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Water Resource Conservation
by Reducing Phreatophyte Transpiration 1
Robert W. Ritzi, Herman Bouwer, and Soroosh Sorooshian2
Through managing the magnitude of depth to the water table
below floodplains supporting phreatophytes, water conservation
may be effected due to resulting reductions in evapotranspiration, while the integrity of the riparian ecosystem is maintained. A model of the phreatophyte-floodplain system has been
developed for analyzing the method's economic feasibility.
INTRODUCTION AND SCOPE
sibility of this conservation method will also be
constrained by the political and legal implications
of affecting a stream-aquifer system including the
possibility of prior appropriations and of instream
requirements.
In the past, water conservation by reducing
phreatophyte transpiration has most often involved
eradication methods, which have presented a conflict between the interest in saving the large
amount of water the plants use, and the interest
in the values that come with preserving a natural
riparian ecosystem. An alternative conservation
method to eradication is presented here, which can
maintain the riparian stand and thus reconcile the
conflict, if the method proves feasible.
A HISTORY OF WATER CONSERVATION BY
REDUCING PHREATOPHYTE Et
After recognizing the relation between certain
species of desert plants and the water table, o. E.
Meinzer (1923, 1927) introduced the term phreato~ (the Greek roots phreatos = well, phuton =
plant) which he defined as "a plant that habitually
obtains its water supply from the zone of saturation, either directly or through the capillary
fringe." Early studies demonstrated this relationship by locating observation wells in phreatophyte
areas and observing their influence on the water
table.
Evidence exists to support a relationship between depth to the water table below phreatophytefloodplain systems, and the evapotranspiration
(E t ) rate from them. By managing the position of
the water table at increased depths, yet within
the range of phreatophyte root extension (saltcedar roots have been excavated to depths of 30
meters), the Et rate can be reduced while maintaining environmental integrity. Thus, hypothetically, pumps or drains would be installed in
floodplain alluvium supporting phreatophytes. The
water table would be lowered to a deeper position,
resulting in a reduction in Et after the phreatophyte root termini have deepened to the new position of the capillary fringe. If the pumped
water can be substituted into regional demand,
Lhen the reduction in Et rate represents salvaged
water.
Later, toward the middle of this century, the
study of phreatophytes in arid areas of the western
United States began to proliferate, no longer because of their usefulness, but because of their water use. Robinson (1958) attributes concern over
water supply problems at this time to a prolonged
drought contemporaneous to a post World War II increase in water demand, and to the spread of one
non-native species, Tamarix chinenses Lour.,
through the stream valleys of the Southwest. Also,
shortly before this time Gatewood et ale (1950), in
studying the Safford Valley, Arizona to determine
possible water salvage for the war industry in
1943-44, used water balance methods to measure evapotranspiration. They were perhaps the first to
draw attention to the potentially large amounts of
water transpired by Tamarix sp.
The scope of the analysis of this conservation strategy here includes the magnitude of possible Et reductions and the cost feasibility. A
modeling methodology is used. Certainly, the fea1 Paper presented at the First North American
Riparian Conference, Riparian Ecosystems and Their
Management: Reconciling Conflicting Uses, April 1618, 1985, Tucson, Arizona.
In 1952, Robinson estimated that phreatophytes
covered 16 million acres in the western states and
consumed 25 million acre-feet of water per year.
While noting that these numbers may be "greatly exaggerated", Van Hylckama (1982) points out:
2 Ritzi is a graduate student and Sorooshian is an
Associate Professor, University of Arizona, Tucson.
Bouwer is with the USDA Water Conservation Laboratory, Phoenix, Arizona.
191
Lowering the water table below floodplains has
been previously investigated. Early analyses were
done in 1956 by the Bureau of Reclamation (reported
by Affleck, 1975), in which a mUltiple regression
equation using air temperature, precipitation, and
depth to ground water was used to predict that a
five-foot lowering of the water table below the
Gila River from Thatcher to G1enbar, Arizona, would
effect water savings of about 7,000 ac-ft annually.
Bouwer (1975) presented iterative and analytical
procedures for computing seepage from streams which
incorporated Et/water-depth relations for application to the phreatophyte problem.
