双层植被结构冠层光谱特性的理论模拟

Theoretical Simulation of Vegetation Canopy's Optical Characteristics with Two-layer Structure

利用双层冠层反射率模型ACRM,模拟不同叶面积指数LAI、含水量Cw和结构参数N下,波长是820nm和1600nm冠层反射率的角度分布。结果说明,该模型能准确模拟出“热点”效应;冠层反射率角度分布对LAI的敏感性小于LAI和Cw以及LAI和N的共同作用。其中LAI和Cw共同作用对波长1 600 nm反射率角度分布的影响非常显著,而LAI和N的共同作用在820 nm略微大于1600 nm。另外,提取冠层含水量的土壤可调节水分指数SAWI受冠层结构的影响也较大。今后在模型选取中应该更好的考虑冠层结构影响。

Several vegetation canopies have a two-layer structure. In forests a moss or lichen layer is on ground surface under grass layer. In field crops a thin weed layer on ground under crop canopy is rather common. Both, optical and structural parameters of these two layers may be rather different and if we use homogeneous canopy reflectance （CR） models for the calculation of the directional reflectance of such canopies using mean （weighted mean） values of phytometrical and optical parameters we may have systematic errors in reflectance values. In this article, based on the two-layer canopy reflectance model ACRM, canopy reflectance angular distributions with wavelength 820 nm and 1 600 nm are simulated under the influence of LAI （leaf area index）, Cw （water content）and N （a structural parameter）. Research results indicate that the hot spot effect can be simulated precisely. The sensitivity of canopy reflectance angular distributing to LAI is less than to LAI-Cw and LAI-N. Reflectance angular distribution at 1 600 nm wavelength is strongly related to LAI and Cw while reflectance at 820nm is appreciable larger than the one at 1 600 nm under LAI and N effect. Besides, using information contained in near-infrared （820 nm）and short-wave infrared （1 600 nm） canopy reflectance we proposed a soil-adjusted water index which is employed to retrieve the canopy water content is also greatly influenced by canopy structure. Canopy structure is worth considering in the process of model selection henceforth.