Comprehensive isotopic characterization of the hydrological processes of the Indus river basin (IRB): A comparison between Upper Indus Basin (UIB) and Lower Indus Basin (LIB)

The Indus river basin (IRB) is one of the most depleted water basins globally, having significant challenges for its water sector. Monitoring of stable isotope composition (delta O-18 and delta H-2) across IRB is a critical aspect that can provide deeper insights for investigating complex hydrological processes. This work analyses the spatial pattern of the isotopic signature using a comprehensive compilation of available datasets of the Global Network of Isotopes in River (GNIR) and Global Network of Isotopes in Precipitation (GNIP), along with the previously published isotopic studies in the Indus basin. Additionally, this work provides a detailed comparison of the isotopic signature of the Upper Indus Basin (UIB), and Lower Indus Basin (LIB). The IRBs water line was found to be delta H-2 = 7.89 x delta O-18 + 13.51, which shows a close similarity with the Global Meteoric Water Line (GMWL), indicating the meteoric origin of the water with insignificant secondary evaporation prevailing across the basin. The Main Indus Channel (MIC) river water line (delta H-2 = 8.88 x delta O-18 + 26.05) indicates a major contribution from the meteoric origin (precipitation/rain) of water (Indian summer monsoon) with minimal effect of evaporation processes. The water line for UIB samples, (delta H-2 = 7.88 x delta O-18 + 11.94) was found to be moderately higher in slope than LIB samples (delta H-2 = 7.17 x delta O-18 + 7.16). However, the slopes of both UIB and LIB river water lines closely approached the slope of GMWL and were consistent with the slope of IRB water line, which indicates similarity in contribution of water sources. The higher slope and intercept in UIB suggest that meteoric water sources contributed to streamflow viz. from snow/glacier with insignificant evapotranspiration, which is also validated by the scarce vegetation cover in the UIB. However, the lower slope and intercept in LIB suggest stream water contribution from significantly evaporated groundwater and precipitation with a complete homogenization of discharge coming from the UIB. Results substantiate that distinct isotopic signatures found in different stretches of the IRB and along the MIC are caused by variations in basin characteristics, hydro-meteorological processes, water mixing, and minor influence of anthropogenic variables.