## 1 Introduction
This will help in representing the spatial and temporal variations of potentiometric surface and flow direction, while considering the hydrogeological characteristics of the area. Groundwater flow modelling provides a tool to assess the groundwater hydraulics, and accounts for the hydrogeological setting and impact of external stress on the groundwater system. A calibrated model helps in simulating and predicting the high temporal water levels subjected to external stresses. It is used in various studies to quantify the impact of human intervention and environmental changes. [PERSON] et al. [14] used the SEAWAT [15] model to assess the climate change and landscape impact on salinization in the Variconi coastal wetland, Italy, due to seawater intrusion. [PERSON] et al. [16] used the two-dimensional FELOW [17] model to estimate the deep inflows and mixing of water in the Riardo plain, Italy, which helped to identify and quantify additional deep recharge terms in the water budget. [PERSON] et al. [18] used a two-dimensional groundwater flow and transport model to assess the influence of hydrological randomness on seawater intrusion in coastal aquifers. [PERSON] et al. [19] used numerical modelling to estimate the freshwater volume and the Reconnaissance Drought Index (RDI) to quantify the aquifer response to drought. [PERSON] et al. [20] assessed the impact of climate change on a coastal aquifer using density dependent numerical modelling. [PERSON] et al. [21] used numerical modelling to estimate the transmissivity of light non-aqueous phase liquid during a bail-down test. In addition to helping to understand the aquifer dynamics, numerical models can also be coupled with optimization algorithms to design monitoring networks. [PERSON] et al. [12] used flow modelling coupled with the probabilistic framework to locate optimal locations for the piezometers. Similarly, [PERSON] et al. [13] used numerical modelling and statistical analysis for designing a groundwater monitoring network. In the Indus Basin, and particularly the Central Indus Basin, groundwater modelling studies have been carried out to study the groundwater dynamics at regional and sub-regional levels. [PERSON] and [PERSON] [22] developed a regional flow model using the FEFLOW model to study the impact of adverse climatic conditions and plausible groundwater extractions on regional groundwater levels in the Upper Chaj Doab of the Indus Basin. [PERSON] et al. [23] calibrated the MODFLOW [24] and MT3D [25] models for the Rechna Doab of the Indus Basin to assess the impact of dry conditions and increased extraction on regional groundwater systems. Similarly, [PERSON] et al. [26] used modelling to explore options for equitable surface water and groundwater distribution to maximize the crop production and manage salinization in the Rechna Doab. In the Lower Indus Basin, groundwater modelling studies have been conducted at small spatial scales to study the groundwater hydraulics and hydro salinity behavior. [PERSON] et al. [27] calibrated the MODFLOW and MT3D models to optimize the operation of selected tube wells in Sindh. [PERSON] et al. [28] studied hydro salinity behavior using the calibrated MT3D model of the Kunner II irrigation distributary, Sindh. [PERSON] et al. [29] calibrated finite element groundwater models to investigate the impact of pumping and canal water levels on the waterlogging in the Khairpur district, Sindh. All of these studies were site-specific and lacked representation of the regional response of the aquifer. In this study, we calibrated a sub-regional MODFLOW model that provided a base tool for regional-level investigation in the freshwater zone in the LIB. This regional-level effort to numerically model freshwater lenses will be fundamental for informing evidence-based decisions for monitoring and managing groundwater systems. The approach presented in this paper is replicable in an irrigated area, where groundwater is used conjunctively with surface water for irrigation.