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

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than 6,710 acre -feet annually or less than one -third of the total available was actually diverted. 2. The full utilization of the Big Thompson River source water would have reduced the Adams Tunnel diversions that were necessary to supply the historical project demands. ' Our analysis indicated that the Adams Tunnel diversions could have been reduced by an average of approximately 15,200 acre -feet annually. Reduced Adams Tunnel diversions would have increased the spills from Granby Reservoir by an average of approximately 13,000 acre -feet annually and would have reduced target flow shortages in the 15 -Mile Reach by an average of more than approximately 3,600 acre -feet annually. ' 3. Our analysis showed that Carter Lake and Horsetooth Reservoir can be operated so that all or nearly all of the in- priority Big Thompson River water can be diverted and made part of the project water supply without reducing the historical yield to the C -BT unit owners. 4. The C -BT Project delivered an annual average 37,800 acre -feet of non - charge water during 1983 -98. These non - charge deliveries occurred in 1983 -87 and 1995 -98 (9 of the 16 years in the study period) and amounted to about 18 percent of the total deliveries. The non - charge deliveries exceeded 100,000 acre -feet in 1986 and 1987 and approached 100,000 acre -feet in 1997. Prior to 1983, non - charge deliveries occurred in only 1962 and 1971. 5. The deliveries of non - charge water to the Big Thompson River, averaging 8,730 acre - feet annually during 1983 -98, were not fully diverted by the water users on the Big Thompson River. Our flow calculations and comparisons indicated that an average of 3,423 acre -feet of this water was not diverted by Big Thompson River water users. Thus, an average of 39.3 percent of the total went undiverted on the Big 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 downstream from the mouth of the Big Thompson River tends to confirm this. Also, our inspection of the flow records for the Cache La Poudre for the times when the non - charge deliveries were being made indicated that similar conclusions can be made with respect to the non - charge deliveries to the Cache La Poudre. 6. Reductions in the non - charge deliveries during 1983 -98 obviously would have reduced the Adams Tunnel diversions and the target flow shortages in the 15 -Mile Reach. Our analysis indicated that the elimination of the non - charge deliveries would have reduced the target flow shortages by an average of approximately 13,300 acre -feet annually when combined with increased utilization of the C -BT Project's Big Thompson River water supply. K