Enhanced understanding of dominant drivers of Water Yield change across China through the improved coupled carbon and water model

Abstract. The rapid environmental changes, including climate change, escalating atmospheric CO₂ concentration ([CO₂]), and vegetation dynamics, have been significantly impacting hydrological processes. Accurately quantifying their contribution to water yield (WY) has become a significant challenge in water resource management and climate adaptation studies. Therefore, this study improved the coupled carbon and water (CCW) model integrating dynamic water use efficiency (WUE) to quantify the CO₂-physiological feedback; furthermore systematically investigated the causes for WY change during 1982–2017 in China using a scenario analysis method based on the improved CCW model. The results showed that the effects on WY from changes in climate, vegetation, and [CO₂] exhibited a significant regional variability. Climate change (especially precipitation change) emerged as the dominant driver, directly affecting over 70 % of China's land area. The vegetation change was the second largest factor, especially in central China, where vegetation change led to a general decrease in runoff. The effect of the escalating [CO₂], which reduced transpiration by inducing stomatal closure, was relatively small. Spatial analysis aligned with isohyetal lines further revealed that vegetation change and [CO₂] exerted greater influence within the 400–1600 mm precipitation range. In addition, the elasticity analysis showed that the sensitivity ranking of impact factors is precipitation (εP = 1.55) > [CO₂] (εCO₂= 0.55) > NDVI (εNDVI = -0.44) for the whole China. Historically, NDVI change has exceeded precipitation and [CO₂] impacts on runoff in some regions due to its higher relative change; however, CMIP6 SSP585 projections indicate that accelerating [CO₂] rise (2.34 % yr⁻¹) will amplify its hydrological effect to a +1.29 % annual WY increase by 2100, surpassing vegetation influences. This study provided theoretical support for water resource management and offered new perspectives for climate change adaptation strategies, vegetation restoration, and water resource management.