Stable water isotope modeling reveals spatio-temporal variability of glacier meltwater contributions to Ganges River headwaters

Himalayan headwater rivers are sourced from glacier meltwater and from Indian summer monsoon (ISM) precipitation, both of which are strongly influenced by ongoing and future climate change. Hydrologic modeling studies indicate that the glacier meltwater contribution is highest during the hot, summer pre-ISM months, but this hypothesis cannot be rigorously tested due to lack of observational data. To provide new spatial and temporal constraints on water source contributions in Himalayan streams, we measured stable water isotopes (O-18/O-16 and H-2/H-1, expressed as delta O-18 and delta D) and temperature compensated electrical conductivity in nested catchments throughout the Upper Ganges Basin across three seasons: pre-ISM (April-June), ISM (July-September), post-ISM (October-December) over three years (2014-2016). For all time points we observed a sharp decrease in delta O-18 and delta D values moving toward higher elevations. To interpret these results, we tested two environmental tracer approaches for water source apportionment. Using a new isotope mixing approach, our analyses reveal large seasonal variability in water source contributions from glacier meltwater, precipitation, and groundwater to streamflow throughout the basin. Electrical conductivity, delta O-18, and deuterium excess were also analyzed through optimum multiparameter analysis (OMPA). Results from our isotope mixing model and OMPA are in close agreement. Glacier meltwater contributions were highest during the ISM and post-ISM seasons, contrary to previous model predictions. In addition to protracted release of glacier meltwater, this result is consistent with "rain-induced" glacier melting during the ISM, which we show can produce 3-11% of total discharge at the glacier snout. Glacier meltwater contributions to Himalayan streams exhibit complex seasonal dynamics; such contributions should be considered when predicting hydrologic responses to climate change in this region.