## 5 Results and Discussion
### Ecological Effectiveness
#### 5.2.1 Comparing Daily Values
Season 13 was also exceptional in two other ways: (1) the FAPAR data includes an anomalously low value near the peak of the growing phase (possibly due to incomplete or insufficient cloud screening), while the GPP data suggests a sustained activity throughout the period. The PDSF model, for its part and in its current implementation, appears to be more sensitive to the distribution of data points during the season than other models. However, when the model inversion does work, it tends to yield very good results. Figure 12 shows the non-linear relation between daily simulated FAPAR and daily GPP measurements across all seasons, as represented by each of the indicated models: remote sensing measurements (in this case, of FAPAR) appear quite sensitive to small variations of GPP when the latter is itself low (i.e., at the  
Figure 11: Superposition of multiple time series, from September 2000 to August 2013: the FAPAR data (small circles, non-dimensional), averaged over a 3 by 3 pixel area around the Skukuza flux tower, the four model fits (smooth colored curves, non-dimensional), each adjusted to fit the remote sensing data, and the daily GPP values derived from the in situ Eddy Covariance (EC) measurements (jagged grey trace, in gC m\({}^{-2}\) d\({}^{-1}\)). start an end of the vegetative seasons), as well as tends to saturate during the peak of the growing phase. This finding may need to be confirmed in other locations but suggests that FAPAR is a sensitive indicator of GPP in the range of 0 to 5 g\({}_{\rm C}\,\)m\({}^{-2}\) d\({}^{-1}\) but then saturates and is not usable to estimate larger carbon exchanges. However, Figure 11 also shows why that relation is complex: daily model values for seasons 4 and 5 are very similar, while daily GPP values in season 5 are almost double the values in season 4. The impact of lacking remote sensing evidence on the shape of the model can also be highlighted by comparing seasons 10 and 11: In those cases, the GPP daily values are comparable but the absence of observational constraint near the peak of the season probably resulted in substantial overestimations by the model in season 11. \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline \multirow{2}{*}{**SOS/EOS**} & **Vegetative** & **Season** & **GPP** & **PDGF** & **PDHyTgF** & **PDLF** & **PDSF** & **Combined** \\  & **Season** & **Length** & **\%** & **r** & **r** & **r** & **r** & **r** \\ \hline
2000–08–31 & & & & & & & & \\
2001–09–10 & 1 & 375 & 99.2 & 0.56 & 0.52 & 0.52 & B & 0.56 \\
2002–10–24 & **2** & 409 & 86.8 & 0.72 & 0.78 & 0.78 & 0.74 & 0.72 \\
2003–08–31 & **3** & 311 & 100 & 0.71 & 0.70 & 0.70 & 0.70 & 0.71 \\
2004–09–18 & **4** & 384 & 100 & 0.81 & 0.82 & 0.82 & 0.82 & 0.81 \\
2005–08–20 & 5 & 336 & 94.6 & 0.92 & 0.93 & 0.93 & B & 0.93 \\
2006–07–15 & 6 & 329 & 24.9 & A & A & A & A & A \\
2007–08–10 & 7 & 391 & 41.4 & A & A & AB & AB & A \\
2008–09–06 & 8 & 393 & 45.3 & A & A & A & AB & A \\
2009–09–16 & **9** & 375 & 82.1 & 0.79 & 0.80 & 0.80 & 0.80 & 0.79 \\
2010–08–27 & 10 & 345 & 100 & 0.69 & 0.72 & 0.72 & B & 0.69 \\
2011–09–06 & 11 & 375 & 100 & 0.80 & 0.82 & 0.82 & B & 0.80 \\
2012–09–01 & 12 & 361 & 100 & 0.82 & 0.80 & 0.80 & B & 0.82 \\
2013–08–26 & **13** & 359 & 100 & 0.56 & 0.56 & 0.56 & 0.55 & 0.56 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Correlation coefficients (\(r\)) between the daily GPP and daily FAPAR estimates from the PDGF, PDHyTgF, PDLF, PDSF, and combined models for each vegetative season and for the area around the Skukuza flux tower. Entry A indicates that fewer than half of the expected GPP values were available, while entry B means that the model could not simulate the available FAPAR data. Figure 12: Scatterplot of daily FAPAR estimates (when the PDHyTgF model successfully simulates vegetative seasons) versus daily GPP (g\({}_{\rm C}\,\)m\({}^{-2}\) d\({}^{-1}\)) values when at least half of the expected values are available (i.e., season 2, 3, 4, 9, 10, and 12).