## 3 Results
### Performance of Satellite-Based Seasonal GPP  
In this study, three fundamental methods contributing to the analysis of the differences in the seasonal performances of the BESS, MODIS, and GOSIF GPP products were carried out. It is worth noting that the BESS and MODIS GPP extracted based on the S_Sd site locations were both invalid data. Firstly, based on the EC-based observations GPP (GPPEC), we assessed the performance across seasonal courses (Figure 3) and the regression relations of the BESS, MODIS, and GOSIF GPP (GPPBESS, GPPMOD, and GPPSIF) (Figure 4) at all ecosystems during 2014 and 2015. From Figure 3, we can find that the performances of the three GPP products varied dramatically with different ecosystems. On the whole, the seasonal variation of the three GPP products was the most consistent with the flux site GPPEC of grassland ecosystems (S_Ar and S_Dsl) but remained largely underestimated for the growing seasons at the other EC sites. Furthermore, the three GPP products had well peaks matching at the S_Dm site and a degree of peak delay in both grassland ecosystems (the S_Ar and S_Dsl sites). Moreover, the GPPSIF performed better than the other GPP products generally, but it was obviously overestimated during the growing season of the S_Ar site. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sites** & **Vegetation Type** & **GPP** & \(R_{e}\) & **NEE** \\ \cline{3-5}  & & & \multicolumn{3}{c}{**g C m\({}^{-2}\) y\({}^{-1}\)**} \\ \hline S\_Dm & Maize cropland & \(1183\pm 39\) & \(454\pm 79\) & \(-728\pm 41\) \\ S\_Ar & Alpine meadow & \(742\pm 74\) & \(421\pm 60\) & \(-321\pm 56\) \\ S\_Zy & Wetland & \(793\pm 228\) & \(324\pm 16\) & \(-469\pm 234\) \\ S\_Dsl & Swamp meadow & \(528\pm 27.33\) & \(208\pm 49\) & \(-320\pm 74\) \\ S\_Sdq & Tamarix & \(665\pm 59\) & \(456\pm 55\) & \(-208\pm 56\) \\ S\_Hhl & Populus euphratica and Tamarix & \(954\pm 205\) & \(728\pm 239\) & \(-227\pm 95\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Annual mean GPP, \(R_{e}\), NEE, and the corresponding standard deviations of different ecosystems based on the six flux sites between 2014 and 2016. Figure 3: Seasonal dynamics of the GPP from flux measurements (GPPEC) and the estimates from the BESS products (GPPBESS), the MODIS (GPPMOD), and the GOSIF (GPPBIF) at the six flux sites between 2014 and 2015. Note: The values extracted from both MODIS and BESS products were null at the Zhangye wetland site (S_Zy). Linear regression analysis was performed to examine the performance of the BESS, MODIS, and GOSIF GPP products on an eight-day period at the five flux sites between 2014 and 2015. As the values of both GPP\({}_{\text{BESS}}\) and GPP\({}_{\text{MOD}}\) were null at the S_Zy site, the performance of these two products could not be further analyzed. As shown in Figure 4, different GPP products varied with different ecosystems. Overall, the three GPP products' performance was underestimated or overestimated in all EC sites and the GPP\({}_{\text{SIF}}\) performed better than the other GPP products. Especially, the GPP\({}_{\text{SIF}}\) had the highest consistency with GPP\({}_{\text{EC}}\) at the S_Dm, the S_Sdq, and the S_Hhl sites, with R\({}^{2}\) values ranging from 0.92 to 0.98 and RMSE from 1.5 to 2.66 g C m\({}^{-2}\) d\({}^{-1}\). However, the GPP\({}_{\text{SIF}}\) (R\({}^{2}\) = 0.95, RMSE = 1.47 g C m\({}^{-2}\) d\({}^{-1}\)) was obviously overestimated at the S_Ar site, having no superiority over the GPP\({}_{\text{BESS}}\) and GPP\({}_{\text{MOD}}\).