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Contributions of Glacial Melt, Snowmelt, and Groundwater to Streamflow During Low-Flow Periods: A Paired Catchment Approach in the Arun Watershed, Eastern Nepal

  • Author(s): Voss, Katalyn Anne
  • Advisor(s): Chadwick, Oliver A
  • et al.
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Abstract

My dissertation research investigates the spatial and temporal variability of water sources in the Arun watershed, a river basin in the eastern Himalaya, using a combination of geochemical and isotopic tools to assess the hydrologic and climatic processes that control water resources in the region. Broadly, I aim to answer the questions: (1) what are the spatiotemporal patterns of the Indian Summer Monson versus Winter Westerly Disturbance storm systems as they contribute to water supply, and (2) how do contributions of snowmelt, glacial melt, groundwater, and rain vary from local-to-regional scales in the eastern Himalaya? I address these questions by exploiting the natural variability in the geochemical and isotopic composition of meteoric water, which is controlled by distinct hydrologic and climatic processes. I explore these data across spatial scales that include the Tibetan Plateau headwaters of the Arun River, a high-elevation glacierized tributary, a low-elevation tributary, and the downstream outlet of the Arun River.

In Chapter Two, I find that deuterium excess emerges as valuable tracer to partition regional precipitation systems while high sulfate levels are correlated to glacial melt source waters. I use these two tracers to qualitatively describe broad patterns in regional water supply for the Arun watershed and its tributaries, and highlight differences that arise from the varying hydroclimatic characteristics throughout the basin. In Chapter Three, I explore the variability in δ 18O and δD stable isotope values along a 6000-m elevation gradient and identify non-linear δ 18O and δD lapse rates in river water controlled by regional precipitation patterns and local mixing with glacial melt. In Chapter Four, I use a combination of dissolved ion concentrations and δ 18O and δD stable isotope values in a mixing model approach to partition streamflow during low-flow periods before and after the monsoon, and I identify the tracers that offer the highest utility to identify water sources in these Himalayan catchments.

The dissertation provides evidence that seasonality and elevation act as strong controls on Himalayan water supply with low-elevation catchments dependent on monsoon rainfall and high-elevation catchments relying more heavily on glacial melt. Contributions of snowmelt to river discharge are seasonally discrete in the pre-monsoon season and spatially constrained to high-altitude regions or headwaters. I elucidate the potential applications of isotopic and geochemical tracers to differentiate snowmelt from glacial melt and to identify seasonal precipitation cycles. This work advances an emerging body of literature focused on Himalayan water resources and provides a practical framework to assess water budgets across local and regional scales. Critically, the dissertation provides insights and direction for future research to accurately quantify water resources in the Himalaya and, ideally, inform water management decisions.

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This item is under embargo until September 6, 2021.