可见光光谱的冬小麦苗期地上生物量估算

doi:10.3964/j.issn.1000-0593(2019)08-2501-06

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摘要：地上生物量是表征冬小麦苗期长势的重要参数之一，对于监测冬小麦苗期长势，预测产量具有重要的实际意义。目前，通过计算光谱指数进行生物量估算是冬小麦苗期地上生物量无损测量的主要方法，但该方法需要一定的平台支撑，在便捷性方面存在一定的不足。为此，利用可见光图像数据获取方便、准确率高的特点，基于冬小麦苗期冠层可见光图像数据开展冬小麦苗期地上生物量估算研究。采用数码相机，采集冬小麦苗期冠层可见光图像并利用Canopeo进行冬小麦冠层与背景的分割。在获取冠层分割图像后，提取了CC（canopy cover）、ExG（excessg reen）、ExR（excess red）、ExGR（ExG-ExR）、NGRDI（normalized green-red difference index）、GLI（green leaf index）、RGRI（red-green ratio index）和RGBVI（RGB vegetation index）共8个可见光图像特征。利用相关性分析进行特征优选，选择与冬小麦苗期地上生物量实测数据相关性较高的图像特征构建估算模型。利用优选的图像特征，分别构建偏最小二乘回归（PLSR）、BP神经网络（BPNN）、支持向量机回归（SVR）和随机森林（RF）模型，开展冬小麦苗期地上生物量估算研究，并定量分析特征数量和播种密度对估算模型准确率的影响。结果表明，ExR，GLI和RGBVI与生物量实测数据相关性较低，因此，将这3个特征剔除。CC，ExG，ExGR，NGRDI和RGRI与生物量实测数据的相关性较高，其中CC，ExG和ExGR与生物量实测数据呈正相关，而NGRDI和RGRI与生物量实测数据呈负相关。利用优选的图像特征构建估算模型，研究结果表明，基于优选的5个图像特征，PLSR的估算准确率最高，模型R²为0.801 5，RMSE为0.0788 kg·m^-2，表明PLSR能够实现冬小麦苗期地上生物量的准确估算。特征数量是影响估算模型准确率的因素之一，随着特征数量的减少，模型估算的准确率逐步下降。利用不同播种密度数据集对估算模型进行测试，结果表明，PLSR在不同的播种密度数据集上均取得了最高的估算准确率，模型R²分别为0.897，0.827 9和0.788 6，RMSE分别为0.062，0.072和0.079 1 kg·m^-2，表明PLSR估算的冬小麦苗期地上生物量数据与实测生物量数据之间具有良好的相关关系。随着播种密度的增加，所有估算模型的准确率均出现下降，而PLSR的准确率下降程度最小。由此可见，基于可见光图像数据，能够实现冬小麦苗期地上生物量的准确估算，为冬小麦苗期田间管理提供参考。

关键词：冬小麦；苗期；地上生物量；可见光光谱；估算

Abstract：Above ground biomass (AGB) is one of the most commonly used traits indicating the growth of winter wheat at early growth stages. It is of great practical significance to monitor the growth and to estimate the yield. The conventional methods involving destructive sampling and manual calculating of the dry weight to measure AGB are prohibitively time consuming and laborious. The non-destructive approach to estimate AGB of winter wheat is through the estimation of vegetation indices (VIs) and regression analysis, which heavily depends on tools such as Remote Sensing and LiDAR. Therefore, the method is subject to specialized knowledge and high-cost. An estimating method for above ground biomass of wheat winter at early growth stages was proposed by using the digital images of winter wheat canopy. The canopy images were captured by a digital camera. The image segmentation of vegetation was achieved by using Canopeo. Based on the segmented images, eight images features, i. e., CC (Canopy Cover), ExG (Excessgreen), ExR (Excess red), ExGR (ExG-ExR), NGRDI (Normalized Green-Red Difference Index), GLI (Green Leaf Index), RGRI (Red-Green Ratio Index) and RGBVI (RGB Vegetation Index), were extracted. Correlation analysis was conducted between pairs of the images features and the biomass measurements. The feature that were highly correlated to the biomass measurements were used to build the estimation model. Results showed that ExR, GLIand RGBVI were not correlated to biomass, resulting in the elimination in the following experiments. The left five features were highly correlated to the biomass measurements. Among the five selected features, CC, ExG and ExGR were positively correlated to biomass measurements while NGRDI and RGRI were negatively correlated to biomass measurements. Based on the selected features, four model, i. e., Partial least squares regression (PLSR), BP neural network (BPNN), Support vector machine regression (SVR) and Random forest (RF), were built to estimate the above ground biomass. The influences of the number of features and the sowing density on the estimating accuracy were analyzed quantitatively. Results showed PLSR achieved the best accuracy based on the selected five features, the R² value was 0.801 5, and the RMSE was 0.078 8 kg·m^-2, indicating that the PLSE was able to accurately estimate the above ground biomass of winter wheat at early growth stages. Thenumber of feature was proved to be an influence of factor. The accuracy of the model decreased with the reduction of the number of features. Experiments on the models by using different sowing density datasets were conducted as well. The results showed that PLSR outperformed the other three models over all the density datasets, the R² values were 0.897，0.827 9 and 0.788 6, and the RMSE values were 0.062，0.072 and 0.079 1 kg·m^-2, indicating that the PLSR the estimated above ground biomass had a goof agreement with the ground truth. With the increase of the sowing density, the accuracy of all the models decreased while the PLSR achieved the minimum reduction. In Summary, the above ground biomass can be estimated by using the digital images, which can provide support to the field management of winter wheat at early growth stages.

Key words：Winter wheat; Early growth stages; Above ground biomass; Visible spectrum; Estimation

收稿日期: 2018-06-20 修订日期: 2018-10-11

中图分类号:

TN941.1

基金资助: 国家自然科学基金项目(31801264)，中国科协青年人才托举工程第四届项目(2018QNRC001)和国家“十三五”重点研发计划项目(2016YFD0300606和2017YFD0300402)资助

通讯作者: 马浚诚 E-mail: majuncheng@caas.cn

作者简介: 张领先，1970年生，中国农业大学信息与电气工程学院教授 e-mail: zhanglx@cau.edu.cn

引用本文:

张领先，陈运强，李云霞，马浚诚，杜克明，郑飞翔，孙忠富. 可见光光谱的冬小麦苗期地上生物量估算[J]. 光谱学与光谱分析, 2019, 39(08): 2501-2506.
ZHANG Ling-xian, CHEN Yun-qiang, LI Yun-xia, MA Jun-cheng, DU Ke-ming, ZHENG Fei-xiang, SUN Zhong-fu. Estimating Above Ground Biomass of Winter Wheat at Early Growth Stages Based on Visual Spectral. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2019, 39(08): 2501-2506.

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