# Validation of Sentinel-2, MODIS, CGLS, SAF, GLASS and C3S Leaf Area Index Products in Maize Crops
## 5 Discussion  
This validation exercise of decametric, hectometric and kilometric LAI products focus on the assessment of time series over maize crops. The selected area of 5 \(\times\) 5 km is flat and relatively homogenous, but the maize varieties planted in the different ESUs are different. The types of maize under study should be the same in future experiments to facilitate upscaling. The spatial homogeneity is an important factor to be considered for the validation of multi-resolution LAI products because the estimation of LAI is scale dependent due to the strong nonlinearity of LAI with reflectance [34; 67]. The small fluctuations in the field-measured time series close to the green peak (Figure 3) were also reported in the earlier paper by [68]. There are two potential factors explaining the fluctuations: (1) The changing weather conditions--LAINet uses multiple observations of direct solar light to construct hemispheric gap fraction, and then calculate LAI based on Beer Lambert law [35]. Therefore, the daily variation of the proportion of scattered sky light will cause fluctuation in the time series, specially under partial cloud cover conditions. (2) The crop management activities, e.g., weeding, irrigation and fertilization, may result in a prompt LAI change. We only used LAINet data for the first half of the growing season given the different definitions of LAI considered to calculate Sentinel-2 LAI (only green elements of the canopy) and LAINet LAI (all elements of the canopy), both photosynthetic active and inactive). The \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **SoS** & **PoS** & **EoS** \\ \hline Reference LAI & 174 & 207 & 252 \\ MODIS LAI & 172 (\(-\)2) & 206 (\(-\)1) & 244 (\(-\)8) \\ GEOV2 LAI & 173 (\(-\)1) & 208 (\(+\)1) & 256 (\(+\)4) \\ GEOV3 LAI & 172 (\(-\)2) & 208 (\(+\)1) & 255 (\(+\)3) \\ EPS LAI & 180 (\(+\)6) & 227 (\(+\)20) & 276 (\(+\)24) \\ GLASS LAI & 173 (\(-\)1) & 216 (\(+\)9) & 256 (\(+\)4) \\ C3S V2 LAI & 170 (\(-\)4) & 201 (\(-\)6) & 246 (\(-\)6) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Timing (day of year, DOY) for the start of season (SoS), the peak of the growing season (PoS) and the end of season (EoS) for the different LAI products. In parenthesis, the bias in days as compared to the reference LAI phenology is indicated. Figure 8: Time series of reference LAI, MODIS, GEOV2, GEOV3, EPS, GLASS and C3S V2 LAI products. The shaded gray area represents the uncertainty of reference LAI. Sentinel-2 LAI before the peak of the growing season (DOY 211) is highly consistent with LAINet LAI (Figure 5b) and clearly improved the performances reported in the previous study [22]. Extending the validation to the entire growing season including senescent period of maize would require having access to ground data from downward looking instruments to account only green elements. Cross-validation reveals that the proposed up-scaling approach based on the recalibration of existing decametric S2 LR Sentinel-2 LAI product with LAINet measurements improved the standard CEOS empirical up-scaling approach. It may constitute an alternative when low ground measurements are available. This methodology may be also applied to other land cover types, but it is limited to the validity of the PROSAIL model used in SL2P algorithm which assumes a turbid medium. In other conditions and over forest areas, in particular, a specific training with a more adapted radiative transfer model would be required. The MODIS, GEOV2 and GEOV3 LAI products perform the best among all the validated hetometric and kilometric products in our study in terms of \(RMSE\) (\(RMSE\) = 0.21, 0.22 and 0.29, respectively) and correlation with reference LAI (\(R^{2}\) = 0.97, 0.98 and 0.96, respectively). The MODIS LAI time series show some gaps close to the maximum peak of the growing season mainly due to cloud and other poor atmospheric conditions [69; 70], which were filtered out through the quality control procedure [47].