Prediction of global water use efficiency and its response to vapor pressure deficit and soil moisture coupling in the 21st century

The response of vegetation water use efficiency (WUE) to soil or atmospheric moisture conditions has been widely identified in the past decades, however, the relationships between soil-atmospheric coupling and global vegetation WUE remain unclear under different CO2 emission scenarios at the end of the 21st century. In this study, we investigated the evolution and characteristics of global soil-atmospheric coupling by the atmospheric vapor pressure deficit (VPD) and soil moisture (SM), and vegetation WUE for both the baseline period (1982–2014) and future periods (2015–2100) under four representative pathways using Coupled Model Intercomparison Project Phase 6 data, and then quantified the impacts of soil-atmospheric coupling on global WUE. Results indicate that the global average vegetation WUE will increase significantly in the future, and the SSP5-8.5 is observed at the fastest growth rate at 0.0902 gC kg−1 H2O/decade. Global soil and atmospheric are projected to become drier, especially in the Amazon and Patagonia plateaus, the Middle and Lower Yangtze River Plains; however, VPD remains relatively stable after 2050 under the SSP1-2.6 scenario, highlighting the effectiveness of active restoration policies on atmospheric drought elimination. It is revealed that over 82.13% of vegetated land exhibits a negative correlation between SM and VPD, suggesting vegetation is more susceptible to coupled soil-atmosphere, and SM-VPD coupling tend to occur during the warm season. Approximately 47–59% of global vegetated land is dominated by SM-VPD under four climate scenarios, mainly in the 20°–60°N and 20°–40°S. The affected area expands under moderate and high emission scenarios, particularly in croplands and grasslands. As climate warms, low-altitude regions will become more susceptible to VPD, and some temperature vegetation may transfer from reliance on SM-VPD to dependence on SM. These findings enrich the understanding of global vegetation WUE trends and provide deeper insights into soil-atmosphere coupling and WUE interactions.