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

002681

arid areas; erosion occurs where susceptible soils are
irrigated on slopes. Passage of water through the soil
profile tends. to leach out nutrients, particularly if
large applications are made for removal of the excess
salts in the soil.
Water and land requirements for irrigation have
been derived by translation of commodity projections
into needs for land, and by application of water-use
factors to those acreages. In the process, adjustments
have been made to compensate for the effects of
technological advances expected to take place in
application of water, control of pests, introduction of
new plant strains, and improvement of fertilizers. The
resulting crop yield rates and water demand values are
highly consistent among the four regions.
The water need for irrigation in 2020 exceeds the
i 965 use by about 29 percent and the land needs
increase by some 25 percent. Land is available but
sufficient water is not.

Irrigation

Status

At the beginning of the study period (1965),
major irrigation development had been going on in
the Pacific Southwest for about 70 years. Few
opportunities to bring good water and suitable land
together at a tolerable cost had been overlooked.
Some 13.3 million acres were in production, using 36
million acre-feet of water annually. Planning agencies
were becoming concerned in the later phase of
development during which substantial acreages were
going out of irrigated farming into urban use.
Water was provided by individual wells and diver-
sions, by local group enterprises, and by tremendous
multipurpose projects exemplified by Hoover Dam
and Shasta Dam, with their complex related systems
for controlling and distributing water.

Needs and Problems

Irrigated agriculture is the largest user of water by
far under all projections. The water supply must be
reiatively constant from year to year as well as
inexpensive if irrigation enterprises are to survive.
This is particularly true in the Pacific Southwest
where irrigable land often is remote from water
supplies. The problem of remoteness can be solved by
the installation of conveyance systems, often at high
cost.
Reservoirs fulfill the vital function of making the
water available in the summer months when it is
needed. The season of greatest precipitation or runoff
occurs in winter in some areas, and in spring or very
early summer in others. The reservoirs also perform
the same function on a year-to-year basis by provid-
ing carryover storage from wet or normal years for
use in dry years.
In the lower reaches of the Colorado River, a large
quantity of salts in the water supply traditionally has
constituted a problem. The increasing return of
drainage from irrigated tracts to the river intensifies
this problem in that area and throughout much of the
Pacific Southwest.
Soil problems common to most irrigated areas are
present in the Pacific Southwest. Uniform distribu-
tion of irrigation water over the fields is difficult to
achieve, and crops become spotty if some areas
receive too much or too little water. Waterlogging of
soils takes place under many soil conditions found in

Plan Response

Water. - All of the four regional framework plans
at base plan level employ the various means of
providing water discussed above. Each one places
emphasis upon multipurpose measures such as reser-
voirs to supply water for several purposes, as well as
to provide floodwater detention capacity and water-
based recreation opportunity.
Figure 8 presents the irrigation water requirements
(depletions) of the regions of the Pacific Southwest
for each of the time frames studied.
The amounts of irrigation water shown for the
Lower Colorado Region do not include desalted
water made available specifically for irrigation. How.
ever, the amounts shown are based on the premise
that the Colorado River will be augmented under the
provisions of P.L 90-537, at least to the level that
wiil relieve the various States of the burden of the
Mexican Water Treaty. This initial augmentation, for
the purpose of providing water to Mexico under the
terms of an international treaty, satisfies decreed
entitlements without shortages under most water
supply conditions. The plan does provide for storage
and conveyance of the augmentation water. Ground-
water overdraft will be reduced but not eliminated.
The plan for the Great Basin provides for more
withdrawal from groundwater bodies than is now the
case, additional imports of surface water from the

45