## 3 Results and Discussion
### Comparative Analysis of Optical Sensor Products and In Situ Data
#### 3.1.2 Temporal Variations of CO2 Flux and Comparison with Ground Observation  
According to the relationship between GPP and CO2 presented in Section 2.3, CO2 (hereinafter referred to as CO2 MODIS) was calculated from MODIS GPP and compared with the CO2 (hereinafter CO2 fluxtower) observed in a flux tower. CO2 MODIS and CO2 fluxtower consist of eight-day products.  
Figure 4 compares CO2 MODIS and CO2 fluxtower from 2014 to 2015 and compared them with the precipitation values observed in the same period. A similar tendency as the SM results in the same period can be seen in this figure after a time lag in the rainfall. As this is related to photosynthesis that occurs after SM affects vegetation activity, so that a certain duration of lag time occurs [38].  
Figure 5 and Table 4 show the statistical analysis results of CO2 MODIS and CO2 fluxtower at two validation points. Figure 5 illustrates a one on one plot of the two results, showing a high correlation in both sites. The result of CO2 flux is somewhat different from that of the SM were all included within the 95% confidence interval, making them statistically significant. The bias, RMSE, gradient of one on one plot, and correlation coefficients were almost similar at both SMC and CMC points, and the SMC results were better due to dense vegetation. Vegetation density is an important factor for the generation of CO2 flux by photosynthesis in the two points. This density can be determined by the leaf area index (LAI). For SMC, LAI shows a value between 6 and 8. For CMC, LAI shows a value between 1 and 2 (Table 1). Although the difference in LAI is somewhat large, CMC is less sensitive to drought status as it is located in the rice paddy terrain where the irrigation system is built [7]. These characteristics narrowed the difference between the results of the statistical analyses of SMC and CMC [31].  
Figure 3: Comparisons between observed and estimated soil moisture at (**a**) SMC and (**b**) CMC site.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Site** & **Bias (m3 m-3)** & **RMSE (m3 m-3)** & **a 1** & **Coefficients of Correlation** \\ \hline SMC & \(-\)3.87 & 8.25 & 0.81 & 0.75 * \\ CMC & \(-\)0.20 & 7.86 & 0.58 & 0.66 * \\ \hline \hline \end{tabular}  
* a: gradients of one on one plot. *: \(p\)-value = 0.05.  
\end{table}
Table 3: Statistical comparison between estimated and in situ measurements of SM.  
Figure 4: Temporal variations of average eight-day CO\({}_{2}\) flux at at (**a**) SMC and (**b**) CMC site.  
Figure 5: Comparison between observed and estimated CO\({}_{2}\) flux using 1 on 1 plot at (**a**) SMC and (**b**) CMC site.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Site** & **Bias (g m\({}^{-2}\) d\({}^{-1}\))** & **RMSE (g m\({}^{-2}\) d\({}^{-1}\))** & **a \({}^{1}\)** & **Coefficients of Correlation** \\ \hline SMC & 0.93 & 12.46 & 0.79 & 0.76 \({}^{*}\) \\ CMC & \(-\)1.37 & 12.52 & 0.74 & 0.72 \({}^{*}\) \\ \hline \end{tabular} \({}^{1}\) a: gradients of one on one plot. \({}^{*}\): \(p\)-value = 0.05.  
\end{table}
Table 4: Statistical comparison between estimated and in situ measurements of CO\({}_{2}\) flux.