Analysis of Colorado-Big Thompson Project Operation With Respect to East Slope Diversions and Non-Charge Deliveries

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209,300 acre -feet annually. In Run C, when the diversions of Big Thompson River water were increased with deliveries of only charge water, the Adams Tunnel diversions were reduced to an average of approximately 173,900 acre -feet annually. Figures 5 and 6 are bar charts that ' facilitate comparisons of the historical annual and monthly average Adams Tunnel diversions with those from Runs A, B, and C. Figure 7 shows and compares the end -of -month contents in Carter Lake and Horsetooth Reservoirs from historical operation and Run A. As one would expect, the end -of -month 1 contents agree quite well on the average. There are departures in individual years, however. In 1990 for example, the two reservoirs were drawn down significantly below their average ' contents. In 1995 -98, Horsetooth Reservoir was kept higher than its average contents. The historical minimum end -of -month contents were 32,100 acre -feet in Carter Lake and 48,500 acre -feet in Horsetooth Reservoir, both in October 1989, which compare to 52,650 acre -feet for ' Carter Lake and 84,600 acre -feet for Horsetooth Reservoir in Run A. ' Figures 8 and 9 allow comparison of the end -of -month contents from Runs B and C with those from Run A. In general, the reservoir levels in Runs B and C are lower during the winter, spring, and early summer to reserve storage capacity to capture Big Thompson River water and ' are near to the Run A levels or higher in the late summer. In Run B, the minimum end -of -month contents were 33,690 acre -feet for Carter Lake and 57,950 acre -feet for Horsetooth Reservoir, both larger than the historical minimum. In Run C, the minimums were 33,620 acre -feet for Carter Lake and 63,890 acre -feet for Horsetooth Reservoir. Appendices B, C, and D are copies of portions of the spreadsheets for Runs A, B, and C, respectively, providing the numerical Ibackup for this part of our analysis. It should be emphasized that the objective in this part of our analysis was to show that ' the project could be operated so as to capture all or nearly all of the Big Thompson River water available under the project water rights without reducing the yield to the project water users ' rather than to develop storage targets that duplicate the historical contents in Carter Lake and Horsetooth Reservoir. In actual operation, different targets could be used for the different hydrologic conditions that can be anticipated through the streamflow forecasts that are made ' regularly during the winter and spring. The incorporation of variable storage targets in Run A would produce reservoir contents that match the historical much more closely. Also, seasonal ' storage targets would be used in actual operation rather than monthly storage targets, which would allow greater flexibility, especially with respect to the necessary project maintenance. Monthly storage targets were used in these spreadsheets to simplify the logic. Variable and ' seasonal storage targets would also reduce the potential impacts on power generation and flat water recreation while allowing the capture of the available Big Thompson River water. ' WEST SLOPE EFFECTS ' The effects of the under - utilization of the Big Thompson River water supply and delivery of non - charge deliveries by the C -BT Project on the flow in the Colorado River in the 15 -Mile 15