## 4 Results
### Correlation between GPP and the Variables
The areas with unconsolidated clastic sediments were shows that have a stronger GPP than areas with carbonated rocks because groundwater cannot easily penetrate between their particles that were so tiny and dense. With regards to land cover, groundwater potential values were higher for paddy fields and urban areas and lower in the mixed forest area. In fact, it is highly probable that there are many wells containing a large amount of groundwater in the mixed-forest covered area, but the frequency ratio was relatively low because 65% of the study area was covered with mixed-forest. In the case of soil texture, the frequency ratio was higher for the D class (very slow infiltration rate) and lower for the C class (low infiltration rate). Sandy soil (D class) has an excellent effect on groundwater penetration because of its high permeability. Conversely, clay soil (A) has a low impact on groundwater accumulation because of its poor drainage capability and permeability. In case of plan curvature, concave areas have a ratio of about 1.2 which is considerably higher than that of convex areas (approximately 0.70). Concave surfaces contain more water, particularly during periods of heavy rainfall. Therefore, areas with concave surfaces are more advantageous than areas with convex surfaces for storing groundwater. The GPP frequency ratio generally increases with increase in linear density. That is, the nearer the linear density is to 0, the lower the GPP generation is. When the value of linear density is larger, the GPP generation is also higher. With regards to the topological factors, such as distance from a fault, lineament, and channel network, the closer the area is to a river, the higher is the likelihood of the groundwater productivity. The longer the distance is, the lesser is the likelihood of groundwater generation. In other words, various linear structures and remote areas were leakier, while the nearby areas had better recharge and higher penetration. The depth of groundwater was highest between 6 m and 12 m. The TWI index is defined as a function of the upstream contributing area per unit and the slope gradient. The results have shown  
that as the TWI value increases, the groundwater productivity ratio also increases. This is because high wetting index demonstrates better groundwater retention capability of an area. The TRI index represents the altitude difference between adjacent cells in grid. That is, the higher the TRI value, the greater is the difference in altitude between adjacent areas. Therefore, low TRI value indicates high groundwater content because the GPP is higher at low altitude differences. A drainage basin represents a catchment area. When the TRI value is between 100 and 105, the GPP is the highest. For mapping the GPP, the LR coefficients of the 18 factors were computed using the SPSS software (Table 3). The LR coefficient represents the probability of occurrence, and this value typically ranges between 0 and 1. If the value of multiple logistic coefficients is calculated to be less than 0, the GPP is low. This is so because GPP becomes smaller than 1 when converted to the corresponding log value. Positive values were obtained for the relative slope position, TWI, lineament density, distance from lineament, drainage basin, and TRI when T productivity was used in the LR model. SPC productivity values were positive for LS factor, lineament density, distance from lineament, depth of groundwater, drainage basin, and TRI. Non-porous volcanic rocks had the lowest T and SPC values for the hydrogeology factor, thereby indicating the lowest impact on GPP. The urban areas had the highest values of land-cover factor. The flat item showed least influence on the GPP corresponding to plan curvature and convergence index variables. Table 4 shows the importance of the values of each predictor variable in the BRT and RF models. The data in the table also explain the correlation between GPP and the related factors. The predictor importance ranges between 0 and 1.