Spatial distribution of δ18O values of water in the Ganga river basin: Insight into the hydrological processes

Knowledge of the spatial distribution of water isotopes in the large river system is useful to comprehend the hydrological processes within the basin. Spatially limited water isotope data in the Ganga river basin (GRB), one of the largest river basins in the world, hinders the understanding of hydrological processes at the basin scale. Therefore to understand the hydrological processes, river water and ground water were collected from the source (Gangotri glacier) to sink (Bay of Bengal) of the river Ganga. The delta O-18 values of river water vary from -14.9%o to - 6.3%o with a general enrichment in O-18 from source to sink. The delta O-18 values of ground water are similar to river water only in the middle stretch of the river, indicating higher mixing of these two water sources in the middle stretch. Comparison of the monthly averaged (three years) delta O-18 values of river water and rain water suggests that temporal variability in the isotopic composition of river water is higher in the middle stretch of the river compared to lower stretch due to lower discharge in the middle stretch. To understand the spatial variability of delta O-18 values in the GRB, delta O-18 values of river water were modeled using latitude and elevation as primary predictors. The residuals of the model suggest that in the upper stretch snow/glacier melt control the delta O-18 values, in the lower stretch Himalayan tributaries control the isotopic composition of the river water and in the middle stretch evaporation dominates which is also supported by delta O-18-delta D plots of the different water inventories.