Vermillion River Watershed Hydrologic Study

The goal of this hydrologic study was to develop a calibrated hydrologic model that would help assess effects of current and future land use on watershed runoff. The rainfall-runoff model was developed and calibrated to actual gaged data. The hydrologic computer model that was used in the analysis was the U.S. Army Corps of Engineers Hydrologic Engineering Center's rainfall runoff model titled HEC-1. The first step in calibration included optimization of unit hydrograph parameters for historic rainfall events at gages within the watershed and in nearby, hydrologically similar basins. The optimized unit hydrograph parameters, watershed characteristics and percent imperviousness used as an index of urbanization, were combined into a regional equation with a multilinear regression analysis. The regional equation was used to predict unit hydrograph parameters for ungaged basins in the watershed. The second step in calibration included adjustment of the loss rates in the basin so that the discharges predicted by the HEC-1 model at the USGS gage location matched the discharge frequency curve. The calibration insures that the 100-year precipitation generates the 100-year flood. The calibrated hydrologic model will be used by the Watershed Management Organization to assess the effects of existing and future land use on watershed runoff.