Hydrograph separation using hydrochemistry mixing models: An assessment of the Langtang River Basin, Nepal
MA: University of Colorado Boulder, 2015.
There are numerous uncertainties in translating cryosphere monitoring information into hydrologic assessments that are useful for understanding downstream water resources, particularly in the Himalayan region of High Asia. At the catchment scale the use of isotopes and conservative tracers allow for hydrochemistry-based hydrograph separation as an approach to quantifying contributions of ice and snow melt end members to river discharge, while simultaneously identifying contributions from rain and groundwater end members. Research to date on hydrograph separation in glacierized catchments is often limited by the failure to separate snow melt from rainfall runoff or snow melt from ice melt, and the synoptic nature of sampling. To address some of these limitations we implement End Member Mixing Analysis (EMMA) in the Langtang River Basin, Nepal using a suite of isotopic and geochemical data from samples collected in May 2012. To our knowledge this is the first time EMMA has been applied in a mid-sized catchment considered to have a glacier melt discharge regime. Three different, published methods for choosing chemistry data input to EMMA are tried and there is wide variability in results: estimates of groundwater contribution to discharge range from 16% to 39% at our lowest elevation site (1479m) and from 25% to 67% at our highest elevation site (3642m) based on method. The remaining percentages of discharge are allocated to high-elevation sources, though we do not see a clear snow melt signal which is potentially due to sampling limitations. Our results show that the subjectivity of EMMA must be considered when interpreting results; however the ability of EMMA to differentiate numerous end members makes it a useful tool in complex systems such as the monsoon-influenced, glacierized catchments of the Himalayas.