Mapping of Sor Depressions and Solonchaks in the Northern Caspian Region Based on Long-Term Landsat Data

Sor depressions (sors) of various origins and hydromorphic solonchaks associated with them are characteristic objects of arid landscapes. In Russia, they are widespread in the southeast of the European part, as well as in the south of Western Siberia and in Transbaikalia. Sors are depressions, at the bottom of which the processes of solonchak formation are actively developing, and often there are permanent or drying up salty and brackish water bodies. The surface of salt marshes is practically devoid of vegetation cover, only a few of the most resistant plant species are able to withstand such high levels of salinity. In recent years, in the southeast of the European part of Russia, desertification processes have intensified up to complete disappearance of vegetation and exposure of shifting sands and blowout basins due to droughts and excessive pasture loads. During satellite monitoring of these processes, open sands are often confused with sors and solonchaks, which are devoid of vegetation due to natural causes, and not due to the impact of adverse factors. For this reason, a technology is needed to separate open sands and deflated areas and natural formations—sors and solonchaks. The paper proposes a method for mapping sors based on the average annual values of the Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) calculated from Landsat data that have undergone the atmospheric distortion correction procedure. Threshold values of these indices are proposed for separating sors from open sands and permanent reservoirs. The identified area of sor depressions and solonchaks in Astrakhan oblast, Stavropol krai, and the Republics of Dagestan and Kalmykia amounted to about 245 thousand ha, which exceeds the area of open sands and deflated territories before the period of intensification of desertification processes in 2019–2022. The developed electronic maps also open up prospects for further study of sors as natural objects, since their genesis and spatial distribution have not yet been sufficiently studied.