## 1 Introduction  
Water resources are indispensable to mankind, society, and countries and have a tremendous impact on people's livelihood as well as national agriculture, industry, and economy. When the water supply from rivers and alluvial aquifers is insufficient and unstable, it is crucial to explore substitute water supply from unexploited areas, such as mountainous regions [1; 2; 3; 4; 5; 6]. However, most mountainous regions are situated in geologically complex terrains (GCTs) with heterogeneous hydrogeological features. For example, Taiwan is located in the subtropical monsoon climate zone and arc-continent collision (ACC) between the Luzon volcanic arc and the Eurasian continent [7; 8; 9]. Due to the rugged terrain and uneven spatial and temporal distribution of rainfall, Taiwan's annual precipitation is about 2.5 times higher than the world's average, but the water supply available to the people per capital is only 20% of the world's average. Thus, exploration of groundwater potential in such a geological environment is a challenge. The high cost of pointwise exploration of groundwater resources and inconvenience of conducting on-site tasks in remote areas are often noticed in engineering practice. Thus, an alternative approach to achieve cost-effective groundwater exploration, and therefore identify groundwater potential sites (GWPS), is of great importance. Integrating remote sensing (RS) and geographic information system (GIS) is a possible alternative [10; 11; 12; 13; 14; 15; 16; 17]. This study lists case studies in bedrock, and three categories of parameters favorable to groundwater (i.e., geology, topography, and remote sensing index), as shown in Table 1[1; 12; 18; 19; 20; 21; 22; 23; 24]. The literature provides useful information on the characteristics of regional hydrogeology and geomorphology. However, certain case study areas were delineated by the boundaries of the administrative area or random rectangle. They were mostly located in terrains with simple geological units and slope degrees. At these local scales, the high-ratio units of each parameter may have more influence on the results, such as rock types and slope degrees; even the adjacent fracture network of bedrock outside the study area was neglected. It needs to consider the hydrogeological model at the scale of the entire drainage system and the regional groundwater flow system. Additionally, most data sources are in vector format with a scale of 1:50,000 or less, such as geological maps and landform maps; certain data can be analyzed as raster files from given spatial resolutions of the digital elevation model (DEM) and remote imagery. In order to achieve consistency of data sources, a raster-based empirical GIS model is recommended to calculate the thematic map and validate it with investigated data. In this paper, the above issues are solved at the scale of catchment management in the GCTs of Taiwan, where the seven categories of terrain were defined by topographic position index (TPI), topographic wetness index (TWI) and slope degree (SD), and the 17 hydrogeological units (HGUs) were delineated from digital geological maps. In addition, the regional lineament and abnormal surface temperature favorable to groundwater were considered and derived from RS. Comparing in-situ well yield data, the accuracy of the interpretation of GWPS varied with rock types and terrains. However, assessing the normalized groundwater potential index (NGPI) in the comprehensive RS-GIS model improved the results to achieve a cost-effective method to identify GWPS.