Accelerating Groundwater-Dominated Storage Decline in the Tigris and Euphrates Basins: Evidence from Two Decades of GRACE/GRACE-FO

The Tigris and Euphrates river basin is among the most water stressed transboundary systems on Earth, yet observationally grounded, basin-resolved assessments of its groundwater have not kept pace with the deepening crisis of the 2020s. Here, we combine mascon solutions of total water storage anomalies from GRACE and GRACE-FO (CSR RL06.3, 0.25° grid) with GLDAS Noah v2.1 land surface storage to isolate groundwater storage anomalies, computed separately for each basin over 2002 to 2026. Anomalies are referenced to a 2004 to 2009 baseline and composited into hydrologic-year seasons. Both basins show sustained and highly significant decline. Total water storage fell at 0.95 and 0.75 cm per year in the Tigris and Euphrates, and groundwater was reduced by 0.83 and 0.65 cm per year (Mann–Kendall p < 0.001, robust to serial correlation). The groundwater residual accounts for about 87% of the total loss. Because it also contains unmodelled surface water, this is an upper estimate, but groundwater remains about two-thirds of the loss under a surface-water sensitivity analysis. The record contains two depletion phases, the 2007 to 2009 drought and a deeper decline after 2021, separated by a transient recovery in 2019 to 2021, with the Tigris depleting faster throughout. Groundwater loss in the Tigris accelerated from 0.88 cm per year over 2002 to 2020 to 2.83 cm per year over 2021 to 2025. Net wet season groundwater storage gain in the Tigris fell from 11.7 cm in 2005 to zero in 2025; across all years from 2003 to 2025, however, the gain shows no significant trend and closely tracks wet season precipitation (r = 0.83), so the 2025 minimum reflects the driest winter of the record rather than a loss of recharge capacity. Spatially, the deficits expanded from near neutral in 2005 to region-wide and severe by 2025 and were deepest along the northeastern Zagros margin. These results extend the established GRACE record of Middle Eastern depletion by more than a decade and show that depletion proceeds through a lowering of the whole seasonal cycle, with dry season losses exceeding wet season gains, which has direct consequences for transboundary water governance.