Inversion model of soil salt content based on WorldView-2 image

Abstract: Soil salinization has become one of the global environmental issues, especially in arid and semi-arid areas. In order to prevent its further deterioration, it is important to monitor soil salinity timely, quantitatively and dynamically. Remote sensing technique has become a promising method to detect and monitor the soil salinity due to its many advantages. The aim of this study was to evaluate the ability of quantitative inversion of soil salt content based on the WorldView-2 images with high spatial resolution. In this paper, Keriya River basin, Xinjiang, China was selected as the study area. Based on the WorldView-2 image data and soil salt content, this paper used 2 kinds of methods including the partial least squares regression (PLSR) and back propagation artificial neural network (BP ANN) to establish the quantitative inversion models of soil salt content. Soil salinity information was extracted from the WorldView-2 data, which was synchronized with field sampling time, and covered an area of 1.2 km × 1 km. The distance between adjacent sampling points was 100 m in east-west direction, and 200 m in north-south direction. Sixty-six sampling points were designed in the study area, and digging depth in soil was 20 cm. Hand-held GPS (global position system) receiver was used to record the coordinates of sampling points, and the soil salt content and soil spectra were measured in the indoor. Spectral radiometric calibration and atmospheric correction were performed on the WorldView-2 data to match the image data with the measured re?ectance spectra. The measurement of soil spectra was conducted using an ASD (analytical spectral devices) FieldSpec3 portable spectro radiometer (American Analytical Spectral Devices, Inc.) at wavelengths from 350 to 2500 nm with a sampling interval of 1.4 nm from 350 to 1000 nm and 2 nm from 1000 to 2500 nm. The edge bands including 350-399 and 2401-2500 nm were removed from the measured spectral data, and the remaining 400-2400 nm spectrum curve was smoothed with Savitzky-Golay smoothing method in software OriginPro. Original soil spectral data were continuum-removed in ENVI 5.1 to analyze the spectral characteristics of soil. Correlation analysis between the original and two-order derivative of measured reflectance data and the soil salinity was performed by using Pearson correlation analysis method, and the significant bands were used to establish the inversion model. The geographic locations and surface re?ectance of the soil samples were obtained precisely from WorldView-2 multi-spectral data. Spectral re?ectance of each band of WorldView-2 data was simulated by calculating a weighted average of the measured re?ectance spectra to reduce the error resulted from the spectral resolution difference of the image derived spectra and measured re?ectance spectra. PLSR model was established, in which the reflectance of 4 bands i.e. B3, B4, B5 and B7 of WorldView-2 image and NDVI (normalized difference vegetation index) and RVI (ratio vegetation index) were selected as independent variables, and salt content was used as dependent variable. Three-layer BP neural network model was established in which the input layer was made up of the reflectance of 4 bands of WorldView-2 image (B3, B4, B5 and B7) and NDVI and RVI, and the number of net neurons was 6; the output layer was a neuron corresponding to the salt content of sampling point. After a lot of tentative computation, the optimal number of neurons in the hidden layer was selected as 8. The results showed that: 1) The prediction accuracy of BP neural network model based on WorldView-2 image data was higher than the PLSR model in the study area, and the coefficient of determination (R2), root mean square error (RMSE) and residual prediction deviation (RPD) were 0.851, 0.979 and 2.337 respectively for the former and 0.814, 1.139 and 2.007 respectively for the latter. 2) The spatial resolution of salinity mapping could be improved by using WorldView-2 images. The NDVI and the RVI were helpful to reduce the influence of vegetation cover and soil moisture on the prediction accuracy. This inversion model established in this paper can meet the needs of monitoring salinization in arid and semi-arid area and promote the further application of WorldView-2 high spatial resolution satellite in the monitoring of salinization.