## 3 Results
### Performances of SIF and NDVI in Grain-Yield Monitoring  
To determine the applicability of the SIF and the NDVI for the grain-yield monitoring, we compared the relationships between the average SIF (NDVI) values and the per-unit grain yields during the growing season over different subregions (Figure 6). Figure 6a shows the correlations of the SIF and NDVI values with the per-unit grain yields over different subregions. The degrees of consistency between the two indicators and the per-unit grain yields indicated the existence of close relationships between the SIF and the per-unit grain yield and between the NDVI and the per-unit grain yield. The PCC values obtained between the SIF and the per-unit grain yield were generally higher than those obtained between the NDVI and the per-unit grain yield, especially over the NEP, NCP, MLRYR, YGP, and ASANC regions. Figure 6b shows the R\({}^{2}\) values obtained via polynomial fitting between the SIF (NDVI) and the per-unit grain yield. Except for in the QTP and MLRYR regions, the R\({}^{2}\) values obtained between the NDVI and the per-unit grain yield were greater than 0.8, while the R\({}^{2}\) values obtained between the SIF and the per-unit grain yield were greater than 0.8, except in the SC and QTP regions. Moreover, except in the SC, SCB, and QTP regions, the R\({}^{2}\) values derived between the NDVI and the per-unit grain yield were greater than those derived between the SIF and the per-unit grain yield. Although the SIF and the NDVI both showed good performances when monitoring the per-unit grain yield, and their performances were similar, the SIF had stronger correlations with the per-unit grain yield than the NDVI, indicating that the SIF could explain grain-yield changes better than the NDVI.  
Figure 5: Correlations between SIF (NDVI) and GPP in regions with different land-cover types and in different seasons: (**a**) growing season; (**b**) spring; (**c**) summer; and (**d**) autumn.  
Figure 6: (**a**) PCC values and (**b**) R\({}^{2}\) values obtained between SIF (NDVI) and the per-unit grain yield during the growing season over different subregions.  
Figure 7 shows the percentage changes in the SIF/NDVI/ per-unit grain yield during 2009-2010 compared with the corresponding mean annual values recorded from 2001-2019. In eight subregions, excluding the NEP, the percentage changes in the per-unit grain yield, SIF, and NDVI showed consistent increases or decreases from 2009-2010. In the NEP subregion, the percentage changes in the per-unit grain yield and the SIF increased from 2009-2010, but the NDVI percentage change decreased. Except in the NEP region, the SIF change rate was larger than that of the NDVI. Therefore, the GOSIF could capture the effects of drought on the grain yield better than the NDVI. As the drought that occurred during 2009-2010 had greater effects over the southern part of China than in other regions in China, the percentage changes in the per-unit grain yield in the NEP, NCP, LP, and QTP subregions did not decrease compared with the corresponding mean annual values. It is worth noting that although the SC region is located in southern China, the percentage change rate in the NDVI and SIF increased. Previous studies showed that the SC region was less affected by drought due to climate background, soil type, and river-network distribution [44; 45] during 2009-2010; therefore, the percentage change rate in the NDVI and SIF increased.