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ASSESSING THE ROLE OF GROUNDWATER IN HIGH MOUNTAIN ASIA’S WATER BUDGET
Dissertation

ASSESSING THE ROLE OF GROUNDWATER IN HIGH MOUNTAIN ASIA’S WATER BUDGET

Sawsan Muhammad Shatanawi
Doctor of Philosophy (PhD), Washington State University
2026
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Sawsan_Shatanawi_dissertation_June5
Embargoed Access, Embargo ends: 07/22/2027 CC BY V4.0

Abstract

Groundwater is a vital component of terrestrial water storage in High Mountain Asia. It provides water for irrigation, sustains ecosystem processes, and supplies fresh water to more than one billion people in the Indus, Ganges, and Brahmaputra Basins. Despite its importance, large-scale land surface models do not represent groundwater dynamics well, leading to major uncertainty in understanding regional water storage changes and in interpreting GRACE-based TWS anomalies.This dissertation addresses this issue with three related studies. First, it assesses the drivers of TWS changes in High Mountain Asia using GRACE data and Noah-MP multi-variant data-assimilation outputs. Results show that soil moisture and snow control short-term TWS changes, whereas groundwater is increasingly dominant in longer-term variability, driven more by irrigation than by climate. Second, a comparison of different models shows that differences in model architecture, not climate forcing, are the main reason for disagreements in simulated groundwater storage. Most models get the timing right but underestimate the magnitude of groundwater changes. Data assimilation improves temporal alignment but cannot overcome structural limitations in groundwater representation. To overcome these structural problems, SawSim is introduced as the first confined groundwater scheme in Noah-MP and, as far as the author knows, the first explicit two-aquifer layer model with lateral flow, river-groundwater exchange, and pumping at this scale in any community land surface model. Tests in Punjab show that SawSim improves groundwater simulations by more robustly representing key processes, and that shallow and deep aquifers respond to pumping in very different ways. SawSim gives a more robust structure for improving both open-loop simulations and GRACE data assimilation in regions that rely on pumping groundwater. This research moves closer to accurately quantifying the contribution of confined groundwater to TWS variability across HMA

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