## 3 Results
### Comparison between Time-Series Phenological Estimation Portfolios for Crop Growth Stages
#### 3.2.1 SIF vs. EVI  
As one of the main objectives of this study is to evaluate the suitability of SIF data for estimating crop growth stages within the agronomic framework, the initial step in this analysis compares the accuracy of estimations using SIF and EVI data. Figure 5 presents the accuracy of the estimation results for each crop growth stages with in situ SIF, TROPOMI SIF, in situ EVI and MODIS EVI. For summer maize, as depicted in Figure 5a, EVI data exhibited superior accuracy in estimating tasseling and silking (T&S) as well as maturity and harvest (M&H) growth stages, especially for M&H, where SIF data cannot predict the date accurately. For these two growth stages, MODIS EVI exhibits lower \(R^{2}\) than in situ EVI. Emerged growth stage is a special case where estimations with ground-based observations (in situ data) performed better than those with satellite data. Conversely, SIF data excelled in the estimation of the fifth leaf, jointing and milk, three of the four key growth stages, surpassing \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **TR** & **DB** & **CU** & \begin{tabular}{c} **GU** \\ **(TROPOMI SIF)** \\ \end{tabular} & \begin{tabular}{c} **GU** \\ **(In Situ SIF)** \\ \end{tabular} & \begin{tabular}{c} **GU** \\ **(In Situ SIF)** \\ \end{tabular} \\ \hline \hline \begin{tabular}{c} Regreen - - \\ \end{tabular} & - & GU & UD & - & UD \\ \hline \begin{tabular}{c} Jointing \\ Heading \\ \end{tabular} & SOS & SOS & GU & - & UD & SD \\ \begin{tabular}{c} Hcal \\ EOS \\ \end{tabular} & POS & MT & SD & SD & - \\ \begin{tabular}{c} Milk \\ EOS \\ \end{tabular} & EOS & SN & DD & DD & RD \\ \
\begin{tabular}{c} Ripening \\ \end{tabular} & EOS & EOS & DM & - & RD & RD \\
\begin{tabular}{c} Harvest \\ \end{tabular} & - & - & DM & RD & RD & - \\ \hline \hline \end{tabular} Note: “-” refers to no suited phenological characterized transition dates.  
\end{table} 
Table 4: Potential best conciliations of phenological characterized transition dates with crop growing stages of winter wheat. Figure 5: Accuracy assessment of estimations with different datasets. Panel (**a**) focuses on maize, while panel (**b**) pertains to winter wheat. The circular arcs in both radar charts represent \(R^{2}\) for each estimation, ranging from 0 to 1. All the growth stages have been estimated with the listed datasets, with dots in the origin of both radar charts referring to \(R^{2}\) of 0.  
the EVI data. Notably, TROPOMI SIF demonstrates favorable performance for fifth leaf and jointing stages, achieving accuracies approximately 0.6 and 0.8, respectively, and provided acceptable estimations for the milk stage with an accuracy exceeding 0.5. In summary, SIF data outperformed EVI data in identifying the key growth stages (three out of four) of maize, with the exception of T&S. TROPOMI SIF also proved useful for estimating most key growth stages of maize with acceptable accuracy. Turning to winter wheat, Figure 5b indicates that SIF data could be effectively employed to estimate most growth stages of winter wheat, except for heading (similar to tasseling in maize, see above), while EVI data performed poorly. The absence of an in situ EVI dataset for winter wheat was attributed to its inadequate performance in growth stage identification. In the winter wheat growing season post-regreen, the crop's greenness remains stable. This leads to an unvarying EVI curve before the peak, posing challenges for phenological modeling and growth stage identification during this phase. Regarding the SIF data, in situ SIF performed slightly better than TROPOMI SIF for all of the KGSs (from jointing to maturity). But TROPOMI SIF, at the same time, can offer decent accuracy for the estimation of jointing, milk and maturity of winter wheat, which could also be adopted for the identification of green and harvest. In summary, according to Figure 5, SIF datasets demonstrated superior performance compared to EVI datasets in identifying crop growth stages for both maize and winter wheat, especially for the KGSs and winter wheat.