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. . digital computer model called the Platte and Colorado Simulation Model (PACSM) to simulate the components, operations, and yields of the water collection system. P ACSM simulates streamflows, reservoir operations and water supply in the South Platte River and Colorado River basins. . The selection of a study period is important because it determines the number and type of extreme hydrologic events which will be simulated and can influence the conclusions drawn from an analysis. For example, a water supply system may perform suitably under average hydrologic conditions, but may fail completely during severe conditions, such as the Colorado drought of 1953-1956. The most important hydrologic event to a water supply study is the critical drought period. That period is calculated as the span from the last time the storage reservoirs are full to the time the reservoir storage is completely depleted and the reservoirs begin to refill, ultimately to their full levels. Since the critical period varies with the sustainability of a given water supply system, the modeled duration must be long enough to encompass critical drought periods for all collection systems studied. A study period from 1947 through 199 I was selected by Denver Water as representative of the long-term conditions for the river basins of concern. Not only is there available detailed streamflow data, but the period encompasses the critical drought periods for all watersheds under consideration. . . . The 1947 through 1991 weather patterns and the resulting streamflows are then applied to existing and proposed operating conditions. For example, P ACSM will simulate the operations of Dillon Reservoir in the 1950s' drought, even though the reservoir was not constructed until the early 19608. Similarly, PACSM can model how potential supply additions would have operated during the study period. This enables water resource planners to better understand how much water would be available to Denver's customers during drought conditions from various water supply options and how these options alter streamflows. . . PACSM simulates the operation of the water collection system to determine the maximum amount of water the system could provide to meet demand without shortages through the study period. Although this water demand is reported as a fIxed number expressed in acre-feet per year, the actual modeled water supply varies from day-to-day, month-to-month, and year-to-year. In a nOOTIal year, water demand in a summer month exceeds demand in a winter month by a factor of about three to one. Also, total annual water demand will be as much as 32% greater in a hot, dry year than in a cool, wet year. These variations in water demand are largely attributable to outdoor uses of water, most notably lawn irrigation. . . For example, over the study period, as simulated by the model under conditions where average system demand were to be equal to Denver's 345,000 acre-foot baseline supply, the delivered water ranged from approximately 375,000 acre-feet in the dry year of 1954 to as low as 295,000 acre-feet in the wet year of 1967. That is, customer demand was 80,000 acre-feet higher in the dry year than the wet year. Likewise, monthly and seasonal variations of as much as three to one from summer to winter are accounted for in the model. . 15