U.S. GEOLOGICAL SURVEY
Fact Sheet 94-074
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Accounting for Consumptive Use of Lower Colorado River Water in
Arizona, California, Nevada, and Utah
By Sandra J. Owen-Joyce and Richard P. Wilson
LOWER COLORADO RIVER
In the Colorado River valley between the east end of Lake Mead
and the international boundary with Mexico (see figure), the river
is the principal source of water for agricultural, domestic,
municipal, industrial, hydroelectric-power generation, and
recreational purposes. Water is stored in surface reservoirs and in
the river aquifer---permeable sediments and sedimentary rocks that
fill the lower Colorado River valley and adjacent tributary valleys.
The hydraulic connection between the river and the river aquifer,
overbank flow prior to building of the dams, and infiltration as the
reservoirs filled allowed the sediments and sedimentary rocks to
become saturated with water from the river. Ratios of isotopes of
hydrogen and oxygen in water from wells indicate that most of the
water in the river aquifer beneath the flood plain and in many
places beneath the adjacent alluvial slopes originated from the
river. The water table in the river aquifer extends from the river,
beneath the flood plain, and under the alluvial slopes until it
intersects bedrock. Precipitation in the surrounding mountains and
inflow from tributary valleys also contribute small quantities of
water to the river aquifer.
Consumptive use of river water in the valley results from
evapotranspiration by vegetation (crops and phreatophytes) on the
flood plain, pumpage from wells to meet domestic and municipal
needs, and pumpage from the river for export to areas in California,
Arizona, and Nevada outside of the river valley. Most crops are
grown on the flood plain; in a few areas, land on the adjacent
terraces has been cultivated. Crops were grown on about 70 percent
of the total vegetated area in 1984. Phreatophytes---natural
vegetation that obtains water from the river aquifer---covered the
remaining vegetated areas on the uncultivated flood plain. Most of
the water used for irrigation is diverted or pumped directly from
the river and reservoirs. Most of the water used for domestic and
municipal purposes is pumped from wells on the flood plain, on
adjacent alluvial slopes, and in tributary valleys. River water also
is delivered to Mexico in accordance with an international treaty.
The Colorado River basin and river aquifer
WHY ACCOUNT FOR CONSUMPTIVE USE?
In the United States, accounting for the use and distribution of
water from the Colorado River is required by law. Water in the river
has been apportioned among seven States and Mexico in accordance
with documents and laws known collectively as "The Law of the
River." The Colorado River Compact of 1922, the Boulder Canyon
Project Act of 1928, and the U.S. Supreme Court Decree of 1964,
Arizona v. California affect Arizona, California, and Nevada. The
Decree is specific about the responsibility of the Secretary of the
Interior to provide complete, detailed, and accurate records of
consumptive use of water diverted from the mainstream---"stated
separately as to each diverter from the mainstream, each point of
diversion, and each of the States of Arizona, California, and
Nevada." According to the Decree, "consumptive use from
the mainstream within a state shall include all consumptive uses of
water of the mainstream, including water drawn from the mainstream
by underground pumping * * *." The Bureau of Reclamation
manages the water resources of the Colorado River. The U.S.
Geological Survey has developed methods for water accounting---the
Lower Colorado River Accounting System (LCRAS) and the accounting
surface. Both methods are tools for river management to enable the
Secretary to comply with "The Law of the River."
LOWER COLORADO RIVER ACCOUNTING SYSTEM
LCRAS was designed to estimate the annual consumptive use of
river water by vegetation from the lower Colorado River and allow
equitable distribution of that use among water users between Hoover
Dam and Mexico. LCRAS uses estimates of consumptive use by
vegetation from a hydrologic water budget and data on vegetation
types and distribution from digital-image analysis of satellite data
to account for consumptive use by point of diversion, diverter, and
State. Consumptive use by vegetation is apportioned to diverters by
using the estimates of evapotranspiration calculated for each
diverter from the image-classification data and water-use rates.
Estimates of evapotranspiration are determined for each of four
areas delineated by dams along the river, which allows areal
variations in temperature to be considered in calculating water-use
rates. Domestic, municipal, and industrial consumptive uses of water
pumped from wells are components of the LCRAS water budget;
therefore, accounting for consumptive use from all these wells
provides a more equitable estimate and distribution of consumptive
use by vegetation to agricultural users.
