Soil salinity inversion in coastal cotton growing areas: An integration method using satellite-ground spectral fusion and satellite-UAV collaboration

Soil salinity is a crucial factor in agriculture, rising salinity undermines cotton (Gossypium spp.) production in coastal areas of China and damages crops in other countries. In this study, we propose an effective integration method using satellite-ground spectral fusion and satellite-unmanned arial vehicle (UAV) collaboration for soil salinity monitoring in cotton growing areas. Firstly, an extreme learning machine (ELM), random forest (RF), and extreme gradient boosting (XGBoost) models were constructed based on UAV images from test areas. The optimal model was selected for soil salinity inversion. Meanwhile, ground imaging hyperspectrum and SENTINEL-2A multispectral images were differentially fused by nonnegative matrix factorization (NMF). Then, taking the inversion results of UAV as the training sample to build convolutional neural network (CNN) model of the fused SENTINEL-2A, the soil salinity distribution map of cotton fields in the study area was obtained by inversion, and the satellite-UAV-ground integrated inversion of soil salinity in coastal cotton fields was realized. The results showed that the spectrum after satellite-ground fusion was closer to the original ground hyperspectrum, the fusion improved the correlation between spectrum and soil salinity, and UAV inversion data possessed great potential for the reference data of satellite inversion. The soil salinity obtained by satellite-UAV-ground integration approach was highly consistent with the measured salinity in the study area (R-2 = 0.805), and the integration approach is suitable for soil salinity inversion in the cotton seedling stage in the coastal area. The satellite-UAV-ground integration approach proposed in this study fully tap the advantages of remote sensing data from different platforms and improved the ability to obtain soil salinisation information in large-scale quantitatively, accurately, and quickly.