The accurate and rapid inversion of soil salinity in regions based on the fusion of multisource remote sensing is not only practical for the treatment and utilization of saline soil but also the main trend in the development of quantitative soil salinization remote sensing. In this paper, the use of a numerical regression method to fuse spectral indexes based on high-spatial-resolution unmanned aerial vehicle (UAV) images and low-spatial-resolution satellite images was proposed to deeply assess the internal relationships between different types of remote sensing data. An inversion model of soil salt content (SSC) was constructed based on high-spatial-resolution UAV images, and the spectral indexes involved in the fusion were selected from the model. Then, a quadratic polynomial fusion function describing the relationship between the spectral indexes based on the two images was established to correct the spectral indexes based on the low-spatial-resolution satellite image (from Sentinel-2A). Then, scenario 1 (the best model based on Sentinel-2A used for the unfused Sentinel-2A spectral index), scenario 2 (the best inversion model based on UAV used for the unfused Sentinel-2A-based spectral index), and scenario 3 (the best inversion model based on UAV used for the fused Sentinel-2A-based spectral index) were compared and analyzed, and the SSC distribution map was obtained through scenario 3. The results indicate that the scenario 3 had highest accuracy, with the calibration R2 improving by 0.078-0.111, the root mean square error (RMSE) decreasing by 0.338-1.048, the validation R2 improving by 0.019-0.079, the RMSE decreasing by 0.517-1.030, and the ratio of performance to deviation (RPD) improving by 0.185-0.423. Therefore, this method can improve the accuracy of SSC remote sensing inversion, which is conducive to the accurate and rapid monitoring of SSC.

Soil salt information from unified soil salinity content (SSC) inversion models based on all samples is insufficient for accurate regional SSC monitoring. Here, a method of building SSC inversion models for different salinization grades is proposed, combined with unmanned aerial vehicle (UAV) and Sentinel-2A images. According to different salinization grades, three groups of samples (mild (M), medium-severe (S), and whole (W)) were obtained. Their SSC spectra characteristics, parameters, and quantitative inversion models were analysed, constructed, and compared, based on UAV images, and substituted into different salinization grade areas in Sentinel-2A. The UAV-based models for M and S outperformed those for W; the same trend occurred after substituted into Sentinel-2A images. The inversion results were closer to the field survey results. Models for different salinization grades achieved better regional inversion than those of the whole. UAV-based SSC models can be applied to satellite imagery to invert regional SSC.

Rapid and accurate determination of soil salt content (SSC) and its spatial distribution are of great significance for the prevention and improvement of soil salinization. Satellite and unmanned aerial vehicle (UAV) remote sensing data have complementary advantages. The fusion of satellite and UAV multisource remote sensing data to improve the accuracy of SSC based on inversion methods has become a hot topic, and the appropriate fusion level of multisource remote sensing data needs to be explored and determined. The objective of this study was to determine the appropriate fusion level of Sentinel-2A Multispectral Instrument (Sentinel-MSI) and UAV image data for SSC inversion by comparing the fusion effect of three levels (spectral data, spectral index, and spectral model). A numerical regression method was employed to analyse the relationship between Sentinel-MSI and UAV image data (MSI-UAV), and MSI-UAV data were fused at different levels. Then, the appropriate fusion level and best inversion model were optimized to realize regional SSC inversion. The results indicate that spectral data fusion was better than spectral index fusion for enhancing the SSC spectral response, with the correlation between spectral indices and SSC increasing by 0.139–0.167 after fusion. After spectral data fusion, the model improved the SSC inversion accuracy most obviously, with a calibration R² of 0.623, validation R² of 0.571, and ratio of performance to deviation (RPD) of 1.821. Therefore, spectral data fusion was found to be superior in enhancing the spectral response of soil salinity and in improving the accuracy of the estimation model. This research optimized spectral data fusion as the appropriate fusion level of MSI-UAV for SSC inversion and formed a set of high-precision MSI-UAV multisource remote sensing fusion inversion approaches for SSC.