# Estimation of Gross Primary Productivity (GPP)
## Results
### Describing SOS and EOS Using VIs

Table 3: Performance of each vegetation index (VI) and filtering methods (HANTS—Harmonic Analysis of Time Series; Polyfit—polynomial; SavGol—Savitzky–Golay) in simulating GPP time-series for each year and season (SOS—start of season; EOS—end of season), separately (see VI definitions in Table 1). RMSE—root mean squared error, in μmol m\({}^{-2}\) s\({}^{-1}\). Figure 4.— Relationship between the start of the season (SOS), in days, derived from Gross Primary Productivity (GPP) and different vegetation indices. (a) NDVI—Normalized Difference Vegetation Index; (b) EVI—Enhanced Vegetation Index; (c) MCARI—Modified Chlorophyll Absorption Ratio Index; (d) gNDVI—Green Normalized Difference Vegetation Index; (e) MTCI—MERIS Terrestrial Chlorophyll Index; (f) ChIPEdge—Chlorophyll red-edge; (g) SIPI–Structure Issensitive Pigment Index; and (h) PSSR—Pigment Specific Simple Ratio. Different colors represent different methods for deriving SOS, blue = first derivative method; green = percentage 10%; red = percentage 20%; black = percentile 10%; pink = percentile 20%. Symbols show different years: stars (a) are for 2016; circles (a) for 2017 and plus (*) for 2018. Figure 5. Relationship between the end of the season (EOS), in days, derived from Gross Primary Productivity (GPP) and different vegetation indices. (a) NDVI—Normalized Difference Vegetation Index; (b) EVI—Enhanced Vegetation Index; (c) MCARI—Modified Chlorophyll Absorption Ratio Index; (d) gNDVI—Green Normalized Difference Vegetation Index; (e) MTCI—MERIS Terrestrial Chlorophyll Index; (f) ChIPEdge—Chlorophyll red-edge; (g) SIPT—Structure Insensitive Pigment Index; and (h) PSSR—Pigment Specific Simple Ratio. Different colors represent different methods for deriving SOS: blue = first derivative method; green = percentage 10%; red = percentage 20%; black = percentile 10%; pink = percentile 20%. Symbols show different years: stars (\(\triangleright\)) are for 2016; circles (*) for 2017 and plus (+) for 2018. The 20% and 10% percentile were the best threshold for estimating GPP metrics for SOS. The distance between the EOS of GPP and EOS of VIs differed the least using a 10% percentile threshold (Table 5). \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Period** & **Year** & **Filtering Method** & **MTCI** & **EVI** & **NDVI** & **ChlRedEdge** & **MCARI** & **PSSR** & **gNDVI** & **SIPI** \\ \hline SOS & 2016 & HANTS & **0.14** & 5.94 & 9.19 & 7.92 & 6.79 & 4.53 & 9.90 & 36.06 \\  & & Polyfit & 3.68 & 9.33 & 10.32 & 11.03 & 8.34 & 7.50 & 13.01 & 42.00 \\  & & SavGol & 2.55 & 8.77 & 12.02 & 11.31 & 9.62 & 5.23 & 14.42 & 41.72 \\  & & 2017 & HANTS & 2.83 & 3.39 & **0.99** & 1.27 & 3.68 & 17.25 & 1.70 & 42.57 \\  & & Polyfit & 5.94 & 6.65 & 2.40 & 1.13 & 5.23 & 15.13 & 5.94 & 43.13 \\  & & SavGol & 3.54 & 4.81 & 1.41 & 1.70 & 6.36 & 12.59 & 3.11 & 42.43 \\  & 2018 & HANTS & 3.54 & 1.13 & 1.70 & 1.27 & 4.38 & **0.42** & 0.99 & 34.08 \\  & & Polyfit & 2.55 & 3.11 & 3.68 & 3.25 & 2.40 & 1.84 & 2.97 & 34.93 \\  & & SavGol & 3.20 & 3.45 & 3.99 & 3.48 & 3.17 & 1.92 & 3.42 & 33.43 \\ \hline EOS & 2016 & HANTS & 12.02 & 20.36 & 19.66 & 19.66 & 12.02 & 12.87 & 19.37 & 69.30 \\  & & Polyfit & 12.87 & 20.51 & 16.97 & 19.80 & 28.43 & 18.10 & 36.91 & 64.35 \\  & & SavGol & 8.20 & 9.33 & 19.80 & 13.72 & 16.40 & **5.80** & 19.80 & 70.43 \\  & 2017 & HANTS & 15.27 & 4.24 & 22.63 & 21.35 & 9.19 & 17.39 & 19.52 & 70.57 \\  & & Polyfit & 8.34 & 7.35 & 18.81 & 17.11 & **2.97** & 8.20 & 20.65 & 58.83 \\  & & SavGol & 17.25 & 11.46 & 30.12 & 25.31 & 14.57 & 17.54 & 30.12 & 68.59 \\
2018 & HANTS & **4.81** & 9.62 & 15.13 & 12.73 & 7.64 & 27.86 & 26.30 & 52.89 \\  & & Polyfit & 6.22 & 10.32 & 16.55 & 14.42 & 29.56 & 24.32 & 28.00 & 51.34 \\  & & SavGol & 8.74 & 12.81 & 19.86 & 17.31 & 22.60 & 32.02 & 35.58 & 44.49 \\ \hline \hline \end{tabular}
\end{table}