COMPARATIVE-ASSESSMENT OF RUNOFF CHARACTERISTICS UNDER DIFFERENT LAND-USE PATTERNS WITHIN A HIMALAYAN WATERSHED

Large quantities of sediments leave the Himalaya through its rivers. T hese rivers are charged with sediments depending on the types of land use in the watersheds. Land use/cover change and hydrology was studied in a watershed in the Sikkim Himalaya. The land use change from fores t and agroforestry to open agriculture has increased by 11% from 1988 to 1992. During the same period substantial areas of dense mixed fores ts have been converted to open mixed and degraded forests as a result of high pressure on natural resources. Stream flow was highest in the rainy season and lowest in summer season in all the streams and all th e three years (1994-1997) of the study. The water quality of streams f rom different microwatersheds varied significantly between seasons and streams. Sediment and nutrient loss was estimated in microwatersheds and soil loss from the total watershed ranged from 4.18 to 8.82 t ha(- 1) yr(-1) during the three-year period of study. The annual total nitr ogen loss estimated at the watershed outlet was at a rate of 33 kg ha( -1), organic carbon 267 kg ha(-1) and total phosphorus 5 kg ha(-1). Th is study suggests that the upland microwatersheds can be hydroecologic ally sustainable only if good forest cover and dense forests with larg e cardamom-based agroforestry are maintained. (C) 1998 John Wiley & So ns, Ltd.