## 5 Discussion
### Verification of the Assessment Performance  
Figure 2a,b shows the assessment results based on Aller's common and pesticide DRASTIC models, which are consistent. Based on the comparison of the two graphs, it can be concluded that Aller's common DRASTIC version is superior to the pesticide version. This conclusion is similar to that of a previous study [34]. The difference between the two versions are mainly the different weights of the soil and the topography. The weights of the soil and topography of pesticide are significantly higher than the original version because physical, chemical and biodegradation reactions occurring in the soil can effectively degrade pollutants. Significant topographical changes can also lead to the loss of surface runoff pollutants and reduce the potential of pollution. Due to the flat terrain of the study area, the impact of topography weight is negligible. Nitrate with specific chemical and physical properties cannot be adsorbed by the soil. This makes reductions in nitrate in the soil very limited. As a result, higher soil and topography weights are no longer valid, and such a result is reasonable. Figure 2c,d shows the results of the improved DRASTIC model evaluation using the MC and GA methods for the weight determination. The results notably differ from those of Aller's common and pesticide DRASTIC models. The consistency between the distribution of the vulnerability indexes and the distribution of the black circle symbols (symbol size represents the nitrate concentration), may indicate the accuracy of the evaluation results. The results derived from the data-driven methods are better than those obtained with Aller's common and pesticide DRASTIC models, which overestimate the areas with high and very high risks. Many pixels with high and very high risks based on the two unmodified versions are assigned to medium- or low-risk areas based on the new results (Table 7). The correlation coefficients can also be used to confirm the assessment performance.  
Based on the comparison of Figures 2d and 3a, the distribution of the changes in nitrate concentration is consistent with the distribution of the vulnerability indexes, which implies that the results of the assessment are accurate on a time scale. Figure 3b quantitatively confirms this result, which again shows that the GA and MC methods are effective. Based on the long-term changes in nitrate concentration and consistency between the assessment results and changes in nitrate concentration, it can be concluded that the level differences can be used to verify the accuracy of the vulnerability assessment in terms of time. Therefore, the accuracy of the current evaluation results can be verified by the correlation coefficients and the long-term reliability of the vulnerability assessment can be determined using the level differences.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Vulnerability Level**} & \multicolumn{2}{c}{**Aller’s Common DRASTIC**} & \multicolumn{2}{c}{**GA DRASTIC**} \\ \hline  & **Percentage (\%)** & \begin{tabular}{c} **Cumulative** \\ **Percentage (\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Percentage (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Cumulative** \\ **Percentage (\%)** \\ \end{tabular} \\ \hline Very Low & 28.26 & 28.26 & 27.37 & 27.37 \\ Low & 25.62 & 53.89 & 39.35 & 66.72 \\ Medium & 23.54 & 77.43 & 17.81 & 84.53 \\ High & 16.83 & 94.26 & 10.88 & 95.41 \\ Very High & 5.74 & 100 & 4.59 & 100 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Results of the evaluation of Aller’s common and GA DRASTIC models.