The objectives of this paper are (1) to show the hydrological effects of changing the spatial resolution of radar-derived rainfall and (2) to show the effect of discretization of radar data on hydrological modeling for flood forecasting. Simulated hydrographs are compared for 2, 5, and 10 km basin grid sizes for the Grand River watershed in Ontario. An eight-level discretization of rainfall is compared with a 256-level scale of rainfall intensities. Spatial averaging tends to reduce the effect of erroneous radar readings and give comparable results with those obtained using finer resolutions. The appropriate level of discretization depends on the area being modeled and appropriate levels are recommended. The level of discretization of radar precipitation data does not significantly affect the generation of hydrographs. Hydrographs based on the radar-derived rainfall generally reproduced measured hydrographs better than those derived from the rain gauge data. Key words: hydrologic models, flood forecasting, weather radar.