Quantifying Water Use Efficiency Across Different Ecosystems Based on Eddy‐Flux Observations

ABSTRACT Water use efficiency (WUE) is an important indicator for assessing the trade‐off between carbon uptake and water loss in terrestrial ecosystems. The WUE is generally calculated as the ratio of gross primary productivity (GPP) or net primary production (NPP) to evapotranspiration (ET) or transpiration (T) in previous studies. However, these traditional WUE formulas only reflect the final water consumption and vegetation productivity, and the intermediate steps of vegetation water utilisation were ignored. Here, based on the eddy covariance (EC) technique, the WUE chain was developed by dividing WUE into four steps: GPP/T, T/ET, ET/SW (SW is the soil water) and SW/TWI (TWI is the total water input) in four typical ecosystems in northwest China. Then, the characteristics of change in the growing season WUE chain for four ecosystems were analysed. Finally, the random forests model and structural equation model were employed to investigate the controlling factors affecting the WUE chain by examining the relationship between the intermediate steps of the WUE chain and major impact factors. The forest ecosystem had the highest growing season WUE (0.89 gC·m −2 ·mm −1 ), followed by cropland (0.30 gC·m −2 ·mm −1 ) and grassland (0.24 gC·m −2 ·mm −1 ), while the smallest was in the desert (0.20 gC·m −2 ·mm −1 ). The controlling factors impacting WUE differed greatly among the four ecosystems by regulating the intermediate steps of the WUE chain. Air temperature mainly controlled WUE change in croplands and deserts by simultaneously regulating GPP/T, T/ET and ET/SW. Leaf area index primarily controlled WUE in grasslands by affecting T/ET and ET/SW. Atmospheric pressure primarily influenced the forest WUE by regulating T/ET. Applying the WUE chain enhances our understanding of the water use process of vegetation and further promotes sustainable water resources management in arid and semi‐arid areas.