## 4 Discussion  
### Overall Discussion  
In this research, the assessment of the dynamics of GPP and the estimation of a GPP model through remote sensing data for a Mediterranean grassland of Central Spain has been carried out. The dynamics of the GPP from an eddy covariance flux tower have been studied during three growing seasons (2018-2020). As it is observed (Figure 2), GPP_T \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Model** & **R\({}^{2}\) (2020)** & **RMSE (2020)** & **R\({}^{2}\) (TS)** & **RMSE (TS)** \\ \hline MODIS\_GPP & 0.81 & 2.49 & 0.80 & 2.24 \\ GPP\_M1 & 0.78 & 2.66 & 0.74 & 2.78 \\ GPP\_M2 & 0.75 & 1.42 & 0.87 & 0.94 \\ GPP\_S1 & 0.87 & 0.99 & 0.89 & 0.93 \\ GPP\_S2 & 0.87 & 1.06 & 0.90 & 0.85 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Coefficient of determination (R\({}^{2}\)) and Root Mean Square Error (RMSE) of the five models studied in the Mediterranean grassland for the year 2020 and for all the time series (TS). Figure 16: Values for the Gross Primary Production Sentinel-2 multiple linear regression model (GPP_S2) versus the tower Gross Primary Production (GPP_T) for the year 2020 (January to July) in the Mediterranean grassland of La Herreria (\(n\) = 25, R\({}^{2}\) = 0.87, ANOVA \(p\)-value = 0.00). depends strongly on SWC having two growing seasons during the year, the main growing season during spring and a secondary growing season during autumn. On one hand, the length and the maximum of the spring season depend on the SWC [60; 61]. In fact, during 2018, the growing season was the maximum, and it was extended until mid-June due to the precipitation in May, which entailed an increase in SWC. On the other hand, the maximum of autumn also depends on the recovery of SWC, and the extension depends on the arrival of the minimum temperatures at the end of autumn [31; 62]. Results showed that MODIS_GPP underestimated GPP throughout the study period except for the minima of summer periods in 2018 and 2019. However, MODIS_GPP was able to capture the seasonal and interannual dynamics of GPP well. Although the MODIS GPP product has been improved in recent years, the sources of error of this product could be the spatial resolution that is not enough to assess finer ecosystem processes [63; 64] or the parameters used in MODIS GPP [65]. The proposed models were based on the eLUE approach proposed by [PERSON] et al. [27] for different savannah ecosystems in Australia, using NDVI instead of EVI. [PERSON] et al. [66] concluded that NDVI showed problems of saturation and a lower dynamic range over the more humid forested sites. On the other hand, they showed that NDVI could present a higher range in values over the semiarid sites, as in the case of our study area. In fact, we obtained the Sentinel-2 EVI time series for our pixels, and it presented more noise than the time series of NDVI. Therefore, the use of NDVI instead of EVI was justified in our case. However, EVI should be considered in other ecosystems due to its higher sensitivity to canopy structural variations and improved resistance to atmospheric conditions [66; 67]. Further research is needed to evaluate the use of other indices in the models, such as the Normalized Difference Water Index (NDWI) [68] or Spectral Shape Indices (SSI) [69]. PARTOA was used to compare the same model proposed by [PERSON] et al. [27] in Australian savannahs in Mediterranean grasslands. This research has already discussed the use of PARTOA instead of the Photosynthetic Active Radiation from the top of the canopy (PARTOC), and they have obtained better results with PARTOA. Therefore, the use of PARTOA in this research was justified mainly for two reasons: (1) To apply and compare the same model as [PERSON] et al. [27] in our Mediterranean grassland, (2) we do not have the measure of PARTOC in our site. In this work, the eLUE approach is valid for a Mediterranean grassland ecosystem. Firstly, in Mediterranean ecosystems, the light use efficiency conversion coefficient (\(\varepsilon\)) shows a strong seasonality [31; 70] as in savannahs.