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• • • • • • Flaming Gorge Draft Environmental Impact Statement Hydrologic Modeling Report R. Clayton and A. Gilmore February 26, 2002 Introduction In October of 2001, a report titled "Flaming Gorge Environmental Impact Statement Hydrologic Modeling Study Report" was distributed to all Cooperating Agencies and Interdisciplinary (ID) Teams working on the Flaming Gorge Environmental Impact Statement. The report described the hydrologic impacts observed in the modeled implementation of the 1992 Biological Opinion (No Action alternative) and the 2000 Flow Recommendations (Action alternative) for the period from 2002 through 2040. Based on comments received from the Cooperating Agencies, ID Teams, as well as the Authors of the 2000 Flow Recommendations, the Flaming Gorge model has been modified to more accurately reflect the intentions of the 2000 Flow Recommendations and the 1992 Biological Opinion. The purpose of this report is to detail these modifications and update the model results so the Cooperating Agencies and ID Teams can conduct their impact analyses. Description of Modifications The Flaming Gorge model was populated with natural inflow data generated from historic river flow and consumptive use records. For the Upper Green River and Yampa River basins, the only records available for consumptive use were recorded as monthly volumes. For this reason, the natural inflow data used to populate the Flaming Gorge model, as well as the model itself, were developed at a monthly timestep. The monthly timestep framework of the Flaming Gorge model limited when operational decisions could be made to the beginning of every month. It became apparent very early in the development of this model that limiting the timing of these operational decisions, which was only an artifact of the model framework, made it more difficult for the model to achieve the target flows and durations specified in the 2000 Flow Recommendations than would be the case in reality. In reality, Flaming Gorge Dam is operated to adjust to changing hydrologic conditions the moment these conditions change. The Flaming Gorge model, however, must wait until the beginning of each month to make these adjustments. Sometimes this caused the daily average releases determined by the model under the Action alternative to be set much higher than necessary to achieve specific targets established for Reach Two. After receiving comments from the Authors of the Flow Recommendations regarding the report issued in October, it became clear that this artifact of the model did not satisfactorily reflect the intended implementation of the 2000 Flow Recommendations.