Illinois State Water Survey

Watershed scale hydrologic simulation models HSPF (Hydrologic Simulation Program – FORTRAN) and SWAT (Soil and Water Assessment Tool) were used to model the hydrology of the 2150 square mile Iroquois River watershed (IRW) located in the east central Illinois. Both models are part of the BASINS modeling system that facilitates pre- and post-processing of data, as well as data input to the models using an ArcView GIS interface and GUI. HSPF has been widely used for different watersheds all over the US. SWAT was added to BASINS in 2001 and is currently under evaluation. Based on the completeness of the meteorological data, a nine year period of 1987-1995 is used for model calibration, and a 15-year period of 1972-1986 for model validation. Time series plots as well as statistical measures such as Nash-Sutcliffe efficiency (NSE), coefficient of correlation (r), and the percent volume error between observed and simulated streamflow values on both monthly and annual bases were used to verify the simulation abilities of the models. Calibration and validation results from both HSPF and SWAT show that the models generally predict daily, and average monthly and annual stream flows close to the respective observed stream flows