Hydrogeomorphic processes in steep forested headwater catchments

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This series of studies covered a wide range of interrelated hydrogeomorphic research topics. Studies in the Wakayama/southern Nara and Miyagawa sites in Japan as well as the Bukit Tarek and Amphoe Mae Taeng catchments in Southeast Asia elucidated linkages between sediment supply and transport in managed forested catchments. In the steeper sites where landslide processes dominated on hillslopes, sediment yields were not only affected by the initial failed volume of landslides but also by the mobility of landslides and debris flows and the storage characteristics of the headwater channels. In the moderately steep catchments in Southeast Asia, road erosion played a major role in sediment transport, underlying the importance of this sediment vector as well as the need to incorporate adequate road planning and construction standards in mountainous terrain. Additionally, the Fudoji and Bukit Tarek studies quantified how mountain roads intercept subsurface flow during storms and reroute surface runoff, thus modifying storm hydrographs. In steep forest catchments in Japan that were clearcut, the timing of many of the landslides and debris flows corresponded to periods of reduced slope stability when root strength was at a minimum ; most landslides occurred 1 to 10 yr after clearcutting with some increases in landsliding up to 25 yr after harvesting. Conceptual models were developed to describe how landslides contribute to debris flow initiation under various conditions in steep terrain. New insights were developed related to hillslope runoff pathways in Mie catchment ; while Hortonian overland flow occurred at small scales, its significance decreased markedly at the hillslopes and small catchment scales. Such effects are now being incorporated in an updated version of the hydrogeomorphic conceptual model of stormflow generation. Advances in small-scale processes included the development of a preferential flow model that can eventually be applied to hillslopes.