Using Hydrochemistry Data to Constrain the Role of Snow and Ice Meltwater in the Langtang Valley, Nepal

The Langtang River Basin of central Nepal is one of the most well-studied valleys of the Himalaya. With an elevation range of 1460m to 7246m and with the Upper Langtang Basin (above 3642m) approximately 40% glacierized, snow and ice melt play a dominant role in the hydrology. Presented here is a synoptic survey of hydrochemistry data from a pre-monsoon field campaign in May 2012. Major chemistry and isotopes are used to conduct hydrograph separations of the Langtang River to quantify the role of meltwater in river discharge. Results using End Member Mixing Analysis (EMMA) showa decline in the contribution of groundwater with increasing elevation - estimated at 24% at 1479m, down to 9% at 3642m. This work isolated contributions of 'high-elevation' water but EMMA did not identify distinct snow and ice contributions. Two versions of EMMA are compared – one including δ¹⁸O in the input tracer set and the second excluding it. EMMA results are compared to a two-component mixing model using δ¹⁸O as the sole tracer to separate groundwater from melt water. Ourobjective is to distinguish potential source waters through hydrograph separation. We show that using hydrochemistry data for EMMA can be an appropriate tool for this.