Spatiotemporal Dynamics and Driving Mechanisms of Ecohydrological Cycles in Global Drylands: A Review Based on Multi-Source Remote Sensing

Drylands, including hyper-arid, arid, and semi-arid regions, are critical components of the global ecosystem, and their hydrological cycles are highly sensitive to climate change. In this review, we compared widely used remote sensing products and integrated multi-source datasets to examine the spatiotemporal patterns of key hydrological variables between 2001 and 2022. We focused on precipitation, evapotranspiration, soil moisture, groundwater, and terrestrial water storage in drylands, and analyzed their driving mechanisms. During the study period, global precipitation increased significantly at 0.62 mm yr−1, whereas precipitation in drylands declined and the extent of drylands expanded. Evapotranspiration rose at 2.26 mm yr−1, with higher values concentrated in drylands. Soil moisture exhibited a significant decreasing trend of 0.20 mm yr⁻1, particularly in arid and semi-arid regions, indicating intensified surface dryness and reduced soil water availability. Groundwater storage decreased markedly at 2.18 mm yr−1, with severe depletion in several dryland basins. Terrestrial water storage also declined at 3.16 mm yr−1, with widespread negative anomalies across drylands. Correlation analyses indicated general positive associations between NDVI and precipitation and terrestrial water storage, and negative associations with evapotranspiration. The spatiotemporal evolution of hydrological processes in drylands was jointly driven by climate change, natural variability, and human activities: global warming intensifies atmospheric evaporative demand and alters circulation patterns, large-scale oscillations modulate precipitation variability, and human interventions reshape the spatial and temporal distribution of water resources. This review provides a comprehensive synthesis of multi-source remote sensing datasets to quantify hydrological changes and their ecological implications in global drylands, offering new insights into the mechanisms of water scarcity and vegetation responses under climate change.