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Description of Existing Model 2 REPRESENTATION OF WATER KIGlITS , 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 Wd.S obtained from the t984 Stream Alpha List, the t988 Water Rights Tabulation (electronic), line diagrams obtained from various sources, and interviews with Water Commissioners and the Division 4 Engineer. Most small irrigation W'ater rights and a few small municipal and domestic rights are represented as aggregated depletions rather than as diversions and return nows (depletions are the net of diversions and'rcturn flows). Thesc riKhls are aggregated by geographical location and by relativc 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 thell' individual priorities. This geog- raphical and legal classification of decrees resulted in dermilion 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 [rom 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 ea.ch 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 Ihal diversions lake place first under more senior w~ter rights. The averaH,t: allocation pallern so obtained W.J.S men used 10 allocate total depletlon~ at each point among the priority classes at that point (this reOects an assumption that each acre.foot diverted supponed the same amount of depletion). A number of instream flow water rights were represented in the model. The CWCB and pri. . V"d.tdy held instn:am flow rights represented arc listed in Tables 4 and 5. In addJ.uon, an instream flow right for 300 cfs, derivcd from the P&M waler right donatcd to The Nature Conservancy, was assumed to exist for the Black Canyon; this right was assumed senior to the Gunnison Tunnel power dec..:rce and served to protect Black Canyon augmentation reJeases (to insure a 300 cfs minimum flow) from Blue Mesa past the East Portal. A few condilional water rights were represented in the model. These were selected from the many existing conditional decrees as being the most significant, either by virtue of their imminent devel. opment or their pOlcntial for impact on basin hydrology and water rights administration. Thc condi- tional decrees represented are listed in Table 6. Table 7 Iisls the ovcrall rankings assigned to the various water rights and water righls groups represented in the model. OPIiIlATIONS OF EXISTING FACIlJ11ES lbe operating procedures of major existing or soon.to.be-developed water supply facilities are represented in lhc basin model. Specific facilities and systems modeled in detail include the Wayne N. Aspinall Unil (formerly known as lhe Curecanli Unil), !he Uncompahgre Pro;ecl, lhe Boslwick Park Pro- jccl, lht Dallas Creck Projecl, and the Project 7 Water AuthorilY Smaller municipal and domcstic sys. terns, such as the City of Gunnison and the Town of Crested Butte, were modeled simply as depletions because of their small size and Ihe spatial and lemporaJ proximity of diversion and return flow pOlnts. ,