## 3 Results
### Seasonal Dynamics of Environmental and Biological Controls  
In order to explore the environmental restrictive factors of ecosystem WUE variability under water-limited conditions, this study illustrated the seasonal variations and statistically evaluated the correlations of WUE and its main components (GPP and ET) with the potential biotic and abiotic controls (Figure 3 and Table 2). Generally, local climate characteristics determine the vegetation growth and functional traits relevant to ecosystem carbon and water cycling. Figure 3 showed that all these variables including Rg, T\({}_{a}\) and T\({}_{s}\), VPD, P and MODIS-derived EVI exhibited strong seasonal trends  
Figure 2: Seasonal variations in gross primary production (GPP\({}_{\text{EC}}\)), evapotranspiration (ET\({}_{\text{EC}}\)), and ecosystem water use efficiency (WUE\({}_{\text{EC}}\)) at the two flux sites, Arou (grassland site) and Daman (cropland site), during the period of 2013 and 2014. The units of GPP, ET and WUE are g m\({}^{-2}\) d\({}^{-1}\), mm d\({}^{-1}\) and g kg\({}^{-1}\), respectively. For x-axis, the data refer to week of year (WOY, eight-day periods) ranging from 1–46. and co-varied with time-series GPP, ET and WUE. As the driving factor of plant photosynthesis and transpiration, R\({}_{\rm g}\) is one of the most important parameters affecting the coupling of carbon and water interactions. Owing to the spatial proximity, R\({}_{\rm g}\) values around the two-year observations were quite consistent at the two flux sites. It remained higher during the summertime but with slight fluctuations. However, the maximum T\({}_{\rm a}\) (mean value during 2013 and 2014) at the Arou site was obviously lower than that of Daman site, at about 12.8 \({}^{\circ}\)C and 22.1 \({}^{\circ}\)C, respectively. T\({}_{\rm s}\) also had similar features, which can be explained by altitude differences (3033 m at Arou vs 1556 m at Daman). During the growing season, the variations in T\({}_{\rm a}\) and T\({}_{\rm s}\) were very close, whereas T\({}_{\rm s}\) was apparently higher in cold winters. As the temperature rose in spring, the vegetation began to grow and the gross photosynthesis rate gradually increased accompanied by plant transpiration. A similar phenomenon as R\({}_{\rm g}\) in the growth period was found in VPD, but the mean VPD of the Daman site during 2013 and 2014 was almost two times that at the Arou site. Fortunately, rain and heat in this region were over the same period with approximately 94.2% and 96.1% of the annual precipitation falling within the growth season for Arou and Daman, respectively, but the overall amount of rain differs significantly between the sites. Among these environmental factors, Pearson correlation analysis (Table 2) revealed that the temperature was strongly and positively correlated to the variability in GPP, ET and the coupling effect-WUE. Particularly, T\({}_{\rm s}\) performed a closer correlation with WUE than T\({}_{\rm a}\) for both sites. The correlations between ecosystem WUE and T\({}_{\rm s}\) can reach 0.853 for the Arou grassland site and 0.734 for the Daman cropland site, respectively. Followed by the natural precipitation (P), P was also an important determining factor for WUE, with the correlation coefficients of 0.802 (Arou) and 0.715 (Daman), respectively. However, it seemed that the water condition controlled GPP more than ET in the arid and semi-arid areas. R\({}_{\rm g}\) and VPD were just the opposite. Both of them were more closely correlated to ET than GPP and ecosystem WUE. Furthermore, as an important biophysical parameter comprehensively reflecting the environmental conditions, time-series MODIS EVI data strongly influenced the seasonal variations in WUE and its components for both grassland and cropland ecosystems (Table 2). With the increase in EVI, ecosystem WUE also increased linearly. Correlation coefficients between site-level WUE and EVI reached up to 0.960 and 0.941 for Arou and Daman, respectively.