叶片辐射等效水厚度计算与叶片水分定量反演研究

在分析叶片水分对叶片反射率光谱影响的基础上,结合叶片内部水、干物质、叶绿素等光谱吸收系数曲线特征,分析了975nm波长水气吸收特征处叶片与光线相互作用原理,并利用Beer定律和945nm,975nm波长光谱反射率差值,推导了975nm波长辐射等效水厚度REWT的计算公式。由于表面反射、杂散射光、非均匀介质和叶片内部多次散射等因素,光在叶片内部的辐射传输不能直接用Beer定律描述,且利用Beer定律计算的REWT与叶片等效水水厚度EWT之间会有较大的偏差。论文设计和获取了多植物、不同水分梯度的叶片光谱获取试验数据,整理和分析了欧盟Lopex93数据,利用这两组独立数据和论文提出的REWT计算公式,比对验证了975nm波长叶片REWT和叶片EWT的统计模型,结果表明：由于光在叶片内部的多次散射,REWT是EWT的3.3倍左右。论文研究结果一方面为叶片EWT定量遥感探测提供了一种快速、简单且有较强通用性的计算方法和模型,另一方面,探测叶片REWT和EWT的定量关系,有助于了解叶片内部的光辐射传输情况,特别是间接了解近红外波段叶片内部的多次散射情况。

Detection of Leaf EWT by Calculating REWT from Reflectance Spectra

The spectral characteristics and the interaction between leaf and light were analyzed based on the optical absorption coefficients of foliar biochemical contents,and the method and equation to calculate the radiative-equivalent water thickness(REWT) of leaves were presented based on the reflectance difference between 945nm and 975nm bands.Because of surface direct reflectance and multi-scatter in the leaf,the REWT derived from Beer principle was quite different from the biochemical equivalent water thickness(EWT).The leaves spectra of different species at different water status were measured by our experiment and Lopex93,and the REWT values at 975nm band were calculated based on the presented method.The relation between leaves' EWT and REWT was analyzed and tested from our experimental dataset and Lopex93 dataset.The REWT was 3.3 times larger than EWT with a significant correlation coefficient(R2) of 0.80 for our dataset and 0.86 for Lopex93 dataset.The results show that:(1) the REWT can be calculated from the reflectance difference between 945nm and 975nm,and it was reliable and significantly correlated with EWT;(2) the relation between REWT and EWT reflected the leaf internal light transfer,and multi-scatter path-length in near-infrared bands was quite different from leaf thickness.