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Each alternative operating stragegy was characterized by a common parameter: the assurance of avoiding releases in excess of the surplus releases scheduled for that strategy or, equivalently, the assurance that the natural flow at Lees Ferry in the coming year would not exceed the volume associated with that particular strategy. Each operating strategy is similar; but, the controlling difference is the degree of assurance assumed. This assurance le~el is stated as a probability that serves to identify each strategy. The 0.70, 0.80, D.90, and 0.99 assurance levels were used in this study. For example, the 0.90 operation strate9Y schedules the appropriate surplus releases to assure that the reservoir system would not exceed the maximum permissible storage level by the end of the water year 90 percent of the time. fn other words, nonscheduled surplus releases could be expected 10 percent of the time. The percentile level operation strategies provide a broad spectrum of alternatives and cover a wide range of operation. If the assurance level is low enough, the strategy's results closely approximate those of the base case. As the assurance level increases, by definition, a higher proportion of surplus releases are made, and the carryover storage in the system is reduced. The annual release from Lake Mead is the first variable computed in the alternative operation strategies. Lake Mead is also the location where the flood control criteria is implemented. The strategy- associated surplus releases from Lake Mead are determined once per year at the beginning of the water year (October 1). For a particular operation strate9Y (.95 for example), the annual release is calcu- lated so that, for 95 percent of the annual inflows, no unscheduled surplus releases will be required to end the water year with the system storage at or below the desired maximum level for this strategy. ft is this annual surplus release from Lake Mead that is to be sche- duled in a beneficial manner throughout the year. Once the annual surplus release has been determined for Lake Mead, the surplus is divided into three categories. First, surplus is given equally to Central Arizona Project (CAP) and Metropolitan Water District (MWD). This surplus, referred to as Category f, cannot exceed 1.3 million acre-feet per year (a limit dictated by CAP and MWD diver- sion capacities and demand schedules). Next, a Category If surplus not to exceed 200,000 acre-feet is given to Mexico. Finally, addi- tional surplus releases (Category ff!) can be made for operational flexibility and power generation. Surplus releases from Lake Powell are computed based on target storage, mean annual flow at Lees Ferry, present contents, Upper Basin depletions, and an assumed minimum objective release. [n no case, are any strategies allowed to alter releases that are required to maintain 602(a) storage. A more detailed discussion of the strategies can be found in Section 3 of the main report. 7