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\ , '<It 0[\0187 MEMORANDUM ,..,..... DIR - SC A$C F I L E . TO: Technical Steering Group FROM: Chuck Brendecke, Ed Armbruster SUBJECT: Refinement of Environmental Baseline and Contemplated Draft model runs DATE: December 22, 1993 We've summarized below the recent modifications we've made to the administrative and operating assumptions for the existing Yampa River basin reservoirs, These changes are reflected in the results of our most recent Environmental Baseline and Contemplated Draft model runs, A discussion of these results is also provided below, GENERAL MODEL ASSUMPTIONS Storage water carried over on each reservoir's start-fill date is charged against the next year's decreed fill amount, The carryover water is charged against the senior-most decree first, then against the junior decrees, Although specific reservoir accounts are maintained throughout the water year, water carried over is re-allocated on the administration date among the various accounts according to specific account priorities, ReserVoir administration dates are modeled as the first day of the month following the month having the lowest average end-of-month storage, This "lowest month" can be expected to change from scenario to scenario depending on various demand and operating assumptions. Under low demand conditions, the choice of start-fill date itself can influence when the average lowest month occurs, As a result, picking the start-fill date is an iterative process, But the start-fill assumption can easily be changed for any or all of the reservoirs, EXISTING RESERVOIRS Stagecoach A third decree has been added to Stagecoach Reservoir, This decree represents storage under the Four Counties water rights that were included in the 1993 application, Thedecree amounts for these Four Counties rights were developed using the Wheeler "correlation method" referred to in the Upper Yampa water right application, The resulting decrees at Stagecoach are now as follows: Four Counties - variable by month and year Average annual amount = 4,595 af Maximum annual amount = 8,960 af Minimum annual amount = 0 af Bear Reservoir (Wessels priority) - 11,614 af/year Pleasant Valley Reservoir - 20,584 af/year In most years, the total decreed storage rights at Stagecoach add up to more than the physical capacity of the reservoir. This causes some, complications in accounting of the two contract pools (municipal and power) and the recreation pool. Because there is additional decreed storage capacity over and above a single reservoir fill, there are instances when the Hydrosphere Resource Consultants 1002 Walnut Suite 200 Boulder, Colorado 80302 . i..~"E.