## 1 Introduction  
Groundwater represents the third most abundant component in the global water cycle, along with oceans and snow (glaciers). Groundwater and glaciers comprise more than 97% of all fresh water on earth [1]. Moreover, they are two of the most important sources of drinking water, used for many different purposes such as agriculture, industry, water supply to the population, etc. [2; 3; 4]. Groundwater research has been gaining importance in recent years due to the growing awareness of the need for groundwater, as well as the increasing decline in its quality and quantity. The numerous issues that have arisen during groundwater research, such as forecasting groundwater level and, consequently, determining the aquifer thickness, have led to the development of various simulation models which describe and predict the groundwater flow [5]. Numerical modeling is a powerful tool that serves to provide a better comprehension of groundwater flow and to obtain information about important groundwater parameters. The most important aspect of the use of such models is acquiring data on the real conditions occurring within the observed environment. Namely, such models provide sufficient information for the efficient management of groundwater resources [6; 7; 8; 9; 10]. The first step in the modeling process is creating a conceptual model. In the next step, this conceptual model is converted into appropriate mathematical expressions which are combined with boundary conditions to form a mathematical model. Although these models are quite complex and characterized by a large amount of input data, their application is extensive. One of the most commonly used numerical modeling software is GMS based on MODFLOW 2005, developed by Aquaveo, LLC in Provo, Utah, USA [11]. A number of authors have used MODFLOW in groundwater modeling for various purposes. Some of them combined it with other models and software for more reliable research results. For example, in [12], the groundwater level was estimated using three models: MODFLOW, ELM, and WA-ELM. Based on the obtained numerical results from all three models, WA-ELM was proven to be a superior model for groundwater level simulation. [PERSON] and [PERSON] [13] analyzed constructed wetlands as alternatives for decentralized wastewater treatment. They used MODFLOW and MODPATH software included in Groundwater Modeling System (GMS) version 9.0.3, based on the finite difference method, to simulate flow in wetlands numerically. The model has proven to be a powerful tool for 3D simulation, allowing the representation of flow distributions, velocities, elevations, and particle trajectories. [PERSON] et al. [14] analyzed arid regions characterized by groundwater withdrawal. They applied a three-dimensional finite difference model (MODFLOW) to a unique aquifer to assess the impact of different groundwater pumping scenarios on aquifer depletion. The study showed that the existing pumping rates could lead to an alarming drop in the groundwater level. Furthermore, Visual-MODFLOW 2000 was utilized for groundwater level analysis in the Purba Midnapur area (West Bengal, India) in [15]. The study was designed to predict the groundwater level in the future usage scenarios for the purposes of better groundwater management. [PERSON] et al. [16] conducted a hydrogeological study to find out the cause of groundwater flooding. The flow model was calibrated for steady and unsteady state using the automatic calibration code, Model-Independent Parameter Estimation (PEST). To evaluate the Jilin urban area's groundwater resources and aquifer system (UUA, Jilin province in northeast China), [PERSON] et al. [17] established a numerical groundwater flow model using GMS based on data from 190 boreholes. Recharge proved to be the most sensitive factor in this model. Based on the supply and demand analysis of water resources, the developed model could finally provide a scientific basis for using groundwater resources sustainably in JUA.