## 1 Introduction  
The assessment of groundwater circulation in natural media, which are complex and irregular systems, needs specific hydraulic, hydrogeological and geometric assumptions and simplifications; this assessment can be carried out by resorting to aquifer modelling techniques, based on the use of different geo-informatics tools that allow for the proposition of possible scenarios of groundwater flow. The investigation of karst aquifers is not easy to carry out due to the complexity and heterogeneity of such natural systems, where groundwater circulation occurs through a well-structured net of cave and conduits [1]. The groundwater modelling in karst aquifers in southern Italy relies on some assumptions which represent the benchmark for the design of the conceptual model. In particular, the methodology followed using the MODFLOW numerical code implementation relies on the Darcy law and laminar conditions of the flow expressed by the Reynolds number, only applicable to the matrix domain of the aquifer. As the karst system is characterized by extreme types of porosity, connected to the presence of voids of different sizes, from the matrix to the conduit/cave systems [2], the Darcy law can be applied to the flow system into the matrix portion, whereas it becomes hardly achievable in aquifers that are strongly fractured and affected by a highly developed net of conduits/shafts, which need different approaches. Among different numerical codes, the tool which better simulates the groundwater flow for the saturated zone, at a regional and subregional scale [3], is the MODFLOW numerical code [4; 5; 6], which allows one to assess the flow path, calibrating the simulated hydraulic parameters on the observed data. Several authors focused on groundwater flow modelling to simulate the aquifer behavior and provide forecast scenarios, through different mathematical and statistical approaches [7; 8; 9]. However, groundwater models are affected by an uncertainty degree, because they represent a simplification of natural and complex hydrogeological systems. [PERSON] [10] proposed a model to simulate the flow within an aquifer that was moderately karstified, for a catchment area of the Swabian Alb, considering a porous-equivalent medium (EPM model). For Barton springs (Texas), which belong to a high karstified aquifer, a regional groundwater model has been proposed by [PERSON] et al. [11]. [PERSON] [12] highlights that the EPM hydraulic approach is suitable if we analyze the regional groundwater flow but that it is not taken into account to examine the hydraulic and hydrogeological aspects at the local scale and near the spring zone. [PERSON] and [PERSON] [13] focused on the effect of the karst/shaft conduit net on the karst aquifer response, proposing a discrete fracture model (DF), assuming that the flow occurs mainly in the conduit net and that no water exchange between the shaft net and aquifer matrix occurs. [PERSON] [14] simulated the groundwater flow in the Trifilia karst aquifer, Greece, providing different scenarios and using MODFLOW both in the steady state and for transient conditions. [PERSON] [8] proposed a simplified approach for groundwater modelling because all models in natural systems are simplifications; then, the number of parameters considered to build the model is the primary measure of its simplicity or complexity. Several groundwater models refer to regional groundwater flow [15], which better depicts the groundwater flow from the recharge area to the discharge zone, describing further the flow pattern in a hierarchically nested composite basin. The groundwater flow theory was widely proposed by [PERSON] [15] and other hydrogeologists and karst hydrogeologists; the theory was physically and mathematically demonstrated. Successively, the theory was applied considering specific tools which allow for the automation of the groundwater flow conceptual model.