基于WorldView-3多光谱和关键环境变量的土壤镉含量反演

    Inversion of soil Cd content using WorldView-3 multispectral and key environmental variables

    • 摘要: 快速准确获取农田土壤重金属含量对区域土地质量评估和粮食安全至关重要。该研究以江西省仙槎河流域小龙钨矿周边农田土壤为研究对象,采用WorldView-3多光谱影像提取光谱反射率并进行光谱变换处理,同时考虑了地形、人类活动和土壤属性等影响农田土壤镉(Cd)含量空间分布的关键环境因子,将光谱、环境变量、光谱与环境变量分别作为模型的自变量,选取了偏最小二乘(Partial Least Squares Regression,PLSR)、支持向量机(Support Vector Machines,SVM)、BP神经网络(Back Propagation Neural Network,BPNN)和随机森林(Random Forest,RF)4种回归算法构建土壤Cd含量预测模型,并利用精度评价指标优选出最佳反演模型。结果表明:仅输入多光谱特征进行Cd含量反演的模型精度总体偏低,R2低于0.2。相比之下,单独输入环境变量的模型精度结果最为理想,最优模型(RF)精度R2可达0.782。然而,融合光谱信息与环境变量共同建模后并未显著提高模型精度,反而导致较优模型(RF)精度略微降低,R2为0.693。研究结果表明,关键环境协变量是决定研究区农田土壤重金属Cd空间分布反演的重要变量,而利用多光谱信息进行土壤重金属反演的能力有限。此外,随机森林模型是预测土壤重金属空间分布的有效手段。

       

      Abstract: Abstract: A rapid and accurate detection of heavy metal content in farmland soils is crucial for land quality assessment and food security. In this study, 203 soil samples were collected from the farmland polluted by Xiaolong Tungsten Mine located in the Xiancha River watershed in Jiangxi Province, southern China. Soil Cadmium (Cd) content was measured using inductive coupling plasma mass-spectrometric (ICP-MS). High-resolution WorldView-3 multispectral imagery was used to extract the spectral reflectance and transformations, including the first order differential reflectance (FDR) and reciprocal logarithm spectra. A correlation analysis was performed to select the sensitive bands suitable for the prediction of soil cadmium (Cd) content. Different from previous studies that merely used the spectral information for modeling, the key environmental factors were also considered as the influence factors of the spatial distribution of Cd content, including the terrain factor (DEM), soil attribute factors (soil organic carbon and pH), and anthropogenic factors (distance to mine and residential area). Partial Least Squares Regression (PLSR), Support Vector Machine (SVM), Back Propagation Neural Network (BPNN), and Random Forest (RF) were used to construct the prediction models of soil Cd content. The best inversion model was selected by the accuracy metrics. The results showed that the transformation of WorldView-3 original spectral data by the first-order differential improved the correlation between spectral data and soil Cd content. However, the prediction accuracy remained low using the inversion model only with the spectral characteristic parameters. By contrast, the environmental covariates alone generated the best accuracy (R2=0.782, RMSE=0.384, MAE=0.294) using RF modelling. Surprisingly, the predictive performance was not significantly improved as expected, when integrating environmental covariates and the spectral information transformed by reciprocal logarithm for modelling, which instead resulted in a slight reduction in the accuracy of the optimal RF model (R2=0.693, RMSE=0.448, MAE=0.336). According to the variable importance ranking, it was found that the relative importance of the five key environmental variables was higher than 74%, which was significantly higher than that of the multispectral bands. Moreover, the model driven by the integration of environmental variables and spectral bands produced a similar spatial distribution trend of soil Cd content to that of the model driven by environmental variables alone from the perspective of spatial prediction. Both models showed that the Cd content in the farmland soil in the study area presented a high degree of spatial heterogeneity, both indicating an increasing distribution trend from the northwest to the southeast. In addition, the soil Cd content showed an increasing trend with the decrease of the distance from the mining area and enriched in the densely populated areas. Despite these similarities, the spatial prediction map with environmental variables alone presented the outstanding strip effect in the southeastern region of the study area. Contrastingly, there was better spatial continuity in the soil Cd map generated by integrating spectral information and environmental variables. These findings indicated that the key environmental covariates were important variables to predict the spatial distribution of heavy metals in farmland soil, whereas the capability of soil heavy metal retrieval using multispectral imagery alone was limited. In addition, the random forest was an effective way to predict the spatial distribution of heavy metals in soil.

       

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