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. I UUI/U0 The first Central Arizona Project (CAP) water supply projections produced by the Colorado River Simulation System (CRSS) (December 1984) portray a significantly different future for the project than did previous studies. This has affected cost allocation and repayment analyses, and raised questions as to what caused the changed projection. The large increases in projected future water supply for CAP are the net result of the many differences in data base, methods of simulation, and modeling assumptions that exist between the CRSS and the Colorado River Storage Project (CRSP) computer models. The most significant of the differences may be explained by one of the following generalizations: 1. More water is entering the river/reservoir system in CRSS than did in CRSP. 2. Mechanisms in CRSS are enabling much of that water to be delivered to CAP. Before elaborating on these differences, it should be noted that comparison of CRSS and CRSP results reveals that CAP water supply is the only major study parameter that differs so significantly. Relative differences in other study parameters are much smaller. This is so because CAP's right to Colorado River water is subordinate to most of the rest of the Lower Basin water users. Any incremental change in the Lower Basin's water supply is likely to have an eventual impact on delivery to CAP. CAP is first to feel the impact of any water shortage or water surplus situation encountered in the simulation of Colorado River operations. Thus any shortage averted or any surplus situation created will go toward increasing the project's average yield. Based on the two studies used for comparison here, it appears that on the average, CAP eventually diverts a little more than half of the additional water available to the Lower Basin in the CRSS study. The remainder of the additional water may flow to Mexico (as the system spills), may be diverted by The Metropolitan Water District (MWD), or may remain in storage. Explanation of the Sources of CAP's Additional Water Supply in CRSS vs. CRSP As previously mentioned, the reasons explaining CAP's additional water supply fall into two general categories. The following expands those general headings to a list of specific significant factors which combine to increase CAP's water supply in CRSS. 1. More water is entering the river/reservoir system in CRSS than did in CRSP.