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

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E h �J 1 u 0 0 n approximately 13,000 acre -feet annually and would have reduced target flow shortages by an average of 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 average 37,800 acre -feet of non - charge water during 1983 -98. These non - charge deliveries occurred in 1983 -87 and 1995 -98 or 9 of the 16 years in the study period, and amounted to about 18 percent of the total deliveries. Annual non - charge deliveries during this period 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 non - charge water delivered to the Big Thompson River during water years 1983 -98 was not fully diverted by the water users on the Big Thompson River. These non - charge deliveries averaged 8,730 acre -feet annually over the 16 -year period and amounted to as much as 46,929 acre -feet in 1986. Our flow calculations and comparisons indicated that an average of only 3,423 acre -feet annually of this water was diverted by Big Thompson River water users. Thus, an average of 39.3 percent of the total was not diverted 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 below 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 water was being delivered 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 these shortages by an average of 13,349 acre -feet annually when combined with increased utilization of Big Thompson River water. 7. Increased diversions of the Big Thompson River water and reduced or precluded deliveries of non - charge deliveries would have enhanced the spring peak flows in the 15 -Mile Reach significantly through increased spills from Granby Reservoir and would have accomplished or largely accomplished the CFOPS goals in this regard. Our analysis indicated that increased east slope diversions would have increased the spring peak flows by an average of 18,498 acre -feet in the middle -range flow years during 18