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

authorized in 1956, developing the waters of the
Upper Colorado Basin. Also, the Congress has autho-
rized for construction the Central Arizona Project
which will deliver Colorado River water to the
Phoenix and Tucson areas of central Arizona, to other
portions of Arizona, and to New Mexico by exchange.

Surface Water Supplies

The Pacific Southwest is predominantly arid and.is
among the driest areas in the Nation. The precipita.
tion - ranging from 2 to 120 inches annually -
produces runoff (undepleted streamflow) ranging
from near 0 to more than 80 inches. Most of the
Pacific Southwest has less than I inch of runoff.
Precipitation and runoff are shown in figure 3. Only
relativelY small areas, in high mountains or near the
coast in the northern part of California, have more
than 20 inches of runoff. Major streams are perennial
but many smaller ones are intermittent or ephemeral.
Yet there are streams with substantial drainage areas
that havebe~n known to be dry for many years.
The mean annual runoff in most river basins is an
index of the amount of water (including ground-
water) that could be developed from that basin on a
sustained-yield basis. The mean annual consumption
. of water from surface sources is generally less than
the runoff because of the impracticability of com.
plete regulation of all floods, and the necessity or
desirability of maintaining minimum flows for in-
stream use.
The mean annual undepleted streamflow in the
Pacific Southwest area is shown in table 4.
More than half of this streamflow occurs in the
California Region north of San Francisco and Sacra-
mento. During extremely dry years the runoff in the
entire area may be only a third of the mean.
Streamflow in the California Region is concen'
trated largely in the north coastal area and the

, '"

. .
" "-"

Sacramento River Basin (including the western slope
of the Sierra Nevada in central California).
In the Great Basin Region, streamflow is concen.
trated largely in the Truckee, Carson, and Walker
Rivers which originate in California and flow into
western Nevada; the Humboldt River in northern
Nevada; the Bear, Weber, and Jordan Rivers in the
Great Salt Lake Basin; and Sevier River in central
Utah. Muchof.the streamflow in the Region is reused
and return flows are an important part of the water
supply. All streams in the Region terminate in
permanent lakes or playas (intermittent lakes) where
the water is mainly consumed by evaporation and
transpiration.
In addition to the annual runoff generated within
the Great Basin Region, 1.1 miJIion acre.feet of
principal streamflow enters the Region from the
California Region, and a net import of about 0.1
miJIion acre.feet is diverted to the Region from the
Colorado River regions.
Most of the runoff in the Colorado River Basin
originates in the high mountain areas that rim the
Upper Colorado Region.The estimated annual virgin
runoff in the Colorado River at Lee Ferry, Arizona -
the compact point between the Upper and Lower
Colorado Basins - has ranged from 5.6 to 24.0
million acre-feet. The lO-year means have ranged
from 11.6 to 18.8 miJIion acre-feet. Consequently,
opinions differ concerning the period of record that
best represents future runoff. The long.term (1906-
1970) mean annual flow is 14.9 million acre-feet. Of
significance is the fact that a period of about 25
years (1906-1930) of predominantly above.average
runoff was followed by a40.year period (1931-1970)
of predominantly below-average runoff.
Below Lee Ferry, tributaries in the Lower Colo.
rado Region would contribute, if undepleted, about
2.4 million acre-feet to the river. This is 700,000
acre-feet less than the estimated runoff for the Region
because of in,region losses before the runoff joins the

Table 4.-Mean annual undepleted streamflow

Streamflow
(Millions of acre-feet)

California
Great Basin
Lower Colorado
Upper Colorado

Pacific-Southwest

~

70.2
6.2a
3.1b

~

94.4

1931-60
1931-60
1914-65
1914-65

a Does not include inflow to the region from the California region.
b Does not include inflow to the region from the Upper Colorado region.

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