A four-component mixing model reveals snowpack melting as early as March during the 2019 hydrological year in the Upper Ganga Basin

Abstract

Ice and snow meltwater are important components of the glacier-fed Himalayan rivers that control the seasonal streamflow pattern. With the ongoing global warming, there remains significant uncertainty regarding how melting Himalayan glaciers will impact the water runoff. Here, we quantified the rainwater, ice meltwater, snow meltwater, and groundwater flow in the river using a δ18O, d, and Cl- -based 4-component hydro-geochemical mixing model. At a weekly resolution, we measured the stable water isotopes (18O/16O and 2H/1H, expressed as δ18O and δD), Cl- and Na+ near the river Alaknanda mouth, in the Upper Ganga Basin over one year (2018–2019). The weekly scale time-series geochemical record captured the subtle variability of ice and snow meltwater flow in the Alaknanda River. The two most significant observation was (i) an onset of snow meltwater was observed as early as in March during the study period contrary to our present understanding that the onset of snow meltwater usually happens in April, and (ii) a significant increase in ice and snow meltwater flow during the peak monsoon season. The early onset of snow meltwater runoff in the Himalayan rivers could be an important concern as this could potentially reduce the flow of snow meltwater during the early summer months when water demand is highest due to intense agricultural activities in the mountainous region. Similarly, an increase in ice meltwater runoff in the monsoon season increases the risks of major flood events. Since weekly scale time-series data of ice meltwater, snow meltwater, rainwater, and groundwater flow in glacier-fed Himalayan rivers over the entire hydrological year was largely missing, this study provides an important dataset to better understand the year-long dependency of the Himalayan rivers on the glaciers critical to improving the accuracy and predictive power of hydrologic models.