## 1 Introduction  
Groundwater is necessary for human survival and development. The exploitation and conservation of groundwater resources are essential in underpinning sustainable aquifer development. In recent decades, extreme rainfall caused by climate change has impacted subsurface hydrologic systems [1, 2]. As climate variability intensifies, groundwater resources may alter, and water shortages may become more pronounced. Due to the frequent drought in many countries worldwide, groundwater resources stored in fractured rock masses in mountainous areas have gradually become alternative water resources to combat this natural disaster [3; 4]. In addition, much of the groundwater, either recent anthropocene groundwater [5] or older fossil groundwater [6], stored under porous and fractured rocks is vulnerable to water quality deterioration and groundwater depletion at geographically varying locations. Therefore, assessing the groundwater resource potential and quality in mountainous areas is a prerequisite. In Taiwan, groundwater resources are traditionally extracted from the plain area commonly composed of porous media. However, severe climate change and subsidence hazards in the plain areas of Taiwan have created restrictions on using such water resources [7]. The mountainous area of Taiwan, which occupies two-thirds of this island area, may offer the potential significance of freshwater compared to groundwater from the plain area of Taiwan. While determining whether aquifers in the mountain region can be considered an essential source of supplementary water resources, the first-ever catchment-scale Groundwater Investigation in the Mountainous region of Taiwan (GIMT) was initiated by the Central Geological Survey, Ministry of Economic Affairs (MOEA) of Taiwan in 2010 [8]. The main objectives of this project are to collect basin-scale hydrogeological data, build groundwater level observation wells for collecting continuous groundwater level changes, and develop a hydrogeological database for Taiwan. Although the project has pointed out the groundwater potentials of various formations from pumping tests performed in the existing boreholes, the groundwater level data continuously collected have not been fully utilized. One of the applications for such data can be to investigate groundwater's spatial and temporal dynamic behavior in mountainous areas, which may further help to determine effective strategies for developing and managing groundwater resources. Long-term groundwater level data can reflect changes in flow and gradients over time and space under natural and artificial effects [9]. Groundwater levels may rise or fall depending on various influencing factors that can be site-specific. Meanwhile, the groundwater level fluctuation pattern is an excellent signal for investigating the potential factors that affect groundwater dynamic behaviors, a hydraulic connection between aquifers, and the interaction between the surface water and subsurface water [10]. Previous groundwater studies pointed out that the groundwater fluctuation potential depends on many environmental factors, such as the lithology and hydraulic characteristics of formations, geomorphology, topography, land use/land cover type, regolith thickness, drainage density, and evapotranspiration [11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. Rainfall and snowmelt are the primary external factors that affect seasonal and annual groundwater level changes [21; 22; 23; 24]. In addition to rain under normal climate conditions, extreme rainfall caused by climate change may influence groundwater level changes. [PERSON] et al. [25] observed the relationship between groundwater level changes and climate changes through observation wells, climate, and hydrological data. The average monthly groundwater level can be assessed as a seasonal cycle dominated by rainfall and snowmelt. The nature of climate change is different and complicated, showing the complexity of the hydrological conditions and the groundwater system.