利用夜间热红外数据求取玉米冠层孔隙率

Investigating the Gap Frequency of a Maize Canopy Using Night TIR Data

孔隙率是光线透过冠层未被拦截到达地表的概率,是描述冠层结构和生物量空间分布的关键变量。已有研究证明,在夜间热红外图像上,玉米地各亮温组分比较稳定,并且温度均值有明显差异,易于区分。本研究的目的是利用2005年8月23日在怀来测得的夜间热红外数据求取中等密度的玉米冠层(LA I=3.64)在不同角度下的孔隙率。结果表明,在沿垄行方向,孔隙率随着天顶角变化缓慢;而在其他方向(45°和90°)变化较快,且变化趋势相似。将测得结构参数代入余涛的热红外方向辐射性模型,模拟出不同方向下的孔隙率。将测量值与模拟值进行对比,二者有一定的相似性,模型能较好地抓住玉米冠层孔隙率的主要特征。针对二者间的差异,初步分析是由测量中观测目标不完全一致、角度控制不精确、测量高度过低以及投影函数和丛生参数的选择等原因引起的。

Gap frequency is the probability that a light unintercepted penetrates through the canopy and reaches the surface under the vegetation, and it is a key variable describing canopy structure and biomass spatial distribution. Previous study revealed that in the night TIR images, the brightness components of maize field was stable and easy to be distinguished. The objective of the research is to investigate the directional variation of gap fraction over a maize canopy（ LAI = 3.64 ） by night thermal infrared （TIR） experimental data which was acquired on August 23rd, 2005 in Huailai county of Hebei Province. The results showed that very little azimuth variation appears for gap frequency except for the observations along row direction, gap frequency declines from the nadir slowly; in other azimuth directions, gap frequency declines sharply. The measured parameters were input into a GORT model, and the gap fractions were simulated at different angles. By comparing the simulated gap frequency with the measured one, the two had good agreement. The model caught the main features of the measured gap fraction of maize canopy and reflected the characteristics of row crop. The differences between them were due to some reasons, such as the disagreement of the objects ; the angle variation was not controlled accurately in experiment; the measurement height was too low; the selection of projection function and clumping parameter et al.