玉米冠层内太阳直接辐射三维空间分布的模拟

Modelling the three dimensional distribution of direct solar radiation in maize canopy

太阳直接辐射在植物冠层内的空间分布特征影响植物生理生态功能 ,是衡量植物群体结构是否合理的重要指标。利用田间实测的玉米冠层内植株各器官的三维空间坐标进行冠层结构分析 ,将冠层内的植株器官表面划分成小面元 ;根据几何光学中光的直线传播原理 ,利用面元沿太阳光线的平行投影和投影深度排序 (Z- buffer)算法计算冠层内面元受太阳光直接照射的情况 ,建立了太阳直接辐射在玉米冠层内三维空间分布的模拟模型。模型可计算出作物冠层内任选植株的器官表面或冠层内地面上的太阳直射光斑 (Sunflecks)分布 ,也可输出选定空间位置或范围上的太阳直接辐射的分布 ,同时可实现模拟结果的三维可视化。根据此模型的模拟结果可对太阳直接辐射在玉米冠层内的空间分布进行各种分析。利用玉米冠层内光斑的三维分布测定试验 ,在光合有效辐射 (PAR)波段对模型进行了检验。

The spatial distribution of solar radiation in a plant canopy influences eco-physiological functions and gives a measure of the light-interception efficiency of stand architecture. A simulation model of the three dimensional (3D) distribution of direct solar radiation in a real maize canopy was developed from precise measurements of 3D canopy structure. The plant organ surfaces in a real canopy were subdivided into multiple facets based on 3D coordinate data obtained by digitizing in the field. The model was based on geometrical optics assuming parallel rays of sunlight. A grid plane, S_(top), was defined above the canopy of the measured plot assuming one ray of sunlight for every grid square on S_(top). The parallel projection of all the canopy facets along the ray of sunlight onto S_(top) was then calculated. The closest facet to each grid square on S_(top) was found using a Z-buffer algorithm to sort the facet distances by projection depth. Because one facet could be projected onto a number of grid squares, the sunfleck intensity of every facet was defined as the ratio of its sunfleck grid number to its total projected grid number. The direct solar radiation intercepted by every facet was obtained from the sunfleck intensity multiplied by the direct solar radiation above canopy. From this model, the distribution of direct solar radiation in a maize canopy (i.e. sunfleck distribution) could be computed for anytime of day. The simulation results were output to data files and as 3D images for more advanced analysis of the sunfleck distribution. A field experiment was carried out for model validation. The three dimensional distribution of PAR (Photosynthetic Active Radiation) in a maize canopy was measured using an AccuPAR (Decagon Inc. USA) and a custom machinery support system designed by us. The statistical results show that the simulated sunfleck ratios at different heights in the canopy were consistent with measured PAR. This model is suitable for applying the facet dividing method to any kind of 3D digitized plant stand.