Spatiotemporal Variability and Trends in Terrestrial Water Storage Over the Ganges–Brahmaputra–Meghna Basin (2000–2020) from Reconstructed GRACE/GRACE-FO Data

Terrestrial water storage (TWS), integrating groundwater, soil moisture, surface water, snow, and ice, provides a comprehensive measure of large-scale hydrological change and freshwater availability. Understanding its spatiotemporal variability is particularly critical in large, transboundary river basins where climate variability and intensive human water use interact. Here, we investigate long-term and seasonal variations in terrestrial water storage anomalies (TWSA) across the Ganga–Brahmaputra–Meghna (GBM) basin using a reconstructed GRACE/GRACE-FO dataset spanning 2000–2020. At the basin scale, TWSA exhibits a statistically significant declining trend of approximately −15.3 mm yr⁻¹, indicating persistent regional water storage loss. However, this basin-average signal conceals pronounced spatial heterogeneity. Sub-basin analysis reveals the strongest depletion in the Ganga Basin (−17.4 mm yr⁻¹), followed by the Brahmaputra (−13.3 mm yr⁻¹), while the Meghna Basin shows comparatively modest declines (−5.3 mm yr⁻¹). Seasonal analysis indicates that water storage losses are consistently stronger during the monsoon season than in winter, suggesting intensified depletion during periods of peak water demand.

Spatial trend mapping identifies the Northwest Ganga Basin as a critical hotspot of terrestrial water storage decline, with extreme and persistent TWSA reductions exceeding −55 mm yr⁻¹, primarily driven by excessive groundwater extraction. The Northeast Brahmaputra Basin represents a second hotspot, exhibiting substantial TWSA decline (≈ −23.8 mm yr⁻¹), with variability primarily governed by hydroclimatic and cryospheric processes rather than intensive human groundwater abstraction. Empirical Orthogonal Function analysis further highlights two dominant modes of variability: a leading mode characterized by long-term depletion centered over the Ganga Basin, and a secondary mode associated with climate-driven variability linked to monsoon dynamics.

These results demonstrate that basin-mean assessments mask critical sub-regional contrasts in water storage change. The pronounced spatial heterogeneity identified here underscores the need for spatially explicit monitoring and region-specific water management strategies to address emerging hydrological stress across the GBM basin.