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These modifications were made according to the facsimile sent to Ross Bethel from Chuck <br />? <br />Brendecke on February 17, 1995 (Job number 226.002). <br />Discussion of the Calibration <br />Stream Gages <br />Both MODSIM and GUNMOD have a tendency to undersimulate the flow at the Gunnison River at <br />Gunnison. This may be a result of understating the required flow regime below Taylor Park <br />reservoir in comparison to historic releases. <br />Both models exhibit the poorest monthly calibrations in the Uncompahgre River. For the <br />Uncompahgre River at Colona, MODSIM oversimulates by 88 percent in October and GUNMOD <br />oversimulates by 165 percent in February. On average, the annual simulation for both models is <br />within 1 percent. The Uncompahgre River at Delta also exhibits large monthly swings; MODSIM <br />oversimulates by 60 percent in December, while GUNMOD oversimulates by 143 percent in <br />February. Annually, however, MODSIM undersimulates the Uncompahgre at Delta by 7 percent and <br />GUNMOD oversimulates by 10 percent. The differences exhibited by both models is a reflection of <br />the assumptions concerning consumptive use and efficiencies in the Uncompahgre Project. <br />Both models tend to oversimulate the flow of the Gunnison River below the Gunnison tunnel by <br />approximately 16 percent. This reflects the undersimulation of flow through the Gunnison Tunnel, <br />which is a direct function of the consumptive use and efficiencies used in the Uncompahgre Project. <br />MODSIM oversimulates the Gunnison River at Delta by approximately 3 percent, and GUNMOD <br />tends to oversimulate the Gunnison River at Delta by approximately 23 percent. <br />Both models tend to oversimulate the Gunnison River at Grand Junction by approximately 2 percent. <br />Key Diversions <br />Simulated MODSIM diversions through the Gunnison Tunnel are typically less than historic, possibly <br />because of understated demands at the Uncompahgr e project canals and/or less storage water being <br />imported through the tunnel because of releases from Ridgway Reservoir. There is also a general <br />tendency to understate diversions at structure headgates (Selig, Loutzenhizer, and North Delta) by <br />approximately 10 percent. This may relate directly to the fact that the model is driven by calculated <br />irrigation demands (acreage x C.U. ÷ efficiency) and does not reflect hist oric excess diversions. <br />The simulated GUNMOD diversion at Fire Mountain canal is consistently overestimated by <br />approximately 20 percent. The Uncompahgre canals are consistently undersimulated by <br />approximately 20 percent. This result is very similar to the MODSIM simulation and may be related <br />to the fact that the models are driven by calculated irrigation demands and do not reflect historic <br />excess diversions. <br />Reservoirs <br />The MODSIM simulation, on average, oversimula tes the Blue Mesa end-of-month storage by <br />approximately 40 percent, and the Taylor Pa rk end-of-month-storage is undersimulated by <br />11 <br />A275 02.23.94 1.15-27 Fosha, Ritsch <br />