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RESEARCH PRODUCT

Solar angle and sky light effects on ground reflectance measurements in a citrus canopy

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Abstract Ground radiometry was used to gather spectral data from different targets of a citrus canopy, in order to analyze the effect of solar zenith angle and proportion of diffuse radiation on spectral reflectance. Results have shown that the variation in solar angle causes significant changes in nadirsensed reflectance from vegetation, which exhibits a marked diurnal pattern with a minimum slightly shifted from the solar noon. This fact is more noticeable in the near-infrared and middle-infrared regions of the spectrum. Furthermore, the visible part of the spectrum has resulted in being highly influenced by the diffuse radiation incident on the canopy, which has been quantified by two different physical parameters: the proportion of diffuse irradiance kd and the sky clearness ϵ. It has been shown that the reflectance factor increases linearly with increasing diffuse radiation, but only below a threshold value, above which the reflectance remains constant. On the other hand, the reflectance dependence on the parameter ϵ has allowed us to identify three well-defined zones of sky light conditions, in which reflectance presents a different behavior.