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

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�1 t 1� C] u ANALYSIS OF THE COLORADO -BIG THOMPSON PROJECT OPERATION WITH RESPECT TO EAST SLOPE DIVERSIONS AND NON - CHARGE DELIVERIES EXECUTIVE SUMMARY This report describes and presents the results of an analysis for the Colorado River Water Conservation District of certain issues associated with the operation of the Colorado -Big Thompson (C -BT) Project during water years 1983 -98. Specifically, the analysis was concerned with (1) the extent to which native Big Thompson River water was diverted and integrated into the project water supply, (2) the extent to which non - charge water has been delivered to the several streams in the Northern Colorado Water Conservancy District service area and diverted and used by the water users, and (3) the effects of these two aspects of the C -BT operation on the flow in the Colorado River in the "15 -Mile Reach ". Initially, the layout and historical operation of the C -BT Project were studied. Then, three EXCEL spreadsheets were developed and applied. The first was used to estimate the water that was available from the Big Thompson River under the C -BT Project water rights and to evaluate the diversion of the non - charge water released to the Big Thompson River. The second spreadsheet was used to simulate the Adams Tunnel diversions and the operation of the east slope project features under three different operational scenarios or runs. Run A corresponded to the historical situation. It used historical diversions of Big Thompson River water, historical deliveries of both charge and non - charge water to the water users, and average historical storage contents for storage targets. Run B involved increased utilization of the Big Thompson River water available under the C -BT Project decrees, historical deliveries of both charge and non - charge water to the water users, and different storage targets designed to reserve storage capacity in the reservoirs so that the Big Thompson River water could be diverted and made part of the project water supply. Run C involved increased utilization of the Big Thompson River water, deliveries of only charge water to the water users, and the same storage targets as Run B. The third spreadsheet simulated the operation of Granby Reservoir and calculated spills from Granby Reservoir, flows in the 15 -Mile Reach of the Colorado River, and the potential for enhancement of spring peak flows as required by the 1999 Programmatic Biological Opinion, which is currently under development as part of the Coordinated Facilities Operations Study. The principal conclusions from our analysis are summarized in the following paragraphs: 1. The C -BT Project did not make full use of its Big Thompson River water source during 1983 -98. Our analysis indicated that an average of 21,862 acre -feet annually of Big Thompson River water could have been diverted during this period. Most of this water, an average of 18,331 acre -feet, was available in May and June. An average of not more 1