Future response of ecosystem water use efficiency to CO.sub.2 effects in the Yellow River Basin, China

Ecosystem water use efficiency (WUE) is pivotal for understanding carbon-water cycle interplay. Current research seldom addresses how WUE might change under future elevated CO.sub.2 concentrations, limiting our 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 CO.sub.2 . Additionally, we further quantitatively decoupled the direct and secondary impacts of CO.sub.2 radiative and biogeochemical effects. Attribution results indicate that WUE in the YRB is projected to increase by 0.36-0.84 gC kg.sup.-1 H.sub.2 O 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, and 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 the understanding of regional ecohydrological processes and provide insights for future predictions of drought impacts on terrestrial ecosystems.