基于波段深度分析和BP神经网络的水稻色素含量高光谱估算

Hyperspectral estimation of rice pigment content based on band depth analysis and BP neural network

  • 摘要: 该文以水稻田间氮肥水平试验为基础,采用单变量的线性和非线性回归方法,建立基于植被指数的水稻色素含量高光谱估算模型。各植被指数对色素含量的估计能力分析结果显示,植被指数在色素含量较大时存在饱和问题,为此尝试将波段深度分析(BDA)与BP神经网络结合,以提高利用高光谱技术对水稻叶片色素含量的估算精度。基于连续统去除处理的水稻冠层高光谱数据(400~750 nm),选取波段深度(BD)、波段深度比(BDR)、归一化波段深度(NBDI)和归一化面积波段指数(BNA)4种波段指数,在此基础上进行主成分分析(PCA)实现降维,然后采用反向传播(BP)神经网络方法对水稻叶片色素含量进行高光谱反演,探讨BDA与BP神经网络结合解决植被指数饱和问题的可能性和有效性。结果表明,波段深度分析突出了光谱吸收特征差异,挖掘了更多的潜在信息,使得光谱曲线的差异性得到增强。BD与BP结合的估算模型对水稻叶片中的类胡萝卜素含量估算精度最高(R2=0.61,RMSEP=0.128 mg·g-1),BNA与BP结合的估算模型对水稻叶片中的叶绿素含量估算精度最高(R2=0.73,RMSEP=0.343 mg·g-1)。对比分析BDA与BP结合的模型和植被指数最佳回归模型的精度,发现波段深度分析建立的BP神经网络模型能较好地解决饱和问题,提高水稻叶片色素含量的估算精度。

     

    Abstract: The estimation accuracy of plant pigment content is low under higher pigment content since conventional vegetation indices tend to be less sensitive to the variance of pigment content. In order to improve estimation accuracy of rice carotenoid and chlorophyll contents with canopy reflectance during all growth stage, we explore the feasibility and effectiveness of combining the band depth analysis (BDA) and back propagation (BP) neural network to solve the problem of vegetation index saturation. With canopy hyperspectral data (400-750 nm), four band indices — band depth (BD), band depth ratio (BDR), normalized band depth index (NBDI) and band depth normalized to band area (BNA) — were calculated via continuum removal processing. Principal component analysis (PCA) was used to reduce the dimensions of hyperspectral data, and determined 10 principle components, which were introduced into BP neutral network as input variables. In the study, canopy hyperspectral reflectance and pigment content measurements were conducted in Meichuan Town of Hubei Province, China. Eight treatments of nitrogen fertilization (0, 45, 82.5, 127.5, 165, 210, 247.5 and 292.5 kg·hm-2) were applied to generate various indices of vegetation and pigment content. Linear and nonlinear regression models were used to quantitatively analyze the vegetation indices and measured pigment content. In addition, coefficient of determination (R2) and root mean square error (RMSE) were used to evaluate the models. All the hyperspectral indices were comparatively analyzed. As a result, BDA showed the differences in spectral absorption characteristics and revealed more potential information to enhance spectral difference. The estimation model combined band index BD and BP had the highest estimation accuracy for carotenoid content in rice leaves, with R2 = 0.61 and RMSE = 0.128 mg·g-1; while the estimation model combined band index BNA and BP had the highest estimation accuracy for chlorophyll content in rice leaves, with R2 = 0.73 and RMSE = 0.343 mg·g-1. Further comparison between BDA & BP models with the best regression model for vegetation index indicated that BP neutral network model based on BDA provided a better solution to saturation problem and a higher estimation precision of rice leaf pigment content.

     

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