Effects of Soil Layer and Permeable Bedrock on Flood and Base Flow Discharges from a Forested Watershed

Abstract

In order to evaluate an ability of a forested watershed for decreasing storm runoff and increasing baseflow discharge, this study analyzed the roles of soil layer and permeable bedrock in rainwater discharge processes by conducting hydrological observations and numerical simulations. Hydrological observations in a forested headwater catchment underlain by weathered granite indicated that saturated throughflow in a soil layer is extinguished just after a storm event while it contributes to a storm hydrograph. On the other hand, base flow discharge from the catchment was fed by the weathered bedrock. Based on these results, numerical simulations for saturated and unsaturated water flow were conducted for analyzing rainwater infiltration and discharge processes in a forested hillslope underlain by a permeable bedrock. Results showed that, while an exfiltration from the bedrock sustains base flow discharge, soil layer behaves as a buffer, moderating the infiltration intensity relative to the rainfall intensity and permitting large amount of rainwater to infiltrate into the bedrock. Thus, the result of this study indicates that measuring effective pore volume of soil layer is not enough for quantifying a water holding capacity of a forested watershed. Instead, evaluation of the buffer function of a soil layer is important.