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r"",\<f)'77 UtJU01 RECOMMENDED RESERVOIR MANAGEMENT CRITERIA FOR LOWER COLORADO RIVER IN THE LOWER DIVISION STATES USBR - Lower Colorado Region 9/27/95 Synopsis Reclamation recommends a shortage criterion of 80% assurance of protecting Lake Mead elevation 1050 feet; a surplus criterion of 80% assurance of not triggering a shortage; and distribution of surplus and unused apportionment as proposed in Report No. 4 of the Lower Colorado River Basin Technical Committee. " ), Backqround Many surplus and shortage strategies have been examined in the past few years. In 1986, Reclamation published a surplus study for avoiding spills in response to. the 1983 flooding that occurred downstream of Lake Mead. A surplus strategy based on a 70% assurance of avoiding flood control releases was recommended to permit increased use in the United States and to reduce excess deliveries to Mexico that occur during conditions of high runoff and full reservoirs. As of yet, no surpluses have been determined in the Lower Basin based on this strategy, which is herein referred to as the "base case." A strategy to deal with drought and shortage was not developed because the reservoir system was full. For planning purposes a fixed Lake Mead shortage trigger elevation was used to protect the Hoover powerplant minimum rated power head elevation of 1083 feet. I As concern developed that future scheduled unused Arizona apportionments might not be sufficient to meet the Metropolitan Water District of Southern California's and Southern Nevada Water System's diversion 'requests, new ideas for meeting their needs became more urgent. The Colorado River Management and Planning Studies (CRMPS) were begun in 1991 to investigate other surplus and shortage strategies. These studies developed operating strategies based on setting an elevation of Lake Mead and protecting that elevation to any given level of confidence. The storage levels in Lake Mead that would trigger shortage and surplus would increase as uses in the Upper Basin increase. In 1994, during consultations for development of the 1995 Annual Operating Plan (AOP), Reclamation presented results of these studies showing that for the current level of use in the Upper Basin there was more than sufficient system storage to meet all beneficial needs in the Lower Basin. Although these studies determined that surplus water was available, Arizona and Nevada (who would bear reductions below their basic apportionments during a shortage) would not support surplus determinations until shortage criteria were also adopted.