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<br />
<br />about 730,000 acre-feet. In 1975, precipita-
<br />'''~tion and runoff returned to nearly normal.
<br />, However, cool temperatures during the growing
<br />;c..) season reduced irrigation demands. Reser-
<br />,.g voir storage reqovered from the previous
<br />...... year's drawdown.
<br />Reservoir eva'poration, also primarily af-
<br />fected by clima~ic conditions, demonstrated a
<br />pattern of varia#on similar to that of irrigation
<br />consumptive us!!.
<br />. Transbasin exports, the second largest
<br />Upper Basin us~, showed the greatest year-
<br />by-year variation)and also the greatest net in-
<br />crease during thl! reporting period. In 1971,
<br />exports totaled ~83,000 acre-feet. In 1975,
<br />exports had rise~ to 815,500 acre-feet
<br />primarily due to the opening of the Boustead
<br />Tunnel in Colorado and Azotea Tunnel.
<br />which outlets in New Mexico.
<br />Thermal powerlwater uses in the Upper
<br />Basin more than qoubled in the 5-year report-
<br />ing period as four; major powerplants went into
<br />operation: San Ju~n (New Mexico) in 1973;
<br />Navajo (Arizona) ip 1974; Jim Bridger (Wyom-
<br />ing) in 1974; andiHuntington (Utah) in
<br />1975. ..
<br />During the 5-ye~r reporting period,
<br />main stem regulatj'ng reservoirs recorded an
<br />Increase of 9,906,;000 acre-feet of surface
<br />storage. As storagei,increased, main stem re-
<br />servoir evaporationkose from 458,000 acre-
<br />feet jn 1971 to 60r.000 acre-feet in 1975.
<br />
<br />LOWER CQLORAOO RIVER
<br />
<br />Water use within the Lower Colorado River
<br />system is increasing~ as a result of additional
<br />irrigated acreage and a fast-growing population.
<br />Irrigated land has inCreased from about
<br />1,285,000 acres in \1971 to 1,440.000 acres
<br />in 1975. Population !In 1970 was estimated
<br />to be.about 2.1 million, and 2.6 million in 1975.
<br />
<br />Mainstream
<br />Table lC-l shows ),vater-surface evaporation
<br />from mainstream reservoirs and channel
<br />losses; table lC-2. tti,e change in surface-water
<br />contents of the resenl,oirs; and table LC-3,
<br />water uses along the ~ower Colorado River
<br />mainstream and flood; plain including water
<br />passing to Mexico. Water passing to Mexico is
<br />
<br />made up of deliveries in satisfaction of the-
<br />Treaty, deliveries made pursuant to Minutes
<br />218, 241, and 242, and regulatory waste.
<br />Mainstream reservoirs gained about 3.4 mil- .
<br />lion acre-feet of surface storage during the
<br />5-year reporting period. Water supplies
<br />necessary to meet the mainstream water use,
<br />including reservoir surface and bank storage,
<br />came principally from the regulated releases
<br />at Glen Canyon Dam.
<br />Annual reservoir evaporation and channel
<br />losses consumed about 1. 5 million acre-
<br />feet. Table LC-9, a water budget below Davis
<br />Dam, results in an estimate of the overall
<br />channel losses in the reach to the International
<br />Boundary. Irrigated land has Increased from
<br />\ about 331,000 acres in 1971 to 351,000
<br />acres in 1975-most of the increase occur-
<br />ring in the Colorado River Indian Reserva-
<br />tion. Municipal and industrial water use, in-
<br />cluding thermal powerplants in Nevada and
<br />Arizona, doubled during the 5-year period.
<br />Much of this demand is within southern
<br />Nevada. Pursuant to Minutes 218 and 242,
<br />saline return flows from the Wellton~Mohawk
<br />Irrigation and Drainage District near Yuma,
<br />Ariz., were bypassed around Morelos Dam at
<br />the International Boundary resulting in a sub-
<br />stantial increase in the water passing to Mexico
<br />in excess of the Treaty requirements. Project
<br />plans to implement the United States measures
<br />required by Minute 242 call for reduction of
<br />bypassed water through improved irrigation
<br />efficiencies, reduced acreage to be irrigated
<br />on Wellton-Mohawk Project lands, and the
<br />construction of a desalting plant converting
<br />drainage water to an acceptable quality for de-
<br />
<br />"
<br />;..,'
<br />
<br />WalerYear
<br />
<br />Avera15e 1971-
<br />1975
<br />
<br />COllltadD Rjver af Estjmated '''''
<br />Compact Poinl, near Tributary
<br />lee ferry, Arizona Inflow to
<br /> MaInstream
<br />(MAF) (MAfl (MAF)
<br />8.61 097 9.58
<br />9.33 0.78 10.l!
<br />10.14 2.12 12.26
<br />8.28 0.85 9.13
<br />212 0.94 10.21
<br />9.13 1.13 iO.26
<br />
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<br />~.
<br />~,
<br />~.
<br />,
<br />~;
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<br />
<br />1971
<br />1972
<br />1973
<br />1974
<br />1975
<br />
<br />17
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