## 4 Discussion  
### Uncertainties in the SIF Measurements  
The SIF retrieval accuracy is dependent on the spectral characteristics of the reflectance and irradiance spectra around the oxygen absorption lines. According to our validation based on EC-based data, the SIF\({}_{760}\)-based GPP at 8:30 on 21 August and 8:30 on 22 August deviate from the EC-based values significantly (dots in the red circles in Figure 6). During our experiment, we measured four canopy positions sequentially from T1 to T4 as illustrated in Figure 2. The spectra of the reference panel were collected eight times: once before and once after each of the four canopy measurements. Incident radiance at 8:30 on 21 August and 8:30 on 22 August was then obtained (Figure 7). The incident radiance showed significant variations during our measurement, especially at the left and right shoulders of the absorption line. Additionally, the measured irradiance did not present a gradual increase trend as expect. This may be attributed to the instability of irradiance in the morning. As light travels a longer path at high solar zenith angle (SZA) in the morning, the influence of atmospheric conditions would be larger than that at lower SZA. Hence, in future studies, it is necessary to consider the influence of air conditions and the affection of imaging geometrical conditions such as the SZA and relative azimuth angle in SIF retrieval.  
Figure 6: Relationships between modeled GPP and EC-based GPP by: (**a**) SIF\({}_{760}\); (**b**) MuSyQ-GPP; (**c**) BEPS; and (**d**) SIF\({}_{686}\) during the experiment period. The dots in the red circle are the values at 8:30 on 21 August, and 8:30 on 22 August. The error bars indicate the range of the four SIF-based GPP values, the red shades represent the 95% confidence bands for the regression functions.  
The data quality (DQ) indicator \(DQ_{\text{s}}\) presented by [PERSON] et al. [84] was adopted to evaluate the stability of the incident radiance. \(DQ_{\text{s}}\) is defined as the percentage of variation between the first and the second irradiance measurement  
\[DQ_{\text{s}}=\left|E_{\text{B}}-E_{\text{A}}\right|/E_{\text{B}}\times 100 \tag{20}\]  
where \(E_{\text{B}}\) and \(E_{\text{A}}\) represents the incident radiance before and after the canopy measurement, respectively.  
In this study, the \(DQ_{\text{s}}\) values of the three bands used in the 3 FLD algorithm were averaged to represent the irradiance stability. As shown in Figure 8, the \(DQ_{\text{s}}\) values were generally higher at morning and evening and lower at noon, the higher values also corresponded with the higher SZA and longer path. For the observations at 8:30 on 21 August and 8:30 on 22 August, the \(DQ_{\text{s}}\) showed the largest values, which indicated that the irradiance changed significantly during these measurement sessions. Therefore, data quality check is needed especially when irradiance demonstrates significant variations, the acquisition that do not meet the quality check should be rejected and no longer considered in the analyses.  
Figure 7: Incident radiance at: **(a)** 8:30 on 21 August; and **(b)** 8:30 on 22 August. T1 to T4 represent the four canopy targets, B and A denote measurements before and after canopy measurement, respectively.  
We used the 3 FLD algorithm to generate SIF in this study. The 3 FLD approach is based on the assumption that the reflectance signal and the SIF signal vary linearly within and outside the absorption band. However, the actual variations of reflectance and SIF signal are not being linear, which causes a violation of the underlying assumption of the 3 FLD method and lead to some uncertainties in SIF retrieval. Additional studies are needed in generating SIF with more advanced SIF retrieval approaches such as the improved FLD (iFLD) method [85] or the Spectral Fitting Methods (SFM) [86].