## 5 Discussion
### Gsa  
Although earlier sensitivity studies have studied aspects of coupled leaf-canopy-atmosphere models along the spectral range e.g., [18; 92; 16], these are \"local\" studies sensitivity studies, i.e., keeping the majority of variables fixed. With \"global\" sensitivity analysis (GSA), all variables are ranged at the same time and interactions are calculated. Variance-based GSA proved to be a powerful tool to determine and study the main drivers that govern TOA radiance as observed by a sensor. This becomes extremely relevant in identifying retrievable biophysical variables [2; 93]. In principle, a high sensitivity value indicates that the input variable is responsible for a significant portion of the output variance and should thus be possible to retrieve e.g., as recorded by an Earth observing satellite. To the best of our knowledge, this is the first time that a GSA was used to decompose the full leaf-canopy-atmosphere radiative transfer of TOA radiance into their driving variables along the 400-2500 nm spectral range at 1 nm resolution. The most remarkable GSA result is the relatively small contribution of the atmospheric variables driving the TOA radiance variance. This indicates that the contribution of vegetation variables is much more important than the contribution of atmospheric variables. In view of mapping applications, it implies that the retrieval of biophysical parameters from TOA radiance should be certainly possible. Moreover, small inaccuracies in the atmospheric data do not affect the sensitivity of the vegetation variable in the TOA radiance [11]. Regarding the atmosphere drivers, H\({}_{2}\)O concentration is the most dominant variable, but it only appears in the water vapor absorption bands; its presence outside these bands appeared negligible as opposed to other drivers. In general, optical sensors do not consider these water vapor absorption bands for biophysical variable retrieval. O\({}_{3}\) concentration has no effect in the GSA, neither at the Chappuis band (400-650 nm), where O\({}_{3}\) has it absorption bands. Outside the water vapour absorption bands, aerosol optical thickness (AOT) is the atmospheric variable with the strongest impact on TOA radiance. Its importance is especially relevant at the lower part of the spectrum (400-500 nm) as combination of high aerosol absorption/scattering and low surface reflectance. This suggests that the retrieval of biophysical parameters would be more feasible in clear atmospheric days and further away in the spectrum. These results can be related to the ones found by [11; 93], who observed that the sensitivities of surface reflectance are comparable to the TOA radiance sensitivities, which implies that atmospheric variables have a rather weak influence in driving variability of the TOA radiance data. Obviously, this is only valid given a cloud-free sky. Both leaf and canopy variables drive the TOA radiance along the 400-2500 nm spectral range outside the water vapour absorption bands. The leaf variable that has the greatest contribution to the TOA radiance spectrum explains more than 50% of the variance in the whole spectrum. Chlorophyll content (Cab) has a dominant impact in the visible region but disappears throughout the red edge as the wavelengths become too large for chlorophyll absorption [94]. Dry matter content (Cm) is dominant in the NIR (750-1200 nm) and water content (Cw) in the SWIR (1400-500 nm). The results are very similar to the ones found by [93]. Regarding the canopy variables, the most important one was the leaf area index (LAI), especially in the visible range and SWIR, and less important in the NIR, also found in other studies [93; 32]. A limitation in the conducted study is that soil brightness coefficient was not included in the GSA. As demonstrated in earlier studies [91], this variable also exerts some influence of a few percent, about equally spaced along the spectral range. The variables that shows hardly any sensitivity, e.g., N, can be safely kept to default values in order to simplify and speed up the GSA [91].