SEDIMENT MOBILIZATION BY SURFACE EROSION IN MIDDLE MOUNTAIN CATCHMENTS OF NEPAL

TV14; Soil erosion is a hot topic in the foothills of Nepal and the entire Hindu Kush-Himalayan region. Particularly topsoil loss from water erosion is considered to be a big problem. As part of the project People and Resource Dynamics in Mountain Watersheds of the Hindu Kush-Himalayan region a number of erosion plots are monitored at an event resolution in two catchments of the Nepal middle mountains. On the basis of catchment wide sediment source mapping, surface erosion including gullying was determined to be the main erosive process in both catchments. In terms of importance as sediment source, surface erosion was likewise determined to be of major importance followed by streambank erosion and occasional landslides. In terms of topsoil loss through surface erosion degraded land produced the highest soil losses throughout the rainy season, followed by rainfed agricultural land, and finally only very small amounts of soil loss were observed on grassland. This is in general agreement with other soil erosion studies in the region. It was further observed that only a few large events are responsible for most of the annual soil loss. The event analysis also stressed the importance of rainfall amount and rainfall intensity for sediment mobilization. Particularly, the 30 minute maximum rainfall intensity, I30max, showed a good correlation with the soil loss that was nearly identical to that for the 10 minute maximum intensity, I10max, on one test catchment and was slightly better than that for I10max on the other test catchment. The importance of these rainfall parameters on degraded land is more pronounced due to the lack of vegetation cover and the subsequent lack of protection from the force of the rain drop splash. The antecedent precipitation conditions generally showed no correlation with the event soil loss from the plots.