Surface soil water content dominates the difference between ecosystem and canopy water use efficiency in a sparse vineyard

Abstract Water use efficiency (WUE) is an important eco-physiological index that reflects the relationship between ecosystem water and carbon cycles. Investigating the ecosystem WUE contributes to a better understanding of the water-carbon coupling mechanism in ecosystems. We explored the responses of the daily ecosystem WUE (WUEe) and canopy WUE (WUEc) to several influential factors, the dominant biophysical factor and the dominant water/carbon flux component of the difference between WUEe and WUEc based on continuous measurements collected via an eddy covariance system, sap flow system, and microlysimeters over two growing seasons from 2017 to 2018 in a sparse vineyard in northwestern China. The results showed that the seasonal averages for WUEe and WUEc were 0.52 g C kg H2O−1 and 1.96 g C kg H2O−1, respectively. WUEe and WUEc responded weakly to the air temperature (Ta) and showed a negative linear correlation to the photosynthetic active radiation (PAR) and vapor pressure deficit (VPD). WUEe decreased significantly with the rising soil water content (SWC) while WUEc showed a weak response. Both WUEe and WUEc exhibited positive linear relationships with the leaf area index (LAI). The results from a structural equation model (SEM) indicated that among the biophysical controllers, SWC and LAI were the top two controlling factors on the difference between WUEe and WUEc, and SWC played a dominant role. The difference between WUEe and WUEc increased with the rising SWC and decreased with the rising LAI. Among the flux components, including soil evaporation (E), transpiration (T), net ecosystem productivity (NEP), and ecosystem respiration (Reco), E dominated the discrepancy between WUEe and WUEc due to the strong control of SWC on E. This study highlights the strong control by the SWC in regulating the ecosystem WUE under a sparse canopy in an arid area.