## 1 Introduction
In the GSFLOW model, the coupling process employs iterative techniques to solve interdependent governing equations for surface water and groundwater flow. The calculations of detailed water balances in time and space are available in the models. The model also allows flexible spatial discretization of the hydrologic response units (HRUs) used for PRMS and the finite-difference grid used for MODFLOW-2005 [11]. Critical connections in the GSFLOW model refer to the unsaturated flow as well as stream interaction modules in MODFLOW. The important processes of SGIs are influenced by land-use change, climate variability, and groundwater exploitations. The interactions of surface water and groundwater play essential roles in hydro-environmental and eco-hydrological systems for a groundwater basin. The effects of agricultural activities such as surface water allocations, groundwater pumping, and irrigation on interactions of surface water and groundwater are very complex, which can significantly change the behavior of surface water and groundwater flow interactions [16]. Understanding the complicated behavior of SGI is very important for regional water resources management. Existing models have been intensively developed for addressing different water resources issues such as irrigation management, SGIs, land-use, climate change and water quality. In many previous studies, the GSFLOW model was applied to simulate the interactions between surface water and groundwater flows for interested watersheds [17; 18]. In view of the complex amounts of input data established in the GSFLOW model, the development of a simulation model for a case study covering a large area with multiple watersheds is a great challenge. The Pingtung Plain groundwater basin (PPGB) is located in southwest Taiwan and bounded by the Taiwan Strait to the south. The PPGB has a relatively large area, a dense river system, and the land surface elevation varies significantly. Due to the combination of natural processes (i.e., tectonic deformation, surficial deposition, and precipitation) and human activities (i.e., land-use and hydraulic infrastructures) applied to the PPGB, the landforms have evolved differently over time in the upstream and downstream areas in the PPGB. The highly non-uniform precipitation is obtained in wet and dry seasons, leading to strong fluctuations of river and groundwater levels. The variation of rainfall is evident in the upstream fan area of the PPGB. Understanding the dynamics of the SGI in the PPGB is the critical element to propose efficient strategies for water resource management. The effects of complex landforms and human activities have to be clarified before the implementation of the IHMs in the PPGB. The present study utilizes the GSFLOW numerical model to assess the SGIs in PPGB. The study aims to characterize the behaviors and dynamics of SGIs and to quantify the influence of interactions induced by the natural and anthropogenic effects. Specifically, the PPGB was divided into the upstream and downstream areas based on the flow patterns of the river system. The water balance for each component in the water cycle was systematically evaluated and quantified according to the detailed interaction dynamics in the PPGB. The calibration and validation of the model are based on three-year observations from January 2015 to December 2017. The evaluation of the developed model relies on different statistical parameters such as the coefficient of determination (R\({}^{2}\)), coefficient of correlation (RE), Nash-Sutcliffe efficiency coefficient (NSE), normalized root-mean-square deviation (NRMSE), and percent bias (Pbias). A scenario of an artificial recharge reservoir was considered to assess the local impact of increased recharge on water balance variations in the simulation area.