Variability in Isotopic Values of Water in a Glacierized Catchment and Implications for Hydrologic Mixing Models

Variability of isotope values in glacier discharge and precipitation over time has implications for the reliability of hydrograph separation in glacierized catchments using a mixing model approach. Isotopic data from synoptic surveys in three different years in the Langtang Valley, Nepal show seasonal shifts in oxygen-18 isotope values (reported as δ¹⁸O) for glacier discharge, liquid precipitation, and surface water samples. From samples collected in November-December 2008, May 2012, and October-November 2013 we present results of isotope values at seventeen sites resampled on multiple occasions over this time period, as well as the elevational lapse rate in monsoon precipitation during the 2012 and 2013 seasons.

 Samples of discharge at the outlet of the debris-covered Lirung Glacier have δ¹⁸O isotope values that range from -9.29‰ in May 2012 to -18.43‰ in November 2013. Samples from discharge of the clean-ice Khimsung Glacier, in the same sub-catchment, have δ¹⁸O values that only range from -13.63‰ in May 2012 to -16.39‰ in November 2013. Depth-integrated snow from a pit at 5184 m in May 2012 has a mean δ¹⁸O value of -8.09‰ while a fresh snow sample from 5100 m in November 2013 has a value of -27.22‰. Samples from a hotspring 20 km do not vary as much, with δ¹⁸O values of -13.3‰ in November 2008 and -13.64‰ in May 2012. Monsoon precipitation collected as bulk samples between June and September 2012 in the Langtang Valley have a decrease in δ¹⁸O from -9.70‰ at 1400 m to -14.88‰ at 4350 m. These values are used in an attempt to unmix Langtang River water samples with δ¹⁸O values ranging from -14.8‰ to -11.83‰ in May 2012 and from -16.78‰ to -13.11‰ in November 2013.

The overlap in river water isotope samples between seasons, the seasonal shift in some source water isotope values, and the isotopic lapse rate in precipitation all present challenges for defining the chemistry of sources that contribute to river discharge and parameterizing mixing models. The variability in mixing model results based on seasonal changes in chemistry values is explored here and the plausibility of different versions is assessed.