Seasonal Dynamics of the Land-Surface Characteristics in Arid Regions Retrieved by Optical and Microwave Satellite Data

Establishing a quantitative relationship between Synthetic Aperture Radar (SAR) data and optical data can facilitate the fusion of these two data sources, enhancing the time-series monitoring capabilities for remote sensing of a land surface. In this study, we analyzed the Normalized Difference Vegetation Index (NDVI) and Shortwave Infrared Transformed Reflectance (STR) with the backscatter coefficients in vertical polarization VV (σ0VV) and cross polarization VH (σ0VH) across different seasons. We used optical and microwave satellite data spanning from the southern Gobi Desert region to the steppe region in northern Mongolia. The results indicate a relatively high correlation between the NDVI derived from Sentinel-2 and σ0VH (RVH = 0.29, RVH = 0.44, p < 0.001) and a low correlation between the NDVI and σ0VV (RVH = 0.06, RVH = 0.14, p < 0.01) in the Gobi Desert region during summer and fall. STR showed a positive correlation with both σ0VH and σ0VV except in spring, with the highest correlation coefficients observed in summer (RVV = 0.45, RVV = 0.44, p < 0.001). In the steppe region, significant seasonal variations in the NDVI and σ0VH were noted, with a strong positive correlation peaking in summer (RVH = 0.71, p < 0.001) and an inverse correlation with σ0VV except in summer (RVV = −0.43, RVV = −0.34, RVV = −0.13, p < 0.001). Additionally, STR showed a positive correlation with σ0VH and σ0VV in summer (RVH = 0.40, RVV = 0.39, p < 0.001) and fall (RVH = 0.38, RVV = 0.09, p < 0.01), as well as an inverse correlation in spring (RVH= −0.17, RVV= −0.38, p < 0.001) and winter (RVH = −0.21, RVV = −0.06, p < 0.001). The correlations between the NDVI, STR, σ0VH, and σ0VV were shown to vary by season and region. In the Gobi Desert region, perennial shrubs are not photosynthetic in spring and winter, and they affect backscatter due to surface roughness. In the steppe region, annual shrubs were found to be the dominant species and were found to photosynthesize in spring, but not enough to affect the backscatter due to surface roughness.