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f.~~iSCRIPTION OF THE GPSIM MODEL 3.1 Model Description A computer model, called GPSIM, has been developed to simulate operations of the Great Plains Reservoir system. GPSIM is implemented using the PC spreadsheet software Lotus 123 and provides a means to quickly examine alternate reservoir opera- tions policies. Reservoir simulation is based on a mass balance equation of the form: S(t+l) = S(t) + I(t) - R(t) - E(t) where: t = time step (month) S(t) = storage (AF) at beginning of time step t. I(t) = inflow (AF) during period t. R(t) = release flow (AF) during period t. E(t) = evaporation (AF) during period t. The equation says that the storage at the beginning of the next' month is computed as the storage at the beginning of the current month plus inflows less losses as release flows and evaporation. Required input data to support the model include the following: 1. Time period: A 35 year period of monthly historical data is used extending from 1950 to 1984. 2. Inflows: The historical record of flows measured at the Kicking Bird gage is used. This record is based on data obtained from the state Engineer and Zorich's (1986) report but has been extended to include the high flow years during the early 1980's. Since 1976 the flows at the Kicking Bird gage are less than they might have been due to the Winter Storage Program in John Martin Reservoir. A total period record has been developed assuming the winter storage Program was not in effect. 3. Releases: The historical record of release flows from the Great Plains Reservoir system (from Nee Noshe and Queen Reservoirs) are used as targets for the operations model. That is, the model will attempt to meet the release target to the extent of available water in the active storage zone. 4. Storage Limits: The maximum and minimum storage levels establish the range of operation of the reservoir. The minimum storage is the dead storage volume, below this level releases 13