Spatiotemporal variations and driving factors of China's ecosystem water use efficiency

Ecosystem water use efficiency (WUEe) is a key metric of the coupled carbon and water cycles. Understanding WUEe variations along the climate gradient (arid to humid) and across different vegetation ecosystems is crucial for assessing the impacts of climate variability on these ecosystems. Based on the Penman-Monteith-Leuning version 2 (PML-V2) dataset from a coupled diagnostic biophysical model, this study investigated the spatio-temporal variations and driving factors of WUEe under different aridity conditions and across vegetation types in China from 2001 to 2020. The results showed that the annual mean WUEe was 1.36 +/- 0.07 degrees C kg-1 H2O in China, and humid regions had higher WUEe than other arid regions except for the extremely arid regions (the top level of aridity classification). Annual WUEe showed an increasing trend of 0.084 degrees C kg(-1) H2O/10a in China, which mainly occurred in humid areas. The forest ecosystem had the highest WUEe (2.48 degrees C kg(-1) H2O), fol-lowed by shrubland (2.11 degrees C kg(-1) H2O) and cropland (1.99 degrees C kg(-1) H2O) ecosystems. Leaf Area Index (LAI) and vapor pressure deficit (VPD) contributed the most to increases in WUEe, which accounted for areal proportions of 63.82% and 77.36% of China, and average relative contributions of 31.80% and 14.73%, respectively. In contrast, air temperature (TEM) contributed the most to WUEe decreases, with an areal proportion of 61.73% and an average relative contribution of 12.42%. Normalized Difference Vegetation Index (NDVI), precipitation (PRE) and downward short-wave radiation (SSRD) showed little differences in the proportions of land area where they either promoted and suppressed WUEe and their respective relative contributions were 16.99%, 14.32% and 8.61%.