*4.1. BRDF Parameters Relationships*

The V parameter shows good correspondence with the NDVI, which is expected considering that it models the volumetric component of vegetation. Therefore, it is not surprising that we find a relationship between them. The R parameter can be physically interpreted as the aerodynamic roughness [29]. The relationship between the aerodynamic roughness and the NDVI is not that evident, in fact, we didn't find any meaningful relationship between the parameters. Given a certain pixel and its geometry, it's reasonable to think that for higher amounts of vegetation, the aerodynamic roughness is bound to change. In fact, Franch et al., studied the possibility of a quadratic evolution of R with the NDVI within a pixel [30].

Considering that the R parameter is associated with a geometrical component, one would expect it to be independent of the spectral band. Experimental results have shown that this isn't true and that there is a difference between both values for medium to high NDVI values [25,31,32]. We did find, nevertheless, a good relationship between the R parameter for both bands, which have a slope relatively close to 1 and an intercept close to 0. For low NDVI values, the R parameters are independent of the spectral band. This was also the case for V parameters with NDVI < 0.1, which as far as we are concerned, has never been observed in previous literature.
