The ratio of transpiration to evapotranspiration and water use efficiency in an irrigated oasis agroecosystem: Different temporal-scale effects

Agroecosystems play an important role in carbon sequestration and water consumption of global terrestrial ecosystems. However, the magnitude, pattern, and regulation of the ratio of transpiration to evapotranspiration (T/ET), and also water use efficiency at ecosystem (WUEe) and canopy (WUEc) scales, remain unclear in agroecosystems in arid areas. In this study, a six-year synchronous observation of water-carbon fluxes and sap flow by the eddy covariance and sap flow techniques was conducted in an irrigated vineyard in northwest China. We found that T/ET, WUEe, and WUEc changed dynamically at the daily (1.0–98.6%, 0.3–4.9 g C kg–1 H2O, 0.4–5.9 g C kg–1 H2O) and monthly (26.0–71.0%, 0.8–1.8 g C kg–1 H2O, 2.3–3.2 g C kg–1 H2O) scales. At the annual scale, the range was relatively narrow, with mean values of 61.7% ± 3.7%, 1.5 ± 0.1 g C kg–1 H2O, and 2.4 ± 0.3 g C kg–1 H2O, respectively (mean ± standard deviation). Biological and environmental factors regulated their dynamics on different temporal scales. Specifically, leaf area index (LAI) and canopy conductance (gc) mainly determined the seasonal variation of T/ET, WUEe, and WUEc at the daily and monthly scales. However, at the annual scale, vapor pressure deficit (VPD) and air temperature (Ta) became the main influencing factors. These results highlight the complexity of water-carbon coupling in agroecosystems and the necessity of considering specific factors at different temporal scales when modeling and managing agricultural water resources. In addition, there is still great water-saving potential from the perspective of WUE in the cultivation of vines in Northwest China.