## 2 Materials and Methods  
### Study Area  
The PPGB is located in southern Taiwan and bounded by the low hills of sediments to the north, the foothills to the west, the Central Mountain Range to the east, and the Taiwan Strait to the south (see Figure 1). The oblique collision between the Philippine Sea Plate and the continental margin of the Eurasian Plate forms Taiwan Island and leads to a series of north-south mountain ranges and active fold-and-thrust belts [19]. The area covers approximately 1300 km\({}^{2}\), land surface elevation varies from the north about 100 m to the south approximately 0 m (see the contours shown in Figure 1a). The climate in PPGB is subtropical and rainfall is alternately affected by typhoons and monsoons in dry and wet seasons. Typhoons and thunderstorms regularly occur in wet seasons and are the leading cause of heavy rainfall. According to climate statistics for the period between 2010 and 2018, the annual precipitation in the PPGB varied significantly from 1431 to 3597 mm, and the average precipitation was 2331 mm. Figure 1 also shows the precipitation stations and the monitoring wells network developed in the groundwater basin. The long-term records of regional precipitation have demonstrated that the rainy season in the groundwater basin is from May to October and the dry season is from November to April in the next year. Specifically, the rainfall in the dry season is mainly affected by monsoons and total precipitation is about 10 percent of that during the rainy season. The Kaoping River flowsfrom north to south and is the largest river along the western foothills with an average slope of 1:43. The coupled natural processes (i.e., tectonic deformation and surficial deposition) and human activities (i.e., land-use and hydraulic infrastructures) altered the landscape in the upstream and downstream fan areas in the PPGB. In the study, the upstream fan is defined as the upstream area of the confluence point (i.e., the Li-lin Bridge) of the Kaoping River, where Ailiao and Laonung Rivers mainly flow in the east-west direction. In the downstream fan area, the flow directions of the Kaoping, Tungkang, and Linien Rivers are generally in a north-south direction to the Taiwan Strait. The downstream fan area is also the region that had faced severe problems in groundwater over-pumping and groundwater contamination [20]. The aquifer of the PPGB mainly consists of unconsolidated Quaternary deposits. The mountainous regions surrounding the PPGB are composed of Tertiary rocks [21]. The rocks of the Suao Group Formation are of Eocene-Oligocene age, located in the eastern part of the plain. The Chaozhou fault, stretching from north to south, separates the Suao Group Formation and the PPGB. Hundreds of meters of recent alluvium formed the groundwater basin. However, the maximum thickness of sediments of the recent alluvium is not determined because drilling logs have not shown the bedrock floor during  
Figure 1: The study area and the selected hydrogeological profiles: (**a**) the Pingtung Plain groundwater basin (PPGB) and the monitoring stations for surface water and groundwater, (**b**) the geological profile for cross-section b1-b5, and (**c**) the geological profile for cross-section a-a’. Note that the area in the upstream of the confluence point (i.e., Li-lin Bridge) of the Kaoping river, Ailiao, and Laonung rivers was defined as the upstream fan in the study. The downstream area of the Kaoping river and Tungkang and Linien rivers was defined as the downstream fan. the installation of wells. This recent alluvium includes three material types, including coarse gravel, alternating gravel and sand, and fine material zones. These unconsolidated sediments are also the primary materials distributed on the surface of the PPGB. The hydrogeological surveys were conducted since 1995. A groundwater monitoring network was also developed based on the characterization of hydrogeological surveys. In the PPGB, the monitoring network was designed to monitor groundwater levels in different aquifer layers.