ACCOUNTING SURFACE
The accounting surface can be used to identify wells that yield
water that will be replaced by water from the river and provides a
uniform criterion of identification, based on hydrologic principles,
for all users who pump water from wells. The accounting surface was
generated for the reach that begins at the east end of Lake Mead and
ends at Laguna Dam by using profiles of the Colorado River and
annual high water-surface elevations of reservoirs, lakes, marshes,
and drainage ditches. The accounting surface represents the water
table of the river aquifer that would exist if the only source of
water to the aquifer were the river. Wells completed in the river
aquifer between the boundaries of the flood plain and river aquifer
with a static (nonpumping) water level at or below the accounting
surface are presumed to yield water that will be replaced by water
from the river. Wells with a static water level above the accounting
surface are presumed to yield water that will be replaced by
precipitation and inflow from tributary valleys. Water pumped from
wells on the flood plain is presumed to be river water.
IDENTIFYING WATER USERS
Accounting for the consumptive use of Colorado River water
requires the identification of all points of withdrawal of
water---diversion from the river by means of canals or pumps in the
river and pumpage from wells on the flood plain and adjacent
alluvial slopes within the boundary of the river aquifer (see
figure). Most points of diversion from the river channel and
reservoirs have been identified and included in the present
accounting procedure; however, this accounting is incomplete because
the existence and location of all wells are unknown. A well
inventory is needed to locate wells, provide current information for
each well, and provide precise position and elevation information
necessary to identify wells that yield water that will be replaced
by river water. About 4,000 wells were inventoried between 1960 and
1993; ownership information, elevation of static water level, use of
water, disposal of unconsumed water, and wellconstruction
information need to be updated. All new sites and those previously
inventoried must be located precisely to ensure that the correct
well can be revisited for future data collection and to ensure that
well ownership is properly identified.
SELECTED REFERENCES
Owen-Joyce, S.J., 1987, Estimates of average annual tributary
inflow to the lower Colorado River, Hoover Dam to Mexico: U.S.
Geological Survey Water-Resources Investigations Report 87-4078, 1 sheet.
-----1992, Accounting system for water use by vegetation in the
lower Colorado River valley: U.S. Geological Survey Open-File Report
92-83, 2 p.
Owen-Joyce, S.J., and Raymond, L.H., 1996, An accounting system
for water and consumptive use along the lower Colorado River, Hoover
Dam to Mexico: U.S. Geological Survey Water-Supply Paper 2407, 3
plates, 94 p.
Robertson, F.N., 1991, Geochemistry of ground water in alluvial
basins of Arizona and adjacent parts of Nevada, New Mexico, and
California: U.S. Geological Survey Professional Paper 1406-C, 90 p.
U.S. Congress, 1948, The Hoover Dam documents: U.S. Congress,
80th, 2d session, House Document No. 717, 936 p.
U.S. Supreme Court, 1964, State of Arizona, plaintiff v. State
of California, et al., defendants: Decree-March 9, 1964, no. 8,
original, 14 p.
von Allworden, B.K., Owen-Joyce, S.J., Sandoval, J.D., and
Raymond, L.H., 1991, Lower Colorado River Accounting System (LCRAS)
Computer Program and Documentation: U.S. Geological Survey Open-File
Report 91-179, 237 p.
Wilson, R.P., and Owen-Joyce, S.J., 1993, Determining the source
of water pumped from wells along the lower Colorado River: U.S.
Geological Survey Open-File Report 93-405, 2 p.
-----1994, Method to identify wells that yield water that will
be replaced by Colorado River water in Arizona, California, Nevada,
and Utah: U.S. Geological Survey Water-Resources Investigations
Report 94-4005, 19 plates, 36 p.
For further information, contact:
Sandra Owen-Joyce
U.S. Geological Survey, WRD
520
North Park Avenue,
Suite 221
Tucson, Arizona 85719-5035
(520) 670-6671
E-mail: sjowen@usgs.gov
or visit the main website:
http://az.water.usgs.gov