Stable isotopic variations (δD and δ18O) in a mountainous river with rapidly changing altitude: Insight into the hydrological processes and rainout in the basin.

Hydrological processes play an important role in stable isotopic compositional variations of the river water. The present study includes spatial distributions of stable hydrogen and oxygen isotopes (δD and δ18O) of a Himalayan River (the Teesta) to trace various hydrological processes and their associated roles on isotopic fractionation of river water. The δ18O compositions of the water samples varies between −12.2‰ and −5.4‰ (mean: −8.01‰ ± 1.81‰) and that of δD between −95.1‰ and −38.5‰ (mean: −57.6‰ ± 15.6‰). The observed compositional variation in δ18O, δD and d values of river water is mainly governed through snowmelt input, evaporation, and recycled moisture contribution. The variation of δ18O and δD values point towards an early melting of snow in the basin and a significant contribution of snowmelt to the river water. The orographic induced precipitation front appears to control the hydrological regimes in a distinct manner as reflected in the trend between d values and elevation. The rain shadow affected northern reaches of the basin are characterized with dry & arid climate triggering evaporation whereas, the windward southern side witness temperature influenced evaporation. The study suggests that the differential degree of recycled moisture contribution to the local precipitation is also a key component in bringing the stable isotopic variation in the river water. This varied magnitude of moisture recycling could result from uneven distribution of evapotranspiration and surface water evaporation. Results found that a rapid change in the altitude of a river basin can give rise to different hydrometeorological conditions, thereby resulting in simultaneous operation of almost all the major hydrological processes and a complex isotopic compositional variation in the river water. Also, the rainout percentage in the Teesta basin is found to be higher owing to this rapid rate of elevation increase. [ABSTRACT FROM AUTHOR]