ERC was the surface water resource specialist tasked with developing a model of the Colorado – Big Thompson (C-BT) and Windy Gap systems to evaluate the integrated operations of those projects and analyze the firm yield and hydrologic effects associated with Windy Gap Firming Project (WGFP) EIS alternatives. ERC used the State of Colorado’s CDSS model and BESTSM (a surface water allocation model) to develop an integrated WGFP model capable of simulating the C-BT and Windy Gap systems and Colorado and South Platte River basins. ERC configured the WGFP model to represent each of the EIS alternatives and simulated the model to generate hydrologic output.  ERC used the model to perform an assessment of Windy Gap water rights, including projected diversions under the EIS alternatives and the effects on other water rights and streamflows in the Colorado River and South Platte River basins. ERC used model output to complete the surface water impacts analysis and associated sections of the EIS document and support evaluations of resources affected by changes in flow and reservoir contents including wildlife, aquatic resources, recreation, stream channel morphology, wetlands and riparian habitat, floodplains, and water quality.