Fox River Watershed Investigation: Stratton Dam to the Illinois River, Phase II, Part 4, Fox River Watershed Hydrology Using the HSPF Model

This report documents the development of the Fox River watershed hydrologic model using the Hydrological Simulation Program - FORTRAN (HSPF). The Fox River HSPF model simulates rainfall runoff processes for watersheds adjacent to the Fox River from Stratton Dam to the mouth of the Fox River at the Illinois River. Runoff is simulated from 31 separate tributary watershed models and is used as an input to the mainstem model. Observed flows at Stratton Dam are used as the upstream boundary condition defining inflow from the Fox River watershed above Stratton Dam. Calibration is based on Hydrologic Response Units (HRUs). These units represent homogeneous areas of land use, soil type, and slope. Parameter values from prior calibration of Blackberry and Poplar Creek models were initially used for the same HRUs in the other 29 tributary watershed models and the mainstem model. However, additional calibration was needed to simulate flow at the Fox River mainstem gages accurately. Calibration was simplified to determine model parameters for dominant HRUs covering more than 10 percent of drainage area. The model parameters for dominant HRUs and reported literature values guided development of model parameters for other major and minor HRUs. Parameter values were developed through calibration of the mainstem model using observed flows from United States Geological Survey gages at Algonquin, South Elgin, and Dayton. The Fox River mainstem model was calibrated for water years 1991–1999 and validated for water years 2000–2003. Simulation results are presented in graphics, and model accuracies were evaluated through graphical comparisons and statistical measures such as the Nash-Sutcliffe efficiency coefficient and coefficient of determination. Exceedence probabilities of simulated and observed flows were compared to assess the simulation accuracy for high-, low-, and middle-range flows at three Fox River mainstem gaging stations. The integrated model simulates flow at the mainstem gages adequately based on specified calibration criteria. Model performance for the two pilot watersheds, Blackberry and Poplar Creek, worsened slightly, but overall performance for all seven gaged tributary watersheds improved.