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Methods for Determining Effects of Controlled Dewatering
of Shallow Aquifers on Desert Phreatophytes
in Owens Valley, California1
Peter D. Dileanis, Farrel A. Branson, and Stephen K. Sorenson 2
Abstract.--The ability of phreatophytic plants to tolerate and survive dewatering of shallow aquifers is being
tested.
At test sites that have been equipped with pumping
wells, soil moisture and plant physiological responses are
being measured as water levels decline.
INTRODUCTION
The U.S. Geological Survey is currently conducting research concerning the possible effects
of ground-water withdrawal on native phreatophytic
vegetation in Owens Valley, Calif.
The project
is being done in cooperation with Inyo County and
the Los Angeles Department of Water and Power.
This paper describes the methods used to measure
the plant's ability to survive changes in water
availability brought about by lowering groundwater levels due to pumping.
O~Tens Valley is situated between the Sierra
Nevada and the White and Inyo Mountains (fig. 1).
The relatively flat valley floor is about 100
miles long and ranges in elevation from about
3,600 to 4,100 feet.
Nountains along the east
and west sides of the valley rise 3,000 to 10,000
feet from the valley floor. Owens Valley receives
an average of only 5 inches annual precipitation
due to the rain shadow east of the Sierra Nevada.
Despite little precipitation, ground water is
plentiful in the valley.
Runoff from the Sierra
Nevada snowpack percolates through the unconsolidated alluvial deposits along the valley margins,
find supplies most of the recharge to the groundwater system.
The water table across much of
the valley floor ranges from land surface to
about 12 feet below land surface.
Ground water
is within reach of the roots of phreatophytic
shrubs and grasses that dominate the valley
floor's plant communities.
Phreatophytes are
plants that habitually rely on ground water by
growing roots down near the water table where
capillary water is readily available (Heinzer,
1927). The phreatophytic plants being tested are:
nevada saltbush (Atriplex torreyi),
greasewood
(Sarcobatus vermiculatus) ,
rubber
rabbitbrush
\
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EXPLANATION
f1i]
Study area
Watershed boundary
Fast-drawdown 5 i te
S 1ow-d rawdown 5 i te
Haiwee
Reservoir
10
!
20
t
10
I
30
'i
40 KILOMETERS
I
20 MILES
Figure l.--Location of study area.
(Chrusothamuns nauseosus) ,
shadscale
(Atriplex
confertifolia) , and big sage ( Artemisia
tridentata) . Two of the plants under study, big
sage
(Artemisia tridentata)
and
shadscale
(Atriplex confertifolia) , while not generally considered to be phrea.tophytes, are thought to be
using ground water in the shallow ground-water
areas of Owens Valley.
These plants continuing
survival on the vall'ey floor may, therefore, be
dependent on ground-water availability.
IPaper presented at
the North American
Riparian Conference,
[University
of Arizona,
Tucson, AZ, April 16-18, 1985].
2Peter D. Dileanis and Stephen K. Sorenson
are Hydrologists, U.S. Geological Survey, WRD,
Sacramento, CA. Farrel A. Branson is a Botanist,
U.S. Geological Survey, WRD, Denver, CO.
In the early 1.900's, the rapidly growing
city of Los Angeles, more than 200 miles south,
looked to Owens Valley as a long-term plentiful
197
supply of water.
The city bought most of the
land in Owens Valley, and in 1913, an aqueduct
was completed which diverted surface water from
Owens Valley to Los Angeles.
In addition, a
series of wells was drilled to supply ground
water to the aqueduct during periods of low
surface-water runoff.
Subsequent extensions of
the original aqueduct and construction of a
second
aqueduct,
completed
in
1970,
have
increased the amount of water being diverted.
EXPLANATION
0
•
Observa tion we 11
Pumping well
•
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A
A cooperative project between the U.S.
Geological Survey, Inyo County, and the Los
Angeles Department of Water and Power began in
1983.
The overall objective of this project is
to develop mathematical models that would be
capable of testing various strategies of groundwater withdrawal designed to mitigate possible
impacts on the valley's vegetation. This project
has several major components including two- and
three-dimensional ground-water-flow models, onedimensional soil-water-evapotranspiration model,
and the controlled drawdown studies described in
this paper.
All of these components will be
integrated into a management/optimization model
which will assist Inyo County and the Los Angeles
Department of Water and Power to effectively
manage
the
ground-water
resources
of
Owens
Valley.
