## 3 Conceptual Models and Numerical Considerations
### Conceptual Model and Parameters for Groundwater Flow
However, the downstream of the Kaoping River (i.e., the southern segment of the Li-lin Bridge station), the Tungkang River, and the Linien River were included in the downstream fan watershed (see Figures 1 and 4). In the study, the entire river system in the conceptual model of the PPGB was divided into 13 stream segments and 166 stream reaches. We fixed the streambed thickness of 3 m. In the PPGB, there are two streamflow stations Li-lin Bridge and Xing She Da Qiao. The Li-lin station monitors three main tributaries of the upper Kaoping, Laonung, and Ailiao Rivers. However, the Xing She Da Qiao station is on the Tungkang River. The time series of river flow observations were the basis for the GSFLOW model calibration and verification. The basic hydrological parameters used in the simulation are the hydraulic conductivity and storage coefficients. In the study area, aquifer parameters had been obtained based on the hydraulic tests conducted during the development of the groundwater monitoring network [22]. Previous site investigations had recognized that the hydraulic conductivity varies significantly from 0.8 to 106 m/d in the PPGB. Most areas in the PPGB consist of gravel and sand deposits, and the values of hydraulic conductivity range from 70 to 80 m/d. Specifically, in the upstream fan area and some regions in the downstream fan areas, the values of hydraulic conductivity are within the interval of 90 to less than 110 m/d. These areas cover the upstream areas of the Kaoping River, the Laonung and Ailiao Rivers, and the upstream basins of the Tungkang and Linien Rivers. In general, hydraulic conductivity values of 10 to 70 m/d occur along the downstream area of the Kaoping River [22]. Areas with low hydraulic conductivity values of the PPGB are in the lowland areas between the downstream Tungkang and Linien Rivers. A typical anisotropic ratio of 1/10 was used in the study to account for the vertical hydraulic conductivity. Because the model considered one conceptual layer, the influence of the vertical hydraulic conductivity might be insignificant. The unsaturated zone flow (UZF) package simulates the water flow from the soil zone to the saturated aquifer (i.e., below the groundwater level). In the MODFLOW model, the physical parameters in the UZF package play an essential role to account for the recharge from the surface water to the groundwater. In the study, the parameters such as the Brooks-Corey exponent in the UZF package, the saturated water content, and the initial unsaturated water content are 3.5, 0.25, and 0.2 [25]. The MODFLOW-2005 model uses the parameters in the UZF packages. However, in the GSFLOW simulation, parts of the UZF features in the MODFLOW have been replaced by the PRMS packages to estimate the detailed evapotranspiration and infiltration dynamics. In the study, we used the PEST module to calibrate the groundwater flow model. The calibration and validation relied on the GMS software environment. We found that PPGB-recharge and PPGB-pumping rates were the main sensitive parameters regarding model calibration.