## 3 Data  
To calculate groundwater productivity, the SPC are set as dependent variable, and various hydrogeological factors, which are known to influence groundwater productivity, are set as independent valuables. The SPC is a quantity which a water well can produce per unit of drawdown. It is normally obtained from a step drawdown test. SPC is expressed as:  
\[S_{c}=\frac{Q}{h_{0}-h} \tag{1}\]  
where  
\(S_{c}\) is the specific capacity ([L\({}^{2}\) T\({}^{-1}\)]; m\({}^{3}\)/day/m)  
\(Q\) is the pumping rate ([L\({}^{3}\) T\({}^{-1}\)]; m\({}^{3}\)/day), and  
\(h-h_{0}\) is the drawdown ([L]; m)  
The SPC of a well is also a function of the pumping rate it is determined at. The SPC of a well is simply the pumping rate (yield) divided by the drawdown. It can be used to identify potential well, pump, or aquifer problems, and accordingly to develop a proper well maintenance schedule. It can also be used to estimate the T of the aquifer(s) tapped by the well's perforations. T is the rate water is transmitted through an aquifer under a unit width and a unit hydraulic gradient. It equals the aquifer's hydraulic conductivity (permeability) times the aquifer thickness. The higher the T, the more prolific the aquifer and the less drawdown observed in the well. In the study, the T was not used. Instead of the T, the SPC was used because of the limited number of the T data [1,12]. Groundwater productivity is governed by many hydrogeological factors, such as surface and bedrock lithology, structure, slope steepness and morphology, stream evolution, climate, soil, vegetation cover, land use, and human activity, but for the study area these relationships have not been verified statistically and quantitatively. In this study, the 27 hydrogeological factors expected to be related to groundwater potential were reviewed with SPC. Finally, 19 hydrogeological factors for Boryeong city and 15 hydrogeological factors for Pohang city were selected according to the opinion of a groundwater expert and applied to the decision tree model (Table 2). The detail influences of the hydrogeological factors for groundwater productivity were analyzed by previous research [13, 14, 15]. To predict ground control problems in underground structures [16], lineament is strongly related to discontinuities such as joints, faults, and folds. For these reasons, lineament was used for structural analysis, analysis of the relationship with geology, and assessment of groundwater productivity. In this study, lineament was detected through interpretation of Landsat TM (Thematic Mapper) imagery and hillshade maps from a DEM (Digital Elevation Model) made by a structural geologist with much experience in such interpretations. Lineament density is useful for understanding the local distribution of lineaments. Lineament density analysis was considered to be the lineament length, frequency and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & **No.** & **Min.** & **Max.** & **Avg.** & **Median** \\ \hline Boryeong city SPC (m\({}^{3}\)/day/m) & 124 & 1.08 & 393.33 & 11.46 & 5.54 \\ Pohang city SPC (m\({}^{3}\)/day/m) & 83 & 1.42 & 111.14 & 16.54 & 6.73 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Statistics of groundwater productivity data of study area. cross points, and the density of frequency and length were also analyzed using the weight of lineament length in the study area. A TIN (triangulated irregular network) was made using the elevation value, and a DEM was made with 30 m \(\times\) 30 m resolution. One input factor, the ground elevation within 300 m, was given a mean value at each grid cell based on the value of neighboring cells within a 300 m radius. Using the DEM, the slope and SPI (stream power index) were calculated. Mean ground elevation and slope within watershed area were obtained after watershed delineation. River density and distance from the river were considered as the indicators for selection of groundwater potential sites because they indirectly indicate the permeability and porosity of the terrain.