Robinson's data served to create active,
even alarmist interest in the phreatophyte
problem ••. ln the literature, salt cedars
have been described as "waterhogs"
(Douglass, 1954), "aggressive" (Robinson,
1965), "greedy" (Douglass, 1965), "insidious" (Sebenik and Thames, 1968), "thieves"
(Robinson, 1952) and "water stealing culprits" (U.S. Information Service, 1965).
These expressions indicate the emotional
and propagandistic attitude that is somet.ilnc.:::; caken towards the phreatophyte
problem.
In light of his estimates, Robinson proposed
the study of water salvage by either 1) replacing
the plants with more beneficial ones, 2) intercepting water upstream of the plants, 3) directly destroying the phreatophytes, or 4) lowering the water table out of their reach and thus killing them.
Thus began the proliferation of research into water
salvage and also into measuring Et to justify water
conservation projects. By 1964, Robinson had published a report of 48 projects relating to the
phreatophyte problem. Bibliographies containing
together over 2,000 titles related to Et and phreatophytes have been compiled by Robinson and Johnson
(1961), Humphreys (1962), and Horton (1973).
RELATIONSHIP
BETI~EEN
Et AND DEPTH TO WATER
A phreatophyte-f10odp1ain system is here
defined as the relationship between an unconfined,
alluvial aquifer, a perennial stream, and phreatophytes with root termini in the capillary fringe
(Figure 1). Et from this system can be thought of
as a function of three interdependent influences
including 1) the availability of energy at the
evaporating surface to supply latent heat demand,
2) the vapour pressure gradient between the water
at the evaporating surface and the bulk air, and 3)
the resistances in the water flow pathways (Hillel,
1982; Slayter, 1967). Thus, in the phreatophytefloodplain system, in addition to factors at the
surface of or external to the evaporating bodies
(atmospheric evaporativity), Et will be a function
of water pathway characteristics in the soil as
well as resistance to water movement within the
vegetation.
Eradication projects to destroy and replace
phreatophytes include those such as Bowser (1952),
Koogler (1952), and Cramer (1952). Fox (1977)
points out that the Los Angeles report of the Phreatophyte Subcomittee of the Pacific Southwest
Inter-Agency Committee in 1969 lists nearly two dozen major clearing projects in Arizona. The best
documented and most recent of the~e (Culler et al.,
1982) is the eradication of some 5,000 acres on the
Gila River floodplain, producing a measured reduction of 480 rom in yearly Et •
It is the increase in pathway resistance in
relation to depth of the water table beneath the
floodplain system (D) that is of interest. In
studying evaporation from bare soils, Gardner
No evidence in the literature has been found
of further attempts to eradicate phreatophytes for
water salvage, probably due to factors including
the rapid regenerative properties of Tamarix sp.,
and the objectionable environmental consequences
including loss of wildlife habitat and bee pasture,
the lowered aesthetic quality, and the increase
caused in erosion. Out of concern for the riparian
habitat, a conference was held in 1977, titled
Symposium on Importance, Preservation and Management of the Riparian Habitat, dedicated to Douglas
C. Harrison whose research quantified the effects
of phreatophyte control on the breeding of birds in
the native riparian woodland of the Verde River.
YEARLY
V=
As an alternative to eradication, substances
have been applied to phreatophytes to reduce transpiration, as proposed by Horton (1976), Ffolliott
and Thorud (1975), and Affleck (1975). Davenport
et al. (1978) measured the Et reduction and environmental effects due to various antitranspirants
(AT). They concluded that ATs safely and significantly reduce Et but were uneconomical to apply.
At their estimated application rate of $135/acre/month to produce a 25% Et savings (.007 ft/day) on
Tamarix sp., the cost is about $634/ac-ft of water
saved/year.
Figure 1.
192
Et
VOLUME
fff ~~~~
dDdA
(1972) do not exist. The data from Gatewood et. a1
(1958) show a relationship; however, their volume/density correction has been questioned (Van
Hylckama, 1974).
(1958) has shown that in the presence of very
shallow water tables, evaporativity determines the
evaporation rate; however, as the depth to the
water table is increased, the increase in pathway
length and tortuosity became controlling factors.
Under the latter conditions, at any given suction
head at the surface, the evaporation rate becomes
less as D increases. The relationship will vary
with soil type, but generally when D exceeds 1
meter, soil evaporation becomes minor to plant
transpiration.
