Characteristics of overland flow generation on steep forested hillslopes of central Japan

SummaryOverland flow generation was monitored in large plots (8 × 25 m) on four hillslopes in a 4.9-ha catchment in Mie Prefecture, Japan. Three Japanese cypress (hinoki, Chamaecyparis obtusa) treatments (including three different understory conditions) and one deciduous forest treatment were studied. For all plots, including deciduous hillslopes, we observed overland flow even for small storm events (<10 mm in total precipitation). The mean runoff coefficients in dense Japanese cypress plots with sparse understory were highest (13.0%) followed by dense Japanese cypress with fern ground cover (6.7%), and coefficients in managed cypress and deciduous forest were 3.6% and 1.2%, respectively. The runoff coefficients tended to be higher during storms that were preceded by dry conditions. High soil water repellency initially occurred in Japanese cypress forests between the litter and mineral soil horizon and might have been partly responsible for overland flow generation. During storms with total precipitation >180 mm, runoff from Japanese cypress plots with dense fern understory exhibited a delayed and higher peak associated with return flow. The dominance of hillslope-scale flow contribution to catchment runoff was also affected by changes in the dominance of overland flow and return flow. Understory vegetation cover and the availability of a litter layer altered the amount of overland flow, which was mediated by soil water repellency and soil moisture. Observations at the hillslope scale are essential for conceptualization of runoff mechanisms and pathways in forested headwaters.