## Results  
### Weather Conditions  
The differences between average \(T_{\mathrm{a}}\) and \(P\) for the period 2013-2016 and the historical averages have been analyzed in Figure 2. The climatic conditions during the study period were characterized by the high average \(T_{\mathrm{a}}\), with the fluctuant annual \(P\) component (Figure 2). Annual mean \(T_{\mathrm{a}}\) values for the period 2013-2016 were all above the mean annual level (1957-2016) whereas the annual \(P\) levels were near the historical average (37.5 mm) in 2013 (34.2 mm), above the historical average in 2015 (70.1 mm) and 2016 (52.1 mm) and below the historical average in 2014 (17.2 mm). In this study, we have also characterized the climatic conditions for the period 2013-2016 and thus analyzed the discrepancies between the respective study-year values with the historical means representing the environmental factors (Figure 3). The annual mean \(T_{\mathrm{a}}\) values were 10.62, 10.47, 10.59 and 10.46 in 2013-2016, respectively. Moreover, the mean values of \(T_{\mathrm{a}}\) during these four growing seasons were 21.45, 21.30, 20.98 and 21.46 \({}^{\circ}\)C, respectively (Table 1). The average GWD in the stand  
Figure 2: A matrix of annual mean (\(\pm\)S.D., vertical bars) air temperature (\(T_{\mathrm{a}}\)) and the annual precipitation (\(P\)) for the study period 2013–2016 at the lower reach of Heihe River Basin. The vertical and horizontal lines are the mean annual \(T_{\mathrm{a}}\) and \(P\) (1957–2016), respectively. was 1.4 m for 2014-2016 and the variations in GWD mainly depended on the amount and the time of the water conveyance activities. Generally, the arrival time of water allocations was concentrated around the months of September and October and the values of GWD decreased rapidly following the entry of water and then increased gradually until the next water allocation. Mean monthly GWD values for the period May to October 2015 were found to be greater than those in 2014 and 2016. The mean monthly \(RH\) values ranged between 13% and 52% for the period 2013-2016, with a relatively lower value in April and May. In terms of \(RH\), the values were similar from July to October ranging from 27% to 37%. In addition, the annual \(E_{p}\) registered the values of 1997, 2062, 2090 and 2087 mm, respectively, while the monthly maximum for each year was found to be about 289.4 mm (July), 293.2 mm (July), 303.9 mm (August) and 285.4 mm (July) in 2013-2016, respectively. Compared to historical mean values, the monthly \(E_{p}\) values were lower in the period of May-October, owing to the lower wind speed values as described in other studies (e.g., [39]), except for the month of October 2014. In contrast to the meteorological variables measured in this paper, the soil hydrological regime was greatly affected by the flood irrigation, as revealed in Figure 4. Daily \(\theta\) values registered similar \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & \multicolumn{2}{c}{**NEP**} & \multirow{2}{*}{**GPP**} & \multirow{2}{*}{**Re**} & \multirow{2}{*}{**GPP/Re**} & \multicolumn{2}{c}{\(T_{\text{a}}\)} & \multirow{2}{*}{\(P\)} & \multicolumn{1}{c}{**GWD**} \\ \cline{3-3} \cline{5-8}  & & & & & \({}^{\circ}\)**C** & & & \({}^{\circ}\)**C** & **mm** & **m** \\ \hline
2013 & & & & & & 21.45 & 34.2 & 1.67 \\
2014 & 297 & 825 & 528 & 1.56 & 21.30 & 17.3 & 1.28 \\
2015 & 427 & 834 & 405 & 2.05 & 20.98 & 70.1 & 1.79 \\
2016 & 278 & 1019 & 742 & 1.37 & 21.46 & 52.1 & 1.16 \\ Average & 334 & 892 & 558 & 1.66 & 21.30 & 43.3 & 1.41 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Yearly net ecosystem production (NEP), gross primary production (GPP), ecosystem respiration (Re), and mean GPP/Re in 2013–2016 at the study site. Also shown are mean air temperature (\(T_{\text{a}}\)), annual precipitation (\(P\)), and groundwater depth (GWD) during the growing season.