Declining water constraints at the cost of water storage for ecosystem on China's Loess Plateau.

Surface greenness alters regional water storage by regulating hydrological processes, thereby modulating water constraints on ecosystem functions and feeding back sustainability. In semi-arid regions, excessive revegetation may exacerbate regional water resource depletion, intensify water limitations on ecosystems, and threaten long-term sustainability. However, these changes have not been adequately assessed. In this study, we employed the Ecosystem Limitation Index (ELI) to differentiate water and energy limitations on ecosystem function. We analyzed its changes and relationship with water storage across China's semi-arid Loess Plateau, where strict revegetation has been implemented since the 2000s. Our results showed a substantial decline in ELI at the plateau scale, suggesting a significant reduction in water limitation. Whereas, ELI increased notably in the eastern and southwestern forest lands, covering 28.4% of the plateau, in contrast to the northwestern grass and desert lands (26.2%), where ELI decreased significantly. The increase in terrestrial water storage (TWS), especially groundwater storage (GWS) availability, is responsible for the declining water constraints on the ecosystem, albeit TWS and GWS substantially reduced with an annual rate of -0.76 cm and -0.49 cm in water thickness, respectively. Spatial trend analysis and Convergence Cross Mapping (CCM) causal detection revealed that the low surface greening rate before 2000 contributed to the regain in TWS and GWS, while the accelerated greening after 2000 has enhanced the shrinkage of regional TWS and GWS. Our findings suggested that the high greening rate of the Loess Plateau over the past decades was potentially achieved at the expense of regional water storage, underscoring the urgent need for water-saving strategies in future revegetation efforts to ensure ecosystem sustainability.