Hydrologic Modeling of the Fox River Watershed: Model Development, Calibration, and Validation

Regional water supply planning efforts in Illinois are attempting to better understand potential impacts of climate change on low flow hydrology and surface water availability for meeting increasing water use. For this purpose, models are being developed for selected watersheds to analyze hydrologic sensitivity to a range of climate scenarios. This paper presents the development, calibration and validation of a hydrologic simulation model for the Fox River watershed using the Soil and Water Assessment Tool (SWAT), which is a basin-scale, semi-distributed model. The Fox Chain of Lakes is modeled as a reservoir controlled at Stratton Dam near McHenry, whose main purpose is to maintain minimum lake levels. A level pool routing algorithm is incorporated into SWAT to simulate the reservoir storage routing. To improve simulations of low flows, modification to SWAT's baseflow algorithm has been made by using a non-linear reservoir approach for groundwater storage-outflow relationship. The model is calibrated for daily streamflows using manual and automatic calibration methods. The Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS), and ratio of the root mean square error to the standard deviation of measured data (RSR) have been used as model performance metrics. The worst NSE and PBIAS obtained for daily simulations were 0.43 and 12.4 %, respectively, which are better than recommended values for satisfactory daily simulations (i.e., ⩾ 0.36 for NSE and within ±25 % for PBIAS). For all calibration gauging stations with the exception of USGS_05548280 near Spring Grove, the NSE values ranged from 0.65 to 0.77, the RSR values were < 0.6, and the worst PBIAS obtained was 12.4 % for monthly streamflow simulations, indicating good model performance. The absolute PBIAS was less than 10 % for all calibration gauging stations but USGS_05552500 at Dayton, Illinois. The model will be ultimately used to simulate various climate scenarios that will help evaluate the water resources of the watershed. Such simulations provide useful information for planning and management of future water supply capabilities.