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While studies over retreat (reduction in glacier mass and snow cover area) of the Gangotri Glacier have received worldwide attention; no research has so far been conducted to quantify the contributions of snowmelt and glacier melt to the total flow of the Bhagirathi (upper Ganga) River emanating from the Gangotri Glacier. In the present study, the fractional contributions of snowmelt, glacier melt and direct runoff to the Bhagirathi River were estimated on ten daily and monthly time intervals for the ablation period (May to September) of 2005 using environmental isotope (delta H-2, delta O-18 and H-3) techniques.

Hydrograph separation was carried out using a two-component and three-component isotope mixing models. The estimated average seasonal (ablation period) contributions of snowmelt, glacier melt and direct runoff to the Bhagirathi River were 59.6%, 36.8% and 3.6%, respectively. Also, the significant temporal variations in their contributions were observed. The monthly proportions of snowmelt in the river flow varied from 42.9% (July) to 91.9% (May), while the monthly ratios of glacier melt ranged between 8.1% (May) and 47.4% (July). The observed temporal variability in the estimated contributions of the snowmelt could be linked to the Snow Cover Area (SCA) of the catchment. The results of the hydrograph separation indicate that the snowmelt dominates the river flow during the ablation period. Environmental tritium (H-3) data also supports this finding. The presence of H-3 in the river (average value: 8.8TU) endorse that it is mainly derived from the melting of relatively modern snow (average value: 12.4TU).

These results suggest that the stable isotopes of oxygen and hydrogen in combination with radioactive H-3 can be effectively used for the identification of the sources of water that sustains the river flow in the glaciated catchment of the Gangotri Glacier, and may contribute to a more robust assessment of the hydrological budget in the glaciated catchments of the Himalayan Region.