Abstract
We developed a distributed runoff model that utilizes a contour-based topographic approximation (TOPOTUBE). The model consists of logical algorithms for water flux and associated routing for Hortonian overland flow, saturation overland flow, and saturated soil matrix flow. The model was applied in a 4.9 ha watershed covered by Japanese cypress forest in Mie Prefecture, central Japan. Changes in infiltration capacity with respect to rainfall intensity and soil water repellency were considered. Because spatial variability of infiltration capacity affects the occurrence and transport of overland flow, we employed detailed field information of soil infiltration capacity based on ground cover distribution. Agreement between observed and estimated catchment storm runoff improved when these spatial patterns of infiltration were considered in the model. In addition to these spatial patterns, further internal spatial variability of infiltration capacity based on the standard deviation of estimated infiltration capacity was included. This assumption means that locally high infiltration pixels are randomly distributed within the hillslopes. Observed and estimated overland flow generation at the hillslope plot scale showed that associating such internal variability of infiltration capacity can further improve the simulation of overland flow generation.