Modeling and quantitative investigation of the groundwater condition of the South Mehyar-Dasht Asman aquifer using the MODFLOW model

Introduction
Today, fresh water sources are very important in human life. During the past years, population growth, development of industries, and agriculture have increased the need for water and subsequently increased the withdrawal from groundwater sources. This increase in withdrawal has occurred without considering the capacity of the aquifers. While logically, the exploitation of an aquifer should be limited to its dynamic storage, in most regions, the static reserves of aquifers have been used to a large extent, and it seems impossible to return these aquifers to their original state. The dynamic storage of the aquifer is actually a part of the storage volume of the aquifer, which has fluctuations, and by examining and determining these fluctuations, it is possible to estimate the permissible amount of exploitation of the aquifer. But the static storage of the aquifer, which is very important, is among the old and stable storage that has been stored for many years. The use of static aquifer storage causes events such as subsidence. The drying up and lack of water in many aqueducts, springs, and extraction wells across the country is one of the consequences of the overexploitation of groundwater resources. In the arid and semi-arid climates of Iran, factors such as climate change and water scarcity, along with overexploitation of water resources, especially groundwater resources, have caused the groundwater levels to decrease. In some areas, we are witnessing irreparable subsidence. As a result, it is necessary to manage and control the extraction of groundwater resources before the crisis occurs and reaches an irreversible level. In this regard, monitoring and controlling the condition of the aquifer using computer models is essential.
 
Materials and Methods
In the present study, the modeling and quantitative investigation of the groundwater condition of the South Mehyar-Dasht Asman aquifer in the Gavkhoni catchment area has been done by the MODFLOW 2005 model and groundwater modeling system )GMS 10.4.5( software. GMS software is one of the few with good performance and has been used in groundwater studies in many countries worldwide. This software simulate groundwater both quantitatively and qualitatively. The MODFLOW (Modular Ground Water Flow) model is used to simulates groundwater flow in aquifers with specific boundary conditions, assuming the necessary values for hydraulic conductivity and other aquifer parameters. The program allows the user to select only the modules needed to study the desired system for specific hydrogeological processes and activate or deactivate a particular part. These features have made the MODFLOW model the most efficient and accessible groundwater model today. For this purpose, the statistics and information from 15 years ending in 2013 of the Iran Water Resources Management Company and topographical information extracted from satellite data have been used to model the groundwater of the South Mehyar-Dasht Asman aquifer.
 
Results and Discussion
After running the model, the parameters of hydraulic conductivity, anisotropy, or HANI, RCH, SY, and parallel lines of the groundwater level were obtained. In the next stage, or the calibration stage, the model was calibrated by the parameters mentioned to minimize the difference between the observed water level and the calculated water level. In the sensitivity analysis stage, the calibrated value of the parameters is systematically changed to determine the model's sensitivity to the parameters. In this stage, it was found that the model received the most influence from the parameters of anisotropy and hydraulic conductivity. To validate the model and measure its accuracy, validation was done. The RMS error was 2.21, which is a good value, but for more certainty, the RMSE was also calculated, and its value was 23 %, which is very appropriate considering the long-term simulation period. Finally, the numbers extracted from the FlowBudget engine based on the calibrated MODFLOW model revealed that during the 12-month simulation period, a particular share of the aquifer's fixed storage was reduced on a daily average basis, resulting in an average drop of 7.5 m in the aquifer's groundwater level during this period. We see that the most significant drop is related to the eastern part of the aquifer.
 
Conclusion
In this research, the South Mehyar-Dasht Asman aquifer was simulated and modeled by the MODFLOW model and GMS software, and the groundwater level was investigated during the simulation period. The model was calibrated to reduce the difference between the observed and the calculated water level. The sensitivity analysis section also examined the influence of the model parameters. Validation was also done to increase the accuracy of the model's performance. The coefficient of determination or R-squared correlation was 0.9971, which seems to be a good number compared to other studies. The results showed that the water withdrawal is greater than the aquifer's recharge, and a specific volume of the aquifer storage is always reduced during the simulation period, which caused a drop of 7.5 m in the groundwater level. The groundwater level of the South Mehyar-Dasht Asman aquifer continuously decreases from the western part to the eastern region, and the lowest groundwater level is related to the east part of this aquifer. Declining groundwater levels, water shortage conditions, and overexploitation, increase the risk of subsidence, which is an irreversible event. To prevent this from happening, the amount of water taken from the aquifer should be proportional to its capacity, and control and management measures should be carried out on a large scale. Computer models and up-to-date methods help to maintain and manage these water resources as well as possible. Factors such as the dryness of the Zayandehroud River, excessive harvesting, and lack of water have led to the aggravation of the adverse environmental effects of the Gavkhoni International Wetland, and this has increased the importance of simulating the existing aquifers in this basin. Due to the fact that no study has been done on the simulation of the groundwater of South Mehyar aquifer-Dasht Asman and also the duration of the simulation period is 15 years, this research can be a basis for future studies and provide the possibility of comparison with other studies.