Factors Affecting Generation of Hortonian Overland Flow in Forested Hillslopes:

Analysis of Observation Results at Three Sites with Different Geology and Rainfall Characteristics

Abstract

To examine factors affecting Hortonian overland flow generation in forested hillslopes, we measured overland flow at plots in Japanese cypress (Chamaecyparis obtusa), Japanese ceder (Cryptomeria japonica), and broadleaf forests of three study sites (Kochi, Mie, and Tokyo), which had different geology and climate. Overland flow occurred at all plots. Runoff ratios of overland flow were different among the three study sites rather than among forest species. By introducing overland flow index for each plot, we investigated relationships between the overland flow index and potential factors affecting overland flow generation. The index was not significantly associated with slope gradient and saturated hydraulic conductivity, whereas the index tended to be smaller as ground cover ratio increasing. This suggests that ground cover reduced rainfall impact to soil surface and thus contributed to preventing formation of structure crust, which inhibited infiltration and enhanced overland flow. The overland flow index increased as increases in silt and clay content in surface soils, implying that silt and clay content was an implicit factor influencing crust formation as a potential component of structure crust.