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

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Adams Tunnel diversions averaged 225,900 acre -feet annually. The values in this table include some Windy Gap Project water beginning in or about 1987. According to USBR records, the diversions of Windy Gap water through the Adams Tunnel averaged slightly more than 6,000 acre -feet annually during water years 1988 -98. Tables 3A, 3B, and 3C respectively show the end -of -month contents in Carter Lake, Horsetooth Reservoir, and Carter Lake and Horsetooth Reservoir combined. Inspection of the tables reveals changes in the way the reservoirs have been operated. Prior to the early 1980s, the reservoirs were generally the fullest at the end of May or June. Since the early 1980s, the reservoirs have been generally the fullest at the end of March and April. At the end of March, for example, the combined contents were an average of more than 30,000 acre -feet larger during 1983 -98 than they were during 1955 -82. This change in reservoir operation limited the opportunities to store east slope water because most of the east slope water became available in May and June, when the reservoirs were already full or nearly full. Figure 3 depicts this change in the reservoir operation graphically. The USBR annual operating plans indicate that the storage of water in Horsetooth Reservoir was restricted due to safety considerations during 1983 -89. Evidently, Carter Lake and Horsetooth Reservoir held some Windy Gap Project water in storage beginning in or about 1988. Finally, Carter Lake and Horsetooth Reservoir contained "carryover storage" during 1986- 89 and 1991 -98. The volumes of this carryover storage were relatively large, averaging more than 75,000 acre -feet and amounting to more than 100,000 acre -feet in 1996 and 1998. The storage of this Windy Gap and carryover storage water in the reservoirs are undoubtedly factors in the change in reservoir operation. Table 4 shows the water that was delivered to C -BT users during 1957 -98. This water, which includes both charge and non - charge deliveries, averaged 215,800 acre -feet annually and ranged from 119,900 acre -feet in 1990 to 322,000 acre -feet in 1989. The average distribution of water was 30.4 percent for the Carter Lake -St. Vrain portion of the project, 23.9 percent for the Big Thompson River and the several turnouts on the Charles Hansen Feeder Canal downstream from the Big Thompson Siphon, and 45.7 percent for the Charles Hansen Supply Canal and Dixon Feeder Canals. During 1983 -98, the total deliveries averaged 212,500 acre -feet annually. Some relatively small volumes of water delivered to users above Flatiron Reservoir (an average of about 500 acre -feet during 1983 -98) are not included in these values. Although water can be released from Carter Lake to the Charles Hansen Feeder Canal, the users who are supplied through the Big Thompson River and Charles Hansen Feeder Canal ' turnouts receive project water primarily as direct diversions through the system without the benefit of regulation in the two main storage reservoirs. ' Table 5 shows the non - charge water that was delivered by the C -BT Project. Non - charge deliveries were made in 1962, 1971, 1983 -87, and 1995 -98. During 1957 -98, the non- charge deliveries averaged 17,300 acre -feet annually. They exceeded 100,000 acre -feet in 1962, 1985, and 1986 and approached 100,000 acre -feet in 1997. During 1983 -98, the non- 1 e