## 2 Materials and Methods
### In Situ Measurements
#### 2.2.2 Diurnal Flux and Meteorological Observations  
The flux and meteorological variables were observed using an eddy covariance (EC) system and the AWS. The AWS was fixed on a stand at the center of our selected field site to collect the meteorological variables, including photosynthetically active radiation (PAR, umol m\({}^{-2}\) s\({}^{-1}\)), air \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**2015**} & \multicolumn{4}{c}{**2016**} \\ \cline{2-7}  & **April 3** & **April 13 \& 14** & **April 24 \& 25** & **April 8 \& 9** & **April 18** & **May 3 \& 4** \\ \hline Growth stage & Erecting & Jointing & Booting & Erecting & Jointing & Flowering \\ LAI & 1.5 & 2.1 & 2.4 & 2.5 & 2.9 & 1.9 \\ Cab (ug/cm\({}^{2}\)) & 59.2 & 62.2 & 61.3 & 55.3 & 53.7 & 57.3 \\ Cw (cm) & 0.0138 & 0.0126 & 0.0158 & 0.0163 & 0.0199 & 0.0177 \\ Cdm (g/cm\({}^{2}\)) & 0.0042 & 0.0040 & 0.0045 & 0.0048 & 0.0049 & 0.0043 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The growth stages and vegetation parameters of winter wheat at the time of ten diurnal experiments in 2015 and 2016.  
humidity (rH, %), vapor pressure deficit (VPD, hpa), soil temperature (T\({}_{\text{soil}}\), \({}^{\circ}\)C), and other input parameters of SCOPE (including R\({}_{\text{in}}\), Ta, p, ea, and u, as listed in Table 2) every 10 s. The AWS output was recorded at 10 min intervals using a CR1000 unit (Campbell Scientific Inc., Logan, UT, USA). Near the AWS, an EC system was installed on a stand to measure the exchange of energy, water vapor, and CO\({}_{2}\) across the canopy-atmosphere interface. The EC system included a 3D sonic anemometer (CSAT3, Campbell Scientific Inc., Logan, UT, USA) for measuring three-dimensional velocity and temperature and an open-path infrared gas analyzer (Li-7500, Li-Cor, Lincoln, NE, USA) that measured CO\({}_{2}\) and H\({}_{2}\)O density. The sensors were installed at a height of 2.5 m above the ground. The main output parameters include the net ecosystem exchange of CO\({}_{2}\) flux (NEE, mg/m\({}^{2}\)/s), latent heat flux (LE, W/m\({}^{2}\)), sensible heat flux (H, W/m\({}^{2}\)), friction velocity (u*, m/s), and the atmospheric CO\({}_{2}\) concentration for model input (i.e., Ca in Table 2). The data were stored in a CR3000 data logger (Campbell Scientific Inc., Logan, UT, USA) and processed with an average time of 30 min at a sampling frequency of 10 Hz. With the obtained half-hour NEE data and the corresponding meteorological variables (including R\({}_{\text{in}}\), Ta, rH, LE, H, u*, and T\({}_{\text{soil}}\)) as inputs, half-hour GPP data could be calculated using the online tool available at the Max Planck Institute for Biogeochemistry (MPI-BGC) website ([[http://www.bgc-jena.mpg.de/~MDIwork/eddyproc/](http://www.bgc-jena.mpg.de/~MDIwork/eddyproc/)]([http://www.bgc-jena.mpg.de/~MDIwork/eddyproc/](http://www.bgc-jena.mpg.de/~MDIwork/eddyproc/))). First, u* filtering was conducted to calculate the u* threshold for identifying conditions with insufficient turbulence and marking those conditions as data gaps to avoid biases in fluxes measured using eddy covariance. Subsequently, gap filling was carried out to fill the gaps in half-hourly eddy covariance data. The gap filling of the eddy covariance and meteorological data were performed with methods similar to [53] while also considering flux co-variation with meteorological variables and flux temporal auto-correlation [54]. Finally, flux partitioning was implemented for partitioning NEE into ecosystem respiration and GPP. Based on the night-time partitioning algorithm [54], respiration is estimated from the night-time and extrapolated to the daytime. Figure 1 exhibits the measured date sets about half-hour Ta, VPD, and PAR observations from ten experiments in 2015 and 2016. It indicates that the weather was sunny and stable during ten of the experiments, except for observations at approximately 11:30 and 14:30 on 25 April 2015, at approximately 13:00 and 14:00 on April 18, and at approximately 15:00 on 4 May 2016, when it was cloudy.