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 (WUE e ) and canopy (WUE c ) 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, WUE e , and WUE c changed dynamically at the daily (1.0–98.6%, 0.3–4.9 g C kg –1 H 2 O, 0.4–5.9 g C kg –1 H 2 O) and monthly (26.0–71.0%, 0.8–1.8 g C kg –1 H 2 O, 2.3–3.2 g C kg –1 H 2 O) 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 H 2 O, and 2.4 ± 0.3 g C kg –1 H 2 O, respectively (mean ± standard deviation). Biological and environmental factors regulated their dynamics on different temporal scales. Specifically, leaf area index (LAI) and canopy conductance ( g c ) mainly determined the seasonal variation of T /ET, WUE e , and WUE c at the daily and monthly scales. However, at the annual scale, vapor pressure deficit (VPD) and air temperature ( T a ) 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. • Sap flow and water-carbon fluxes are measured synchronously for 6 years in an oasis agroecosystem. • The ratio of transpiration to evapotranspiration ( T /ET) is 61.7% ± 3.7% during growing season. • Water use efficiency (WUE) at the ecosystem scale is 1.6 times than that of the canopy. • Gas exchange metric and WUE explain 73.7% of the multi-dimensional space variation of plant traits.