Impacts of Elevated Atmospheric CO2 Concentrations on Vegetation Water Use Efficiency in China: A Modelling Study in an Endorheic River Basin

Increased CO₂ concentrations promote vegetation leaf growth and evapotranspiration (ET); however, they reduce stomatal conductance and reduce vegetation water consumption, a relationship that remains incompletely explored in the previous studies. Therefore, uncertainty remains regarding whether elevated CO₂ concentrations increase ET or reduce water consumption in vegetated areas. This study investigates the impacts of elevated atmospheric CO₂ concentrations on vegetation water use efficiency (WUE) in the ecologically fragile Heihe River Basin using the Community Land Model 5.0 (CLM5.0). From 2020 to 2099, under two carbon emission scenarios (RCP4.5 and RCP8.5), the simulations show that increased CO₂ concentrations promote vegetation gross primary productivity (GPP) but have a less significant effect on ET. WUE generally increases under both scenarios, with higher values under RCP8.5 (~2.41). By 2099, the difference in WUE between RCP8.5 and RCP4.5 is approximately 0.9. Forests exhibit the highest WUE, followed by cropland and grasslands. The study highlights that although elevated CO₂ concentrations enhance WUE, excessive CO₂ may lead to reduced water availability for crops, posing challenges for agricultural production. These findings provide insights into vegetation carbon sink potential under future carbon emission scenarios, emphasizing the need for balanced CO₂ concentrations to maintain optimal vegetation growth and water resource utilization.