基于小波特征和冬小麦生理参数的土壤水分高光谱模型优化

    Optimization of the soil moisture model based on hyperspectral inversion by integrating wavelet features and growth parameters of winter wheat

    • 摘要: 土壤水分是影响作物生长的关键因子,在精准灌溉中估算土壤含水率有重要意义,结合作物生理参数与叶片光谱特性,能够在一定程度上增强土壤含水率遥感监测模型的稳定性。为了提高土壤含水率遥感监测模型在冬小麦多种物候期的适用性以及迁移能力,该研究通过连续小波变换增强光谱对叶片不同生化生理指标的响应后,通过变量投影重要性分析方法对冬小麦叶片含水率、叶绿素、叶面积指数敏感的光谱特征进行特征筛选,结合偏最小二乘回归构建土壤含水率模型,并与土壤含水率所选特征建立的监测模型在独立年份数据与不同传感器之间进行比较。结果表明,土壤含水率变化显著改变了冬小麦叶绿素以及叶面积,进而影响了小麦冠层光谱,小尺度小波变换可以增强冬小麦冠层光谱和土壤含水率的相关性(相关系数的平方由0.46提升至0.61)。综合基于地面非成像数据集和机载成像数据集进行的模型验证结果,基于叶绿素所选小波特征在2021年高光谱非成像数据集和2022年机载成像数据集构建的土壤含水率监测模型表现最优,其中基于分解尺度1的叶绿素小波特征构建的模型效果最好,其在独立非成像数据集验证中决定系数为0.541,均方根误差为2.42%,在成像数据集验证中决定系数为0.687,均方根误差为1.92%。因此,通过冬小麦叶片叶绿素与连续小波变换选取的光谱特征进行土壤含水率监测的适用性更强,可以进一步提高土壤含水率监测模型的准确性及稳定性。

       

      Abstract: Soil moisture is one of the most important factors to affectaffecting the crop growth. An accurate estimation on of the soil moisture content can greatly contribute to the precision irrigation in modern agriculture. Hyperspectral data with more radiation information can provide a feasible solution to the crop soil moisture. However, the monitoring model driven by the soil moisture content can be varied in the different sensors and environments. In this study, a soil moisture monitoring model was constructed to combine the wavelet characteristics of the winter wheat canopy spectrum and leaf physiological parameters. The test site was located in the Yangling District, Shaanxi Province, China (34°17'42'N, 108°04'02'E) in the semi-humid and arid climates. Four water treatments were set up in the experiment, with three replicates and a total of 12 experimental plots. There were the consistent experimental treatments in 2021 and 2022. Non-imaging spectral data and soil moisture content under four water treatments were obtained on March 23, April 8, and April 30, 2021. The winter wheat canopy imaging and non-imaging hyperspectral data, leaf moisture content, leaf area index, chlorophyll, and field soil moisture content were then collected on February 25, March 28, April 2, April 13, April 20, April 21, May 2, May 11, and May 16, 2022. The Savitzky-Golay (SG) was used to smooth the wheat canopy spectrum. The Mexican hat wavelet family (the wavelet function in the family was Mexh) was selected as the governing function to perform an 8-scale (21, 22, 23, ···, 28) continuous wavelet transform on the spectral data. After that, the optimal wavelet transform scale was determined to analyze the correlation between the wavelet coefficients at different scales and soil moisture, chlorophyll, leaf area index, and leaf water content. The variable projection importance analysis was implemented to obtain the wavelet features sensitive to different physiological and biochemical indexes. Finally, the partial least squares (PLS) regression was used to monitor the soil water content of winter wheat roots. The results showed that the changes of in chlorophyll and leaf area were represented by the main influencing factors of soil moisture content on the canopy spectrum of winter wheat. The small-scale wavelet transform was enhanced the connection with the spectrum and soil moisture. In addition, it was feasible to monitor the soil moisture content of winter wheat using the physiological indexes from the wavelet features. The selected features were more applicable than those of the soil moisture content. A comparison was made on the accuracy of the models under different features. Among them, the soil moisture monitoring model with the wavelet features using chlorophyll in different year data 2021 hyperspectral non-imaging data set and 2022 imaging different scale data set performed best (R2=0.541-0.687), were better than the SMC driven model (R2=0.274-0.572). In summary, the spectral characteristics of winter wheat leaf chlorophyll and continuous wavelet transform were more applicable for the soil moisture monitoring. The finding can provide a strong reference to further improve the accuracy and stability of the soil moisture monitoring model.

       

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