Effect of climatic drought on surface soil salinity in Kashan Plain

Stability of soil structure is one of the most important indicators of land degradation sensitivity and soil quality. Also, soil salinization is among of the main causes of land degradation in arid and semi-arid regions and the critical factor limiting agricultural production (Arasto and Akhiani, 2018). Soil salinity affects plant growth and in this case, the importance of soil is determined due to increasing population growth and the emergence of new needs for more food (Karam et al., 2018). Drought as a natural disaster and inevitable phenomenon has been seen in a wide range of countries, especially countries located in arid and dry regions of the world (Saeidipour et al., 2018). 

In order to investigate the role of drought in intensifying soil salinity in Kashan Plain, drought periods were studied by standardized precipitation index (SPI) in six time periods in the statistical period 2000 to 2017. The results indicated that the years 2000, 2008, 2015, and 2016 were the reference years with the maximum intensity of drought. Salinity changes in reference years were analyzed using the salinity index (SI) obtained from satellite images, ETM + sensors through ENVI software (version 7.4). Then, the ground reality map of soil salinity was obtained by field sampling, laboratory studies, and interpolation in ArcGIS software (version 10.4.1). According to the root-mean-square error (RMSE) criterion, the inverse distance weighting (IDW) method was selected as the most suitable interpolation method in spatial mapping of drought intensity. 

The results of the correlation analysis showed that there is a significant relationship between the actual and intermediate salinity values ​​at the level of 1% with a correlation coefficient of 0.968. This suggests that the obtained model is a good estimator for soil salinity prediction. The results also showed a significant correlation at the 1% level between drought and EC using the Spearman method. The results showed that increasing drought will increase the salinity amount, although this relationship was found inverse for 2016. 

The characteristic arid zone has a variable climate; so that these climatic fluctuations have made it prone to high sensitivity. Over the past few years, reduced rainfall and increased temperature in the Kashan Region have been the main causes of soil salinity. Along with the occurrence of meteorological drought, over exploitation of groundwater and the entry of solutes through precipitation are intensified the soil salinity over the study area.