Effect of Melamchi Water Supply Project on Soil and Water Conservation in the Indrawati River Basin, Nepal

摘要

The Indrawati River basin, located about 50 km North-East of Kathmandu lies in the central region of Nepal. The river originates from the Himal region of the Mahabharat range (5,863 m asl). From its origin, Indrawati flows southwards to meet Sunkoshi River, one of the major rivers in Nepal at 626 m asl. The basin spans across Sub-tropical to Alpine climatic zones and overlaps three districts, Sindhupalchok, Kavreplanchok and Kathmandu of Nepal. The catchment area of the basin is 124,000 hectares and 40 percent of the basin is covered by forest. The average annual rainfall at higher elevation (Sarmathang) is 3,874 mm, while it is about 1,128 mm at Dolalghat, lower elevation Zone. The average annual potential evapotranspiration for the basin is 953.91 mm and the temperature ranges from 32.5°C to about 5°C. The average relative humidity is about 70% and varies from 60% in the dry season to 90% in the rainy season. The population density of the basin in 1991 was 165/km~2 and the projected figure for 1998 is about 175/km~2. Ninety-six percent of the population is involved in agriculture. Farm size per household (0.9 ha on an average) is very small. Major crops grown in the basin are rice, wheat, maize, potato, mustard and millet. A major attraction of the basin is its National Park at the head of the basin covering sizeable area and is one of the most favourite trekking routes of the tourists visiting Nepal. Water in the basin is used for irrigation, for drinking by both humans and animals, consumed by forest and vegetation, to extract hydropower and to operate water mills. In steep terrain the variation of rainfall is high. The effects of high rainfall on soil erosion and landslide are tremendous. At the same time, over grazing by livestock, intensive crop cultivation and encroachment of forest and cultivation of marginal lands have caused soil erosion and severe landslides in the basin. Majority of the soils in the basin are red soils (Rhodustalfs) that are highly susceptible to erosion and landslides. Agricultural practices that are adopted in the basin are also supportive to increase the rate of soil erosion. At the same time, construction activities of development infrastructures have also challenged the dynamic ecosystem of the basin. Recently, the Government of Nepal is implementing Melamchi Water Supply Project (MWSP). Melamchi River basin is one of the sub-basins of the Indrawati River basin. Kathmandu valley, the capital city of the Kingdom of Nepal, has been suffering from the shortage of drinking water for a long time. The population of Kathmandu valley is 1.1 million and the growth rate is about 4.82 percent per annum. The shortage of water supply has affected public health and economic activity in Kathmandu. The government carried out many studies to meet the water demand of Kathmandu valley. These studies formed the basis for the implementation of the MWSP. An environmental impact assessment (EIA) study of the Project has been carried out and the impacts of MWSP on the environment of Indrawati River basin cannot be overlooked. The present study is also an attempt of documentation of the effects of MWSP on soil and water in Indrawati River basin.