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

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charge deliveries averaged 37,800 acre -feet annually or about 18 percent of the total delivered. These deliveries were made when Granby Reservoir on the west slope was expected to spill, and were not counted against the delivery allotments made to the C -BT unit holders. Although the Northern Colorado Water Conservancy District and USBR refer to this non - charge water as "delivered ", it appears that the water was, for the most part, released to the several streams in the project area and turned over to the Water Commissioner for distribution under the priority system. We evaluated whether these deliveries were made when shortages were occurring to the water users in the stream system. Such shortages, which were not limited to those of just C- BT unit holders, would have occurred when the water users on the stream were using the entire native water supply available to them and were needing or wanting additional water. In other words, there should have been a dry section on the stream when non - charge deliveries were made. As will be discussed later, the non - charge deliveries to the Big Thompson River averaged 8,720 acre -feet annually during 1983 -98, and our analysis indicated that the non - charge deliveries were not fully diverted by the water users. Big Thompson River water is diverted into the Olympus and Dille Tunnels either as ' "skim" water or "in- priority" project diversions. Depending on the physical availability and power requirements, skim water is diverted, run through the C -BT system and thereby used to ' generate power, and then discharged back to the Big Thompson River at the Big Thompson Powerplant or the Big Thompson Wasteway. In general, skim water is diverted on a given day at a variable rate and returned to the Big Thompson River the next day at a uniform rate. Although ' some records for the in- priority project diversions are available from the USBR and the Colorado Division of Water Resources, they do, not appear to be complete. In order to estimate the in- priority diversions of Big Thompson River water, the C -BT operations on the Big Thompson River during 1983 -98 were evaluated as shown in Table 6. ' The C -BT diversions of Big Thompson River water (column 5) were calculated by subtracting the Adams Tunnel diversions (column 2) from the total diversions into the Olympus and Dille Tunnels (columns 3 and 4). These diversions of Big Thompson River water averaged 53,550 acre -feet annually. As indicated above, some of both Big Thompson River water and west slope water brought through the Adams Tunnel was discharged back to the Big Thompson River ' through the Big Thompson Powerplant and Wasteway. The total discharges back to the Big Thompson River (column 8) averaged 88,960 acre -feet annually. The west slope water discharged back to the Big Thompson River (column 9), which is accounted for in the USBR ' records as deliveries of C -BT Project water to C -BT users, averaged 42,120 acre -feet annually. The remaining discharges to the Big Thompson River (column 9), therefore, were Big ' Thompson River water and averaged 46,840 acre -feet annually. Since the total diversions of Big Thompson River water averaged 53,550 acre -feet annually and the returns of Big Thompson River water averaged 46,840 acre -feet annually, the Big Thompson River water retained in the C -BT system (column 11) averaged 6,710 acre -feet annually. The negative value indicated for 1992 would seem to indicate an out -of- priority diversion of Big Thompson River water. The Adams Tunnel diversions in column 2 do not agree exactly with those from Table 2 because the values in column 2 were obtained from the daily CDSS records rather than the monthly USGS