Attribution and Response of Water Use Efficiency to Future Changes in the Yellow River Basin

Ecosystem water use efficiency (WUE, defined as the ratio of primary productivity to evapotranspiration) has garnered significant attention in recent years for its role as a vital indicator of the interplay between carbon and water cycles. Numerous studies have underscored the substantial impact of elevated CO₂ concentrations on WUE through changes in climate and land surface properties. However, the relative contributions of these factors and their interrelations remain less clear. This study delves into the linkage between WUE and the water-energy exchange dynamics within the Yellow River Basin, employing the Budyko framework model as a foundation. We propose and validate a linear Budyko model tailored to WUE, demonstrating satisfactory physical performance (R² = 0.60-0.73). Building on this, we construct an attribution framework for WUE, grounded in the Budyko model and global climate models (GCMs), to quantitatively disentangle the impacts of climate and land use change, as well as to elucidate the mechanisms underlying CO₂-induced radiative and biogeochemical effects on WUE. Our attribution analysis suggests that the WUE of the Yellow River Basin is anticipated to increase by 0.36-0.84 (g C/kg H₂O) in future scenarios, with climate change being the predominant driving force (77.9%-101.4%). The study also uncovers variations in the response of WUE to different drought conditions within the basin. Specifically, we observe an increase in WUE under moderate drought conditions, whereas a decline is noted in most areas as drought severity escalates. Under high emissions scenarios, WUE exhibits a more pronounced sensitivity to drought, particularly under extreme conditions. The outcomes of this research contribute to our understanding of how future CO₂ concentration increments, under varying scenarios, may induce changes in WUE in the Yellow River Basin. Moreover, they reveal the prospective response mechanisms of vegetation to drought events within the basin, offering a scientific basis for the formulation of regional ecological strategies and water management practices.