Estimating Runoff In Glacier-Covered Watersheds In The Nepal Himalaya Using Area-Altitude Distributed Models
Racoviteanu, Adina E 1 ; Armstrong, Richard 2 ; Alford, Don 3
1 Dept. of Geography and Institute of Arctic and Alpine Research, University of babyֱapp
2 National Snow and Ice Data Center, University of babyֱapp
3 1831 Poly Drive, Billings, Montana
Glaciers are a component of the hydrologic regime of many large mountain ranges and regions, including the Himalayas. However, the hydrologic regime of Himalayan catchment basins, and the role of glaciers in the hydrologic regime of this mountain range is not well understood. As a result of a general unavailability of hydrologic, climatic and topographic data for Himalayan catchment basins, the application of hydrologic concepts and models developed for mountain catchments in Europe or North America is difficult, and sometimes, misleading. Current concerns over the impact of the retreat of Himalayan glaciers on water supplies poses an urgent need to resolve questions related to specific aspects of the water budget cycle, and to determine the impact of glacier change on regional-scale water supplies. The present study focuses on assessing and quantifying the role of glaciers in the hydrologic regime of the Nepal Himalaya. The overarching goal is to estimate the potential impact of a continued glacier retreat in this region.
The methodology developed for this study involved establishing a relationship between the area-altitude distributions of catchment basins and glaciers, and the water and energy exchange gradients. Two area-altitude distributed process models were developed: an orographic runoff model and a glacier melt model. The orographic runoff model was based on the relationship between mean specific runoff, and the mean altitude of each basin. The area-altitude distribution of streamflow was calculated for 1000 meter belts, as the product of the specific runoff depth and the area of the belt. The glacier melt model was based on glacier melt from 100 meter area-altitude belts for the glacierized portion of each catchment and involved: defining an “ablation gradient” (the rate of increasing specific ice melt with decreasing altitude in the ablation zone, taken as 0.6 to 1.4m/100m in the Himalayas); defining the mean maximum altitude of the 00 C isotherm during the ablation period (~5400 m); determining the volume of ablation as the product of specific ice melt values taken from the ablation gradient and the area-altitude values of corresponding belts in the glacier ablation zone. Topography was defined by digital elevation data sets acquired from the Shuttle Radar Topography Mission (SRTM) for both models, and glacier outlines were obtained from the International Center for Integrated Mountain Development (ICIMOD), Nepal.
Preliminary results indicate that the contribution of glacier annual melt water to annual streamflow in the Nepal Himalayas varies among catchment basins from 2-13% of the total annual flow volume, measured at low altitude hydrometric stations. This represents 2-3% of the total annual streamflow volume of the rivers of Nepal. These results suggest that neither the timing nor the volume of the streamflow of rivers of Nepal will be affected significantly by a continued retreat of the glaciers.
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