The role of glaciers in stream flow from the Nepal Himalaya

Abstract. 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 been accompanied by few analyses describing the role of glaciers in the hydrologic regime of these mountains. This is, at least in part, a result of the relative inaccessibility of the glaciers of the Himalaya, at altitudes generally between 4000–7000 m, and the extreme logistical difficulties of: 1) reaching the glaciers, and 2) conducting meaningful research once they have been reached. It is apparent that an alternative to traditional "Alpine" glaciology is required in the mountains of the Hindu Kush-Himalaya region. The objectives of the study discussed here have been to develop methodologies that will begin to quantify the role of complete glacier systems in the hydrologic regime of the Nepal Himalaya, and to develop estimates of the potential impact of a continued retreat of these glacier, based on the use of disaggregated low-altitude data bases, topography derived from satellite imagery, and simple process models of water and energy exchange in mountain regions.

While the extent of mesoscale variability has not been established by studies to date, it is clear that the dominant control on the hydrologic regime of the tributaries to the Ganges Basin from the eastern Himalaya is the interaction between the summer monsoon and the 8000 m of topographic relief represented by the Himalayan wall. All the available evidence indicates that the gradient of specific runoff with altitude resulting from this interaction is moderately to strongly curvilinear, with maximum runoff occurring at mid-altitudes, and minima at the altitudinal extremes. At the upper minimum of this gradient, Himalayan glaciers exist in what has been characterized as an "arctic desert".

The methodologies developed for this study involve the relationship between area-altitude distributions of catchment basins and glaciers, based on Shuttle Radar Topography Mission (SRTM3) data and water and energy exchange gradients. Based on these methodologies, it is estimated that the contribution of glacier annual melt water to annual stream flow into the Ganges Basin from the glacierized catchments of the Nepal Himalaya represents approximately 4% of the total annual stream flow volume of the rivers of Nepal, and thus, is a minor component of the annual flow of the Ganges River. The models developed for this study indicate that neither stream flow timing nor volume of the rivers flowing into the Ganges Basin from Nepal will be affected materially by a continued retreat of the glaciers of the Nepal Himalaya.