The Modelling Of Three Dimensional Distribution Of Photosynthetically Active Radiation In Maize Canopy

The radiation distribution in crop canopy is an important micro-environmental element conditioning directly or indirectly many aspects of crop growth and crop field ecology. It complicatedly varies three dimensionally (3D) and is influenced by phytometrical characters of the canopy. As the interactions of 3D crop canopy structure and its functions are more and more recognized in modern crop system researches, the simulation models of 3D radiation distribution in crop canopy should be desired in a wide extent, such as physio-ecological modeling of crop (or any kind of plant colonies) production system, interpretation models for vegetation remote sensing and virtual plant models. For modelling of three dimensional distribution of Photosynthetically Active Radiation (PAR) in crop canopy, the important contents of this thesis are as following. (1) Based on 3D digitized crop structure, the facet analyzing method to serve with radiation distribution model had achieved. Based on the principle of geometric optics, a set of models have been developed to simulate the 3D PAR distribution in maize canopy by calculating the light propagation and interceptions among the facets in crop canopy. The Projection-Zbuffer model was constructed to simulate the 3D distribution of direct sunlight and sunflecks in maize canopy. The incident diffuse radiation distribution model was constructed using Turtle algorithm, which simulate the 3D distribution of incident diffuse PAR in crop canopy by calculating the viewable sky rate. The incident total PAR in canopy can be achieved by synthesizing the simulated direct solar radiation and incident diffuse radiation. (2) The 3D visualization model to present the simulation results of the distribution of sunflecks, diffuse PAR and total incident PAR in canopy into 3D images was realized. (3) Some analysis about the distribution characteristics of diffuse and direct solar PAR radiation in maize canopy was realized based on the simulated result. Under sunshine conditions, the PAR conditions in maize canopy are mainly governed by direct solar PAR. The direct solar PAR varies sharply and the diffuse PAR changes much less. (4) A 3D measurement of PAR distribution in maize canopy was realized for 3D model validation. Comparing the measured total PAR and the simulated incident PAR in maize canopy, they agreed consistently very well for vertical variation of PPFD (Photosynthetic Photon Flux Density), PAR transmittance and sunfleck area at different depths. (5) The 3D measurement of PAR distribution in fully developed maize canopy shows that, (a) PAR in canopy extinct with accumulated LAI by exponential law with very low PAR transmittance in canopy bottom. But the extinction coefficient varies evidently with different solar time and weather conditions. (b) PAR transmittance in sun flecks varies very slightly in whole canopy but there are much fewer sun flecks in the bottom of canopy. (c) PPFD and maximum PAR transmittance in deep canopy is badly decreased in cloudy days. The 3D PAR distribution model developed in this study is based on explicit physical principles and precise calculations. The input and output are also determinate variables. It is suitable for any kind of plant, when its 3D structure data are provided, for short wave radiation including PAR and ultra-violet, which have very low reflectance and transmittance in green plant canopy.