Investigation methods being developed
for use in this study, and particularly the
information obtained concerning vegetation responses to water-table drawdown, will be useful
in other areas of the Western United States where
the need for water by growing population centers
conflict with other uses of available water
resources.
o
Fence
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0
35~--~7~5--~1~50~--~37.00~----~lO~070----~2~OO~O----~37~5~O-----50~O--O
B
DISTANCE FROM PUMPING WELLS, IN FEET
Figure 2.--Fast-drawdown site near Bishop:
(A) Areal view showing
placement of wells and sampling stations (125-foot transects
within fenced enclosures); and (B) cross section showing depth
to water as of February 25, 1985.
EXPLANATION
o
Observation well
•
Pumping well
•
•o
DESCRIPTION OF TEST SITES
~0
USGS
vegetat ion study enclosure
Two types of test sites were established,
each designed to investigate different aspects of
water-deficit stress caused by controlled dewatering.
One type, called a fast-drawdown site,
is designed to rapidly lower the ground-water
level 25 to 30 feet by pumping from a small
cluster of wells.
This pumping results in a
cone of depression in the water table.
Sampling
stations are 125-foot transects,
located at
increasing distances away from the wells (fig. 2).
Observation wells were drilled adjacent to all
sampling stations to monitor ground-water levels.
Two fast-drawdown sites were established, the
first located about 4 miles south of Bishop, and
the second about 3 miles east of Independence
(fig. 1).
The second type of site, called a
slow-drawdown site, is designed to lower water
tables in annual increments of about 6 feet.
A
constant water table level is maintained under
the test sites by pumping the six wells surrounding the site (fig. 3).
A drawdown of
about 6 feet was made the first year with an
additional 6-foot drawdown scheduled for the
end of the 1985 growing season.
Two slowdrawdown sites were established, the first about
3 miles north of the fast-drawdown site near
Bishop, and the other about 0.5 mile west of
the fast-drawdown site near Independence.
All
sites are located in typical areas of relatively
undisturbed
phreatophytic
vegetation.
Soil
LADWD-I nyo Co.
fl)
vegetation study
ene 1asure
150' x 85'
85' x 85'
A
Fence
o
•
B
::t:
~
15 _ _ _ _ _ _ _ _ _ _ _ _ _ £!.£j~~~~i:l~.r---1e~
2D~------------------------------------------
_______
Figure 3.--Slow-drawdown site near Independence:
(A) Areal view
showing sampling station (125-foot transect within fenced
enclosure); (B) cross section showing depth to water.
types are different between the Bishop and
Independence area test sites. Variability in soil
types was desirable so that plant responses can be
linked closely to the variety of soil conditions
in the valley.
198
METHODS OF INVESTIGATION
Vegetation cover at each station is measured using
the point quadrat method (Goodall, 1952) along
each 125-foot linear transect. A linear array of
long pins spaced 0.5 foot apart are held in a
frame above the plant canopy and perpendicular to
the ground. As each pin is low'ered, every ob jec t
the pin contacts enroute to the ground is recorded
in one of the following categories:
plant
species, non-living plant material (standing or
detached mulch), and bare ground. The point frame
is moved along the entire length of the transect.
A summary of all the first contacts made by all
pins is used to estimate the percentage of cover
of the individual species and plant community at
each transect.
Records of all pin contacts give
estimates of leaf density and cover repetition.
As the water level declines below rooting
zones of the plants, increasing water-deficit
stress is expected to occur.
At the fastdrawdown sites, a series of plant stresses is
expected to occur, ranging from insignificant
changes at control stations farthest from the
wells to stress severe enough to cause death to
plants at stations closest to the wells.
The
slow-drawdown sites are testing the ability of
the plants to extend roots to a lower water table
if water tables are reduced slowly.
In order to
determine the effects of dewatering on the plants,
four methods of investigation are being used.
Soil Sampling
Plant Growth and Phenology
The survival of plants in a particular
location is controlled largely by the physical
characteristics of the soil, and by the amount
and availabili ty of moisture in the soil.
In
order to determine soil and soil-moisture characteristics, soil samples are collected monthly from
March through October at all transects.
The
samples are collected using a specially designed,
2-inch diameter hand auger.
Successive. 10-cm
samples are collected from the surface down to
near the water table.
