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<br />. . <br /> <br />'. <br /> <br />" <br /> <br />Description of Existing Model <br /> <br />2 <br /> <br />RclUrn flows from certain water uses are also represemed in the model as inflows; these 20 in- <br />flows are computed internally by the model based On user-supplied lag factors. The specific return flow <br />faclors vary depending on type of water use and amount diverted and were adjusted during calibration. <br /> <br />, <br /> <br />InOow ForecastS <br /> <br />The model simulates the forecast.based operation of Taylor Park. Blue Mesa. and Ridh'W"c.lY reser. <br />voirs. Forecasts of influws to these reservoIrs were developed for input to the model using historical <br />forecasts and snowcourse records. Histoncal forecasts for Taylor Park and Blue Mesa were obtained <br />from the USBR Sail Lake; 110 historical forecasLS existed for Ridgway at the rime of model develupment <br />These forecasts were used. along with snowcourse records to derive regression models which predicted <br />forecasts from snowcourse data. These regression models were then used to synthesize inflow forecasts <br />for years when historical forecasts were nOt available. <br /> <br />REPRESENTATION OF WATER RIGIITS <br /> <br />Gunnison basin water rights are represented in the model in one of [wo ways. Major water <br />rights which significantly influence river administration or which transfer water from one sub.basin to <br />another are modeled individually. These water rights, which are listed in Table 2. are modeled as diver. <br />sions, consumptive use demands, and return nows. Data for representation of water rights was obtained <br />from the 1984 Stream Alpha Ust, the 1988 Water Rights Tabulation (electronic), line diagrams obtained <br />from various sources, and intervie\VS with Water Commissioners and the Division 4 Engineer. <br /> <br />Most small irrigation water righLS and a few small municipal and domestic rights ilrc rcpresented <br />as aggregated depletions rather than as diversions and return flows (depletions are the net of diversions <br />and return nows). These rights ilre aggregated by geographical location and by relative priority. Four <br />priority classes were defined using the. priorities of major water TIghts to divide the classes. The small ir- <br />rigation decrees were then distributed into Lhese classes based un their individual priorities. This geog- <br />raphical and legal classllication of decrees resulted in definition o( 212 separate demand poinls (or rep- <br />resentation of small irrigation water rights. Table 3 lists the priority intervals used in aggregating smaller <br />rights. <br /> <br />I <br /> <br />The depletions associated with each of these aggregated demand points were derived from con. <br />sumptive use calculations, surveys of irrigated acreage under specific ditches, diversion records, and <br />detailed operating stu,ljes of seJected dHches. The depletions arc allocated be[\veen priority classes at <br />each aggregation point based on the distribution of decreed rights and historical diversions at each <br />point. Historical diversions at each point were allocated among the: priority classes based on the as- <br />sumption that diversions take place first under more senior watcr rights. The average allocalion pattern <br />so obtained was then used to allocate LOtal depletions at each point among the priority classes at that <br />point (thiS reflects an assumption that each acre. foot diverted supported the same amount of depletion). <br /> <br />A number of instream flow water rights were represented in the model. The CWCB and pri- <br />vately held in stream flow rights represented arc listed in Tables 4 and 5. In addition. an instream flow <br />right for 300 cfs. derived from the P&M water right donated to The Nalure Conservancy, was assumed to <br />exist for the Black Canyon; this right was assumed senior to the Gunnison Tunnel power decree and <br />served to protect Black Canyon augmentation releases (to insure a 300 cfs minimum flow) from Blue <br />Mesa past the East Pon,,!. <br /> <br />A feW" conditional water rights were n:presenLed in I Ill' model. These were seleCted from the <br />many existing conditional decrees as being the most signil1c3m. either by \lrtue of their IInmlllcnt devel- <br />opment or their potenLial for impacL on hasin hydrology and water rights alhmniSlration. The condi- <br />tional decrees rcprc~el1tcd are listed in Table G. <br /> <br />, <br />