SHADOW-BAND CORRECTIONS FOR PHOTOSYNTHETICALLY-ACTIVE RADIATION UNDERCLEAR AND OVERCAST CONDITIONS

The diffuse component to the incident photosynthetically active radiat ion (PAR) is commonly estimated using a shadow-band to obscure the sol ar disk from the measurement sensor. The obstructed fraction of the sk y radiation is either ignored or estimated using the shadow-band corre ction factors existing in the literature. A prior work reported on equ ations developed to describe the PAR sky radiance under overcast and c lear sky conditions. Here we seek to use these distribution functions to determine shadow-band correction factors specific to the PAR. Numer ical integration of the shadow-band exclusion of the sky radiance was made for a range of latitudes, shadow-band dimensions, and sky conditi ons. Results showed that the correction factors in the PAR are typical ly 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-ban d dimensions. Correction coefficients varied annually to a greater ext ent than diurnally. The correction coefficients for clear sky conditio ns based on the PAR anisotropic sky radiance distributions were typica lly 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 co rrection coefficients. The isotropic PAR correction coefficients best estimated the shadow-band measurement error for clear sky conditions w hile the anisotropic PAR correction coefficients best estimated the er ror for translucent overcast skies. The error in diffuse radiation est imation after correction of the shadow-band measurements can be reduce d to approximately 1-2% for both sky conditions across the solar zenit h range of 20-70 degrees. (C) 1997 Elsevier Science B.V.