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MODEL VERIFICATION Because the HEC-3 simulation model has no parameters to calibrate, only veri- fication to gaged streamflow was used to determine the accuracy of its predictive capabi I i ty for the Yampa River basin. Therefore, a model simulation representing historical conditions with negligible reservoir operations was compared to stream- flow records at three streamflow-gaging stations for 50 water years (1927-76). The comparisons between simulated historical and measured mean annual discharges at the three streamflow-gaging stations are shown in figures 4 through 6. Simulated historical discharges were within 5 percent of measured discharges at control point 39 (Yampa River near Steamboat Springs, Colo.) and control point 42 (Little Snake River near Lily, Colo.), and within 20 percent at control point 18 (Yampa River near Maybell, Colo.). The decrease in accuracy for certain locations is partly due to the uncertainty in accurately representing historical irrigation di- versions in the model. On the basis of these simulations, it is concluded that the model has been partly verified for the study area. MODEL SIMULATIONS Because the HEC-3 model is limited to a 50-year interval, model simulations were made for the 50-year period of water years 1927 through 1976. This period was chosen because it included a wide range of climatic conditions, including the droughts of the 1930's and the 1950's. Thirty-four simulations were made to determine streamflow at the 47 control points in the model. The first simulation determined historical conditions without any proposed transmountain diversions or reservoir development. For the second simulation, the assumption was made that only the two transmountain diversions would be in operation. In each simulation, mean, median, and 80-percent exceedence flows, in cubic feet per second, were determined for each month at each control point. Statistically, median flows for a given month can be expected to be ex- ceeded once every 2 years, on the average, and the 80-percent exceedence flows can be expected to be exceeded 4 out of every 5 years, on the average. Simulated historical monthly streamflows at the 47 control points throughout the Yampa River basin were determined as follows: A. Historical conditions: 1. Historical conditions without any proposed diversions. 2. Historical conditions with 100 percent of proposed trans- mountain diversions. B. Reservoir-development options 1 through 4: 1. Allocation of 25 percent of total active reservoir storage for agricultural use without any transmountain diversions, and including 100 percent of industrial and municipal diversions. 2. Allocation of 25 percent of total active reservoir storage for agricultural use with 25 percent of proposed trans- mountain diversions, and including 100 percent of industrial and municipal diversions. 14