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<br />r <br /> <br />., <br /> <br />On31J9 <br /> <br />, <br /> <br />.' <br /> <br />. <br /> <br />SUMMARYDESCRIPTION <br />. OF <br />EXISTING GUNNISON BASIN MOnEL <br /> <br />INTROQjJCll0N <br /> <br />This document Is a summary description of the existing basin model based on the executive <br />summary and excerpts from Task Memorandum No.5 of the Colorado Water Resources and Power De- <br />velopment Authority's Phase 1 Study. A diagram of the model network and tables listing Important data <br />and Information supporting the summary are attached at the back of this document. <br /> <br />MODEUNGAPPROACH <br /> <br />The basin model was developed using a network modeling system and runs on IBM-compatible <br />microcomputers. In the model, the basin Is characterized as a network of llnks representing inflows, <br />stream: reaches, diversions, reservoirs, and demands. The priorities placed on various links in the <br />network are derived from water rights considerations. These priorities control the allocation of water to <br />various uses represented in the model. Operating rules for various facilities are Implemented through <br />special computer code which adjusts the priorities assigned to specific links, such as those which <br />represent reservoir storage or releases. <br /> <br />The model operates on a monthly time step over a 1952-83 study period. In each month, the <br />model solves for an optimal allocation of available water supplles according to the priorities placed on <br />various links by the user. Reservoir carryover storage and lagged return flows are the connections be- <br />tween one month's solution and the next. A diagram of the model network is attached to the back of <br />this document. <br /> <br />S11lDY PERIOD HYDROLOGY <br /> <br />Files of model inflows at 59 sites were derived by U.S. Bureau of Reclamation (USBR) staff in <br />Grand Junction. These inflows are of three types: 1) virgin flows, 2) gaged flows, and 3) Imputed reacl'l <br />gains. Virgin flows at gaged sites were derived from historical flow records corrected for upstream leser. <br />voir operations, basin imports and exports, and irrigation depletions. Historical irrigation depletions <br />were estimated from consumptive use rates derived from the CRSS database, acreages obtained from the <br />USBR, the SCS, and the Gunnison County assessor, and from detailed operating studies of selected <br />ditches. Virgin flows at ungaged sites were developed from virgin flows at "reference gages" corrected <br />for area and elevation differences; Table lUsts these reference gages. <br /> <br />Gaged flows were used to represent inflows from the North and Smith Forks of the Gunnison <br />and the inflows to Ridgway Reservoir. Reach gains imputed from gaged flows were used to represent ac- <br />cretions along the Uncompahgre River above the South Canal and along the Gunnison below Delta. <br /> <br />Return flows from certain water uses are also represented 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 />factors vary depending on type of water use and amount diverted and were adjusted during calibration. <br /> <br />',,- <br />