Specular and anisotropic reflectance effects on the relationship between remotely sensed data and wheat canopy chlorophyll concentration

Considerable variations have been noted in the relationship between remotely sensed data and canopy chlorophyll concentration. This paper investigates two sources of variation; specular and anisotropic reflectance. Reflectance spectra and polarised reflectance spectra were collected for a wheat canopy and both the NDVI and red edge position derived. Strong correlations were observed between both measures of reflectance and canopy chlorophyll concentration, however, there was considerable variation in the strength of the relationships with change in view zenith angle. Partial correlation analysis revealed the cause of this to be significant change in the proportion of illuminated canopy viewed at each view zenith angle. A ‘minus specular’ version of the NDVI was generated but found to have little impact on its relationship with chlorophyll concentration at all view zenith angles. These results have implications for future sensor design and methodologies, as well as contributing to our understanding of radiation/canopy interactions.