Examining Runoff Production Using Radar-Rainfall and Physiographic Databases

The lack of rain gages is problematic for studies of runoff production in many basins, as localized phenomena can seriously affect runoff production. Without taking into account the small scale space-time variability of rainfall, it is impossible to examine with certainty the partitioning of rainfall into runoff. In this study, we make use of a new WSR-88D derived hourly rainfall database, covering the entire Mississippi basin, which was produced for the GCIP program. In this preliminary study we examine the roles of space-time rainfall patterns and basin structure in explaining the quantity and timing of runoff from 11 basins throughout Illinois with drainage areas ranging from 62 to 932 sq. km from Water Years 1996-2000. The study will later be expanded to basins throughout the upper Midwest. Factors being considered include the effects of river network structure, both in plan (i.e., the width function) and in elevation (basin hypsometry) on routing; the effects of land-use, land-cover and soils, including the presence of drainage tiles on infiltration and interflow; the effect of geology on baseflow; and the effects of land-use, land-cover, and vegetation on evapotranspiration and pre-storm soil moisture. Statistics used to characterize the hydrographs, which are derived from hourly USGS streamflow data, include peaks, dispersion and skewness, volume, and relationships between flow and rainfall quantities such as runoff ratio. This new radar-rainfall data set combined with physiographic data using GIS facilitates the study of rainfall-runoff relationships in unprecedented detail over a large number of basins, distributed over a wide geographic area, using a consistent methodology.