Seasonal variation in stable isotope compositions of surface waters from Upper Sutlej River Basin: Estimation of the moisture source

The stable isotopes of water, ¹⁸O and ²H, are impacted by climatic events that give them a distinct fingerprint of their source. Investigating the origin of river water requires this fingerprint as a precursor. On an annual basis and at the global level, the flow of moisture from the oceans and its return via rainout and runoff is similar to a dynamic equilibrium. Rivers in the Himalayan region have their moisture source in various end members which include glacier/snow melting, rainfall/runoff, and groundwater/springs. Sutlej River is one such river that travels across the Himalaya and receives its waters from all the aforementioned regions. 105 water samples from 36 different locations have been collected from the Upper Sutlej River Basin in the pre-monsoon, post-monsoon, and lean seasons to study the isotope system of surface water in the basin. A seasonal cycle with high δ¹⁸O and δD values (‰) during the pre-monsoon (March to May; −14.42, −114.94), intermediate values during the winter (lean season) (December to February; −12.63, −105.10), and low values during the post-monsoon (October to November; −12.13, −101.6) is observed. The river falls in the western Himalaya that receives precipitation both from the Indian Summer Monsoon (ISM) as well as from the Western Disturbances (WDs). The intercept and the d-excess values in the water samples fluctuate due to the variable contributions from these two moisture sources and the related rainfall in different seasons which are generally higher than the global meteoric waters. The 168-hour back trajectories in different seasons using HYSPLIT model converging at a height of 4,200 m a.s.l. (mean elevation of the Upper segment of the catchment) for moisture source identification have shown that winds mainly blow from south or south-east with moisture source from the Arabian Sea and the Bay of Bengal in summer and monsoon seasons, whereas in winter and spring seasons winds blow mainly from the west bringing moisture from the Central Asian and Eurasian water bodies through Western Disturbances. The results of HYSPLIT model and isotopic analysis indicate a dominant contribution of Western Disturbances and glacier melt in the upper segment of the basin which is consistent with recent data on glacier retreats in the Himalayan region.

Keywords: Himalayan Rivers, Sutlej River, Stable Isotopes, Western Disturbances, Indian Summer Monsoon.