Liang Weili, Kosugi Ken’ichirou, and Mizuyama Takahisa. Lab. of Erosion Control, Division of Forest Science Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwake-Cho Sakyo-Ku Kyoto 606-8502, Kyoto, Japan
In forest soils, macorpores originated by animals or plant roots are usually found, contributing to soil heterogeneities. Previous studies indicated that rapid infiltration occurs in a surrounding area of a tree stem, and such preference flow can not be adequately described by Richards equation for explaining ordinary water movement in saturated and unsaturated soils. Although it is important to understand these heterogeneous infiltration phenomena for modeling runoff generation and predicting shallow landslides, detailed observations are still lacking. The purpose of this study is to clarify effects of a tree stand on rainfall infiltration processes at a forested hillslope based on detailed observations of soil moisture distributions. We selected a Tall Stewartia (Stewartia monadelpha) as an observed tree and monitored soil water contents at depths of 10, 20, 30, 40, 50 cm by capacitance meters (Sentek, EasyAG-5p) at 10 locations around the tree. Results showed that, no matter what soil moisture conditions were, the water content at the depth of 50 cm showed rapid increases before an average wetting front reached to the depth, at the location 25 cm downstream from the tree stem. When rainfall intensity increased, similar phenomena were observed also at the location 50 cm downstream from the tree stem. It was presumed that such rapid water penetrations were caused by stemflow which infiltrated along main roots of the trees as bypass flow, contributing prompt rising of local ground water table.