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Dry-Year Stream~a~e Comparison <br />The 1976 through 1999 period includes individual wet, dry, and average years. The year 1977 <br />was a very dry year in the basin. Figure 4 shows the average of the simulated streamflows for <br />the seven streamgages shown in Table 1 for 1977. As shown, the Previous CRDSS analysis <br />generally predicts annual streamflow that is about 2 percent higher than measured. The <br />Enhanced analysis predicts annual streamflow that is about 7 percent lower than measured. <br />These results are consistent with the Enhanced approach. During dry years, irrigation water <br />requirements are generally higher than in average or wet years, because less of the crop potential <br />consumptive use is satisfied from rainfall or water stored in the soil reservoir. The result is more <br />efficient use of diverted water, therefore, less water returning to the river, and lower predicted <br />streamflow. <br />StateMod uses this same methodology in reverse to estimate baseflows, therefore intuitively the <br />predicted streamflow should be the same as historic. During the previous CRDSS modeling of <br />the Yampa River basin, certain areas experienced relatively large "negative gains" during <br />baseflow generation. These "negative gains" are set to zero, essentially adding water to the <br />system. When variable efficiency is implemented, the system becomes tighter and, as noted in <br />the LRE Variable Efficiency Task 1.3 Memorandum (August 2001), there are less negative <br />gains, therefore less water available to the system. With the use of variable efficiency, <br />distribution of gains to ungaged tributaries and the locations where return flows enter the river <br />become more critical to calibration. <br />VariableEff Taskl-5 5 of 17 November 5, 2001 <br />