Evaluating a three dimensional model of diffuse photosynthetically active radiation in maize canopies.

Diffuse photosynthetically active radiation (DPAR) is important during overcast days and for plant parts shaded from the direct beam radiation. Simulation of DPAR interception by individual plant parts of a canopy, separately from direct beam photosynthetically active radiation (PAR), may give important insights into plant ecology. This paper presents a model to simulate the interception of DPAR in plant canopies. A sub-model of a virtual maize canopy was reconstructed. Plant surfaces were represented as small triangular facets positioned according to three-dimensionally (3D) digitized data collected in the field. Then a second sub-model to simulate the 3D DPAR distribution in the canopy was developed by dividing the sky hemisphere into a grid of fine cells that allowed for the anisotropic distribution of DPAR over the sky hemisphere. This model, DSHP (Dividing Sky Hemisphere with Projecting), simulates which DSH (Divided Sky Hemisphere) cells are directly visible from a facet in the virtual canopy, i.e. not obscured by other facets. The DPAR reaching the center of a facet was calculated by summing the amounts of DPAR present in every DSH cell. The distribution of DPAR in a canopy was obtained from the calculated DPARs intercepted by all facets in the canopy. This DSHP model was validated against DPAR measurements made in an actual maize (Zea mays L.) canopy over selected days during the early filling stage. The simulated and measured DPAR at different canopy depths showed a good agreement with a R (2) equaling 0.78 (n=120).