Applications of hydrochemistry data from the glacierized Langtang Valley, Nepal and Lahaul-Spiti Valley, India

Alāna Wilson (1,2), Naveen Pandey (3), Mark Williams (2), AL Ramanathan (3), Rijan Kayastha (4), Adina Racoviteanu (1)

Abstract
Hydrochemistry data, in particular oxygen and hydrogen isotope values, prove useful in conducting hydrograph separation work in catchments where multiple sources contribute to river discharge. The Langtang Valley, Rasuwa District, Nepal has been the site of intermittent synoptic hydrochemistry sampling by our group between 2008 and 2014. Hydrochemistry sampling in the valley of the Chhota Shigri Glacier, Lahaul-Spiti Valley, Himachal Pradesh, India has been conducted for eight years and isotope sampling for hydrograph separation was conducted in 2013 and 2014. This paper explores variability in glacier discharge, snow, groundwater, and river water chemistry and how chemistry data can be used to interpret changes in melt dynamics over time. Langtang Valley and Chhota Shigri data are compared here for an improved understanding of both potential similarities and potential differences in regional variability of glacier and snow melt patterns and the contribution of meltwater to downstream discharge. Using δ18O data from Langtang Valley, in the pre-monsoon of 2012 the contribution of melt water to discharge at a distance of 15 km downstream of glacier termini has been quantified to be 21%, and in the post-monsoon season of 2008 melt was quantified to be 63% of discharge at the same location. The isotopic lapse rate in monsoon-season liquid precipitation in the Langtang Valley in 2012 was found to be -1.8‰ per kilometer elevation change. The isotopic lapse rate of fresh snow collected along an elevational transect of the Chhota Shigri glacier in 2013 was -3‰ per kilometer. This suggests that constraining isotopic gradients of precipitation is important for interpreting surface water isotopic values. Since precipitation makes important contributions to discharge, hydrograph separation work must consider these gradients. Hydrograph separation and isotopic gradient results from 2014 for both sites will be presented.