Pronounced decline in Iran's terrestrial water storage from GRACE and GRACE-FO associated with climate and unsustainable land-use change

Iran. This research investigates the spatiotemporal variability of total water storage anomalies (TWSa) across Iran's six major river basins from 2002 to 2022. It considers GRACE and GRACE-FO data with precipitation, evapotranspiration, land use land-cover (LULC) changes, and the normalized difference vegetation index. This combined approach analyzes the association between TWSa fluctuations and key climatic and anthropogenic factors at basin and country scales. Results reveal a nationwide decline in TWSa (Sen’s slope = −2.06 mm yr⁻¹; p = 0.07), with statistically significant decreases in the Caspian Sea (−3.77 mm yr⁻¹) and Lake Urmia (−3.46 mm yr⁻¹) basins. Declines in other basins are consistent but not statistically significant. Although individual hydroclimatic variables did not show statistically significant monotonic trends, the generalized additive model identified them as considerable explanatory factors for TWSa variation. Extensive LULC changes such as cropland expansion, urbanization, deforestation and grassland conversion, have further reduced landscape hydrological resilience, exacerbating storage deficits. The findings underscore that water scarcity in Iran results from the interplay of climatic variability and unsustainable land management, highlighting the need for targeted adaptive strategies to enhance water security amid persistent environmental pressures. • First basin-scale TWSa study in Iran reveals impacts beyond climate trends. • Land use changes is the main drive of water loss. • TWSa plus Earth data offer vital tools for tracking water use in data-poor regions. • Lasting water security needs cuts in extraction, land use reform, and smart policies.