Quantitative delineation of water sources in the river Ganga using stable water isotopes

Quantitative delineation of water sources in a large river system is essential for the sustainable use of water. In the present study, we have tested two different methodologies to estimate the contribution from different water sources in the river Ganga. The first model uses stable isotopes and physicochemical parameters of water to delineate the contribution of glacier-melt, groundwater, and surface runoff in different stretches of the river Ganga. The end member-based mixing model provides glacier-melt contribution of similar to 31.2%, similar to 5.5%, and similar to 0.5% in the upper, middle, and lower stretch of the river Ganga. The model showed maximum contribution from groundwater (similar to 66.3%) in the middle stretch and surface runoff (similar to 57.2%) in the lower stretch of the river Ganga. However, the uncertainties in the estimates from the three-component mixing model were significantly high due to temporal variability in the end member values. To provide estimates with lower uncertainty, an alternate method (discharge dependent budget estimation [DDBE]) has been proposed which delineates the contribution of groundwater and surface runoff in small segments of the river. The DDBE model confines the calculation for budget estimates to smaller segments of the river and hence leads to lower uncertainties in the results, providing improved systematic estimation of water budget for large river systems. In the present study, estimates from the DDBE model suggest that groundwater contributes similar to 79% of the additional water whereas the contribution from surface runoff is similar to 21% in the middle and lower stretch of the river Ganga. The DDBE model was successfully applied in 5 small segments along the stretch of the river Ganga and the methodology used shows great potential for systematic delineation of water sources in large river system.