The samples are put into
an air-tight plastic bag along with a 5.5-cm disk
of filter paper, sealed in a metal can, and incubated at 20°C for at least 1 week to allow the
filter paper to come to moisture equilibrium with
the soil.
The wet soil and filter paper are
weighed, oven dried, and weighed again. Gravimetric soil-mositure content and moisture content of
the
filter
paper are calculated from these
weights.
Water potential is calculated using the
calibration curves of R. F. Miller and F. A.
Branson (U.S. Geological Survey, written commun.,
1985). Because of the auger barrel design, a reasonably constant volumetric sample is taken each
time, making it possible to closely calculate bulk
density, void capacity, and volumetric water
content of the soil.
Water-deficit stress may affect the growth
and development of plants.
At each station, a
representative plant of each of the dominant shrub
species has been selected and three branches were
labeled with color coded wire.
Terminal growth
on these branches is measured biweekly or monthly
throughout the growing season.
In stressed
plants, less metabolic energy is available for
vegetative growth,
resulting in lower growth
rates.
The phenology of the marked plants is
observed and recorded at the same time growth measurements are made.
For this study, 10 phenological stages are recognized from beginning leaf
growth through dormancy.
Xylem Water Potential
Negative water potentials occur in the xylem,
or water conducting tissues, of vascular plants
when loss of water by transpiration through the
stems and leaves becomes greater than water gained
from absorption by roots.
As water is depleted
from the soil, the water potential gradient between soil, plant, and atmosphere increases resulting in lower potentials for the water moving
through the plant.
Heasurements of physical and moisture characteristics of soil are essential in determining
soil moisture availability to plants.
The amount
of water in the soil column, as well as the force
wi th which the soil retains the moisture, determines how much moisture is available to plants and
what forces the plant must exert in order to use
that moisture.
Soil characteristics also are essential to the more applied question of plant survivability with declining water tables. Knowledge
of the soil characteristics allows estimates of
how much moisture will drain from the soil when
water tables decline, and how much remains for
plant use. These answers will, in part, determine
if the plant will have enough moisture in the newly drained soils to grow deeper roots or ~'1hether
the amount of water will be so limiting that the
plant will not survive.
Measurements of xylem water potential are
made monthly during the growing season to document
water-deficit stress and to determine the minimum
water potentials the plants can tolerate.
In
1984, measurements were made ·at midday when water
potentials are normally at their mlnlmum.
In
1985, additional measurements will be made before
dawn when water potentials are at their maximum.
These measurements will define the range of waterdefici t stress the plants experience throughout
their diurnal cycle.
Xylem water potential is measured using a
pressure bomb (Ri tchie and Hinckley, 1975) •
A
small leafy branch is cut from the plant and
sealed in a chamber with the cut end of the stem
.protruding from a gas-tight rubber stopper. Pressurized air is metered into the chamber until
fluid appears at the surface of the cut stem. The
positive pressure that is needed to force fluid
back to the end of the branch in the pressure bomb
is equal in number, but opposite in sign, to the
negative pressure of potential in the xylem at the
time the branch was cut.
Vegetation Transects
Vegetation
cover
characterfstics
of
the
plant community at each site is expected to
change as the plants are subjected to increased stress resulting from water table drawdown.
199
assist in the effective management of the groundwater resources in Owens Valley.
SUMMARY
By lowering water-table elevations, phreatophyte plant responses are being studied in relation to moisture availability in the soil.
Plant
responses are determined by measuring species
population,
vegetation
cover,
plant
growth,
phenological development, and xylem water potential.
The amount and availability of moisture
in the soil to plants is determined by measuring
volumetric
water
content
and
soil-moisture
potential.
LITERATURE CITED
Goodall, D. ~l.
1952.
Some consideration in the
use
of
point quadrats for analysis of
vegetation. Australian Journal of Scientific
Research
Series
B-Biological
Sciences,
v. 5, p. 1-41.
Meinzer, O. E.
1927.
Plants as indicators of
ground
water.
U. S.
Geological
Survey
Water-Supply Paper 577, 95 p.
Data from this study will be used to determine the minimum soil moisture required by the
phreatophytes and will be used in conjunction
with ground-water-flow models being developed by
the U. S. Geological Survey.
These models will
estimate the effects of ground-water withdrawal
on plant survivability w·hich can be used to
Ritchie, G. A., and T. M. Hinkley.
1975.
The
pressure
chamber
as
an
instrument
for
ecological research.
Advances in Ecological
Research 9: p. 165-254.
200