## 3 Results and Discussion
### Potential and Apparent Underlying Water Use Efficiency  
Table 2 shows the annual \(uWUE_{p}\) for the five sites. It is evident from Table 2 that among the three natural vegetation sites, Xitaizi shows the highest average annual \(uWUE_{p}\) value of 23.22 g C hPa\({}^{0.5}\)/kg H\({}_{2}\)O, followed by Arou and Sidaoqiao with an average annual \(uWUE_{p}\) value of 12.34 g C hPa\({}^{0.5}\)/kg H\({}_{2}\)O and 10.02 g C hPa\({}^{0.5}\)/kg H\({}_{2}\)O, respectively (Figure 6).  
The weekly \(uWUE_{a}\) of Xitaizi ranges from 2.44 to 18.67 g C hPa\({}^{0.5}\)/kg H\({}_{2}\)O, with an average of 9.54 g C hPa\({}^{0.5}\)/kg H\({}_{2}\)O, the highest among the three vegetation sites when compared with Arou and Sidaoqiao, which range from 1.01 to 21.63 g C hPa\({}^{0.5}\)/kg H\({}_{2}\)O with an average of 10.15 g C hPa\({}^{0.5}\)/kg H\({}_{2}\)O and 2.07 to 8.65 g C hPa\({}^{0.5}\)/kg H\({}_{2}\)O with an average of 5.95 C hPa\({}^{0.5}\)/kg H\({}_{2}\)O, respectively (Figure 7). From the results, it is clear that the annual \(uWUE_{p}\) and weekly \(uWUE_{a}\) are higher for the deciduous broadleaf forest as compared to grass and shrub. Apart from the difference in vegetation types among the three sites, the difference in precipitation also contributes to the variation in annual \(uWUE_{p}\) and weekly \(uWUE_{a}\) for the three sites. From Table 1, it is seen that the long-term average annual precipitation is highest at Xitaizi (541.0 mm), followed by Arou (445.0 mm) and Sidaoqiao (116.8 mm).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Site**} & \multicolumn{4}{c}{\(uWUE_{p}\) (g C hPa\({}^{0.5}\)/kg H\({}_{2}\)O)} \\ \cline{2-6}  & **2013** & **2014** & **2015** & **2016** & **Average** \\ \hline Arou & 13.27 & 11.51 & 12.25 & - & 12.34 \\ Daman & 25.17 & 26.14 & 26.94 & - & 26.08 \\ Sidaoqiao & 9.23 & 9.66 & 11.15 & - & 10.02 \\ Xinier & 24.00 & 19.23 & 20.61 & 19.87 & 20.61 \\ Xitaizi & - & - & 24.33 & 24.89 & 23.22 \\ \hline \hline \end{tabular}
\end{table}
Table 2: A comparison of annual \(uWUE_{p}\) for the five sites.  
Figure 6: Annual \(uWUE_{p}\) for the five sites.  
Annual \(uW\mathit{UIE}_{p}\) and weekly \(uW\mathit{UIE}_{a}\) are positively related to precipitation, as higher precipitation is associated with higher net primary productivity (NPP), leading to higher water use efficiency.  
The average annual \(uW\mathit{UIE}_{p}\) for Daman site is 26.08 g C hPa\({}^{0.5}\)/kg H\({}_{2}\)O, which is much higher than the 20.61 g C hPa\({}^{0.5}\)/kg of Xinier site. However, the average weekly \(uW\mathit{UIE}_{a}\) is 10.15 g C hPa\({}^{0.5}\)/kg for Daman, which is lower than the 12.82 g C hPa\({}^{0.5}\)/kg at Xinier and might be due to the difference in irrigation schemes between the two sites (Figure 7). In Daman, flood irrigation is conducted only four times during the growing season, which causes a dramatic distinction in soil water and ET between irrigated days and non-irrigated days and leads to a high inter-annual variation of \(uW\mathit{UIE}_{a}\). In contrast, much drip irrigation is applied at Xinier during the growing season once a week or less than once a week (during the poll stage), comprising 11 irrigation events in total. Consequently, the variation in soil water and ET at the Xinier site is much smaller compared to the Daman site, which leads to a lower variation in \(uW\mathit{UIE}_{a}\).