FIGURE 45. Profiles of MC, SMC, CMR, and pF at shaded site 64UM under mesquite trees on the San Carlos River flood

plain. Sampled May 14, 1964.

5. When water is exuded from roots, nutrients (salts) 

are also exuded.

6. Moisture transfer from one part of a tree to another 

part of a tree in response to a difference in evaporative stress of the microclimate can be of significant magnitude and can occur rapidly (Daum, 

1967).

In this paper, translocation of soil moisture is shown 

in data from natural, undisturbed sites. The saltcedar 

tree appears to maintain the moisture in the soil mass 

permeated by its roots at hydraulic equilibrium with 

the water table, even though some moisture is used by 

other plants. This equilibrium would seem to be a 

natural consequence of translocation of moisture 

through the root system of the plant.

An efficient translocation mechanism with the concomitant ability of the tree to sustain the soils near 

equilibrium with the water table at all times may be a 

primary characteristic of a phreatophyte. A plant that 

utilizes soil moisture when the water table is within 

reach of its roots may not be a true phreatophyte. 

Cotton wood trees (fig. 44) and mesquite trees (fig. 47) 

both use soil moisture, and, as Meinzer indicated, they

can exist without a shallow water table; therefore, they 

may not be true phreatophytes.

CONCLUSIONS

One question explored by this study was: Will the 

transpiration decrease due to removal of saltcedar 

trees be partly, or entirely, offset by increased evaporation from the soil surface when shading is eliminated ? The mechanisms mvolved are movement of soil 

moisture in response to thermal gradients, and capillary rise from the water table.

When transpiration by saltcedar trees is reduced or 

eliminated, the mean water table will rise until increased outflow in the stream and increased evaporation balance the reduction in transpiration. If water 

moves to the surface by capillary action, a possibility 

under present conditions at several sites investigated, 

any decrease in depth to water will increase the potential for capillary flow to the surface and for consequent 

increased evaporation. Theoretical considerations and 

data obtained in this study indicate that capillary flow 

may be limited to a maximum height of 2 meters above 

a water table. In sands and gravels, capillary rise may
E48 GILA RIVER PHREATOPHYTE PROJECT

be limited to nearer the water table because the diameters of the pores are generally much greater than the 

effective diameter of a meniscus at the capillary limit. 

Evaporation may be limited to the depth of diurnal 

temperature fluctuations; this depth is about 0.6 meter 

at the unshaded sites studied. The loss of moisture by 

increased evaporation due to elimination of shading can 

be restricted by keeping the depth to water greater 

than the total depth of diurnal temperature fluctuations plus the height of capillary rise, or about 2.6 

meters.

When grasses planted to replace the phreatophytes 

become established, the water conserved will be the 

difference between present transpiration by the trees 

and transpiration by the grasses less any increase in

evaporation because of a shallower water table. If 

the water table is permitted to rise, salts may acc^mulate at the soil surface, and beneficial use of the land 

may be impaired.

Moisture movement in response to thermal gradients 

is from warm to cold. Figures 8, 9, 10, and 11 show 

that warm to cold thermal gradients are downward 

during the spring and summer and upward during the 

fall and winter. Although the measuring stations were 

destroyed before a full year of record could be obtained, partial data show that at the unshaded sites 

the downward gradient was established earlier in the 

spring and remained until later in the fall than at the 

shaded site. Also, the downward gradients at the unshaded sites were steeper than the downward gradients

0.2 0.3 0.4 

MOISTURE-WEIGHT RATIO

234 

pF (MOISTURE STRESS)

FIGURE 46. Profiles of MC, SMC, and pF at shaded site 64UM under mesquite trees on the San Carlos River flood plain.

Sampled July 20, 1964.
SOIL-MOISTURE AND ENERGY RELATIONSHIPS ASSOCIATED WITH RIPARIAN VEGETATION E49

at the shaded sites. Probably, most of the moisture that 

moves from the belt of soil water by thermal gradients 

is deposited in the intermediate belt or in the capillary 

fringe; from there it moves to ground water. Loss of 

moisture by evaporation occurs when the warm to cold 

thermal gradient is upward during the summer nighttime hours and during the fall and winter seasons. If 

the water table is deep, this loss may be minimal. If 

the water table is shallow, evaporation may be appreciable because the evaporated water is replenished by 

capillary flow.

Saltcedar trees appear to maintain the moisture in 

the soils permeated by their roots in hydraulic equilibrium with the water table. Consequently, the moisture contents at moderate depths are greater where

WET

 *

*

*1

*

*

*

*1

*

*

*

*

EXPLANATION

May 14, 1964

July 20, 1964

-1 01234 

DEPARTURE FROM EQUILIBRIUM (ApF)

FIGURE 47. Profiles of ApF for shaded site 64TJM under 

mesquite trees on the San Carlos River flood plain. Sampled 

May 14 and July 20, 1964.

saltcedar roots are present. When the trees are removed, the possible increase in evaporation due to 

elimination of shading may be offset by a decrease in 

evaporation when moisture contents at shallow depths 

are no longer sustained by translocation of moisture 

through the tree root systems.

Riparian vegetation may obtain moisture from the 

unsaturated zone above the water table or from the 

saturated zone below the water table, depending upon 

the species. Saltcedar and willow use very little moisture from the unsaturated zone, but cottonwood and 

honey mesquite use considerable moisture from the 

unsaturated zone. These trees are all usually classed 

as phreatophytes. However, their moisture utilization 

characteristics are different. A new classification cr a 

new definition of phreatophyte may be in order.

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