Locating Hydrologically Unsustainable Areas for Supporting Ecological Restoration in China's Drylands

China has undertaken extensive ecological restoration (ER) projects since the late 1970s in drylands, dominating the greening of drylands. The greening, especially ER-induced, can affect regional water availability and even cause hydrological unsustainability (i.e., lead to a negative shift in ecosystem water supply and demand balances). However, there is still limited research on accurately identifying the hydrologically unsustainable greening areas (GA) in China's drylands. Here, we developed an ecosystem water supply-demand indicator, namely, the water self-sufficiency (WSS), defined as the ratio of water availability to precipitation. Using remote sensing and multisource synthesis data sets combined with trend analysis and time series detection, we conducted a spatially explicit assessment of the hydrological sustainability risk of greening in China's drylands in the context of ER projects over the period 1987–2015. The results showed that 17.15% (6.36 × 10⁴ km²) of the GA faced a negative shift in the WSS (indicating hydrological unsustainability), mainly in Inner Mongolia, Shanxi, and Xinjiang provinces, driven by evapotranspiration. Moreover, 29.34% (1.09 × 10⁵ km²) of the GA, whose area is roughly double that of hydrologically unsustainable GA, exhibited a potential water shortage with a significant WSS decline (−0.014 yr⁻¹), concentrated in Inner Mongolia, Shaanxi, and Gansu provinces. The reliability of our findings was demonstrated through previous studies at the local scale and an analysis of soil moisture changes. Our findings offer precise grid-scale identification of the hydrologically unsustainable GA, providing more specific spatial guidance for ER implementation and adaptation in China's drylands.