Seasonal variations in water use efficiency during the 2022 extreme drought in the Yangtze River Basin: effects of drought timing and severity

Water use efficiency (WUE) is critical for understanding the carbon-water coupling of ecosystems, yet its response to extreme droughts remains debated. This study applied the CEVSA-ES model to simulate WUE across the Yangtze River Basin from 2020 to 2022, assessing the effects of extreme drought. The results reveal a significant 12.67% decrease in WUE during summer, while extreme and severe drought areas in autumn experienced an 11% increase in WUE. Seasonal WUE variations were linked to evapotranspiration (ET) and gross primary productivity (GPP): in summer, increased ET led to WUE decline, while reduced ET in areas affected by autumn drought resulted in WUE increase. In contrast, areas with moderate to light drought in autumn saw continued ET increase and WUE decline. These findings suggest that WUE responses to drought depend on drought duration and severity: WUE tends to decline in the early stages of drought and increase in later stages, with the transition timing influenced by drought severity. This study underscores the need to account for the spatiotemporal heterogeneity of drought and its complex regulatory mechanisms on WUE in predicting future climate change effects.