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5.2 Green River Basin Plan -Little Snake River Spreadsheet Model <br />The State of Wyoming undertook a statewide water planning process in 1999, producing the <br />Green River Basin Plan as its first major basin plan in 2001. The final plan report includes a <br />Basin Use Profile, a characterization of current water uses by sector; Available Surface <br />Water and Groundwater Determination, an investigation of hydrology which involved <br />developing spreadsheet models for the four major sub-basins of the Green River, one of <br />which was the Little Snake; Demand Projections, an economically based estimate of future <br />needs by sector; and discussion of Future Use Opportunities, the product of user meetings to <br />identify potential projects and enhancements of the water supply, along with a review of <br />institutional considerations. <br />The Little Snake spreadsheet model comprised three separate spreadsheets with different <br />hydrologic and demand input, representing conditions under Normal, Wet, and Dry states of <br />the basin. Each spreadsheet shows one twelve-month sequence, but the input for the model is <br />based on historical data from the Normal, Wet, or Dry years within the period 1971 through <br />1998. This underlying data, including streamflow, diversions, crop demand, and historical <br />efficiency, has been incorporated in the Wyoming portion of the CDSS model of the Little <br />Snake River. Much of the data and details are not contained in the final Basin Plan report, but <br />in technical memoranda produced as part of the Green River Basin Plan. These are available <br />at Wyoming's website for the Green River basin plan, <br />http://waterplan.state.wy.us/plan/ rg een/ rg een_plan.html <br />Specific information from the Green River Basin Plan, on which CDSS Yampa model input <br />has been based, includes the following: <br />• Irrigated acreage, for two individual ditches, and for sub-tributary aggregations of <br />ditches <br />• Crop irrigation requirement based on climate data and regional crop mix was <br />available for Normal, Wet, and Dry years; each year within 1971-1998 was <br />designated as Normal, Wet, or Dry, and so a time series of irrigation water <br />requirements by model structure could be developed for those years <br />• Monthly historical diversions for First Mesa and Westside Canals, and the Cheyenne <br />diversion project <br />• Normal year monthly efficiencies for First Mesa and Westside Canals <br />• Average monthly depletions for municipalities of Baggs and Dixon, and average <br />monthly efficiencies for these diversions <br />• Maximum efficiency for irrigation structures <br />Yampa River Basin Information 5-6 <br />