Hydrologic Model of the Vermilion River Watershed for Streamflow Simulations

In continuation of the efforts made by the Illinois State Water Survey to develop a detailed hydrologic and water quality simulation model of the entire Illinois River Basin, a hydrologic simulation model was developed for the Vermilion River Watershed (one of the major tributaries of the Illinois River) to simulate streamflows using available climatic data. The model was developed using Hydrologic Simulation Program – FORTRAN (HSPF, version 12) under the BASINS (Better Assessment Science Integrating Point and Nonpoint Sources, version 3.0), a multipurpose environmental analysis system developed by the U.S. Environmental Protection Agency (USEPA). The watershed was sub-divided into 25 smaller, hydrologically connected sub-watersheds and their stream reaches. Hydrologic Response Units (HRUs) were created within each sub-watershed based on landuse and hydrologic soil groups. Streamflow data from two USGS streamflow gaging staions in the watershed and climate data from six representative staions for 1970-1995 was used. Model was calibrated using data for 1987-95 at the USGS gage at Pontiac, IL and then verified using 1972-1986 data from the same station, and using 1972-1995 data from the USGS gage at Leonore, IL. Model simulated the monthly streamflows with correlation coefficients and Nash-Sutcliffe Efficiency (NSE) of close to or greater than 0.8 during calibration as well as verification periods. Flow-duration curves of the daily observed and simulated streamflow data indicated good simulation for all flow conditions, except for some very low flow periods. The flood year of 1993 was under-simulated by the model whereas some very low flow years were generally over-simulated.