Unintended Groundwater Depletion in Inner Mongolia Driven by Grazing Ban-Induced Irrigation Expansion

Vegetation greening is significantly altering hydrological cycles in arid regions. However, current research often emphasizes direct hydrological effects, largely neglecting the underlying policy-driven mechanisms. By integrating GRACE/FO, MODIS LAI, and ISIMIP3a data (2003–2024), we reveal distinct hydrological responses across the Mongolian Plateau. Results show that Inner Mongolia, China experienced 2.73×10⁴ km² grassland restoration and 5.64×10⁴ km² cropland expansion, compared to 1.92×10⁴ km² and 1.25×10⁴ km² in Mongolia. Despite a synchronous precipitation increase (3.87 and 2.45 mm/yr), TWS trends diverged sharply: severe depletion in Inner Mongolia (-2.11 km³/yr) versus a marginal decline in Mongolia (-0.25 km³/yr). Deriving groundwater anomalies via water balance residuals confirms that groundwater depletion in Inner Mongolia (-2.73 km³/yr) is the primary driver of the TWS decline. Notably, ISIMIP3a natural simulations maintain a coupled TWS-precipitation increase, contrasting with the stark decoupling observed in reality; this divergence identifies policy-driven irrigation as the primary cause. We urge integrating anthropogenic processes into models and adopting adaptive policies to enhance regional resilience.