## 5 Discussion  
This study provides an evaluation of the effectiveness of individual Sentinel-2A MSI bands and spectral indices for discriminating burned areas. It supports the findings of previous studies that the NIR provides high burned-unburned discrimination in a variety of ecosystems including, for example, boreal forest [23; 78; 79], savanna [23; 75; 80] and temperate forest [36]. As observed in other studies, the visible bands provide low burned-unburned discrimination [23; 75; 81; 82]. This is primarily because the visible reflectance spectra of non-photosynthetic vegetation is similar to the spectra of burned vegetation and also to soil that may be revealed post-fire [12; 83; 12]. Few studies have considered the red-edge wavelength region for burned area mapping because it is not present on sensors commonly used for fire monitoring. The two MSI red-edge bands demonstrated relatively high burned-unburned discrimination and this was previously observed using Medium Resolution Imaging Spectroradiometer (MERIS) data [84]. Given the performance of the Sentinel-2 MSI red edge bands, which will also be present on the Sentinel-3 Ocean and Land Colour Instrument (OLCI) sensor [74], the results of this study suggest they could be combined, for example, by an efficient model inversion [85]. The performance of the NDVI and NBR spectral indices considered in this study was not generally comparable with the better performing spectral bands in the sense that the spectral index separabilities were more variable among the five sites. The majority of previous studies concerning the suitability of spectral indices for burned area mapping have been specific to a given biome or land cover type. As stated in [71; 86] (boreal forest), [77] (savanna), [87] (grass-shrub), the performance of NDVI is limited by vegetation senescence or growth, and the NDVI has been shown to provide poor discrimination in savannas [75; 76; 77]. The NBR was developed originally for burned area mapping [33], and not for satellite-based assessments of fire/burn severity although it is used widely for this purpose [88; 89; 90; 91]. The MSI NBR provided comparable or higher separability than the NDVI and the NBR has been observed by others to provide high burned-unburned discriminative capability, e.g., [33] (mediterranean forest), [34] (boreal forest), [80] (savanna), [92] (heatland). The reported separabilities for the spectral indices were likely more variable among the five sites than for the individual bands because the spectral indices combine bands from spectral regions with different sensitivities to factors including the vegetation type and condition, the soil background, atmospheric contamination, and the degree of char and ash deposition [14]. Therefore, caution in the interpretation of the exact cause of the different separability results is necessary, particularly given the complexity of the post-fire surface trajectory discussed in the introduction. This study only considered the effectiveness of individual Sentinel-2A MSI bands and spectral indices for discriminating burned areas in a radiometric sense. Further work to consider the relative advantages of the 10 m bands, particularly the 10 m NIR band, for improved spatial resolution burned area mapping is recommended, although we note that the 20 m and 10 m bands have different wavelengths and so are not directly comparable. Burned area mapping algorithms usually take advantage of the discriminative power of multiple spectral bands, for example, by thresholding spectral indices. In this study, we reported separability analyses in the conventional manner with respect to single indices and bands. This provides meaningful insights into which individual indices and bands are appropriate for burned area mapping. The effective use of multiple bands and/or indices for burned area mapping is dependent on the algorithm used. The design of algorithm-specific experiments is beyond the scope of this study, but is a recommended subject for future research.