METHODOLOGY
For an analysis of the system, constant evaporativity is assumed, and thus annual Et volume for
a given community is only a function of D:
In addition to reduced soil evaporation, the
literature suggests although the data are limited,
that with other factors remaining constant, phreatophytes will transpire less water with increasing
D. Excavation studies have shown that phreatophytes concentrate root termini in the capillary
fringe (Tomanek and Ziegler, 1963; Gary, 1965).
Thus, with an unrestricted water supply, the plants
can be thought of as always transpiring at the potential rate, which will lessen with increased root
depth. A rigorous explanation for the relation of
this phenomenon to resistances in plant pathways
for water transport is not yet known. It should be
noted that certain phreatophytes may exist in the
absence of a water table. Turner (1974) made a
distinction between facu1ative phreatophytes and
obligate phreatophytes. The latter need uninterupted access to the water table, while the former including Prosopis sp. and Tamarix sp., may survive
indefinitely in the absence of saturated soil as
zerophytes. However, this analysis will be concerned only with systems having a capillary fringe
within rooting range.
Et Annual Volume
where:
= Jll Et(D) dD dA
(1)
A = surface area of the floodplain
(L2 )
In the absence of well defined Et/D relationships, we can assume them and examine their sensitivity. An empirically based equation giving a
sigmoidal curve is used to model the Et/D relationship:
(2 )
(LIT)
a,C
= empirical constants
(dimensionless, L)
In a survey of field studies measuring the water consumption of riparian vegetation, ten were
found having Et/D data. The majority involve applying water balance methods to evapotranspirometers where D is varied between 1 and 3 meters.
From these, some cross sectional data are plotted
in Figure 2. The data from Van Hy1ckama (1974)
suggest a linear Et/D relationship, having a correlation coefficient above 0.95; however, extrapolating to the abcissa gives the absurdity of zero Et
for Tamarisk with D greater than 4 meters. Thus
Bouwer (1975) has suggested a sigmoidal model. Unfortunately more data outside of the D range of 1
to 3 meters such as that shown from Harr and Price
Curves for various a,C (C in meters) are plotted in Figure 3. It is assumed that root adjustment is instantaneous with D. In addition, it is
assumed that the evaporation rate from the water
surface of the river occurs at rate equal to EtMAX.
The latter assumption will tend to over-estimate
Et , because the evaporating surface of plants may
greatly exceed the land area beneath their canopy,
and this greater surface area more than compensates
for the additional resistance in the water vapor
pathway (Slayter, 1967). Examination of Figure 3
shows that because the Et/D relationship is nonlinear, lowering the water table from 5 feet below
ground surface to 6 feet below will cause greater
reductions in Et than lowering it an equal amount
but from 25 feet below to 26 feet below. Thus,
certain segments of the Et/D curve can be thought
of as being more effective in terms of the reduction in Et effected by a unit change in D.
Figure 2.--E t /D data.
Figure 3.--E t /D Models.
2.5
Annual soltcedar (Tamcrix chjneosis Lour.) water use in
evapotranspirometers for 1961-1963, from Von Hylckam, 1974
~-------
2.0
Annual greoswood (Sorcobatus vermjculgtus) community
water use measured insitu for 1970, Harr a Price,1972
Annual baccharis (Beechons sp.) water use in evopotronspirometers for 1943-1944, from Gatewood et aI., 1950
1.5
Ql
c
E
Ql
>-
.....
Ql
1.0
E
W 0.5
I
---6-------- -------------- -
-6- - -
OL__~~___ L_ _~_ _~~_ __LI__~I___LI__~I__~I~~I__~I__~__~
7
10
12
13
14
15
10
D (meters)
DEPTH
193
15
TO WATER
20
(meters)
25
30
If Dupuit assumptions are made with respect to
ground water flow, a two dimensional finite difference model for quifer simulation (Trescott et aI,
1976) can be used, and equation 2 included as a
source term to evaluate various dewatering scenarios. Et was made a function of average D at
the 2 previous iterations to assure stability of
the finite difference scheme.
along with an energy cost of $0.08/kw-hr. Based
results by Campbell & Lehr (1973), at 50%
efficiency pump power consumption is 1.5 kw/HP.
Results are compared in Table 3.
