Spatio-temporal variations in oxygen and deuterium isotope of different water sources in Sikkim Himalayas: An understanding towards regional scale basin hydrology

The glaciated Himalayan region is the third largest storehouse of freshwater in its frozen form that feeds several mega river systems. Especially the north-eastern Himalayas, where the snow and glacial stock depend on various moisture sources. This study presents a detailed investigation of water sources in the Teesta watershed in terms of oxygen (818O) and deuterium (8D/82H) values. The study helped understand the Spatio-temporal variability in the isotopic composition of water and moisture sources. Progressive depletion of 818O values in the stream water as moving towards the non-renewable glacial source indicates orographic ascent attributed to cooling and subsequent rainout. This effect is profound in the case of the western tributary above Chungthang, the Lachen Chu river. The computed d-excess values suggest the dominance of mid-latitude westerlies in the high-altitude perennial streams of the west and north Sikkim. Also, a deterministic Monte Carlo-based three endmember 818O-d-excess mixing model is developed to quantify the contribution of source endmembers such as snowmelt, rainfall, and glacial melt in the river streams and waterfalls of the Rathong river as well as Lachen Chu and Lachung Chu rivers above their confluence at Chungthang. With the model being iterated 10,000 times for each waterfall and stream sample, the results have been obtained at the mean, 25th, 75th and 95th percentiles. The model results in the mean glacial ice-melt contribution between 40% and 47% in different seasons, with the standard deviation between & PLUSMN; 3.5% and & PLUSMN; 10%.