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I! I' I, STUDY APPROACH The study has been conducted in consultation with the Work Group. The study approach has been to explore alternative concepts, make Colorado River Simulation System (CRSS) model runs and comparisons, and recommend concepts for more detailed investigation or for implementation in development of Annual Operating Plans (AOP). Reclamation has been responsible for study management and for conducting all CRSS modeling exercises and presenting the data results to the Work Group. The Work Group reviewed study results and worked with Reclamation to reformulate the model studies needed to address the questions raised . The results of each run were compared with a base case analysis. Following the comparison, issues and concerns were revisited to determine if additional modeling runs were needed. Over the course of the study, hydrologic and water use changes occurred and the model studies were revised to address specific questions. The three most significant changes were the 1987 through 1992 drought which drew the initial storage conditions down; the most recent Central Arizona Project (CAP) projected water delivery schedule which extended the time frame when CAP would be fully diverting their apportionment until year 2035 and extended out projections of the year that the lower basin would be using less than the full apportionment of 7.5 maf ti1l year 2009; and the inclusion of unmeasured return flow. From modeling the effects of these changes the studies recognized that the surplus strategy needs to factor in both the level of storage and the level of total basin depletion. During the spring of 1993 Reclamation worked with the Basin States conducting studies of banking alternatives using the CRSS-EZ model. CRSS-EZ is an annual time step model that is less complex, and more user friendly than CRSS. Use of the model as a planning tool offers several practical advantages as multiple alternatives can be easily considered, displayed and studied. And the effects of changes to initial storage conditions, scheduled forecast depletions, or other conditions can be quickly assimilated and discussed. Following the reduction in CAP depletion schedule and inclusion of unmeasured returns, the surplus studies used the CRSS-EZ model to examine the multiple alternatives and issues. CRSS-EZ Model: The CRSS-EZ Model Simulates the river system on an annual basis Uses methods of the CRSS monthly time step model that can be implemented annually Surplus is determined on January I, not Oct I. 8