Study the Effect of Natural and Anthropogenic Factors on The Ground Water Falling in The Minab Plain

Introduction
Groundwater is the main source of fresh water in arid and semi-arid regions, making it resilient to the lack of precipitation. Previously, groundwater was considered an unlimited or, at a minimum, entirely renewable natural resource. However, in recent decades, primarily because of population growth and industrial development, it has been under irreparable pressure, which has resulted in groundwater depletion in most parts of the world, in particular in Iran. In addition to the mentioned factors, climatic events, climatic changes caused by global warming and drought sequences, geological and topographical characteristics are also effective at raising and lowering ground water levels. Several studies have been conducted to investigate factors affecting the depletion of aquifers. Studies such as the investigation of climatic factors and aquifer extraction on changes in the level of the Maharluo lake, changes in groundwater quantity and quality in Sistan and Balochistan and factors affecting the depletion of aquifer storage in Darab have shown that the human and climatic factors have been effective in depleting groundwater. Studies in China have stated that socio-economic and agricultural development, land use changes, and dam construction are the most important factors in depleting groundwater resources. The Minab Plain is one of the fertile areas of Hormozgan province, where excessive groundwater extraction has reduced groundwater levels in the area. This plain also plays an important part in the transfer of water between basins from Minab to Bandar Abbas, so that a large quantity of water is pumped and transferred daily. Therefore, it is important to study the variables that affect the depletion of groundwater reserves in this plain. Considerable research has been carried out in the Minab plain to investigate changes in hydroclimatic variables, land use and groundwater. However, no research was conducted on the effect of these factors on groundwater loss. The aim of this research is to analyze the trend of hydro-climatic variables and groundwater level, to investigate land use changes, to detect the trend of groundwater extraction, water transfer operations from Minab plain to Bandar Abbas and, to investigate its effects on depletion in the Minab plain.
 
Material and Methods
The study area in the present study is the Minab Plain which the Esteghlal Dam was built on this river in 1983, with a water intake volume of 250 MCM. The data used in the research including the groundwater level, precipitation, discharge, and temperature. The number of 47 piezometric or observation wells have measured water level variations in the Minab plain since 1986, and 20 rain gauge stations have been distributed around case studies, of which 10 active rain gauge stations, six evapotranspiration stations and four hydrometer stations are used with a long statistical period. In the current study, the Mann-Kendall non-parametric method was used to analyze the trend. The point change was determined by the Pettitt’s test. Landsat satellite time series images were used to extract land use classes. Considering the existing land uses in the region, five land uses were considered: agricultural, rock, rangeland, urban and waterbodies. The images were analyzed in three time periods of 1991, 2005 and 2021 using Landsat satellite images.
 
Results and Discussion
The results indicate that during the 1986-2020 statistical period, the average level of groundwater in the plain fell by 10.19 m. The calculated Mann-Kendall statistic is -6.69, indicating a significant negative trend and, according to the Pettitt test statistics, 2013 was reported as a time of change. With respect to the trend of hydroclimatic variables, temperature and evapotranspiration are increasing at all stations, while precipitation and flow are trending downward which is not statistically significant. In addition, at most stations, change points were from 2001. The results of the land use change survey showed that rangeland decreased by 22%, and irrigated agricultural land and urban land increased by 2.2 and 2.8 times, respectively. In investigating the effect of human factors on groundwater depletion levels, it showed that the number of wells increased from about 100 to 796 wells, 16 wells were drilled in this plain to provide drinking water to Bandar Abbas, and the inter-basin water transfer with a discharge flow of 1200 lit/s, started since 2002 has put double pressure on the storage. The other reason for groundwater depletion is the decrease in aquifer feeding because of the decrease in precipitation, increase in temperature and evaporation, and decrease in infiltration of water from the Minab River, especially after the operation of Esteghlal dam in 1984. The amount of aquifer feed due to runoff infiltration is 12/77 MCM, or 22% of the total aquifer feed. Finally, it can be concluded that natural factors, including a reduction in precipitation and runoff and an increase in temperature, and non-natural factors such as building dams, increasing the allocation of aquifers for agriculture, domestic and industrial purposes, inter-basin transfer water, the increase in agricultural land and residential areas led to a double pressure on groundwater storage in the Minab Plain, resulting in a marked depletion of groundwater levels in the Minab Plain.