A Model for Managing Reservoir Water Releases*

Two surface reservoirs in California's Salinas Basin are used to increase recharge of valley aquifers. Under the assumptions of zero priee elastieity of demand for groundwater, unregu-lated groundwater withdrawals, and the objective of economic efficieney, the management criterion is to maximize the present value of expeeted aquifer recharge where the value of a unit of recharge is the associated reduction in pumping eosts. A cyclieal Markov dynamic programming management model is developed; optimal strategies ate obtained. Results indi. cate the optimal strategy is relatively stable within the realistie range of alternative ground-water demand growth rates. The marginal value function for reservo/r eapacity is obtained by parametric analysis. p RESENTED ttERE is a particular model for optimal management of surface reservoir releases to increase water supply. Optimal strategies are developed where the effect of a decision is uncertain but where the set of possi-ble outcomes and the associated probabilities are known. Uncertainty exists because reservoir inflows are not known in advance of the release decisions. Although the inflow which occurs during a period is not known at the beginning of the period, a probability distribution for the set of possible inflows is assumed known. Ir the year is divided into T periods, the probability distribution would not be expected to be the same for all periods within the year. For ex-ample, precipitation expected uring a summer month likely will not be the same for a winter month. Consequently, there is an annual cyclical change in the distribution of the stochastic variable. This element is critical to this study since intra-year as well as inter-year aspects of management a e considered. Sa l inas Basin Salinas Basin is located in the central coastal region of California. It contains 3,950 square miles of mountain and foothills that serve asa watershed for the 239,000 acres of alluvial and benchlands which are the site of most economic activity. The Basin's major hydrologic feature