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• Both the Previous CRDSS analysis and the Enhanced analysis predict streamflows and <br />diversions during the representative wet-year (1984) that are essentially the same as <br />gaged. <br />• Reservoir calibration does not appear to be better or worse during varying hydrologic <br />conditions for either the Previous CRDSS or Enhanced current analysis. This is likely the <br />result of the operating rule used in the Historic analyses that release stored water to meet <br />targets. <br />• The Enhanced analysis predicts consumptive use estimates for explicit structures that are <br />closer to the consumptive use estimated by the Yampa River Basin StateCU analysis in <br />dry, average, and wet hydrologic periods. <br />• The Enhanced StateMod simulation using the variable efficiency and soil moisture <br />accounting options better represents the interaction between diversions, irrigation water <br />requirements, and return flows. The Enhanced results more accurately represent the crop <br />consumptive use results from the StateCU modeling efforts in the Yampa River Basin <br />and are consistent with the methodology used in Colorado by engineers to determine <br />historic consumptive use for water right transfers and augmentation planning. <br />About 20 percent of the structures in the Yampa River Basin have late irrigation season <br />calculated efficiencies equal to the 60 percent maximum efficiency set for the basin. A <br />more detailed study of basin-wide conveyance and application efficiencies may result in a <br />lower maximum efficiency. A preliminary analysis showed that a lower maximum <br />efficiency would likely predict streamflows closer to gaged during dry periods. <br />The Enhanced analysis predicts the largest diversion shortages in Water District 54. The <br />calculated efficiencies for most of the structures in Water District 54 are at the 60 percent <br />maximum. If a lower maximum efficiency was determined to be more representative of <br />conditions in the basin, more of the diverted water would be returned to the river and <br />made available for downstream diversions. A preliminary analysis showed that this <br />would likely reduce the shortages in this water district, and increase the consumptive use. <br />The methodology used to develop "estimated" diversions for aggregate structures uses <br />monthly irrigation water requirements divided by monthly basin-wide average <br />efficiencies. These average efficiencies tend to determine inflated "estimated" diversions <br />during dry and average years because they assume a full water supply was obtained. This <br />method also tends to determine smaller "estimated" diversions during wet years. Ideally, <br />aggregate "estimated" diversions would be determined using available historic data, <br />appropriate data filling techniques and, potentially, efficiencies that vary by month and <br />year. As shown in Water District 54, some sub-basins generally operate at a higher <br />efficiency than others do, therefore at a minimum it may be appropriate to determine <br />aggregate "estimated" diversions using average monthly efficiencies determined by water <br />district rather than the total basin. <br />Recommendations <br />The variable efficiency and soil moisture accounting algorithms are believed to provide a better <br />representation of the interaction between diversions, irrigation water requirements, and return <br />flows. The fact that calibration did not significantly improve with this approach may be an <br />VariableEff Taskl-5 16 of 17 November 5, 2001 <br />