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I I I I I I I I I I I I I I I I I I I 1. The model operated on monthly time steps, and included an algorithm to reflect the variations in daily flow rates during the month. 2. Return flows and tributary inflows for that portion of the watershed below the USGS streamflow gauge at the mouth of the Big Thompson Canyon were included in the total quantity of water to be distributed each month. 3. Available water was allocated in order of priority; if a shortage condition existed the model divided the available water among those currently in priority. 4. Average monthly irrigation demands would remain the same in future years as they have in the past for each ditch. Details concerning the model operation and parameter values may be found in the Technical Appendix. The historic Big Thompson streamflows from January 1979 through September 1984 were used to verify the reliability of the model. The historic streamflow data was used as input and simulated values for average annual diversion rates from the Big Thompson River for irrigation purposes were compared to measured values obtained from the State Engineers Office. Table 8-2 compares the simulated to observed values; the level of accuracy obtained with the model was consistent with the study objectives. The 1979 through 1984 period was selected for calibration of the model primarily because of the range of streamflow conditions observed on the Big Thompson River during this interval. Annual streamflow values (corrected for CBT operation of the Dille Tunnel and Charles Hansen Feeder Canal) at the Drake gauge (located at the mouth of the Big Thompson Canyon) are presented below: 1979 1980 1981 1982 1983 1984 Annual Streamflow (100 Ac-Ft) 1624 2231 722 1240 2264 1512 Ratio of Annual Streamflow to Average Annual Flow (%) Year (water year) 129 177 57 98 180 120 8-10