Evaluation of an ice ablation model to estimate the contribution of melting glacier ice to annual discharge in the Nepal Himalaya

This study focuses on the contribution of annual glacier ice melt to streamflow along two rivers in two watersheds situated in the monsoon-influenced part of the Nepal Himalaya (Trishuli and Dudh Kosi basins). We used a simple elevation-dependent ice ablation model based on glacier areas from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and IKONOS remote-sensing data combined with hypsometry from the Shuttle Radar Topography Mission (SRTM). Long-term hydrologic measurements were used to calculate the percent contribution of the glacier ice melt component of the water balance to discharge at various elevations and distances from glacier outlets. Glacier ice melt was positively correlated with the basin glacierized area and contributed 58.3% to annual flow in the small Langtang Khola watershed in the Trishuli basin (43.5% glacierized area) and 21.2% in the Hinku watershed in Dudh Kosi basin (34.7% glacierized area). Of this, 17.7% and 4.1% of streamflow, respectively, was due to the contribution of debris-covered glaciers in Langtang Khola and Hinku. The contribution of glacier ice melt to measured discharge decreased substantially toward lowland locations in both study sites, i.e., 9.5% of the streamflow measured at Betrawati (600 m) and 7.4% at Rabuwa Bazaar (470 m), about 50 km from glacier termini. Glacier ice melt contribution decreased to 4.5% of annual discharge further downstream in Trishuli basin (at 325 m, about 75 km from glacier termini). At low elevations, debris-covered tongues contributed a small percent (1.1% and 3.0%) to measured discharge at Betrawati and Rabuwa Bazaar stations, respectively. We independently evaluated the ice ablation approach with synoptic sampling of stable water isotopes (O-18 and D) collected during the post-monsoon season to quantify the contribution of various sources of water to river flow. Mixing models showed groundwater to be an important component of river flow within only tens of kilometers of the glacier outlets in the post-monsoon season.