DISCUSSION
For the Et/D models and pumping scenario used,
the steady-state results of Et reduction range from
20 to 45%. For perspective, if Robinson's 1952 estimate of 25 million ac-ft/yr for phreatophyte consumption were reduced by 20%, the resulting saving
would amount to over three times the annual amount
of water to be provided by the Central Arizona Project (assuming annual deliveries of 1.5 million acft, Arizona Department of Water Resources, 1980).
In defining parameters and boundary conditions
for the model, it is assumed that the floodplain
aquifer can be represented by a prism of homogeneous and isotropic material symmetric in shpae to
the river. Average dimensions are based on thje
Gila River, Arizona (Culler et al., 1982) including
width, thickness, topography and channel dimensions. Yearly averages for streamflows, recharge
and underflow are used, thus ignoring the seasonality of these components. Dirchlet boundary conditions are used at all aquifer boundaries, and a
head dependent source term is used at the river.
Recharge and discharge wells normal to the river at
its ends, are used to simulate underflow.
The results show that a maximum reduction in
Et for this scenario occurs with the model having
a,C values of 2.5,4 m with a pumping rate of 2.2
ft 3 /s. Increasing the pumping rate by 22% from 1.8
to 2.2 ft 3 /s causes a 4 to 7% increase in Et savings, and yet the energy costs for any rate are
under $50/ac-ft of water saved. The volume of
water produced by pumping is 4 to 6 times the volume of water saved by reducing Et • A continual
pumping rate of 2.2 ft 3 /s represents roughly a
3,000 ac-ft/ yr per 4,000 ft of river water supply
at a cost below $10/ac-ft. Clearly these costs are
low when compared to other large southwestern water
projects such as the Central Arizona Project, for
which estimates of actual water costs have been as
high as $300/ac-ft.
RESULTS
The prepumping, steady state water balances
resulting from using Et/D models with a,C of 4,
2.5 and 1.5, 3.5 are compared to estimates for the
Gila river in Table 1. Both, while representing
different systems, fall within the range of estimates made for the Gila.
With pumping wells arbitrarily spaced 4,000
feet apart and 1,300 feet from the river on either
side, it was possible to pump each well at a maximum of 2.2 ft 3 /s without the wells going dry when
drawdown was calculated in a 1-foot well radius.
The effects of this pumping scenario on the water
table were observed after one year and again after
steady-state conditions had bveen reached. Table 2
summarizes the computed Et savings with the varied
pumping rates, duration, and values of a,C.
The cross sections of the floodplain in Figure
4 show that river flow is slightly maintained for
a,C of 2.5,4 at maximum D, but is not for the other
system. These results are important from an environmental standpoint. Clearly, the phreatophyte
system is maintained in either case; however, the
non-phreatophyte component of the riparian ecosystem would be altered. Thus, pumping rate must
be constrained by instream water requirements to
maintain environmental integrity.
Costs of water saved by lowering the water table include well field installation, operation, and
maintenance costs. However, when both fixed and
maintenance costs are amortized over the life of
the pumping system, they become negligible to energy costs. In light of this and the fact that fixed
cost estimates are subject to large variation, only
energy requirements will be analyzed.
It is interesting to note that the maximum reduction in the Et rate for a given set of conditions is approached or reached by the end of the
PUMPING WELL
LAND
Power plant and pump efficiency were assumed
to be 50%. Electric pumping plants were assumed
---SURFACE
Table 1.--Alluvium Water Balance At Steady-state
Conditions. Numbers are in ft 3 /s
a,c=4,2.5
Et
-10.32
Leakage
5.56
4.81
Recharge
Underflow ±1.0
1.5,3.5
-11.98
7.14
4.81
±1.0
As calculated by
Hanson & Culler
Scale (feet)
-7.28 to -12.52
2.92 to 8.08
3.64 to 5.22
2.57
Figure 4.--Floodplain cross sections.
194
tions including Et rate, canopy response, and root
depth changes are crucial to understanding the
feasibility of this approach. Unfortunately, despite the tremendous effort in past phreatophyte
research, this still remains a complex and expensive proposition. New advances in remote sensing
of Et (Reginato et a1., 1985) and in psycrometric
methods (Gay, 1979) might prove useful for Et measurements.
