## 3 Conceptual Models and Numerical Considerations
### Conceptual Model and Parameters for Groundwater Flow  
The reports of Taiwan CGS defined four aquifer layers with a total thickness of more than 200 m in the PPGB [22]. The groundwater monitoring network had shown a few interactions between the first layer and the deeper layers in the downstream fan area. Monitoring data also indicate that the first aquifer layer near the coastal line exhibits a good connection with the ocean. It is recognized that the topsoil influence and dominate the characteristics of the SGIs. In the PPGB, most pumping wells are installed in the first layer. In the study, the conceptual models then considered the first layer of the PPGB aquifer system for the simulations. The MODFLOW model used 40 rows and 90 columns with a uniform cell size of \(1\times 1\) km\({}^{2}\) for the entire PPG (see Figure 4). Figure 3: The calibration and validation workflow of the numerical model in the study. The transient flow simulations of MODFLOW-2005 involve 36 time steps ranging from 1 January 2015 to 31 December 2017. However, the time step of the MODFLOW in the GSFLOW simulations will become a day to resolve the daily variations of SGIs. In the MODFLOW model, the initial condition was specified based on the results of a steady-state simulation for the month of December in 2014. The steady-state model had been calibrated based on the monthly averaged data in December 2014, which is a month earlier than the starting time of the simulations. In the study, the altitude, topographic slope, and river network are distributed differently in the upstream and downstream fan areas. In the GSFLOW model, we considered two sub-watersheds to obtain the SGIs for upstream and downstream fan areas (see Figure 4). The upstream and downstream fan areas were divided based on the Li-lin Bridge. Note that the two sub-watersheds for upstream and downstream fan areas share the same groundwater model for the entire PPGB. The MODFLOW-2005 model enables the consideration of different types of boundary conditions, including the default no-flow boundary condition, the specified hydraulic head boundary condition that can vary within or between stress periods, and the constant hydraulic head boundary condition that holds a fixed head value in all stress periods [14]. In the study, the hydrogeological boundaries for the entire PPGB domain are the western foothills, the northern low hills, the eastern mountain central range, and the southern Taiwan Strait. The no-flow boundaries are specified along the western foothill, the northern low hills, and the eastern central mountains. The boundary condition along the shoreline of the Taiwan Strait was the constant hydraulic head boundary condition. The specified hydraulic head boundaries are assigned to the inlet valleys of the rivers on the boundaries (see Figure 4). Note that the hydraulic head values at the river inlets used the observations of water levels from the nearest  
Figure 4: The conceptual model and boundary conditions for the specified simulation domains. The dashed line defines the boundary for upstream and downstream fan areas. monitoring wells. Such specified hydraulic head boundary conditions allow assigning hydraulic head values that change over the simulation time steps. In the study, the aquifer thickness for the PPGB follows the definition of aquifer layers reported by Taiwan CGS [22]. In general, the aquifer thickness varies within the range of 60 to 100 m.  
The river system in the study area consists of three main tributaries, namely the Kaoping River, the Tungkang River, and the Linien River. All the rivers drain into the Taiwan Strait. Based on the definition of upstream and downstream fan areas in the study, the upstream of the Kaoping River (i.e., the northern segment of the Li-lin Bridge station) and along with the Aliao and Laonung Rivers form the upstream fan watershed (see Figure 1 and the dashed line in Figure 4 for the boundary).