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

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' Thompson River. While this analysis considered only water uses on the Big Thompson River, the fact that calls were not occurring on the South Platte River on the days when this undiverted non - charge water was being delivered would indicate that it was not being diverted or used on the South Platte River either. Our inspection of the records for the flow in the South Platte River below the mouth of the Big Thompson River on these days tends to confirm this. Also, our ' inspection of the Cache La Poudre flow records for the days when non - charge water was being delivered indicated that similar conclusions could be made with respect to the non - charge deliveries to the Cache La Poudre. ' OPERATION OF EAST SLOPE FACILITIES TO MAINTAIN HISTORICAL C -BT YIELD L �I Full utilization of in- priority Big Thompson River water will require changes in the way the C -BT Project is operated. In particular, the filling of Carter Lake and Horsetooth Reservoir will need to be delayed so that the full or nearly full volumes of the Big Thompson River water that become available under the priority system can be captured in the two reservoirs and made part of the project water supply. In order to demonstrate that all or nearly all of this Big Thompson River water can be captured and made part of the project water supply without reducing the yield to the project water users, the operation of C -BT's east slope features during the 1983 -98 analysis period was studied. An EXCEL spreadsheet was developed to facilitate this study by simulating the operation of these features under different diversion and storage patterns. This spreadsheet operates on a monthly basis; its primary inputs are in- priority Big Thompson River diversions from the previously- described calculations, historical deliveries to project water users, and monthly, end -of -month storage targets. The spreadsheet calculates the west slope water that is to be brought over to the east slope through the Adams Tunnel in a procedure that attempts first to supply the historical deliveries to project users and then to accomplish the monthly storage targets, given the relevant system losses and capacity constraints. When the combination of Big Thompson River and Adams Tunnel water is not sufficient to supply the historical deliveries, water is released from storage. When the combination of Big Thompson River and Adams Tunnel water is more than sufficient to satisfy the historical deliveries, then water is placed into storage in the two reservoirs under criteria intended generally to equalize the storage in the two reservoirs in the proportion of their unfilled storage capacities. Conveyance and reservoir efficiencies are also inputs into the spreadsheet and were derived from the records for the actual system operation during 1983 -98. Three different spreadsheets or "runs" were developed to simulate the three different situations outlined below: Run Conditions A Historical east slope diversions with non - charge deliveries B Increased diversion of east slope water with historical non - charge deliveries C Increased diversion of east slope water with delivery of only charge water 13