WSP02037

• File Number: 8200.700
• Description: Colorado River Basin - General Publications
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 11/1/1979
• Author: US Forest Service
• Title: Managing Vegetation to Increase Flow in the Colorado River Basin - US Forest Service
• Water Supply Pro - Doc Type: Report/Study

00071)1

is insufficient waler surplus to the apportion-
ments. Together. these commitments totaled 16.5
million acre.feet. a not unrealistic demand on
Upper Basin water yield based on records prior to
1922. Since then. however. undepleted flow at lee
Ferry has averaged less than 14 million acre-feel,
\',:hich many believe is about all that can be ex-
pected ove; the long run. Long-term streamflow
records. reconstructed from tree rings by Stockton
and Jacoby (1976). indicate that for the past 450
years, flow al Lee Ferry has averaged 13.5 million
acre-feet. Their analysis showed that only one
other period. the early 1600's, was as wel as the
first 30 years of this centurv.
While present mainstwam waler lIses of about 12
million acre. feet per year in both Upper and Lower
Basins ilre below the long-term trend in average
annual water supply, Ihis supply is less than the
apportionments and the ~fexican delivery obliga-
tion. Future water needs for development of the
Basin's energy resources ilnd other uses have been
forecast 10 exceed supplies by aboul the year 2000.5
Water yield within Lower Basin watersheds is
about 3.] million acre-feet per year, much ofwhich
is used in Arizona. Additional large amounts of
water are pumped from ground water 10 supply
agriculture and municipal-industrial needs.
Ground water overdraft in the Lower Basin aver-
ages nearly 2.5 million acre-feet per year. To sus-
tain the current use rate will require continued
overdraft of ground water, even with complelion of
the Central Arizona Project. unless the supply can
be augmented.

"USDI. Water frx EIIergy Management Team Report on waler
for energy m the Upper Ct:JkNado River Basm 1974 71 P

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Increasing Waler Yield
by ~Ianagement of Vegetation and Snow

Of lhea\'erage 190million acre-feet (14.2 inches)
of rain and snow that fall each vear on the C'.olorado
River Basin, more than 90% of it evaporates. With
such large amounts of water being returned to the
atmosphere, e\'en a slight reduction in this loss
would leave substantiallv more water for
streamflow. If. for example" evapotranspiration
over the entire C..olorado River Basin could be re-
duced by only 1%, the surface water supply would
increase on the average bv 1.75 million acre-feet
annually. However. the opportunity to signifi.
cantly reduce evapotranspiration b}' management
of vegetation and sno\\>' is limited to certain types of
cover. Only about 16% (26 million acres) of the
basin is vegetated well enough or has sufficient
snO\..' to be suitable for water yield improvement
measures.
Vegetation is controlled largely by climate.
Areas of low rainfall dominate th(! I.ower Basin,
where vegetation is sparse and drought resistant.
Precipitation generally increases with elevation
and latitudc, until at mid to upper elevations,
brush, pinyon.juniper, and associated types are
replaced by forests. Eventually, the forests give
way to the cold. harsh climate of the alpine zone,
where only low-grnwing forms of vegetation sur-
vivc. Precipitation varies from less than 5 inches
annually in the deserts to more than 50 inches in
the wettest mountain areas. Potential evapotrans.
piration tends to be inVCrSI!ly related to precipita-
tion, because energy for evaporation declines with
increasing elevation, latitude, and cloudiness.

Progressive
IO-year average

Average
11896-1978)

FIgure 2.-Annual undlpleted (virgin) !lows ot the Colorado River II lee Ferry. Arizona,
1896-1918.

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