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which represent streams, inflows (natural runoff), reservoirs and diversions that exist in the basin. The model simulates the amount of water flowing in each link (stream reachy and the amount of water contained in each reservoir while taking account of consumptive use by various water users. 7his is done on a monthly basis for each year being modeled. The selection of the years used in the modeling is discussed in the following section. 3.3 STUDY PERIOD HYDROLOGY The first steps in developing the basin model were to select the historical period to be studied and to compute virgin flows (flows wfiich would have occurred without the effects of stream regulation or diversions) for that study period. The study period consisted of 384 consecutive months from October 1952 through September 1983. This period was selected because it is statistically similar to the total period of record for three key streamflow gaging stations in the basin and includes a variety of hydrologic conditions. Nearly all the significant dry years in the period of record were included in the selected study period. Separate data files of monthly virgin flows were derived for 59 locations in the basin. Virgin flows at gaged sites were derived frc�� historical flow records corrected for upstream reservoir operations, basin imports and exports, and irrigation depletions. Historical irrigation depletions were estimated from climatic data, consumptive use calculations, diversion records, and detailed operating studies of selected ditches. Uirgin flows at ungaged sites were developed from virgin flows at gaged sites corrected for area and elevation differences. 3.4 REPRESENTATION OF WATER RIGHTS Gunnison basin water rights are represented in the model in one of two ways. "Select" water rights which significantly influence river administration or which transfer water from one sub-basin to another are 3-2