光谱技术结合水分校正与样本增广的棉田土壤盐分精准反演

    Inversing soil salinity in cotton fields using spectroscopy sample augmentation and moisture correction

    • 摘要: 棉田土壤盐分的精准反演对于棉花的种植管理具有重要意义。水分和盐分作为主要环境因素,共同影响棉田土壤的波谱特征,两者之间的耦合关系直接影响土壤盐分的检测分析。为了提高基于光谱技术构建的模型对棉田土壤盐分信息解析的准确性与可靠性,该研究联用可见/短波近红外(400~1 000 nm)和长波近红外(960~1 693 nm)技术,采集不同含水率与含盐量的新疆地区土壤样本的光谱;结合外部参数正交法(external parameter orthogonalization,EPO),校正不同标样集与不同波段光谱中的土壤含水率干扰信息;引入基于不同卷积步幅的深度卷积对抗网络(deep convolutional generative adversarial networks,DCGAN),进行样本增广与质量评估;参考三层残差神经网络设计一维卷积神经网络RNet,最终构建基于EPO-DCGAN-RNet的优化模型,用于棉田土壤盐分的反演。结果表明,与传统机器学习方法和基于VGG或EfficientNet结构一维卷积神经网络相比,该研究提出的EPO-DCGAN-RNet方法能够有效地滤除水分对盐分反演的影响、提高模型对特征波段的挖掘能力、降低深度学习算法对样本量的依赖性,并能得到更优的模型预测性能。EPO-DCGAN-RNet的建模集R2和均方根误差分别为0.942、115.420 μS/cm,验证集R2和均方根误差分别为0.910和136.472 μS/cm。研究结果可为新疆棉田土壤盐分快速精准检测提供理论指导和技术支持,有助于促进盐碱地区棉花种植的水肥科学管理。

       

      Abstract: Rapidly inverting soil salinity is crucial to the soil water and salt migration for the prevention of the secondary salinization. However, the accuracy and efficiency of soil salinity inversion models are hindered to the coupling relationship between water and salt in soil, particularly for time-consuming and labor-intensive soil collection. This study aims to reduce the interference of soil moisture for obtaining better sample diversity and further improving the robustness of soil salinitu inversion models by using spectral technology. A total of 113 normal and 115 saline soil samples were collected in the Xinjiang cotton fields. These samples were further subjected to different levels of wetting treatment. and subsequently 467 soil samples with varing salt and moisture contents were obtained. Soil salt content was calibrated using the conductivity of soil leaching solution. Spectral data of samples was captured using an ASD ground object spectrometer (400-1000 nm) and a near-infrared spectrometer (960-1693 nm). The soil moisture was also corrected using the external parameter orthogonalization (EPO). Additionally, deep convolutional generative adversarial networks (DCGAN) with different transposed convolution stride strategies were designed to evaluate the sample set using Fréchet Inception Distance (FID) scores. Machine learning models were employed to invert the soil salinity, including partial least squares regression (PLSR), random forest (RF), and one-dimensional convolutional neural network (1D-CNN) models using VGG (VNet), EfficientNet (ENet), and ResNet (RNet) architectures. The results demonstrated that the EPO can effectively reduce the interference of moisture on the salinity, indicating the better prediction performance of different models. RNet out performed PLSR, RF, VNet, Enet, RNet, and exhibited the best performance to predict the soil salinity in cotton fields. The lightweight residual neural network without attention mechanism was more suitable for one-dimensional hyperspectral data. There was an increase in the convolution stride and kernel length of the deep convolutional adversarial generative network. The better samples were obtained for the hyperspectral data with long sequences. The superior FID scores were achieved in the generated augmented sample set using Generator B (designed with the larger convolution stride and kernel size), compared with the rest. Specifically, the FID scores were reduced by 7.9% and 13.4%, respectively, compared with GA and GC. The weight distribution of attention was optimized after expanding the training set by DCGAN. The stability and accuracy of the model were further enhanced to predict the soil salinity under certain constraints on training samples. The EPO-DCGAN(GB)-RNet (called EPO-DCGAN-RNet) model was achieved in the superior RMSE and R2 values of 136.472 μS/cm and 0.910, respectively, on the validation set, compared with the EPO-SG-RNet (using SG filtering denoising) and EPO-RNet (without sample augmentation). Furthermore, 1D-CNN with Grad CAM was employed to identify the characteristic bands of soil conductivity in the soil leaching solution of cotton field. In summary, an accurate inversion model EPO-DCGAN-RNet was constructed for inversing soil salinity in the cotton fields using spectral technology. Water correction and sample augmentation were incorporated for the soil salt composition. The improved model has the promising potential to the salt-tolerant cotton varieties and irrigation strategies using slightly salty water in cotton fields.

       

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