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<br />Projected Plan
<br />
<br />A proposed operation plan for water }<'ar 1991 for major
<br />reservoirs of the Colorado River system was formulated and
<br />distributed to rep[esentati~s of the Colorado River Basin
<br />States in November 1990. This plan was prepared in
<br />accordance with the Operating Criteria published June 4,
<br />1970, in compliance with Section 602, Public Law 90-537. The
<br />plan reflects operation for flood controL river regulation,
<br />beneficial consumptive uses, hydroelectric power generation,
<br />water quality control, enhancement of fish and wildlife,
<br />recreation, and Colorado River Compact requirements.
<br />
<br />The 1991 operation plan reflects the effects of below a"'rage
<br />reservoir inflow during 1988, 1989 and 1990. However,
<br />recognizing the system storage that is available and the
<br />beneficial water needs of the basin states, requests for
<br />Colorado Ri"'r water by holders of water deli",ry contracts
<br />with the United States and by holders of othcr water rights
<br />recognized by the decree in Arizona v. California. will be
<br />satisfied during calendar }<'ar 1991 up to 7,500,000 acre-feet
<br />of beneficial consumpli~ use in the Lower Basin.
<br />
<br />The Colorado Ri"'r Basin has experienced below normal
<br />precipitation for the last few }<'ars resulting in depleted
<br />reservoir storage, very dry soil moisture conditions. lowering
<br />v.rater tables, and below normal streamflows. Because of these
<br />conditions, the operation plan for water year 1991 emphasizes
<br />the conservation of reserwir storage by providing minimum
<br />releases necessary from Colorado River basin reservoirs to
<br />satisfy water rights according to "The Law of the Ri"'r".
<br />
<br />For 1991 operations, three reservoir inflow scenarios 'Were
<br />de ",loped and analyzed. The projected monthlyinflo\\5 were
<br />based upon current hydrological conditions and the following
<br />assumptions: (I) reasonable maximum, based upon the
<br />annual wlume of inflow which would be exceeded about 10
<br />percent of the tiroe. (2) most probable, based upon annual
<br />volume of inflow which would be exceeded about 50 percent
<br />of the time; and (3) reasonable miniroum, based upon the
<br />annual wlume of inflow which would be exceeded about 90
<br />percent of the tiroe. Each of these scenarios was adjusted for
<br />current soil moisture deficiencies throughout the basin;
<br />therefore, each is lower in magnitude than the historical upper
<br />decile, mean, and lower decile inflo\\5. The National Weather
<br />Service computer model used to adjust the scenarios \\'as the
<br />Extended Streamflow Prediction model. This model
<br />calculated the projected unregulated inflo\\5 for water }<'ar
<br />1991 above Lake Powell as 14,702,000, 8,655,000, and
<br />5,391,000 acre-feet for the three scenarios, respecti",ly.
<br />
<br />The plan for water year 1991 calls for a total Glen Canyon
<br />release of 8.23 million acre-feet under all three assumed
<br />inflow conditions. Hoo",r Dam releases (including pumping
<br />from Lake Mead) will be sufficient to satisfy up to 7,500,000
<br />acre-feet of reasonable beneficial consumptive use
<br />requirements by mainstream users in the LO'Wet Basin during
<br />calendar year 1991 in accordance with Article 111 of the
<br />Operating Criteria and Article Il(B)( I) of the Decree in
<br />Arhona Y. California. Because Arizona and Nevada wiU not
<br />fully consume their respective apportionme.nts pursuant to
<br />Article II(B)(I), California will be allowed to utilize
<br />apportioned but unused \\I3.ter from these States, provided
<br />that the calendar year 1991 consumptive use by mainstream
<br />Lower Basin users does not exceed 7,500,000 acrc.feet.
<br />
<br />Special long-range studies using Reclamation's Colorado
<br />Ri"'r Siroulation System (CRSS) were conducted to test the
<br />iropact on the risk of future shortages if California, during
<br />calendar year 1991, were allowed to use water apportioned to,
<br />but unused by, Arizona and Nevada. The studies were
<br />conducted using the entire hydrologic record (1906 through
<br />1985), utilizing 80 hydrologic sequences indexed by one }<'ar.
<br />These studies siroulated 50 years into the future to test the
<br />risks of shortages to all users of Colorado River water for
<br />beneficial purposes and evaluated several specific
<br />parameters. These studies indicate that for the next 20 years
<br />there are: (I) no increase in the risk of shortages; (2) reduced
<br />risk of flood control releases; and (3) expected reduced exccss
<br />deli'teries to Mexico.
<br />
<br />Because of the large vacant storage space in the Colorado
<br />Ri"'r system reserwirs at the beginning of water }<'ar 1991,
<br />no flood control releases are anticipated from Hoover Dam
<br />pursuant to the Hoo",r Dam Flood Control Rcgulations, and
<br />no releases are contemplated from Glen Canyon Dam to
<br />a\Uid anticipated spills or to equalize active storage in Lakes
<br />Mead and Powell. Water releases from each of the Colorado
<br />River s)Stem reservoirs will be in accordance v.ilh existing
<br />minimum flow. reservoir operating criteria, target storage
<br />elevations, and, with the exception of Fontenelle, all releases
<br />will pass through the powerplants. The resulting operation
<br />will provide benefits to all the authorized project purposes at
<br />each of the reser'\Oirs.
<br />
<br />The projected operation for most probable runoff conditions
<br />for the major reservoirs in the Colorado Riycc Basin for water
<br />year 1991 is described in the following pages. Charts showing
<br />projected releases, inflows and storage, for the three assumed
<br />hydrologic conditions. are presented with each reservoir
<br />operation.
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