Seasonal variation in stable isotope compositions of waters from a Himalayan river: Estimation of glacier melt contribution

Abstract

AbstractStable isotopic compositions (δ18O and δD) in water samples collected from Parbati River in Himachal Pradesh, India, during 2002–2005 were measured to delineate the contributions from different sources in different seasons. A seasonal cycle with high δ18O and δD values (in ‰) during the spring (March to May; −9.2, −58.6), intermediate values during the winter (December to February; −10.1, −65.6), and low values during the south‐west monsoon (July to September; −10.9, −71.8) is observed. The d‐excess values (15.2 ± 2.1‰) are higher compared with the global meteoric waters indicating significant contribution (~26%) of moisture carried by western disturbances. Higher δ‐values during the spring are ascribed to enhanced contribution from snow melt. The lower δ‐values during the monsoon are due to various Rayleigh effects (altitude, continental, and amount effects) and large‐scale convection effect on rains. A three component‐mixing model using the isotopic data and some plausible end member isotopic values was applied to identify contributions to the river discharge from different sources such as groundwater or subsurface base flow, glacier/surface snow melt, and the monsoon rain. Meltwater from glacier and snow combines with the base flow in spring season. South‐west monsoon rain and glacier melt along with the base flow constitute the monsoon discharge. The post‐monsoon season and winter are dominated by groundwater contribution. About 80% of the discharge is contributed by glacier melts in spring season. In the rainy monsoon season, glacier/snow melt contributes ~41% of the discharge. The mean annual glacier melt contribution to the river water is estimated to be 44 ± 15%. The present estimate along with some previous studies suggest that glacier contribution to the river discharge increased from ~35% to 50% during the period 1990 to 2011. This is consistent with recent data on glacier retreats in the Himalayan region.