Stable water isotopic evidence for the moisture source and composition of surface runoff in Ladakh, upper Indus river basin (UIRB)

Stable water isotopic analysis of oxygen and hydrogen (δ¹⁸O and δ²H) in stream waters, glacier melt and precipitation were used to investigate hydrological pathways and residence times in snow and glacier dominant mountainous sub-basins of Ladakh (UIRB). A significant spatial and temporal variation in precipitation, stream water and glacier melt samples of Ladakh (UIRB) was observed. The δ¹⁸O and δ²H values of precipitation, stream water and glacier melt varied from −16.8‰ to −11.2‰, ₋16.8‰ to ₋11.2‰ and −15.3 to −11.4‰ for δ¹⁸O and −124‰ to −83‰, ₋124‰ to ₋83‰ and −112 to −70‰ for δ²H. The depleted isotopic values observed in January and enriched in July with drop in the month of august is attributed to temporal changes in ambient temperature, precipitation amount and change in source of moisture. The average altitude effect of stream water is much higher than precipitation and glacier melt. Monthly δ¹⁸O and δ²H values of glacier melt were enriched May and depleted in September. The Local Meteoric Water Line (LMWL) of stream water in Ladakh (UIRB) including tributaries and nallahs is δ²H = (7.8±0.1) ˟ δ¹⁸O+ (15±1.9) with lower slope than GMWL but higher than LMWL of Ladakh (UIRB). The results suggest that the snow and glacier melts dominantly controls the annual stream flow with average snowmelt of (29%), glacier melt (39%) and rainfall (21%) respectively. The estimated MRT varied from 7.2 to 13.8 months, the longest MRT for the Suru and Nubra sub-basin is ascribed to its complex topography (increasing snow lag) and large size than the other sub-basins. The MRT for the Dras sub-basin is shorter (7 months) owing to its simple topography and carbonate terrain, which facilitates the runoff.