Yampa River Basin Water Resources Planning Model Final Report

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1.0 EXECUTIVE SUMMARY The Yampa River Water Resources Planning Model (Yampa Model) was developed jointly by the Colorado Water Conservation Board and the Division of Water Resources as part of the Colorado River Decision Support System (CRDSS). The objective was to develop a monthly water allocation and accounting model which includes 100 percent of the basin's consumptive use and would be capable of making comparative analyses for the assessment of historical and future water management policies. 1.1 Background The Yampa River basin in northwestern Colorado comprises an area measuring nearly 7,660 square miles. The basin includes portions of two counties (Routt and Moffat) and supplies an annual flow of approximately 1,623,000 acre -feet to the Green River at the Colorado -Utah state line. The river basin contains reservoirs, transbasin diversions, and large power generation stations. The administration of portions of the Yampa river is extremely complex. The area from the Upper Bear River down to the Stagecoach Reservoir is the most complex. The Upper Bear River contains four reservoirs (Stillwater, Yamcolo, Allen Basin, and Stagecoach) along with two significant transbasin diversions (Stillwater Ditch and Sarvis Ditch) and one relatively minor one (Dome Creek Ditch). The reservoirs provide water to a number of divertors through a complex set of operational exchanges, water use agreements, and informal administration policies. _ 1.2 Approach The Yampa Model was developed using StateMod, the State of Colorado's Stream Simulation Model, and is operated on a monthly time step from water years 1975 to 1991. It was developed in two steps as follows: Phase II, completed in July, 1996, modeled 246 key diversion structures, 9 reservoirs, and 73 instream flow rights. Key structures were identified using the following approach: A list of diversion structures, based on the structure's net absolute decreed amounts, was compiled and ranked form largest to smallest. • The list was shortened by including those structures that represent 75 percent of the basin's net absolute decreed amounts. • Meetings were held with division and district engineers in the basin and the preliminary list of structures was refined to include structures that were considered important by the district engineers. Also, some structures were removed from the model because they historically diverted significantly less than the decree amount, they were abandoned or removed, or they had infrequent diversion records. Phase IIIa enhanced the previous model to account for 100 percent of the basin's consumptive use. This enhancement featured the following changes from the Phase II model: Twenty -seven aggregated irrigation structures were added to represent 449 structures supplying 24,960 acres of irrigated land. These lands compose all land in the 1993 survey which were not explicitly modeled in the Phase II version of the model. Executive Summary 1 -1