## Result  
### Annual and Seasonal Trends of GPP Calculated from LUEopt Products  
Figure 3 exhibits the spatial distribution of the annual and seasonal trends of GPP nationwide while Figure 4 shows the GPP trends at the basin scale. During the last thirty-four years, the GPP of terrestrial vegetation in China has shown an increasing trend, consistent with the studies of [PERSON] et al. [32] that focused on multiple sets of GPP data. This can be explained by the effect of CO\({}_{2}\) concentration and air temperature rise on vegetation production in recent years [51]. On the other hand, in the 21 st century, the Chinese government has issued a series of policies to restore the ecological environment, such as the Grain to Green Program [52; 53]. These policies may facilitate an increase in GPP [54; 55]. However, specific to basin scale, certain differences are detected in the trends of varying river basins at different time scales.  
Annually, except for the SEB which showed a decreasing trend of \(-0.19\) g C/m\({}^{2}\)/year), the remaining basins all exhibited an increasing trend. Among them, the HRB showed the largest average ascending trend (3.43 g C/m\({}^{2}\)/year), and the YRB ranked the second highest (2.67 g C/m\({}^{2}\)/year), followed by the SRB, the HuRB, the YERB, and the SWB, with the four being nearly equal (2.19, 2.07, 2.06 and 1.82 g C/m\({}^{2}\)/year). Because of the large variability within pixels (the standard deviation is 5.48 g C/m\({}^{2}\)/year), the PRB showed an increasing trend of only 1.05 g C/m\({}^{2}\)/year. With an inherently small mean GPP value, the CB only showed a slight increase (0.24 g C/m\({}^{2}\)/year). Relatively significant upward trends were mainly distributed in the HRB, the YERB, and the border areas of the YRB, the SWB, and the PRB. Meanwhile, the significant downward trends were mainly distributed in the eastern region of the PRB. Our results are consistent with the conclusions of [PERSON] et al. [12].  
Seasonally, the four northern basins (SRB, CB, HRB and YERB) revealed an increasing trend in spring, summer and autumn, despite a stagnant period in winter, which was probably because of the low air temperature and SRAD in winter that leads to the stagnation of plant photosynthesis [56]. In the HuRB and the YRB, which are located in the central part of China, a significant increasing trend was detected in spring. Although there was a downward trend in summer, the ascending trends in the other seasons rendered the annual trend of the two basins to be rising. In the SEB and the PRB, statistically significant decreasing trends were found in summer. This can be attributed to the fact that high air temperature is more likely to occur in summer, which inhibits vegetation growth and is not conducive to increases in GPP [57]. Due to the increasing trend of GPP in spring and winter, the annual trend of the PRB exhibited an increasing state. The SEB was the only basin with a downward annual trend because of the decreasing trend in summer (\(-2.56\) g C/m\({}^{2}\)/year).  
Nevertheless, the SWB maintained an upward trend consistent with the other three seasons in summer when the other basins in the south of China generally showed a downward trend. This can be explained by the fact that the SWB area has a high altitude and the terrain is mainly mountainous, which is not favorable to having extremely high air temperatures in summer [58]. On the whole, the noticeable discrepancies of the GPP trends in the nine river basins mainly occurred in summer, which mostly dominated the annual trend pattern of the basin.  
Figure 3: Spatial distribution of GPP trends at annual and seasonal scales from 1982 to 2015. The values in the figure represent the change in GPP cumulative value in a year (or a season) compared with the last year (or the last same season) in the unit area. Values with statistical significance at a 95% confidence level are displayed with cross symbols. The nan value is displayed in white.