## 4 Results
### Mapping of Groundwater Potential
#### 4.5.3 Application of the NGPI to Predict Groundwater Potential and Verification of In-Situ Data  
Since the very high level of NGPI was above 0.63, the threshold of the NGPI for identifying GWPS and non-GWPS was determined to be 0.63. To apply and inspect the NGPI for groundwater exploration, the well yield data of 48 sites in central Taiwan were employed as training data. The value achieved an accuracy of 83.3%. According to previous experience, 24 sites in southern Taiwan were predicted. The estimated results achieved an accuracy of 87.5%, while that for all the tested sites was 84.7% (Table 5). On the other hand, using a threshold of 0.63 to predict more well yields (higher than 300 or 600 L per minute) as GWPS resulted in lower accuracy. Therefore, the definitions of GWPS were met by the degree of 0.63-0.97 NGPI (very high) and verified with in-situ data. The verified NGPI value of 0.63 is a significant threshold for identifying local and regional water supplies in the follow-up groundwater exploration in the ACC.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**No.** & **Region** & **Site** & \begin{tabular}{c} **Well Yield** \\ **(L/min)** \\ \end{tabular} & \
\begin{tabular}{c} **GWPS or** \\ **Non-GWPS 1** \\ \end{tabular} &
\begin{tabular}{c} **Regolith** \\ **Depth (m)** \\ \end{tabular} & **HGU** & **Terrain** & **NGPI** \\ \hline
66 & & B104-07 & 738 & GWPS & 45 & BRSP & Vm & 0.67 \\ \cline{2-8}
67 & & B105-01 & 153 & GWPS & 15 & WFSS & Sf & 0.70 \\ \cline{2-8}
68 & & B103-05 & 840 & GWPS & 7.5 & WFSS & Vbf & 0.70 \\ \cline{2-8}
69 & & B103-01 & 840 & GWPS & 32.2 & BRSP & Vm & 0.73 \\ \cline{2-8}
70 & & B103-02 & 900 & GWPS & 31.7 & BRSP & Vm & 0.73 \\ \cline{2-8}
71 & & B103-03 & 300 & GWPS & 25.9 & BRSP & Vbf & 0.73 \\ \cline{2-8}
72 & & B103-04 & 840 & GWPS & 5.2 & BRSP & Vm & 0.77 \\ \hline \multicolumn{8}{c}{Average value} & 203.9 & - & 15.9 & - & - & 0.60 \\ \hline \hline \end{tabular}  
* A GWPS is defined as having a well yield of greater than 60 L per minute; otherwise, it belongs to non-GWPS where groundwater is insufficient for water supply. Abbreviations: near roof (Rf), at ridge (Rg), steep slope (Ss), flat slope (Sf), valley or creek bottom (Vb), alluvial fan of downstream the valley (Vbf), main riverbed deposit (Vm).  
\end{table}
Table 4: _Cont._  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Region** & **NGPI** & \begin{tabular}{c} **Well Yield** \\ **(L/min)** \\ \end{tabular} & \begin{tabular}{c} **Number of Sites** \\ **(Predicted)** \\ \end{tabular} & \
\begin{tabular}{c} **Number of** \\ **Sites (Actual)** \\ \end{tabular} &
\begin{tabular}{c} **Accuracy** \\ **(\%)** \\ \end{tabular} & **Total (\%)** \\ \hline
**Central Taiwan** & \(\geq\)0.63 & \(\geq\)60 (GWPS) & 22 & 17 & 77.3 & \\ (Training data) & \(<\)0.63 & \(<\)60 (non-GWPS) & 26 & 23 & 88.5 & \\ \hline
**Southern Taiwan** & \(\geq\)0.63 & \(\geq\)60 (GWPS) & 12 & 12 & 100 & \\ (Predicted results) & \(<\)0.63 & \(<\)60(non-GWPS) & 12 & 09 & 75.0 & 87.5 \\ \hline
**Whole study area** & \(\geq\)0.63 & \(\geq\)60 (GWPS) & 34 & 29 & 85.3 & \\ (\(>\)60 L/min as GWPS) & \(<\)0.63 & \(<\)60 (non-GWPS) & 38 & 32 & 84.2 & \\ \hline
**Whole study area** & \(\geq\)0.63 & \(\geq\)300 (GWPS) & 34 & 14 & 41.2 & \\ (\(>\)300 L/min as GWPS) & \(<\)0.63 & \(<\)300 (non-GWPS) & 38 & 36 & 94.7 & 69.4 \\ \hline
**Whole study area** & \(\geq\)0.63 & \(\geq\)600 (GWPS) & 34 & 11 & 32.4 & \\ (\(>\)600 L/min as GWPS) & \(<\)0.63 & \(<\)600 (non-GWPS) & 38 & 38 & 100 & 68.1 \\ \hline \hline \end{tabular}  
* The prediction accuracy of all the tested sites to identify GWPS with well yields exceeding 60 L per minute was about 84.7%. Thus, the reliability of the synthesized map of NGPI was improved.  
\end{table}
Table 5: Statistical results of NGPI, well yield, GWPS, and non-GWPS.