Assessing the impact of climate change on three decades of soil salinity dynamics in the Bajestan Playa, Northeast Iran

Playas are endorheic sediment basins in drylands that are temporally filled with water. During dry seasons, their surfaces are generally covered with a thick crust of clay-rich clastic material and soluble salts. Strong winds can mobilize that fine-grained material, including the salts, from the playa surfaces as aeolian dust that strongly affects the surrounding ecosystems and human livelihood. During recent decades, climate change strongly altered the salinity regimes of many playas, leading to an increase of surface salinity. During this study we investigated the impact of climate change on soil salinity dynamics in the Bajestan Playa, Northeast Iran, over a period of three decades (1992-2021). The studied region is particularly exposed to the "Sistan winds of 120-Days" during the dry season in summer. Therefore, it is one of the main dust source areas in eastern Iran.

In order to better understand and predict the geochemical composition of aeolian dust, it is necessary to monitor and map regional salinization processes. Thus, we applied a multi-disciplinary approach that encompased remote sensing, field-based ground truthing, and climatic data analysis. Remote sensing analysis on Landsat data (first week of July of all studied years) were carried out on three generations of sensors (TM, ETM+, OLI 8) that were uniformly corrected for atmospheric and geometrical conditions. During a field campaign in July 2021, a total number of 130 soil sampleswere collected from the upper 20 cm of soil of areas that represent seven soil salinity classes that were preliminary identified based on multispectral remote sensing analysis and regional geological maps. In addition, the electrical conductivity (EC) was subsequently measured on the soil surface samples in the laboratory. Based on these field measurements and the remote sensing analyzes, we were finally able to derive twelve soil salt indices. Two among all indices (S12 and S13) showed the most satisfactory calibration accuracies between the field and remote sensing-based EC values. Finally, the index SI2 was applied to the Landsat images for temporal and spatial quantitative soil salinity mapping. Furthermore, to validate the impact of climate change on the salinity changes, the Standardized Precipitation Index (SPI) and the Mann-Kendal Index were calculated based on temperature and precipitation data at different time scales of the last 60 years.