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联合多种衍生光谱特征的冬小麦叶绿素含量估算

Estimating chlorophyll content of winter wheat using multiple derivative canopy spectra

  • 摘要: 叶绿素含量是评估作物长势的重要指标,准确估算叶片叶绿素含量对指导田间管理具有重要意义。高光谱遥感是大面积无损探测叶片叶绿素含量的重要手段。与原始冠层光谱反射率相比,冠层衍生光谱通过对原始光谱反射率数学变换处理显著降低了噪声的影响,提升了光谱特征对作物生理生化参量的敏感性。不同衍生光谱特征对生理生化参量的敏感性波段存在差异,与单一衍生光谱特征相比,充分利用并联合多种衍生光谱敏感性特征,有望进一步提升叶绿素含量探测精度。为此,该文提出了一种联合多种衍生光谱特征探测冬小麦叶绿素含量的方法。通过4a连续田间试验获取冬小麦4个关键生育期(拔节期、抽穗期、开花期和灌浆期)的冠层光谱反射率和叶片叶绿素含量,对原始光谱反射率进行一阶导数反射率变换、标准正态变量变换、去趋势校正和连续统去除变换形成4种衍生光谱特征,比较了原始光谱反射率与4种衍生光谱特征对叶片叶绿素含量敏感性差异。通过递归消除法与4种机器学习算法(随机森林回归、随机梯度提升回归、支持向量回归和核岭回归)结合,分别对单一光谱特征数据和多种光谱特征组合数据选取叶绿素含量敏感性光谱特征,分别构建基于单一光谱特征和联合多种衍生光谱特征的叶绿素含量探测模型。结果表明,逐波段衍生光谱特征与叶绿素含量决定系数大于原始光谱反射率,其中连续统去除变换特征在478 ~ 725 nm波段范围内决定系数R2 > 0.6。在基于单一光谱特征数据估算模型中,以衍生光谱特征驱动机器学习估算叶绿素含量精度优于原始光谱反射率,其中以30个一阶导数反射率驱动随机梯度提升回归算法构建的模型精度最佳,在全部数据中估算精度(决定系数determination coefficient,R2=0.841, 均方根误差 root mean square error,RMSE=3.309 μg/cm2)优于采用原始光谱反射率构建最佳模型估算精度(R2 = 0.814和RMSE=3.602 μg/cm2)。联合25个不同衍生光谱特征组合驱动随机梯度提升回归模型进一步提升了叶绿素含量估算精度,并降低了模型所需光谱特征数量,在全部数据中估算精度为R2 = 0.847和RMSE=3.246 μg/cm2。研究结果提升了叶绿素含量估算精度,降低了模型的复杂性,为利用高光谱数据估算作物叶绿素含量提供参考。

     

    Abstract: Leaf chlorophyll content is one of the most crucial indicators to evaluate the crop growth status in agricultural production. Hyperspectral remote sensing can be expected for the non-destructive detection of the chlorophyll content in leaves. The original spectral reflectance can be mathematically transformed into the derived spectrum. The noise interference can be reduced to enhance the sensitivity of the spectral information to biochemical parameters. Different band features of the derived spectra can also exhibit varying sensitivity to biochemical parameters. The accuracy of the chlorophyll content estimation can be improved to fully combine the sensitive features of multiple derived spectra. In this study, multiple derived spectral features were combined to estimate the chlorophyll content in winter wheat. Field experiments were carried out on the canopy spectral reflectance and leaf chlorophyll content of the winter wheat at four key growth stages (joint, heading, flowering, and filling stage) over four consecutive years. The original spectral reflectance was transformed to generate four derivative spectral features, including the first derivative reflectance (FDR), standard normal variable (SNV) transformation, de-trend (DT) correction, and continuum removal (CR) transformation. A comparison was made on the sensitivity difference between the original spectral reflectance and the four derived spectral features to leaf chlorophyll content. The sensitivity spectral features of the chlorophyll content were selected to eliminate the recursive feature for both single and multiple spectral features. Four machine learning algorithms (random forest, stochastic gradient boosting, support vector, and kernel ridge regression) were combined to construct the chlorophyll content estimation models, according to the single and multiple derivative spectral features. The results indicated that the determination coefficient (R2) between derived spectral features and chlorophyll content exceeded that of the original spectral reflectance. In the range of 400~436 nm, the R2 value of the derived spectral feature SNV was between 0.514 and 0.567, which was much higher than that of the rest. In the range of 437~752 nm, the R2 value of the CR was between 0.330 and 0.707. And the value of R2 was greater than 0.6 in the range of 478~725 nm. In the range of 725~850 nm, the SNV and DT shared the largest R2 values (between 0.358 and 0.219) in the different band ranges. The maximum R2 of the five spectral features for the OR, FDR, SNV, DT, and CR were 0.575, 0.612, 0.688, 0.423, and 0.707, corresponding to the wavelengths of 681, 629, 702, 712, and 697 nm, respectively. There was a high accuracy of the machine learning model that was driven by the derivative spectral features to estimate the chlorophyll content. The better performance was achieved, compared with the original spectral reflectance (thirty spectral features were used to combine with the support vector regression). The highest accuracy was achieved in the random gradient lifting regression driven by 30 first-order derivative reflectance. The optimal model (R2=0.841 and RMSE=3.309 μg/cm2) was also obtained with the support vector regression, compared with the original spectral reflectance (R2=0.814 and RMSE=3.602 μg/cm2). Furthermore, 25 derived spectral features were combined to reduce the number of input spectral features. The stochastic gradient lift regression model was further improved to increase the accuracy of the chlorophyll content estimation, including two CR and SNV, six DT, and fifteen FDR spectral features. The estimation accuracy was R2=0.847 and RMSE=3.246 μg/cm2. Multiple derivative spectral features were combined to drive the machine learning algorithms. The finding can provide a strong reference to further improve the estimation accuracy of the chlorophyll content for a less complex model. The improved model can also be expected for the aerial and aerospace hyperspectral remote sensing observation platform for the large-scale, accurate estimation of chlorophyll content in winter wheat leaves.

     

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