Assessing spatially distributed infiltration capacity to evaluate storm runoff in forested catchments: Implications...

We assessed spatial distribution of infiltration capacity under forest canopies to simulate runoff processes in a steep small catchment in Japan. High resolution LiDAR data were used to assess light availability to understory vegetation, which in turn was converted into an understory vegetation cover ratio. Spatially distributed infiltration capacities based on data from field rainfall-runoff experiments were incorporated into the TopoTube model using a relationship between infiltration capacity and vegetation cover. Effects of spatial distribution of infiltration capacity and its influence on hydrological connectivity and runoff processes were examined based on numerical simulations in the following three infiltration scenarios for seven natural storms: (1) spatially distributed infiltration capacity; (2) uniform infiltration capacity using the weighted mean of the spatial distribution in the drainage basin; and (3) uniform infiltration capacity using the arithmetic mean of field measurements. Although saturated subsurface flow dominated storm runoff in all simulations, Hortonian and saturated overland flow were significant for storm runoff response in smaller catchments (headwaters and zero-order basins). Simulations using spatially distributed infiltration capacities showed more Hortonian overland flow compared to using uniform infiltration because of areas with low infiltration located along channels. On the other hand, no differences in runoff were apparent between spatially-distributed and uniform infiltration scenarios in larger catchments. Our study also indicates that differences in contributions of subsurface and groundwater flow at the catchment scale need to be examined based on observations and simulations in nested catchments. Our findings suggest that including detailed patterns of vegetation ground cover are important for understanding hydrogeomorphic connectivity within catchments.