# Validation of Sentinel-2, MODIS, CGLS, SAF, GLASS and C3S Leaf Area Index Products in Maize Crops
## 5 Discussion
The MODIS LAI is generated based on a simple compositing approach of the daily LAI estimates in an 8-day window whilst the other analyzed products use more elaborated compositing techniques either at the level of LAI estimates (GEOV2 and GEOV3) or input reflectances (EPS, GLASS and C3S V2) with longer and adaptive temporal windows as described in Section 2.2. The similarity of GEOV2 and GEOV3 time series can be explained because the two versions of products are retrieved from the same sensor (PROBA-V) and preprocessing chain, and both are based on the use of a similar NNs retrieval approach and smoothing temporal filters and compositing [51]. The GLASS product showed the smoothest temporal evolution but clear artifacts in terms of phenology (Figure 8) with a delay in the second half of the growing season after the peak of LAI as compared to the reference time series. This may be introduced by an over-smoothing in the reprocessed MODIS reflectances which are used as input of the GLASS algorithm (Section 2.2.6). The EPS product showed clear deficiencies to reproduce the seasonality of reference LAI time series with a delay in its phenology (Figure 8) and an underestimation for LAI > 3 (Figure 7). The underestimation of EPS LAI product may indicate an early saturation of the retrieval algorithm [11]. The C3S V2 LAI product also showed systematic differences with reference LAI both for the seasonality: advanced phenology, and LAI magnitude: systematic underestimation of the reference LAI values for the entire range 0-3.5 LAI and especially for low LAI values (<0.5) at the start of growing season and high LAI values (>3.0) at the peak of growing season and after senescent (Figure 8). [PERSON] et al. [71] attributed the high uncertainty of LAI derived from TIP model for the small and high values to the observational uncertainties and the saturation effects, respectively. Note, however, that the GEOV2 and GEOV3 products derived from the same PROBA-V data, as C3S did not show these artifacts and they improved the C3S V2 product when compared with the reference LAI both in terms of the LAI magnitude and phenology. The hetometric and kilometric LAI products showed an earlier SoS as compared to the reference LAI from decametric Sentinel-2. This may be explained by the spatial heterogeneity and the presence of species with earlier growing seasons in the study area. The main planting crop in the study area is maize, but since it is planted by individual farmers, it is inevitable to be covered by other crops planted in a small range or weeds. The coarse resolution satellites may result more sensitive than high resolution satellites to the presence of species with an earlier SoS [72]. The comparison between LAI products and with reference LAI time series show important differences in terms of phenology with apparent limitations for EPS, C3 SV2 and GLASS which exhibit systematic differences in the timing of phenological metrics as compared to other products and reference LAI. However,-phenological ground measurements are not available in our study area. Further validation and comparison with ground-based phenology will be conducted in future studies to directly validate the satellite phenological metrics.