The impact of reservoirs on dryland vegetation and its regional variation.

Dryland ecosystems are highly sensitive to variations in water availability, and reservoirs, as major anthropogenic interventions in surface-water systems, may exert complex effects on vegetation dynamics. This study investigates the impacts of reservoirs on vegetation greenness (NDVI) and fractional vegetation cover (FVC) across global drylands during 1984-2020 using residual analysis combined with a random forest model. Monthly changes in reservoir storage (ΔS) were used to characterize reservoir-associated hydrological dynamics, while climatic variables were incorporated into the random forest models to control for their influence when quantifying reservoir impacts on vegetation. Results indicate that the global-average vegetation responses associated with reservoir storage dynamics are small in magnitude (generally <10-4), but substantial spatial and ecological heterogeneity is observed across climatic regions and vegetation types. Reservoir-associated storage dynamics are generally associated with enhanced vegetation in arid and semiarid regions but reduced vegetation indicators in semihumid regions. Across vegetation types, cropland, wetland, and grassland generally show positive responses, whereas forests tend to exhibit negative responses, with most effects below 10-5. Scenario simulations based on hypothetical perturbations in reservoir storage dynamics further indicate that vegetation responses vary with the magnitude of monthly storage changes, with arid and semiarid regions generally showing stronger responses. Overall, these findings highlight the need for region- and vegetation-specific reservoir management, particularly to avoid excessive reductions in downstream water availability in arid and semiarid regions where vegetation is highly sensitive to water deficits.