Measurement and distribution of runoff and soil losses from selected watersheds in the Hindu-Kush Himalayan region

Abstract

The sediment loads of Himalayan Rivers are amongst the highest in the world, resulting in problems such as siltation of reservoirs, blockage of river channels and poor quality of water supplies. The aim of the study was to develop total sediment calculation and distribution on different land uses and thus to provide information for planning the quantification of sediment loads to rivers and reservoirs. Hydrological, meteorological and soil erosion data were collected from four watersheds, the Jhikhu Khola, the Hilkot, the Bhetagad and the Xizhuang, all of which featured several types of land use. Runoff and sediment losses were monitored. Total annual rainfall in the region ranged between 972 to 2400 mm. About 44 to 66 percent of rainfall occurred from June to August, which is mostly the Monsoon season in the region. Results show that runoff and soil losses were higher in the months of May to September, when rainfall duration and intensity were higher in all the watersheds. Large rainfall events contributed most of annual runoff and soil loss in the area. The highest annual runoff was generated in the Jhikhu Khola watershed under all conditions of land use while it was very low in the Bhetagan and Hilkot watersheds.

The highest runoff and soil loss was recorded from degraded land plots because there less vegetation was available and the soil was loose and susceptible to erosion. In agricultural plots runoff and soil erosion was higher during the periods when there was no crop canopy and soil was not compacted after ploughing. During the months when crop cover was good there was less runoff and soil erosion. In pasture and forest plots runoff and soil loss was minimal throughout because of good vegetation and higher concentrations of organic matter that increased water infiltration and reduced runoff and soil loss.