Multisource observations reveal different roles of tropical vegetation in terrestrial water regulation

Tropical vegetation plays a vital role in global ecosystem services, with one critical aspect lying in its hydrological functions of water cycle regulation. Climate change and accelerated human interventions threaten the stability of tropical vegetation, associated with profoundly hydrological changes particularly in recent decades. Despite various studies on land-atmosphere feedback using earth system models, the regulation of terrestrial hydrological components remains unclear over tropical regions, due primarily to inherent limitations of models in accurately simulating terrestrial water storage (TWS) and runoff. Here, we combine multisource observations to reveal a disparity pattern in storage-runoff interactions over tropical regions for the past two decades. Using satellite-based Landsat optical archives, Global Ecosystem Dynamics Investigation, GRACE gravimetry, and gauge-based runoff database, we show that large-scale forest degradation and cropland expansion have weakened moisture recycling over the eastern tropical South America and eastern tropical Africa (Region I), indicated by a significant decrease in net precipitation input (precipitation minus evapotranspiration). This further causes declines in both TWS and streamflow, shown as a pattern of “less storage and less runoff” due to vegetation degradation. In contrast, over the western tropical South America, western tropical Africa, and tropical Asia (Region II), we did not find marked changes in land cover but a significant increasing trend in vegetation greenness and leaf area index. This is associated with a significant increase in net precipitation input and an enhanced moisture recycling. The increased water input over Region II causes an increase in TWS but a decline in streamflow, shown as a pattern of “more storage but less runoff” due to the decrease in rainfall-runoff generation induced by vegetation growth. The disparity patterns between Region I and Region II highlight different responses of tropical terrestrial water system to a changing environment. Unlike most past studies relying on land surface or earth system models, this study leverages strengths in advanced observation techniques to explore different mechanisms underlying changes in the tropical terrestrial water system. Findings from this study provide valuable supplements to the current model-based analysis, and inform adaptive strategies for changes over tropical regions.