## 4 Data Analysis
### Seasonal Variations in SIF, NDVI, and GPP  
In order to monitor the maize during two consecutive growing cycles, the SIFSpec system was first installed on the tower at the DM site on 13 May 2017 when the maize had just sowed. However, the instrument only began operating normally on 6 June 2017, and two obvious measurement problems occurred--one from 12 to 19 July 2017 and one from 9 to 21 August 2017. These were the result of the breakdown of the SIFSpec system hardware. In 2018, the instrument was mounted prior to the corn  
Figure 10: The diurnal response of SIF to PAR at both O\({}_{2}\)–A (left) and O\({}_{2}\)–B (right) bands at the DM site on four different days during the 2018 growing season. The blue and orange points indicate morning and afternoon, respectively.  
sowing in 2018 and no obvious measurement problems occurred during this growing cycle. Therefore, almost continuous measurements were collected at the DM site during these two growing cycles, especially in 2018. A total of 208 days of observations was obtained at the site during 2017 and 2018.  
The SIFSpec measurements were acquired roughly every 3-5 min from 07:00 to 19:00 local time. This time interval is made up of the predefined three minutes as well as the time needed to complete a whole measurement cycle, which includes the collection of one irradiance spectrum and two radiance spectra. The IT optimal time before each sampling and the time for shutter switches make up the rest of the 3-5 min. Thus, more than 250 spectra were collected on clear-sky days. The SIF retrievals were averaged into half-hour values after quality control. In order to explicitly present the seasonal variability in SIF in tandem with PAR, NDVI, and GPP, all variables were processed into daily averages. The long-term, continuous measurements made by the SIFSpec system, as well as flux data covering two almost complete growing seasons at the DM site, are depicted in Figure 11.  
The NDVI is a good indicator of the vegetation growth. The variation in SIF at the O\({}_{2}\)-A and O\({}_{2}\)-B bands with NDVI are presented in Figure 11a,b, respectively. Overall, the seasonal dynamics of SIF values at both the O\({}_{2}\)-A and the O\({}_{2}\)-B bands are consistent with the variation in NDVI. A better correlation between the SIF and NDVI can be observed at the O\({}_{2}\)-A band than that at the O\({}_{2}\)-B band; the corresponding correlation coefficients are 0.58 and 0.37, respectively. The SIF values tend to show distinct rising trends earlier than NDVI at the growing stage of the 2018 season.  
A pleasing synergistic variation in the GPP data and SIF at both bands can be observed. Compared with the SIF at the O\({}_{2}\)-B band, the SIF at the O\({}_{2}\)-A band is more significantly correlated with GPP, with a correlation coefficient of 0.81. The increases and decreases in SIF and GPP are both consistent with canopy growth and senescence during the two growing seasons. As for the O\({}_{2}\)-B band, a much better variation of SIF with GPP can be observed for 2018 than for 2017. The reliable correlation between SIF and GPP indicates the ability of SIF to track the photosynthetic activity of vegetation.  
Figure 11: Seasonal variations in SIF at both the (**a**) O\({}_{2}\)–A (blue) and (**b**) O\({}_{2}\)–B (red) bands. Also shown are Normalized Difference Vegetation Index (NDVI) (orange) and flux gross primary production (GPP) (green) data obtained by the SIFSpec system at the DM site.