基于高光谱数据的盐荒地和耕地土壤盐分遥感反演优化

    Optimizing the inversion of soil salt in salinized wasteland using hyperspectral data from remote sensing

    • 摘要: 盐荒地作为研究区的"临时盐库",其土壤盐分远高于研究区平均水平,因此探究不同土地利用类型土壤盐分的光谱响应差异以及对盐分遥感模型的影响,是实现不同土地类型土壤盐分反演值更加接近真实值的重要途径。该研究以河套灌区永济灌域为例,针对耕地和盐荒地土壤分别进行原位高光谱测定(FieldSpec 4 Hi-Res,ASD),对光谱数据进行多种光谱变换(基础数学变换、导数变换及光谱指数)后,分别基于特征波长和特征光谱指数构建单一土地类型盐分反演模型(耕地(Agricultural Land,AL)、盐荒地(Salinized Wasteland,SW))和整体盐分反演模型(耕地+盐荒地(Agricultural Land + Salinized Wasteland,AL+SW)),对比分析2种建模方式下的模型精度,提出区域土壤盐分遥感反演的最佳建模方式。结果表明:AL、SW和AL+SW中土壤样本数据的平均含盐量分别为5.09、13.42和7.09 g/kg,且在各等级盐分区间内,SW的光谱反射率均大于AL,其中轻度盐化土、中度盐化土和重度盐化土的光谱反射率平均差值分别为0.040、0.020和0.034;光谱变换和光谱指数均能有效改善不同土地类型中土壤盐分与光谱的相关性。相比基础变换(倒数、对数、根式等),导数变换不仅增大了敏感波长的范围,还使得特定波长处相关系数得到显著提升。不同土地类型中基于特征光谱指数的模型精度均高于基于特征波长的模型;单一土地类型盐渍化反演模型明显提高了区域土壤盐分的反演精度,单一土地类型盐渍化反演模型中(AL、SW模型)各变换下光谱指数模型平均决定系数相比整体模型(AL+SW模型)由0.50提高到了0.61,其中基础变换、一阶导数和二阶导数模型平均R2相比整体模型分别提高了0.06、0.11和0.17,同时,基于最优光谱指数的单一土地类型盐渍化反演模型平均R2相比整体模型由0.74提高到了0.92。因此,当区域中存在盐分相差较大的多种土地利用类型时,对不同土地利用类型单独构建土壤盐分反演模型能确保反演结果更接近实际情况。

       

      Abstract: Salinized wasteland can be served as the temporary salt reservoir with the much higher salt content beyond the average level. Therefore, the high-precision inversion of soil salinity can be realized to explore the difference in the spectral response of soil salinity in different land use types and its influence on the remote sensing model. In this study, a typical salinization region (Yongji of Hetao irrigation district in China) was chosen as the study region. The salinized wasteland was relatively scattered and mostly concentrated around the agricultural land. The salt content in the salinized wasteland was much higher than that in the agricultural land. Firstly, in-situ hyperspectral measurement (FieldSpec 4 Hi-Res, and ASD) was carried out for the agricultural land and salinized wasteland in April from 2018 to 2020. Secondly, the spectral data was subjected to the various spectral transformations, including the fundamental transformation (original, reciprocal, logarithm, and radical transformation), derivative transformation (the first and the second derivative), and spectral index (normalized differential soil index, difference soil index, and simple ratio soil indices), respectively. Thirdly, the multiple stepwise regressions were used to acquire the characteristic bands and spectral indices. Lastly, the single land type salt inversion model (Agricultural Land (AL), Salinized Wasteland (SW)), and the overall salt inversion model (Agricultural Land + Salinized Wasteland (AL+SW)) were constructed using the characteristic wavelength and characteristic spectral index, respectively. The model accuracy under different modeling was evaluated using the coefficient of determination (R2), and Root Mean Square Error (RMSE). As such, the best modeling was proposed for the regional soil salinization. The results showed that the average content of soil salinity in the samples of AL, SW, and AL+SW model was 5.09, 13.42, and 7.09 g/kg, respectively. Specifically, the SW spectral reflectance was greater than that of the AL in each wavelength range of different grades of salt zone. Among them, the average differences were 0.040, 0.020, and 0.034 in the slightly, moderately, and strongly saline soil, respectively. Spectral transformation and spectral index were effectively improved the correlation between the soil salt and spectrum in the different land types. Compared with the fundamental transformations (reciprocal, logarithm, and root), the derivative transformations significantly increased the range of sensitive wavelengths for the high correlation coefficient at specific wavelengths. The accuracy of models with the characteristic spectral index was much higher than that with the characteristic wavelength in different land types. After the first derivative transformation, the average R2 of AL, SW, and AL+SW regression models increased compared with the wavelength regression model. The average R2 of AL, SW, and AL+SW regression models also increased after the second derivative transformation. The salinization inversion model of single land type significantly improved the inversion accuracy of regional soil salt. A significant increase from 0.50 to 0.61 was found in the average R2 of the spectral index model under each transformation in the single-land type salinization inversion model (AL, and SW), compared with the overall model (AL+SW model ). The average R2 values of the fundamental transformation, the first, and the second derivative models were 0.06, 0.11, and 0.17 higher than that of the overall model, respectively. At the same time, there was the increase from 0.74 to 0.92 in the average R2 of the single land type salinization inversion model using the optimal spectral index, compared with the overall model. Therefore, the inversion models of soil salt for the different land use types can be expected to ensure the inversion closer to the actual situation, particularly for the various land use types with the large differences in salinity.

       

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