Assessing groundwater level declination in Meighan Playa Catchment

In recent decades, following the exploitation of groundwater, especially in areas with limited surface water and in which groundwater is the greatest resource in meeting the needs of water demands, the trend of reducing the volume of aquifers has increased. Meighan watershed, which includes Meighan wetland, is one of the most important plains of Iran that supplies a significant part of Arak city's drinking water and many agricultural wells. Since the drought causes a rapid reduction in surface flows, declines the ground water level, intensifies the wind and water erosion, changes the quality of water resources, causes ground settlement and especially, leads to the problem of salinization of drinking and irrigation water, it is necessary to evaluate the effects of drought and manage its risk.

The study area of Meighan is located in Central Iran with 2854.63 km2 area. To investigate the possible drinking water crisis in the region, water level decline of agricultural wells, and the risk of ground settlement in the groundwater measurement network, the observed groundwater level and absolute height of groundwater level data were collected in 46 piezometric wells in the statistical period of 1986-2006 and 2006-2020. For each year, an average isopieze map was drawn. Then by deducting the maximum and minimum values in each well, the amount of loss in each year was determined. These data were used to draw maps of annual decline. The reason can be the lack of proper feeding of the aquifer due to the decrease of precipitations and drought and as a result of the reduction of the aquifer storage due to the excessive exploitation.

From 1971 (the first data collection) to 2004, with the increase in the number of deep and semi-deep wells, the discharge rate of the wells has increased by a total of 531.5 mm3. In 2009, despite the increase in the number of wells, the discharge rate of wells has decreased by 153.9 mm3 in 6 years. In the last data collection in 2020, the number of wells and their discharge rate have decreased which indicates drying up of the wells due to the decline in the groundwater level in many areas of the plain which has led to the disuse of wells.Reviewing the decline maps of 1986-2006 and 2006-2020 indicate that the northern and eastern areas of the plain do not show significant changes and have a relatively constant trend. However, as it is shown in isopieze maps, in the western regions and in the south of the plain, groundwater level has had the highest decline. From 2006 to 2020, in the north and northeast, as well as in the south, west and southwest of the basin, there are areas with the highest rate of decline.

According to the analysis of rainfall data and the absolute height of groundwater level of Arak Plain, it can be concluded that the amount of rainfall does not directly affect the decline of the groundwater level. Research results indicate that before 1986, the hydraulic slope of the Arak plain aquifer conforms to the level of the plain and the Meighan playa has been a natural drainage of the plain. However, during the period of 1986-2006 with the increase and concentration of exploitation wells in the western and southwestern parts of the plain, the hydraulic slope of the aquifer has inclined towards the western regions. From 1986 to 2006, it can be concluded that the maximum groundwater level decline of 20 meters occurred in the western and southwestern areas of the basin and extended to the eastern areas of the basin. However, from 2006 to 2020, the decline classes have increased significantly, such that in the west, southwest, north and northeast of the basin, an increase in the groundwater level decline up to 70 meters has been observed.