## 1 Introduction  
The geological environment of mountainous areas in Taiwan is characterized by activerogenesis and abundant rainfall, resulting in complex hydrogeological characteristics of the rock strata [1]. Understanding the hydrogeological characteristics is of importance for constructing and designing engineering works in fractured rock mass [2]. Moreover, collecting such information can help establish a hydrogeological site descriptive model [3, 4]. While engineers conduct such an investigation, a series of single-borehole hydrogeological surveys, including borehole drilling, laboratory tests for rock samples, and conventional hydraulic testing, have to be carried out. This survey method has the advantage of less workforce and lower costs. Nevertheless, the survey accuracy may not be satisfied for dealing with more complicated geological conditions. To obtain better survey results, engineers then plan more intensive survey points, as well as advanced and cross-hole hydrogeological tests, to achieve this task. The improved survey technologies include multilevel monitoring approaches [5], tracer tests [6, 7], cross-borehole flowmeter tests [8], double-packer hydraulic test [7, 9], distributed temperature sensing [10, 11, 12],borehole radar [13; 14], hydraulic tomography [15; 16], seismic [17; 18], and fractured rock passive flux meters [19; 20; 21]. Although these methods with case studies have proved that more information in identifying fractured rock network systems can be obtained, the survey's cost is not affordable for budget-constrained projects. Whether or not there are cheaper alternatives that can provide the same information was the focus of this study. To reduce investigation costs, one of the possible methods is to simply use existing data to perform a value-added analysis of the data, to reveal comprehensive hydrogeological information around a borehole. This study examined available data from past research projects in Taiwan mountainous regions. The existing data showed that the fracture network properties mainly relied on investigations of individual borehole scale. Based on the sparse data, the hydrogeological site descriptive model was then developed. As an example, the groundwater resource exploration project in the Taiwan Mountain Region initiated by Taiwan Central Geological Survey [22] was a valuable study in seeking alternative water resources for combating drought. The purpose of this study was to understand the flow behavior of groundwater in the mountain region and to locate sites with high potential groundwater resources. However, the determination of the potential sites only relied on borehole-scale hydraulic testing data. From the engineering design point of view, if two- and three-dimensional hydrogeological information can be explored based on the existing data from single boreholes, the interpretation of the groundwater resource potential is more reliable and practical in helping the information needed for actual engineering development and design. The typical use of the existing hydraulic test data, measured by double-packer hydraulic testing with a fixed interval of 1.5 m, was to provide information on the hydraulic conductivity. However, hydraulic testing using a constant head test lasts for a long period. During the testing, variations of water injection rate into rock mass may reveal fracture network properties (e.g., fracture network pattern and degree of connectivity) in an extended area from the testing borehole. A few studies attempted to use the concept of flow geometry or flow dimension to understand geometries of fracture network systems [23; 24]. Their studies also reported that the flow dimension was expected to be low in a relatively spare fracture system; the flow dimension was expected to be high in a relatively dense fracture system. Indeed, most traditional flow models, such as the Theis equation [25], for flow toward a borehole assume a radial flow.