Simulating runoff behavior in an urbanizing watershed
Land-use change in urbanizing watersheds has significant impacts on hydrolic processes and stream quality. Geographic information system (GIS) processing of spatial information is now commonly used to parameterize hydrologic models. We use and evaluate a system developed for the US Environmental Protection Agency (US EPA), coupling Hydrologic Simulation Program Fortran (HSPF) with a commonly used GIS (ArcView) to assess the effects of land-use change on watershed behavior. We extend this anlaysis to investigate relationships beetween runoff ratio and baseflow as a function of percent impervious cover and percent soil saturation for upper Gwynns Falls watershed, Baltimore, MD, USA. Hydrologic model output is used to define summary expressions to describe these relationships and condense complex system behavior. The summary expressions for the runoff ratio and baseflow relationships show that in upper Gwynns Falls from pre-urbanized times to 1990: (1) baseflow had declined by as much as 20%; and (2) only small changes in runoff ratio had occurred. The summary expression for the runoff ratio relationship indicates the existence of a threshold percent impervious cover (?20%), above which the runoff ratio changes more dramatically. By 1990, the percent impervious cover for upper Gwynns Falls (?18%) has not yet exceeded this threshold and may explain why only small changes in the runoff ratio had occurred.
hspf runoff ratio baseflow urban hydrology stormflow