Loess Plateau Vegetation Restoration Enhances Water Vapor Transport to Mitigate Drought in Downwind Drylands

Drylands are confronting dual challenges of intensifying drought and insufficient water vapor supply, which threaten regional ecosystem stability and food security. As a critical moisture source, terrestrial evapotranspiration (ET) may buffer such risks by enhancing regional water vapor cycling; however, the role of ET increase induced by vegetation restoration in supplying water vapor to downwind drylands remains poorly understood. This study investigates the drought-mitigating effect of ET increase driven by vegetation restoration (initiated by the Grain-for-Green Program since 2001) on China’s Loess Plateau, focusing on its impact on downwind dryland regions. Results indicate that since 2001, vegetation restoration on the Loess Plateau has substantially increased annual mean ET. Simulations using a Lagrangian-trajectory-based PyTraject method show an expanded water vapor transport contribution range, with the northeast direction as the primary pathway, indicating a notable increase in water vapor supply. By integrating a Copula model with extreme water vapor deficit scenario analyses, we identified key convergence zones where water vapor export from the Loess Plateau significantly alleviates drought severity in downwind dryland areas. Further analysis reveals that regions more strongly influenced by this water vapor transport exhibit lower actual drought occurrence probabilities—particularly in May–June, when ET increase from vegetation restoration can reduce the probability of severe drought in downwind dryland regions by up to 7%. This study demonstrates that under vegetation restoration, the Loess Plateau plays a stable and sustained regulatory role in supplying water vapor to downwind drylands, thereby enhancing drought resilience and supporting ecosystem stability and food security in these regions.