Use of a hydrologic mixing model to examine the roles of meltwater, precipitation and groundwater in the Langtang River basin, Nepal

Understanding the hydrology of glacierized catchments is an important step in assessing the vulnerability of water resources to a changing climate. While there have been increased efforts recently to understand the dynamics of Asia's cryosphere, glacier melt dynamics and hydrograph separation of river discharge are open questions. A multi-year, multi-seasonal dataset of water chemistry from the Langtang Valley, Nepal, is used to explore water sources and flow paths that contribute to Langtang River discharge. Differences in hydrochemistry of samples from debris-free Khimsung Glacier and debris-covered Lirung Glacier demonstrate the effect of debris cover on glacier outflow. Additional data show seasonal transitions in the composition of Langtang River discharge. End-member mixing analysis (EMMA) using geochemical and isotopic tracers suggests that reacted meltwater contributes the majority of flow during most of the year, with the exception of the summer when unreacted meltwater and precipitation dominate streamflow. We hypothesize our dataset is missing characteristic monsoon water and utilize a Late May river sample as a proxy for precipitationin-fluenced groundwater in the EMMA. Results offer insight into the plausibility of flow sources and pathways in the basin.