## 3 Results and Discussion
### Spatio-Temporal Variation of Terrestrial Ecosystems' GPP, ET, and WUE  
The maps presented in Figure 6 are the GPP distributions from the MODIS 17 product in the dormant and growing seasons in 2007 and 2008. We found that the GPP of the terrestrial ecosystem in South Korea was ranged from 1 to 70 gC\(\cdot\)m\({}^{-2}\).8-day\({}^{-1}\) and its annual mean was between 21.91 and 25.91 gC\(\cdot\)m\({}^{-2}\).yr\({}^{-1}\). The estimated GPP was higher near the coastline than in the inland areas [46, 47] and we detected a trend that the value of R\({}_{\rm s}\), or daily incoming shortwave radiation, was overestimated in areas near the coastline, while the VPD values were underestimated in the inland areas during winter season [46]. However, a large amount of vegetation production detected in the summer was due to the fact that the R\({}_{\rm s}\) had the most significant impact on the estimated GPP in the areas near the coastline and the VPD was more significantly affected in the inland areas [46, 47]. The VPD was underestimated in the summer in the entire region except the inland areas, because the GPP values were overestimated in the inland areas (Figure 4) [46]. The MODIS GPP values were lower than the flux tower ground measurement data, because of the underestimation of input data including VPD, R\({}_{\rm s}\), and \(T_{\rm min}\) (Figure 7). In Figure 7, we found decreasing trends of MODIS-based GPP on growing season, because it was affected by atmospheric effects such as cloud effect which reflect the inaccuracy of the MODIS products. Many methods were developed in previous studies to improve the accuracy of MODIS dataset. [PERSON] _et al._[28] introduced the smoothing method to improve MOD 17 GPP products. In Figure 8, the MODIS-based GPP was fitted well with the flux tower measured GPP. The fitted lines between the MODIS-based GPP (y) and the flux tower  
Figure 5: Temporal variation of saturated vapor pressure deficit (VPD), ET, and GPP at GDK site in South Korea. (**a**) VPD and flux-based GPP; (**b**) VPD and flux-based ET. measurement GPP (x) were: y = 1.22x \(-\) 7.02 with correlation coefficient (R) = 0.85, Bias = 2.94 gC\(\cdot\)m\({}^{-2}\) and root-mean-squared-error (RMSE) = 14.12 gC\(\cdot\)m\({}^{-2}\) in 2007 and y = 1.28x \(-\) 11.66 with R = 0.94, Bias = 3.63\(\cdot\)gC\(\cdot\)m\({}^{-2}\) and RMSE = 9.04 gC\(\cdot\)m\({}^{-2}\) in 2008 (Table 2). In [PERSON] _et al._[22] which compared MODIS GPP product with the products of GPP using the RHESys model at same study site, we found that results of validation showed almost similar patterns. The spatial distributions of the ET retrieved from the MOD16 from summer to winter in 2007 and 2008 are shown in Figure 9. The estimated ET values were approximately 5 to 35 g-m\({}^{-2}\)-8-day\({}^{-1}\) in the flat western region of Korea and approximately 10 to 70 g-m\({}^{-2}\)-8-day\({}^{-1}\) in the eastern mountainous forest region during 2007 and 2008. Even though we found that the ET values in the western region were substantially underestimated, the general pattern of the MODIS 16 ET values in South Korea were in agreement with the highest ET values over the eastern forested regions and the lowest ET values in inland region [50]. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**GDK** & \multicolumn{2}{c}{**Average**} & **Bias** & **RMSE** & **Regression Slope** & **Correlation Coefficient (R)** \\ \hline
**GPP** & 2007 & 21.70 & 2.94 & 14.12 & 1.22 & 0.85 \\ (gC-m\({}^{-2}\)-8-day\({}^{-1}\)) & 2008 & 24.63 & 3.63 & 9.64 & 1.28 & 0.94 \\ \hline
**ET** & 2007 & 13.85 & 2.52 & 9.84 & 1.07 & 0.76 \\ (g-m\({}^{-2}\)-8-day\({}^{-1}\)) & 2008 & 15.84 & 4.70 & 10.41 & 1.26 & 0.83 \\ \hline \end{tabular}
\end{table} 
Table 2: Statistical comparing observed and estimated GPP and ET. Figure 8: Relationship between MODIS-based GPP and flux tower measurement GPP. Figure 7: Time series of observed and estimated GPP. The distribution of the ET showed similar trends with GPP distributions [11, 18, 19].