Assessment of soil erosion in the Nepalese Himalaya : a case study in Likhu Khola Valley, Middle Mountain Region

Abstract

Soil erosion is a crucial problem in Nepal where more than 80% of the land area is mountainous and still tectonically active. Although deforestation, overgrazing and intensive agriculture, due to population pressure, have caused accelerated erosion, natural phenomena inducing erosion, such as exceptional rains, earthquakes and glacial-lake-outburst flooding in the high Himalayas are also common. It is important to understand the erosion process under normal conditions and to assess the magnitude of problem so that effective measures can be implemented. Results provided by running a soil erosion assessment model (Morgan et al., 1984) in a GIS environment show that annual soil loss rates are the highest (up to 56 tonnes/ha/yr) in the areas with rainfed cultivation, which is directly related to the sloping nature of the terraces. The lowest soil losses (less than 1 tonne/ha/yr) are recorded under dense forest. In the degraded forest, the soil loss varies from 1 to 9 tonnes/ha/yr and in the grazing lands it is estimated at 8 tonnes/ha/yr. The rice fields seem to trap the sediments brought from up-slope. Erosion rates are higher on the south facing subwatershed than on the north facing one. The index of structural instability of the topsoil, calculated by the amount of dispersible clay content, seems not to vary so much among the soils, whether developed on gneiss or micaschist, the main rock types of the study area. Under normal climatic conditions, soil losses can be considered low although in heavy monsoon, with exceptional rain, the situation might be different. The study shows that soil erosion can be modelled in the mountainous region and that the results confirm the soil loss data obtained by means of experimental erosion field plots in the area, and the study of suspended sediment delivery from small catchments.