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, . If the strategy were based on assurance of spill avoidance, the amount of assurance or assurance level would be input into the model as a percent probability. For example, an assurance level of 0.70 determines a Spill Avoidance Space adequate to capture a flow that has not been exceeded 70 percent of the time of 1906 to 1990. Increasing the assurance level, increases the design magnitude of the Spill Avoidance Space required, decreasing the threshold elevation and generally increasing the likelihood of a surplus determination while decreasing the likelihood of spill. The Assurance of Spill Avoidance method takes uses into account while the Surplus Threshold Capacity method does not. SHORTAGE PROVISION The assumed shortage threshold elevation provides a level of protection against the potential for the Lake Mead storage to drop below an assumed target objective. Objectives such as the minimum power pool elevation, or the elevation of the SNWS intake, or the elevation corresponding to 7.255 maf of storage were considered within the investigation. If the storage in Lake Mead fell below the shortage threshold elevation determined to protect the minimum objective, then shortages would be determined. Modeling assumed that CAP shortage delivery would be reduced to 800 leaf, and that SNWS depletions would be reduced by 4 percent of Arizona's reduction, or about 30 leaf. If the storage in Lake Mead continued to fall the next year, shortages would continue to be determined as described above. Model runs checked for the occurrences of CAP deliveries at the reduced 800 leaf delivery. INITIAL CRSS MODEL RUNS The first model run was designed to identify the total amount of flood control releases that would flow to Mexico if there were no provision for surplus. Surplus provisions were then set at 0 Percent Assurance of A voidance of Excess to establish the most reliable case (lowest probability of shortage) for providing the states their basic allocated apportionment. The quantity of excess water arriving at the border with Mexico that occurred, with the reservoir system using the most reliable provisions, represented the MINIMUM amount of excess that "potentially" could be made available for surplus use by the Lower Division States without effecting the risk of shortage. Under the most reliable case no storage elevation was set to trigger a shortage determination, allowing the physical capabilities of the system to govern the timing of shortages. And the surplus assurance level was set at 0 percent probability of avoiding an excess release (water is surplused only for flood control). By reducing the shortage elevation threshold, shortages would take place only when Lake Mead is to the lowest outlet gate. By setting the surplus assurance level to 0 percent, releases greater than that required for normal deliveries would only occur as the direct result of flood control releases. Under these assumptions the model runs identified conditions which yielded the lowest probability of shortages. 14