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Evaluation of the Accuracy of Radar-based Precipitation for Use in Flow Forecasting for the Fox Chain of Lakes

This study evaluated the use of radar-derived daily precipitation values for estimating watershed precipitation in the Fox River watershed in northeastern Illinois and southeastern Wisconsin, and for the potential use in preparing real-time streamflow forecasts for the Fox River. Two types of radar-derived precipitation estimates based on data from the National Weather Service (NWS) WSR-88D radar network were evaluated. These estimates were developed using a 4-km grid resolution and computed using: 1) radar data alone and 2) composite or multi-sensor precipitation estimate based on radar data and real-time hourly precipitation gage observations. These gridded precipitation estimates were obtained from the National Center for Environmental Prediction (NCEP) and were compared to point measurements of daily precipitation from precipitation gages in the vicinity of the Fox River watershed for the period February 2002–September 2004. Multi-sensor precipitation estimates were found to be a considerable improvement over the gridded radar estimates during all seasons. In comparison to the daily gage data, however, the multi-sensor precipitation estimates were on average 25 percent lower throughout the year. Multi-sensor daily precipitation estimates from four storm periods were input to the Fox River Forecast Model to evaluate their potential application in hydrologic forecasting. Only one storm event produced flooding conditions on the Fox River in the period for which radar data was available. The remaining three storm events analyzed in this report produced only moderately high flows. Simulated flow forecasts using the multi-sensor precipitation values were less accurate than companion forecasts using observed data from precipitation gages. Based on the limited number and size of storm events available for simulation, this study was unable to conclude whether or not the multi-sensor precipitation values would be useful for forecasting floods. Until more comparisons can be conducted using data from future major flood events, flow forecasts for the Fox River should continue to be developed using near real-time data from precipitation gages. The present tendency of the multi-sensor precipitation data to underestimate rainfall amounts also makes their use generally inappropriate for longer term hydrologic simulations.