Future response of ecosystem water use efficiency to CO₂ effects in the Yellow River Basin, China

Abstract. Ecosystem Water Use Efficiency (WUE) is pivotal for understanding the carbon-water cycle interplay. Current research seldom addresses how WUE might change under future elevated CO₂ concentrations, limiting understanding of regional ecohydrological effects. We present a land-atmosphere attribution framework for WUE in the Yellow River Basin (YRB), integrating the Budyko model with global climate models (GCMs) to quantify the impacts of climate and underlying surface changes induced by CO₂. Additionally, we further quantitatively decoupled the direct and secondary impacts of CO₂ radiative and biogeochemical effects. Attribution results indicate that WUE in the YRB is projected to increase by 0.36–0.84 gC·kg^-1H₂O in the future, with climate change being the predominant factor (relative contribution rate of 77.9–101.4 %). However, as carbon emissions intensify, the relative importance of land surface changes becomes increasingly important (respective contribution rates of -1.4 %, 14.9 %, 16.9 %, and 22.1 % in SSP126, SSP245, SSP370, SSP585). Typically, WUE is considered a reflection of an ecosystem's adaptability to water stress. Thus, we analyzed the response of WUE under different scenarios and periods and various drought conditions. The results show a distinct "two-stage" response pattern of WUE to drought in the YRB, where WUE increases under moderate-severe drought conditions but decreases as drought intensifies across most areas. Furthermore, GCM projections suggest that plant adaptability to water stress may improve under higher carbon emission scenarios. Our findings enhance understanding of regional ecohydrological processes and provide insights for future predictions of drought impacts on terrestrial ecosystems.