Table £ .--(;omputed Et savings with aquifer dewatering using
the eight-well scenario
a,C
=
1.5,3.5
Pump rate (ft 3 /5)
1.11
Duration
1 yr
Et
rate (ft/yr) :
Pre-pumping
Post pumping
Savings
Reduction
4.4467
3.6024
0.8443
18.99%
4,2.5
1.8
2.2
Z.Z
9.13 yrs
(steadystate)
1 yr
60.9 yrs
(steadystate)
4.4467
3.5850
0.8617
19.38%
4.4467
3.3936
1.0531
23.68%
4.4467
3.3554
1.0913
24.54%
1.8
2.2
1 yr
(steadystate)
3.8290
2.3964
1.4326
37.42%
1 yr
(steadystate)
3.8290
2.1238
1. 7052
44.53%
A survey of classifications of riparian-phreatophyte communities has shown that the variation in
them in the Southwest is relatively small (see for
example Campbell, 1970; Marks, 1950; or Haase,
1972). Ideally, a distinct Et/D relationship could
be defined for each community, or perhaps a small
number of curves for each as they vary with climate
or elevation.
first year. This results from the assumption of an
instantaneous root adjustment by the plants. How
root systems of phreatophytes will respond to dewatering is not well known. Tamarisk roots have been
measured to grow 30 inches in the first year to the
bottom of phytometers by Merkel and Hopkins (1957)
and Tomanek and Ziegler (1963). The latter authors
have suggested that maximum root depth may be determined by the position of the water table at a
critical time in the life of the plant. Unfortunately, time series data for root extension do not
exist. If the roots of one generation of plants
are in fact not capable of recovering to a new position, it may take generations of plants with increasing root depth before steady steady state is
reached.
LITERATURE CITED
Affleck, R.S. 1975. Potential for water yield
improvement in Arizona through riparian vegetation management. 238 p. PhD dissertation,
University of Arizona.
Aguado, E., and I. Remson. 1980. Ground-water
management with fixed changes.
J. Water
Resources Planning Management Div., Am. Soc.
Civ. Eng. 106(WR2):375-382.
Arizona Department of Water Resources. 1980.
(Oct.) Colorado River operation and supply
studies. [unpublished report]
Bouwer, H. 1975. Predicting reduction in water
losses from open channels by phreatophyte
control. Water Resources Research,
2(1):96-101.
Bureau of Reclamation. 1973. Progress report phreatophyte investigation - Bernardo evapotranspirometers. 33 p. Middle Rio Grande
Project Office.
Campbell, C.J. 1970. Ecological implications of
riparian vegetation management. J. Soil Water
Conservation.
25:49-52.
Campbell, M.D, and J.H. Lehr, 1983, Water Well
Technology. McGraw-Hill. p. 325.
Cramer, S.F. 1952. Activities of the South
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Amer. Geophys. Union. 33(1):77-80.
Culler, R.C., et a1. 1982. Evapotranspiration
before and after clearing phreatophytes, Gila
River floodplain, Graham Co., Arizona. 67 p.
The achieved 45% reduction is Et is not the
maximum possible for the systems evaluated. To
compute a maximum, future hydraulic modeling efforts can be lilnked to management models to optimize pumping scenarios (see for example Maddock,
1973 and Aguado and Remson, 1980).
CONCLUSIONS
The positive results of this analysis will
hopefully encourage continued research into managing water table positions in phreatophyte-floodplain systems as a feasible method of water resource conservation.
Clearly the nature of the Et/D relationship
needs to be better defined. Insitu measurements of
phreatophyte response to altered water table posi-
Table 3.--Computed energy requirements and costs based on the eight-well
scenario
a,C =
4,2.5
1.5,3.5
Pump tate (ft 3 /s)
1.8
1.8
2.2
2.2
1.8
2.2
-----
Duration
1 yr
9.13 yrs
(steadystate)
1 yr
60.9 yrs
(steadystate)
1 yr
(steadystate)
1 yr
(steadystate)
158129
12650
47.52
86102
6888
19.71
135990
10879
26.16
KW-HR/YR-PUMP:
$/YR-PUMP:
$/AF:
92375
7390
35.89
93051
7444
35.42
154071
12326
47.99
195
Marks, J.B. 1950. Vegetation and soil relations
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Graham Co., Arizona. 27 p. U.S. Geological
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196