Prolonged drought strongly suppresses temperate grasslands actual evapotranspiration: Insights from eddy covariance observations upscaling

Despite frequent droughts in East Asian grasslands, most studies have concentrated on their impacts on carbon balance, leaving limited understanding of actual evapotranspiration (ET a ) responses under prolonged drought conditions. The Chinese Grassland Transect (CGT), located in the transition zone between croplands and grasslands, spans alpine grasslands (AG) and temperate grasslands (TG) and provides a natural laboratory for examining drought impacts on water dynamics. In this study, monthly data from 18 eddy covariance sites were used to train and evaluate 24 machine learning models, and the best-performing model was applied to estimate ET a across AG and TG. Results showed a persistent increase in ET a in AG, whereas TG experienced significant declines during 1999–2011 due to prolonged drought associated with reduced precipitation (P). The variability of P in TG was largely controlled by the El Niño - Southern Oscillation (ENSO), whereas in AG, P was primarily influenced by the South Asian monsoon system and the region was less affected by drought impacts due to the Tibetan Plateau. Interannual variability in net ecosystem productivity (NEP) closely tracked the water balance, with NEP in TG more strongly influenced by water input (P), whereas NEP in AG was more strongly influenced by water loss (ET a ). These findings reveal contrasting ET a responses to prolonged drought along the CGT and highlight the key roles of ENSO and monsoon systems in regional hydrology.