Hydrology and nutrient flux in an agrarian watershed of the Sikkim Himalaya

Abstract

A hydrological study was carried out in an agrarian watershed located in the Sikkim Himalaya. The Mamlay watershed has an elevational range of 300-2650 m above msl, with a total are of 3014 ha. The drainage of the watershed is dendritic type and the stream texture is fine on the higher elevation that gradually becomes coarse at the valley. The discharge was greatest in rainy season and smallest in summer. Sediment concentration of 325 mg/l was recorded during rainy season at the watershed outlet. Overland flow was greatest in open agricultural (cropped) are (9.6% of the precipitation) and smallest in cardamom agroforestry (2.17%). Soil loss of 477 kg/ha was recorded in open agricultural area, while it was very low (6 kg/ha) in dense mixed temperate natural forest. Large quantities of nutrients were lost through soil erosion from the open agricultural area in comparison to agroforestry systems. Agroforestry systems should be promoted in most of the areas where open agricultural practices are followed. The overland flow, soil and nutrient loss, and precipitation partitioning in different pathways in natural forest and agroforestry systems suggests that these land uses promote conservation of soil, water, and nutrients.