In Field Assessment on the Relationship between Photosynthetic Active Radiation (PAR) and Global Solar Radiation Transmittance through Discontinuous Canopies

Ricardo Oyarzún12*, Claudio Stöckle1, Joan Wu1, and Matthew Whiting3

In many crop models, the process of radiation transmittance through the canopy is normally described as an exponential attenuation process (Beer’s Law equation), which is assumed to be valid for canopies covering the ground with a random spatial distribution of leaves. However, for discontinuous canopies, where a distinctive row pattern of plant exists, there is a more complex situation because of the presence of gaps between individual plants. This must be accounted for when characterizing radiation relationships for these kinds of systems, in particular when short time-scales are of interest. Photosynthetically active radiation (PAR) transmittance (t) is more commonly studied and reported than global solar radiation (Sg) transmittance (tSg). However, both PAR and Sg are important in radiative transfer sub-models used in plant growth simulation. In this work simultaneous measurements of tSg and tPAR under discontinuous canopies were performed, and the hourly changes in radiation transmittance for PAR and global solar radiation were characterized. Two methods were assessed to transform between tSg and tPAR. The two methods yielded similar results for low values of transmittance, but disagreement occurred for higher values of transmittance. The method based on a fixed value for the ratio of extinction coefficients for PAR and Sg outperformed the method based on a linear relationship between PAR and tSg with average relative errors (RE) of 7.97% vs. 13.29% and 2.84% vs. 7.77% for hourly and daily time-scale, respectively.

Keywords: radiation interception, extinction coefficient, crop simulation model.
1Washington State University, Biological Systems Engineering Faculty, Pullman, Washington, 99163, USA.
2Universidad de La Serena, Facultad de Ingeniería, Benavente 980, La Serena, Chile. Corresponding author (royarzun@userena.cl) 3Washington State University, Irrigated Agriculture Research and Extension Center, Prosser, Washington, 99350, USA.