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Without any excess deliveries, what would
<br />happen in the Lower Basin? System gains
<br />between Lake Powell and Lake Mead are
<br />estimated to be in the range of 600,000
<br />to 750,000 af/year (Tipton), providing
<br />an inflow to Lake Mead of 8.85 to
<br />8.95 maf/year, or approximately 9.0 maf/
<br />year. In an average year, the demands on
<br />Lake Mead are:
<br />California, Nevada,
<br />and Arizona 7.5 maf /year
<br />Mexico 1.5 maf/year
<br />Evaporation and
<br />system losses 1.0 to 1.5 maf/vear
<br />Total demands 10.0 to 10.5 maf /year
<br />Under this scenario, which also represents
<br />an average flow year under current storage
<br />capacity, demands on Lake Mead exceed
<br />the supply by 1.0 to 1.5 maf/year. If the
<br />Lower Basin no longer received any
<br />excess flows from the Upper Basin, the
<br />Lower Basin would ultimately always be
<br />short by this amount, and under the Law
<br />of the River, the Central Arizona Project,
<br />Nevada, and perhaps Mexico would bear
<br />the brunt of the shortage.
<br />Although storage in the Upper Basin is
<br />large, it is not unlimited. Even with a
<br />mean flow of 13.5 maf/year, wet cycles
<br />would still increase the volume of Lake
<br />Powell, triggering equalization releases to
<br />Lake Mead, and dry cycles would trigger
<br />shortages of greater than 1.0 maf /year.
<br />However, excess releases will be far less
<br />frequent than under the 15 maf/year mean
<br />flow under which we currently operate
<br />the system.
<br />Short or Long -Term Planning?
<br />As the basin states face the challenge from
<br />Secretary Norton to address river shortage
<br />conditions, officials face a fundamental
<br />choice: adopt a strategy to take us through
<br />the current drought using Reclamation's
<br />assumed 15 maf/year mean at Lee Ferry,
<br />or assume a lower mean flow as supported
<br />by Tipton and the tree ring record and plan
<br />for the long -term future. If we assume
<br />the 15 maf/year mean at today's level of
<br />depletions in the Upper Basin, Lake Mead
<br />and Lake Powell will ultimately recover.
<br />But how bad will the situation get before
<br />recovery occurs and when can we expect
<br />it to happen? If we assume a 13.5 maf/
<br />year mean, the Upper Basin states will
<br />have to recognize they are at or near full
<br />development. And Lower Basin states will
<br />have to prepare to permanently reduce
<br />consumptive uses by at least a million
<br />acre -feet per year.
<br />Eric Kuhn, the general manager of the Colorado
<br />River Water Conservation District, can be reached
<br />at ekuhn @crwcd.org. The District is a political
<br />subdivision of the State of Colorado, covering the
<br />entire Colorado River Basin within Colorado outside
<br />the San Juan and Upper Dolores River Basin.
<br />References
<br />Stockton, C. W, and G. C. Jacoby, 1976. Long -term
<br />surface- waters supply and stream flow trends in
<br />the Upper Colorado River Basin, Lake Powell
<br />Research Project Bulletin No. 18, Inst. of
<br />Geophysics and Planetary Physics, University
<br />of California at Los Angeles.
<br />Tipton and Kalmbach, Inc., 1965. Water Supplies of
<br />the Colorado River, July 196.5. Prepared for the
<br />Upper Colorado River Commission.
<br />United States Department of Interior, Colorado
<br />River System Consumptive Uses and Losses
<br />Report. The 1996 -2000 final report has not yet
<br />been issued but table UC -1 through UC -8 is
<br />available on the USBR Upper Colorado River
<br />Region, website at www usbrgov /uc/library/
<br />envdocs/reports/crs/crsul.html.
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<br />March /April 2005 • Southwest Hydrology • 21
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