## 4 Discussion
### Overall Discussion
Therefore, it seems that, in Mediterranean grasslands, the hypothesis that FAPAR and \(\varepsilon\) are synchronized and related both with spectral indices, in our case NDVI, is also fulfilled. This fact is due to the patterns of Mediterranean grasslands that are conditioned by the dynamics of the Leaf Area Index, which depends on environmental variables, such as SWC [71; 72]. However, in this research, the correlation between eLUE and MODIS NDVI was much lower (R\({}^{2}\) = 0.35) than the one obtained by [PERSON] et al. [27] between eLUE and MODIS EVI in savannahs (R\({}^{2}\) = 0.81). The reason was probably due to the different ecosystems that have been compared. In fact, in one type of semi-arid Savanna (Acacia woodland) studied by [PERSON] et al. [27], this relationship was also lower. The Acacia semi-arid woodland is more similar to our Mediterranean grassland ecosystem with small bushes, trees, and herb species than the other types of studied savannahs. In this type of savannah, the eLUE model failed to capture the maxima and the minima of the GPP, but in our eLUE MODIS model 1, the model infra-estimated the values during all the study period (GPP_M1, Figure 4). The model improved clearly (with a lower RMSE) when SWC and T\({}_{\text{min}}\) were considered (GPP_M2, Figure 6). Pasture phenology was linked to the dynamics of soil water availability, water being the main limiting factor during the growing cycle. Thus, during summer, annual herb species died, and only green trees and bushes could contribute to GPP due to their longer roots that could absorb water from deeper layers of the soil [73; 74]. This fact was the reason for a value greater than 0 g C m\({}^{-2}\) day\({}^{-1}\) during this season. Therefore, the inclusion of SWC in the model helped to capture the minima during summer. This factor was also relevant in the savannahs of Australia, especially in the semi-arid ones [27; 75]. In the Mediterranean climate, it is known that SWC is an essential parameter of GPP (Figure 2) [72; 76]. On the other hand, temperature is only a limiting factor during winter [31; 77], although some species could be active during this season [61], but the inclusion of this factor in the model contributed to capturing the GPP minima during winter. In addition, both factors affected the capture of the maximum GPP during spring. The ability of Sentinel-2 indices to model the GPP of grasslands has been demonstrated. [PERSON] et al. [78] also concluded that several Sentinel-2 vegetation indices, especially those based on red-edge data, followed the phenological variations of two grasslands in Australia. The higher temporal (5 days) and spatial resolution (10 m) of Sentinel-2 have served to improve the model results. In fact, only by increasing these resolutions did the GPP_S1 adjust better than the MODIS GPP_M1. In this sense, [PERSON] et al. [35] showed that the combination of high spatial and temporal resolution sensors, such as Sentinel-2, improved biomass estimation in grassland ecosystems. Besides, [PERSON] et al. [34] also showed the potential of Sentinel-2 compared with other remote sensing data to assess the phenology of ecosystems. In addition, [PERSON] et al. [79] showed that Sentinel-2 has a positive impact on land cover/use monitoring due to its higher temporal and spatial resolution. In this work, the main wind directions of the satellite's overpassed time were taken into account when selecting the pixels for the study. This footprint estimation approach was done due to the location of the flux tower, which is influenced by mesoscale thermally driven flows (mountain breezes) due to the presence of the Guadarrama Mountains, located quite close to the station [48; 49]. According to these studies, the dynamics of the planetary boundary layer and these breezes influenced the CO2 mixing ratio of the study area. Therefore, it was important to consider the main direction of wind when selecting the Sentinel-2 pixels that would influence the observed and estimated GPP. In this sense, [PERSON] et al.