光谱指数用于叶绿素含量提取的评价及一种改进的农作物冠层叶绿素含量提取模型

The Assessment of Spectral Indices Applied in Chlorophyll Content Retrieval and a Modified Crop Canopy Chlorophyll Content Retrieval Model

对目前提出的光谱指数用以提取叶片叶绿素含量的适应性进行了分析和评价。通过分析,解释了为什么研究者得出这些指数与他们的观测样本叶绿素含量有显著的相关的结论以及为什么某个研究者提出的某个指数和叶绿素含量间的关系用于其他样本时会失效。此外,改进了一个农作物冠层叶绿素含量的提取模型,通过独立实测数据验证,效果较好,认为是可以用于其他地区农作物叶绿素含量提取的模型。

Vegetation chlorophyll content is a key component in ecosystem function. Study on vegetation chlorophyll content retrieval is carried out in 3 directions in which developing or modifying spectral index to retrieve canopy chlorophyll content is a direction which may be a compromise between multiple stepwise regression and inverting physical models because it has some physical meaning while is easier than the latter. At leaf level, we analyzed the applicability of the spectral indices when applied to chlorophyll content retrieval. The reasons that why some studies found these spectral indices are highly correlated with chlorophyll content of their observed samples and why these relationships can not be applied to other peoples＇ observed samples are explained in this paper. A recent study developed a semi-empirical model to retrieve canopy crop chlorophyll content which combines spectral index TCARI and soil adjusted index OSAVI. According to this study, the chlorophyll content is determined by the slope of the intersected isolines whose independent variable y is the value of TCARI and the dependent variable x is the value of OSAVI. So a semi-empirical model was derived which is a logarithmic function of TCARI/OSAVI value and through validation with observed corn canopy reflectance and chlorophyll content, this model gives promising results. In this paper, we give some modifications for this semi-empirical model. First, the intersection point of the isolines is considered which was taken as the origin while actually is not, i. e, the chlorophyll content is determined by（ TCARI-a/ OSAVI-b） with a and b being the Y-coordinate and X-coordinate of the intersection point respectively in TCARI-OSAVI space. Second, a reciprocal function is thought to be more appropriate than a logarithmic one. Considering these two points, a modified model is given in this paper. With our observed corn canopy reflectance and chlorophyll content, this modified model gives better results.