Recent concerns related to the potential impacts of the retreat of Himalayan glaciers on the hydrology of rivers originating in the catchment basins of the Himalaya have not been accompanied by any analysis describing the role of glaciers in the hydrologic regime of these mountains. This note presents an estimate of glacier runoff from the altitudinal belt above 5000 meters above sea level for the Karnali, Narayani and Sapta Kosi basins of the Nepal Himalaya, based on a comparison of specific runoff (depth/unit area) and the distribution of surface area with altitude in those catchments. Specific runoff for the altitudinal belt occupied by the glaciers of these catchments was estimated to be between approximately 500 - 1000 mm, based on analysis of published data from the Nepal Department of Irrigation, Hydrology and Meteorology. The surface area of this altitudinal belt was determined by planimetry of 1:500,000 maps. Glacier areas were taken from the literature.
It was assumed that runoff for altitudes above approximately 5000 m is produced by the melting of glaciers, perennial snow fields or transient snow deposits, Based on a value of 1000 mm for the mean annual specific runoff for the 5000-7000 m altitudinal belt, the total estimated runoff from the belt within the catchment basins was slightly more than 27,000 million cubic meters annually, or slightly less than 20% of the total annual discharge of the rivers of Nepal. Runoff from the glacierized portion of the three catchments was calculated to be slightly less than 5200 million cubic meters annually, representing approximately 4% of the estimated 145,000 million cubic meters flowing on average into the Ganges River from Nepal each year.
