## 3 Results
### Performance of Upscaled Daily SIF to Track GPP at the Seasonal Scale  
As demonstrated above, the effects of changed weather conditions on the differences between the instantaneous scale and daily scale can be reduced by upscaling \(\mathrm{SIF}_{\mathrm{inst}}\) to the daily scale using PAR data. As the GPP is a product of APAR and LUE, the improvement in the SIF-APAR relationship caused by SIF upscaling using the PAR-based method may produce a better correlation between \(\mathrm{GPP}_{\mathrm{daily}}\) and SIF. Figures 10 and 11 display the seasonal cycles of \(\mathrm{GPP}_{\mathrm{daily}}\) and upscaled \(\mathrm{SIF}_{\mathrm{daily}}\), respectively, using two methods at the XTS and DM sites. To obtain a better understanding, the \(\mathrm{SIF}_{\mathrm{T30}}\) and GPP data  
Figure 8: Scatter diagrams between daily absorbed PAR (APAR) with \(\mathrm{SIF}_{\mathrm{inst}}\) (**a**,**d**) and upscaled \(\mathrm{SIF}_{\mathrm{daily}}\) using the cos-based (**b**,**e**) and PAR-based (**c**,**f**) methods on sunny and cloudy days, as indicated by the colors, at the XTS (upper row) and DM (bottom row) sites. Figure 9: Scatter diagrams between the daily APAR with \(\mathrm{SIF}_{\mathrm{inst}}\) (**a**) and the upscaled \(\mathrm{SIF}_{\mathrm{daily}}\) using the cos-based (**b**) and PAR-based (**c**) methods with four latitudes, as indicated by the colors, at the XTS (upper row) and DM (bottom row) sites. The overall \(\mathrm{R}^{2}\) and CV are shown in black text labels per panel. on some representative days are emphatically shown in the figures. We did not exhibit the seasonal cycles of the \(\text{SIF}_{\text{inst}}\) values because they were almost the same as those of the cos-based upscaled \(\text{SIF}_{\text{daily}}\). In general, the upscaled \(\text{SIF}_{\text{daily}}\) using both methods can reliably track the seasonal changes in \(\text{GPP}_{\text{daily}}\) at the two sites. Specifically, the \(\text{GPP}_{\text{daily}}\) and upscaled \(\text{SIF}_{\text{daily}}\) using the two methods all exhibit similar seasonal cycles across the growth seasons for wheat and maize; they all increase at first with crop revival and growth and then decrease with the crop senescence. However, for several days that experienced substantial changes in weather (e.g., the 125 th day in 2017 and the 136 th and 137 th days in 2018 at the XTS site (see Figure 10) and the 178 th, 183 th, 229 th, and 230 th days at the DM site (see Figure 11)), the diurnal cycles of the PAR-based SIF agreed better with the diurnal GPP changes and, thus, the PAR-based upscaled \(\text{SIF}_{\text{daily}}\) values were more accurate than the cos-based upscaled \(\text{SIF}_{\text{daily}}\). This result made the seasonal cycles of PAR-based upscaled \(\text{SIF}_{\text{daily}}\) more consistent with the seasonal \(\text{GPP}_{\text{daily}}\) than the cos-based upscaled \(\text{SIF}_{\text{daily}}\). Note that both the upscaled \(\text{SIF}_{\text{daily}}\) by the two methods and the measured \(\text{SIF}_{\text{daily}}\) will underestimate \(\text{GPP}_{\text{daily}}\) on those cloudy days (see Figures 10 and 11), especially for the data at the DM site. Figure 11: Comparison between the seasonal cycles of \(\text{GPP}_{\text{daily}}\) with the upscaled \(\text{SIF}_{\text{daily}}\) using the cos-based and PAR-based methods during the growth season for maize at the DM site in 2017 and 2018 (**c**). (**a,b**) \(\text{SIF}_{\text{T30}}\) and GPP values on several cloudy days within the green dashed frames in (**c**). Figure 10: Comparison between the seasonal cycles of \(\text{GPP}_{\text{daily}}\) with the upscaled \(\text{SIF}_{\text{daily}}\) using the cos-based and PAR-based methods during the growth season for wheat at the XTS site in 2017 and 2018 (**c**). (**a,b**) \(\text{SIF}_{\text{T30}}\) and GPP values on several cloudy days within the green dashed frames in (**c**). Figure 12 reports the quantitative assessment of the SIF-GPP correlations at half-hour and daily intervals for C3 and C4 crops at the XTS and DM sites, respectively.