Changes in groundwater storage in the southern plain of Western Nepal: insights from remote sensing, modeling, and field studies.

Groundwater (GW) levels in many parts of the world are depleting, including the Western Terai region of Nepal in the Indo-Gangetic Plain, yet knowledge base on the extent of change in storage and availability of the resource is limited. This study intends to characterize the state of GW storage potential and change in storage in the southern plain of Western Nepal by integrating field-based, model-based, and remote-sensing-based approaches. Borehole lithologs were used to evaluate storage potential. Observed GW level data, along with Gravity Recovery and Climate Experiment (GRACE) satellite data, were used to characterize the storage change. Primary data were collected to assess annual GW abstraction. The static storage potential of a deep confined aquifer is estimated in a range of 8-800 Mm3, and that of an unconfined shallow aquifer, is 7500-15,000 Mm3. Regional terrestrial water storage (2004-2022) is trending downward. Locally, storage change (2008-2023) across municipalities ranges from negligible (0.1 m, Tikapur Municipality) to high (3.3 m, Gauriganga Municipality). Average annual GW abstraction for agriculture and domestic needs is estimated as 378.39 Mm3 and 44.38 Mm3, respectively. Based on insights from field and model-based studies, as well as techniques documented in literature, it is recommended to consider non-structural strategies for the conservation of GW. The findings of this study are constructive for better planning and future development of GW resources in the area.