Response of ecosystem water use efficiency to extreme drought and wet events in the Loess Plateau, China

Variations in ecosystem water use efficiency (WUE) are central to understanding the terrestrial-atmosphere carbon-water coupling. However, the response of WUE to extreme climate events, such as intensified droughts and wet events, remains poorly understood. This study investigates the variations in WUE across the Loess Plateau of China from 2001 to 2020, focusing on how extreme drought and wet conditions influence WUE and the underlying drivers. Using remote sensing data of Gross Primary Productivity (GPP), Evapotranspiration (ET), and gridded climate data interpolated by ANUSPLIN, we reveal several key findings. First, extreme wetness has a more widespread impact on WUE (80.67 %) than extreme drought (57.86 %) during the study period. In addition, an unexpected increase in WUE during extreme wet years compared to dry years across all three major biomes (grassland, shrubland, and forest). Second, asymmetrical variations in GPP and ET contribute to the differences in WUE between wet and dry years. In wet years, WUE increases due to a combination of increased GPP and reduced ET, whereas in dry years, grassland and shrubland WUE rises due to less negative GPP relative to ET, forest WUE decrease attributed to decreased GPP and increased ET. Third, precipitation and temperature exert differing influences on WUE across biomes. In wet years, both factors contribute more significantly to WUE than in dry years, with precipitation being the dominant driver in grassland and shrubland, while temperature plays a more consistent role in forests under both extreme conditions. Our findings suggest that extreme wetness may be less detrimental to ecosystem WUE than extreme drought, particularly in water-limited regions. This study provides novel insights into how terrestrial ecosystems modulate carbon-water exchanges under hydroclimatic extremes, informing predictions of ecosystem resilience and function in a changing climate.