Desertification intensity affected by groundwater and land subsidence in Maharloo-Bakhtegan watershed

Introduction
Groundwater, one of the most important sources of water supply, has faced a considerable decrease in quantity and quality in recent decades due to harvesting more than their natural recharge. Over-extraction of groundwater causes subsidence, soil salinity, reduction of the base flow of rivers due to the drying up of springs, the salinity of groundwater, and in a general view, land destruction and the creation and expansion of desert areas. Desertification is one factor that threaten human life, which causes the destruction of natural resources. Therefore, knowledge of desertification processes is important and necessary to reduce the severity of this phenomenon and prevent its spread by providing suitable management solutions and methods. The Iranian Model of Desertification Potential Assessment (IMDPA) is efficient to evaluate the intensity of desertification. In this model, nine key criteria including climate, land-geomorphology, soil, vegetation, economic-social, agricultural water, groundwater, erosion, and urban development technology are considered. According to the reports about the spread of desertification factors in the Maharloo-Bakhtegan watershed located in Fars province, the objective of the present study is to investigate the impact of the water table drop and variations in the groundwater quality and land subsidence in desertification and land degradation as well as to prepare relevant maps in the mentioned watershed.
 
Materials and Methods
Maharloo-Bakhtegan watershed is one of the closed watersheds of Iran, which is a subcategory of the Central Plateau and has 27 study areas. A zoning map of the risk of water table drop was prepared using the related tables that classify the watershed area into four classes: low, medium, severe and very severe. In the same way, the data of quality degradation indicators of water resources including electrical conductivity (EC) and sodium surface absorption ratio (SAR) were classified; then, EC and SAR zoning layers were prepared. Then, the final desertification intensity map was obtained from the integration of maps of the intensity of qualitative and water table drop of groundwater resources, as well as the land subsidence map as a soil criterion. Finally, the zoning map of desertification intensity was prepared using the final risk classes of desertification.
 
Results and Discussion
The EC map showed that 37.2% of the watershed is in the low or insignificant class, 24.8% is in the medium class, 9.5% is in a severe class, and 28.5% of the watershed area is in the very severe class. So, it can have a significant effect on increasing the intensity of desertification. However, the zoning map of the SAR showed that more than 99% of the watershed area has a value less than 18 and is placed in the negligible or low class. The zoning of the water table drop also shows that 65.6% of the studied watershed is placed at the low or insignificant class, only 3% is in the medium class, 5.7% is in a severe class, and 25.7% is in the very severe class. The map of different levels of land subsidence showed that more than 97% of the watershed area either has no subsidence or is placed in a low and negligible level. The desertification intensity map also showed that 83% of the watershed area has low and moderate desertification intensity and 17% of the watershed area has severe and very severe desertification intensity. Therefore, the general situation of the watershed is not critical in terms of the intensity of desertification, but the areas with a moderate degree of destruction are subject to transition to extreme class due to the change of land uses that are expanding today as well as the increase of agricultural areas in the watershed.
 
Conclusion
The present study was conducted with the aim of investigating the effect of the decrease and change in the quality of underground water and land subsidence in desertification and land degradation in Maharlu-Bakhtegan watershed. The results showed that the electrical conductivity factor of underground water can have a significant effect on the intensity of desertification. On the other hand, land subsidence index was not prioritized in determining the intensity of desertification. Also, the intensity of desertification in a major part of the watershed was in the low and medium classes, and with the change of current uses and the increase of the cultivated area, it is recommended to make the necessary plans to prevent the progress of the intensity of desertification in the mentioned areas. In this direction and considering the role of human activities in the drop of the underground water level, the quality drop of the underground water and finally the subsidence of the land, measures such as the preparation of technical guidelines for the use of underground water, the use of pressurized irrigation systems, the proper implementation of the balancing plan of the Ministry of Energy and the implementation Management plans are recommended to preserve the vegetation of forests and pastures and to modify the cultivation pattern. Also, the zoning maps of indicators and intensity of desertification prepared in this study can be used by experts to prepare the land and provide suggested solutions and apply appropriate management methods with the aim of preventing the expansion and advancement of saline and desert lands. Conducting research in the field of indicators and other factors affecting desertification, such as evaporative formations in the region, the gentle slope of the water table in the plain, the presence of semi-permeable or impermeable layers, the lack of underground water flow from upstream and the infiltration of salty water from salty rivers into the aquifers of the river in the watershed. Maharlo-Bakhtagan is also recommended.