# Estimation of Gross Primary Productivity (GPP)
## Results
### Describing SOS and EOS Using VIs
The results of the same analysis applied to EOS are represented by scatter plots in Figure 5. A good performance in predicting SOS of GPP for all threshold methods and years was obtained for MTCI (Figure 4), points were the closest to the 1:1 line. For MCARI, NDVI, EVI and ChlRedEdge, the first derivative method failed in estimating SOS of GPP in 2016 for all filtering methods (Figure 4). Points for EOS for MTCI and EVI were the closest to the 1:1 line (Figure 5) indicating good performance in predicting EOS of GPP. Except for the first derivative method in 2016 and 2018, MCARI showed good performances too. Among other VIs, NDVI, gNDVI, ChlRedEdge and SIPI showed a clear tendency to predict the EOS later than GPP. The analysis of the perpendicular distance to the 1:1 line confirmed that HANTS and Polyfit were the most suitable filtering methods for SOS, while for EOS it was not possible to identify only one method (Table 4). For SOS, the MTCI showed the minimum distance to 1:1 line (3.10 days (SD \(\pm\) 3)), followed by ChlRedEdge (4.7 days), NDVI (5.08 days) and EVI (5.18 days). However, ChlRedEdge and NDVI showed very high variability (SD \(\pm\) 4.23 and SD \(\pm\) 4.25, respectively) among the different filtering methods (Figure 3 and Table 4). All other VIs showed larger distances, indicative of poor performance in predicting the SOS of GPP phenology. For EOS, MTCI and EVI showed a small distance to 1:1 line (10.41 days (SD \(\pm\) 4.18) and 11.78 days (SD \(\pm\) 5.48), respectively). Bigger distance values were found for other VIs (Table 4). Furthermore, the distance to the 1:1 line was VI-dependent and varied strongly with the filtering method (Table 4). However, the distance analysis confirmed that MTCI predicted well the SOS in 2016 (2.12 days (SD \(\pm\) 1.81)) and the EOS in 2016 and 2018, the year with limiting SWC at the end of the growing season (Figure 3; 11.03 days (SD \(\pm\) 2.49) and 6.59 days (SD \(\pm\) 1.99), respectively). MCARI predicted well GPP at SOS in 2017 (the year of the copping; 1.37 days (SD \(\pm\) 0.29), gNDVI (1.40 days (SD \(\pm\) 0.84) at the SOS in 2018, while NDVI showed the best accuracy at the EOS in 2017 (7.68 days (SD \(\pm\) 2.49; Figure 3, Table 4). \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**RMSE**} \\ \cline{2-10}
**VI** & \multicolumn{4}{c}{**2016**} & \multicolumn{4}{c}{**2017**} & \multicolumn{4}{c}{**2018**} \\ \cline{2-10}  & **HANTS** & **Polyfit** & **SavGol** & **HANTS** & **Polyfit** & **SavGol** & **HANTS** & **Polyfit** & **SavGol** \\ \hline _SOS_ & & & & & & & & & \\ NDVI & 3.23 & 1.75 & 2.32 & 0.12 & 0.30 & 0.13 & 1.31 & 1.72 & 1.65 \\ EVI & 1.98 & 2.07 & 1.59 & 0.52 & 0.90 & 0.76 & 0.85 & 1.22 & 1.19 \\ MTCI & 0.09 & 0.49 & 0.41 & 0.49 & 0.82 & 0.53 & 0.92 & 0.70 & 0.74 \\ ChlRedEdge & 2.32 & 2.36 & 1.55 & 0.36 & 0.11 & 0.24 & 0.74 & 1.15 & 1.06 \\ MCARI & 3.23 & 3.14 & 2.73 & 0.60 & 0.80 & 0.97 & 1.09 & 0.66 & 0.74 \\ PSSR & 1.42 & 1.28 & 0.83 & 4.03 & 2.20 & 2.96 & 0.80 & 1.18 & 1.14 \\ SIPI & 0.43 & 0.27 & 0.55 & 1.11 & 1.35 & 1.21 & 2.30 & 2.67 & 2.20 \\ gNDVI & 3.76 & 5.65 & 3.98 & 0.20 & 0.62 & 0.68 & 1.09 & 1.49 & 1.51 \\ _EOS_ & & & & & & & & & \\ NDVI & 9.50 & 5.16 & 7.03 & 6.73 & 4.85 & 6.56 & 4.80 & 3.85 & 4.16 \\ EVI & 5.65 & 6.48 & 6.14 & 1.12 & 1.21 & 1.95 & 2.84 & 2.01 & 2.20 \\ MTCI & 5.97 & 4.30 & 2.74 & 3.68 & 2.78 & 3.40 & 1.69 & 1.80 & 2.15 \\ ChlRedEdge & 8.69 & 8.91 & 3.72 & 5.08 & 3.92 & 4.92 & 2.30 & 2.32 & 2.23 \\ MCARI & 8.14 & 11.14 & 9.72 & 2.50 & 1.86 & 3.48 & 1.99 & 2.26 & 2.15 \\ PSSR & 4.78 & 3.17 & 2.95 & 5.89 & 3.46 & 5.16 & 8.81 & 7.38 & 7.33 \\ SIPI & 5.94 & 3.05 & 3.18 & 3.31 & 2.85 & 2.92 & 8.24 & 8.27 & 7.98 \\ gNDVI & 9.88 & 14.33 & 13.47 & 4.66 & 5.53 & 6.54 & 5.82 & 4.67 & 5.19 \\ \hline \hline \end{tabular}
\end{table}