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Description of Existing Model n., ,1'1',"1, ii' I! ..uAJ ..~iM w oJ 2 , Return flows from certain water uses are also represented in the model as inflows; these 20 in- flows are computed internally by the model based on user-supplied lag factors. The specific return flow factors vary depending on type of water use and amount diverted and were adjusted during calibration. InOow Forecasts The model simulates the forecast-based operation of Taylor Park, Blue Mesa, and Ridgway reser. voirs. Forecasts of inflows to these reservoirs were developed for input to the model- using historical forecasts and snowcourse records. Historical forecasts for Taylor Park and Blue Mesa were obtained from the USBR Salt Lake; no historical forecasts existed for Ridgway at the time of model development. ,/ These forecasts were used, along with snowcourse records to derive regression models which predicted forecasts from snowcourse data. These regression models were then used to synthesize inflow forecasts for years when historical forecasts were not available. REPRESENTATION OF WATER RlGIfIS Gunnison basin water rights are represented in the model in one of two ways. Major water rights which significantly influence river administration or which transfer water from one sub-basin to another are modeled individually. These water rights, which are listed in Table 2, are modeled as diver. sions, consumptive use demands, and return flows. Data for representation of water rights was obtained from the 1984 Stream Alpha List, the 1988 Water Rights Tabulation (electronic), line diagrams obtained from -various sources, and interviews with Water Commissioners and the Division 4 Engineer. Most small irrigation water rights and a few small municipal and domestic rights are represented as aggregated depletions rather than as diversions and return. flows (depletions are the net of diversions and return flows). These rights are aggregated by geographical location and by relative priority. Four priority classes were defined using the.priorities of major water rights to divide the classes. The small ir. rigation decrees were then distributed into these classes based on their individual priorities. This geog- raphical and legal clasSIfication of decrees resulted in definition of 212 separate demand points for rep- resentation of small irrigation water rights. Table 3 lists the priority intervals used in aggregating smaller rights. The depletions associated .with each of these aggregated demand points were derived from con- sumptive use calculations, surveys of irrigated acreage under specific ditches, diversion records, and detailed operating studies of selected ditches. The depletions are allocated between priority classes at each aggregation point based on the distribution of decreed rights and historical diversions at each point. Historical diversions at each point were allocated among the priority classes based on the as. sumption that diversions take place first under more senior water rights. The average ~llocation pattern so obtained was then used to allocate toral depletions at each point among the priority classes at that point (this reflects an assumption that each acre.foot diverted supported the same amount of depletion). A number of instream flow water rights were represented in the model. The ewCR and pri. vately held instream flow rights represented are listed in Tables 4 and 5. In addition, an instream flow right for .300 cfs, derived from the P&M water right donated to The Nature Conservancy, was assumed to exist for the Black Canyon; this right was assumed senior to the Guimison Tunnel power decree and served to protect Black Canyon augmentation releases (to insure a 300 cfs minimum flow) from Blue Mesa past the East Portal. A few conditional water rights were represented in the model. These were selected from the many existing conditional decrees as being the most significanl. either by virtue of their imminent devef~ opme-nt or their potential for impact on basin hydrology and water rights adminiscra'tion. The condi~ tional decrees represented are listed in Table 6.