A three-dimensional model of the effect of stemflow on soil water dynamics around a tree on a hillslope.

In forested stands, precipitation is intercepted by the canopy and partitioned into throughfall and stemflow as diffuse input and point input, respectively. Therefore, the water reaching the forest floor is not spatially uniform. Although there are many numerical models that simulate precipitation redistribution processes, the rainwater concentrated by stemflow is usually disregarded. In this study, we performed detailed observations of soil water dynamics during a storm event and developed a three-dimensional model of the effect of stemflow on soil water dynamics around a tree on a forested hillslope. In the stemflow infiltration process, water flowed rapidly through a deep layer, causing irregular changes in the vertical soil water content. This process is very different from the vertical rainfall infiltration process, in which the wetting front expands slowly from the upper layer to the deeper layer. Thus, simulations using the conventional net precipitation input assumption are likely to contain large errors as a result. The model proposed in this study, which considered the point input characteristic of stemflow and parameterized stemflow as a source flux spring in the soil layers, showed adequate spatial and temporal variations in soil water dynamics and closely agreed with observations. Applying the variable source term in the Richards equation to stemflow is a new approach and makes it possible to represent the root-induced bypass flow infiltration process around a tree growing on a hillslope.