GIS-based modeling of runoff source areas and pathways

Abstract. The application of runoff models that rely on calibration to future land use and climate conditions is restricted to situations where the reaction of Hydrologic Response Units to environmental change is known. This limitation and the ensuing uncertainty of model results can be avoided when a risk-based approach to landscape and runoff analysis is taken. GIS-based landscape analysis provides the possibility of assessing the risks associated with non-linear responses of Hydrologic Response Units to changing rainfall and land use. In this paper, a runoff module designed for IDRISI-Andes to calculate runoff amount and routing for single or multiple rainfall events on a hillslope at small catchment scale is presented. The module is raster-based and uses layers with topographic and hydrological parameters to calculate a spatially distributed output layer of surface runoff. Conceptually, the module extrapolates point data of infiltration capacity onto a field or hillslope. A spatially distributed runoff map is calculated based on the addition of layers with rainfall data and the routing of runoff through pathways connecting pixels in a digital elevation model. Unlike outlet-based runoff modeling, the need for parameterization of the catchment is kept to a minimum. The application of the RUNOFF module in a test area in the Eifel region of Germany indicated that runoff from grassland is sensitive to small increases in rainfall intensity and soil compaction. The spatial patterns of infiltration capacity also contribute significantly to the non-linearity of the test area reaction to changing rainfall and soil hydrologic properties.