## 3 Materials and Method  
### The Study Area  
The island of Taiwan (Figure 3) covers an area of 35,873 km\({}^{2}\), and its central geographic coordinates are 23\({}^{\circ}\)58\({}^{\prime}\) N and 120\({}^{\circ}\)58\({}^{\prime}\) E. In Taiwan, 25 central government-governed rivers originate in the steep topography, which has an elevation ranging from sea level to 3952 m. The island is 394 km long and 144 km wide. The geological sections [7; 59] from east to west are delineated by major tectonic structures  
Figure 2: Hydrogeological characteristics of mountainous regions and illustration of groundwater exploration using remote sensing.  
and faults, including the Coastal Range (CR), Longitudinal Valley (LV), Taroko and Yuli Belt (TY) of the East Central Range, the Hsueshan Range (HS) and the Backbone Range (BR) of the West Central Range, the Western Foothills (WF), and the Coastal Plain (CP). In the evolution of plate tectonics in Taiwan, the Tananao Schist complex is composed of a metamorphic pre-Tertiary basement of the Eurasian passive margin, which has evidence of past orogenic events in recent metamorphic events. For instance, the TY belt was strongly deformed and metamorphosed in the Mesozoic period. Due to the Cenozoic arc-continent collision and orogen located on the boundary of Eurasia and the Philippines Sea plates, the slate belt in the HS and BR is connected with the Chinese passive continental margin, the CR corresponds to the accreted Luzon arc, and the LV is considered a suture zone between the arc and continental margin. In the western foreland basin, the WF at lower altitudes has been accreted and deformed with syn-orogenic sediments, and the CP is part of the present foreland basin. As a result of this orogenic movement, the terrain of Taiwan consists of approximately 1/3 plains and 2/3 mountain regions (Figure 3). The plain areas (e.g., CP and LV), where the formations of unconsolidated rocks are divided into aquifers and aquitards according to the hydrogeological characteristics, are delineated into 9 major groundwater areas and are the locations of most of the major cities. In the mountain regions (e.g., WF, HS, BR, TY and CR), the formation of consolidated rock mainly includes sedimentary, metamorphic, and volcanic rocks ranging from Eocene to Pleistocene with complex fracture networks of folds and faults.  
Due to the limited number of investigations to date, the central and southern mountainous regions of Taiwan were chosen as the study area. The area is about 19,098 km\({}^{2}\) in size, and it is 280 km long and 110 km wide. The eastern and western watersheds originate from the Central Range, below the  
Figure 3: Distribution of study area and in-situ sites implemented from 2010 to 2017. (**a**) Geology of Taiwan (after [7,59]); (**b**) topography of Taiwan. From east to west, Taiwan is delineated by major tectonic structures and faults, including the Coastal Range (CR), Longitudinal Valley (LV), Taroko and Yuli Belt (TY) of the East Central Range, the Hsueshan Range (HS) and the Backbone Range (BR) of the West Central Range, the Western Foothills (WF), and the Coastal Plain (CP). The major drainage in each watershed was extracted by a threshold value of flow accumulation.  
elevation of 3952 m, and include 11 rivers: the Daija, Wu, Jhuoshuei, Pachang, Zengwun, Gaoping, Sizhong, Hoping, Hualien, Xiuguluan, and Beinan rivers. The 118 drilling sites and 72 groundwater wells were distributed in different topographic terrains and in each geological section of the WF, HR, BR, TY, and CR, as shown in Figure 3.