A comparison of three different canopy radiation models commonly used in plant modelling.

Three radiation models are compared. They are the two-stream approximation, Goudriaan's radiation model and Beer's law. If direct beam and diffuse radiation are considered separately with the appropriate extinction coefficients, Beer's law can be used to estimate the fraction of absorbed visible radiation quite accurately, as compared with the other two models. However, Beer's law always overestimates the fraction of absorbed near-infrared radiation because of the significant loss of scattered radiation in the near-infrared radiation waveband; the error can be up to 50%, compared with the two-stream approximation. The error is systematic and is not reduced by integrating over a day or longer. It can be even larger if absorption of direct beam and diffuse radiation are not considered separately. Compared with the two-stream approximation, Goudriaan's model underestimates the amount of absorbed visible and near-infrared radiation, and consequently net photosynthesis, and overestimates sensible heat fluxes of the canopy when Goudriaan's model is used to estimate the amount of radiation absorbed by sunlit and shaded leaves within a canopy in the CSIRO Biosphere Model. However, the mean differences in the calculated hourly fluxes of net canopy photosynthesis, latent and sensible heat are less than 5% for a wheat crop during the whole growing season.