Application of GeoWEPP for Evaluating Sediment Yield in a Mountain Area: Agatsuma Watershed, Japan

Abstract

Sediment discharge monitoring can be used to detect sediment disasters in upstream areas in order to enable prompt countermeasures. Sediment disaster signals should be easily differentiated from ordinary sediment discharge. This study examined the application of the geospatial interface for the Water Erosion Prediction Project (GeoWEPP) for the assessment of baseline sediment discharge in a mountain watershed to allow early detection of upstream sediment disasters. When compared to detailed observation-based data from the Agatsuma watershed, GeoWEPP successfully reproduced continuous sediment discharge in subwatersheds of varying size, topography, and land use. GeoWEPP parameter settings corresponded to actual conditions and processes involved in water and sediment dynamics. Additionally, the introduction of a restrictive layer and improved settings for evapotranspiration rate were critical for predicting surface runoff and subsequent surface erosion in hillslope sections. The depth to non-erodible layer was important for determining the overall sediment discharge at the study site, as it was the only parameter that varied over time and could not be obtained from the actual depth of bed material. This parameter should be interpreted as a conceptual index that represents the spatial distribution of bed material and its erodibility, and requires an initial adjustment period. Therefore, GeoWEPP calculations should eliminate the first year in order to obtain optimal results in a mountain watershed with a significant depth of bed material in a channel section.