Shadow-band corrections for photosynthetically-active radiation under clear and overcast conditions

Abstract

The diffuse component to the incident photosynthetically active radiation (PAR) is commonly estimated using a shadow-band to obscure the solar disk from the measurement sensor. The obstructed fraction of the sky radiation is either ignored or estimated using the shadow-band correction factors existing in the literature. A prior work reported on equations developed to describe the PAR sky radiance under overcast and clear sky conditions. Here we seek to use these distribution functions to determine shadow-band correction factors specific to the PAR. Numerical integration of the shadow-band exclusion of the sky radiance was made for a range of latitudes, shadow-band dimensions, and sky conditions. Results showed that the correction factors in the PAR are typically smaller than reported in the literature for the entire solar short-wave band. The clear sky and overcast sky correction coefficients were influenced by the declination, solar zenith angle, and the shadow-band dimensions. Correction coefficients varied annually to a greater extent than diurnally. The correction coefficients for clear sky conditions based on the PAR anisotropic sky radiance distributions were typically smaller than those computed assuming isotropic sky radiance. While the overcast sky radiance distribution was developed for two classes of overcast conditions, there was negligible difference between the correction coefficients. The isotropic PAR correction coefficients best estimated the shadow-band measurement error for clear sky conditions while the anisotropic PAR correction coefficients best estimated the error for translucent overcast skies. The error in diffuse radiation estimation after correction of the shadow-band measurements can be reduced to approximately 1–2% for both sky conditions across the solar zenith range of 20–70°.