WSP06362

• File Number: 8210.470
• Description: Pacific Southwest Interagency Committee
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 11/1/1971
• Author: PSIAC
• Title: Pacific Southwest Analytical Summary Report on Water and Land Resources based on Framework Studies of Four Regions - November 1971
• Water Supply Pro - Doc Type: Report/Study

Water Resources

water from both surface and groundwater sources
and the mean annual remaining streamflow for 1965
conditions of development were estimated by each
of the regions.
In the California Region, the gross annual deple-
tion in 1965 was 25.2 million acre.feet (MAP). This
estimate included all consumptive uses and losses
related to servicing all agricultural, domestic, and
industrial purposes. The California Region depletions
include about 5 MAP of Colorado River water not
included in the regional streamflow, and, in addition,
a net groundwater overdraft of about 2.6 MAP
annually. Of the remaining regional streamflow, 39.6
MAP represents the potential available for future
development.
The total depletions of the Great Basin are esti.
mated at 6 MAP annually. The Great Basin Region
supply available for depletion includes 1.1 MAP of
natural inflow from the California Region and 0.1
MAP imported from the Upper Colorado Region.
The natural depletion of 1.1 MAP from nonirrigated
wet meadows is included in the Great Basin deple.
tions. The remaining streamflow that could be devel.
oped is about 2.2 MAP per annum. However, if the
Great Salt Lake, Pyramid Lake, and Walker Lake are
to be maintained at about the 1965 year-end area
and stage, a combined equivalent of principal stream
inflow of 2.3 MAP a year would be required. To
maintain the lakes at these levels, other principal
stream inflow should be about 1.8 MAP, 0.37 MAP,
and 0.15 MAP for the Great Salt Lake, Pyramid Lake,
and Walker Lake, respectively. Modified inflow to the
lakes for the reference period, 1931-60, was 1.5,0.24,
and 0.08 MAP, respectively. The average annual
deficit thus at the 1965 level of development was
about 0.5 MAP.
The 1965 depletions of the Lower Colorado
Region were estimated at 5.8 MAP, of which 2.5
MAP comprise groundwater overdraft. The Upper
Colorado Region 1965 depletions, including regu.
lating reservoir evaporation, totaled 3.5 MAP.
The apparent remaining water supply in the
Colorado River in 1965 was estimated at about 2.1
MAP. This results from a depletion of 3.5 MAP in
the Upper Colorado Region (including evaporation
losses and imports), and in depletions in the Lower
Colorado Region of about 1.3 MAP, a net export to
the California Region of about 5.0 MAP, delivery
of 1.5 MAP. to Mexico, and 2.5 MAP of main stem
and channel evaporation losses and system spills.
Large withdrawals of groundwater began some.
what later than withdrawals (diversions) of surface
water. Groundwater "mining" - depletion of ground~
water in amounts that exceed replenishment - has
occurred in many parts of the Pacific Southwest. The
greatest mining has occurred in southern Arizona,
California, and Nevada. In some localities overde-

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velopment at groundwater is restricted by law, thus
preventing large declines of the groundwater level and
consequent problems.
In the California Region, overdrafts have occurre<i
in south coastal valieys but these have been largely
corrected by use of floodwater and imported water to
augment the natural recharge. Most of the remaining
overdraft is in the southern part of the San Joaquin
Valley.
In the Great Basin Region, estimated groundwater
withdrawal in 1965 was 1.0 million acre.feet. In
certain cases concentrated pumping has caused local.
ized overdraft and water is being pumped from
one.time groundwater storage. This depletion is esti.
mated to be 97,000 acre.feet per year. About
600,000 acre-feet of groundwater is considered to be
reasonably available to meet projected uses on a
sustained basis.
In the Lower Colorado Region, serious overdraft
has occurred chiefly in the Phoenix.Tucson area in
Arizona and also in the Las Vegas area in southern
Nevada. The regional overdraft represents a large
proportion of the total annual use.
Among the undesirable effects of the long.
continued water level declines that accompany over-
draft are: (I) depletion of streams in the area (long
reaches of the Lower Gila River in Arizona had no
flow from 1941 to 1965), (2) increasing costs of
constructing wells and pumping water, (3) degrading
quality of water with depth and with reuse,
(4) induced recharge from a polluted or saline source,
and (5) structural damage or flooding caused by land
subsidence. If pumping is not restricted, only higher
economic uses ultimately will be accommodated.
Although such overdrafts have been prime factors in
the economic development of parts of the Pacific
Southwest, continuation of these practices obviously
cannot be endured indefinitely. The mean annual
quantities of groundwater withdrawals and overdraft,
adjusted to 1965 conditions of development, for each
of the regions are:

Withdrawal Overdraft
(Millions of acre.feet)

California 14.8 3.2
Great Basin 1.0 0.1
Lower Colorado 5.1 2.5
Upper Colorado . . 0.1 .
Pacific Southwest . . 21.0 5.8

Problems of Future Development

The most serious problems of future water supply
development concern:
1. The insufficiency of streamflow in the Colo-
rado River to meet projected demands;