**4. Discussion**

The purpose of this study was to discover the highly sensitive variables of a groundwater simulation model adjacent to an ARL using short-term rainfall. The model was established by MLP with past 10-day short-term events. Normalized rainfall (R1 to R5), rainfall intensity (RI1 to RI5), and groundwater data were selected as model variables. The R<sup>2</sup> was used for model accuracy assessment [18]. A sensitivity analysis (SA) was conducted to evaluate the importance of each model input. The study results are discussed as follows.

First, during the model establishment, the MLP model was suggested for development [5,40,41]. The best fit R<sup>2</sup> of the MLP model was 0.914 (Well 3) and the lowest R<sup>2</sup> in MLP was 0.759 (Well 5). This model uses rainfall as an input; the rainfall converges in the ARL and the river then infiltrates into the groundwater layer. The farthest well (Well 5) from the Lin-Bien River and the ARL responded more slowly than the other wells with a lower R2. Comparing Well 5 with a similar location well (Well 6), the well screen of Well 6 (13–25 m, above sea level) was shallower than that of Well 5 (−17 to 5 m, above sea level). Therefore, the rainfall response in Well 6 was faster than in Well 5.

Second, a sensitivity analysis was conducted for input importance evaluation. The highest and second highest inputs in Well 1 were R2 and RI3; in Well 2, they were R2 and R3; in Well 3, they were R4 and R2; in Well 4, they were RI2 and RI1; in Well 5, they were R5 and R4; in Well 6, they were RI4 and R2; and in Well 7, they were R4 and R5. The rainfall amount (R) was more sensitive than rainfall intensity (RI) in this research area.

Finally, when considering the SA results for the Lin-Bien River catchment area (Figure 2) and the relevant hydrogeological profile (Figure 3), a slight difference between "within the catchment" and "outside the catchment" can be found. Wells 1 to 3 are in the study catchment and adjacent to Lin-Bien River and the ARL, and responded faster in past 3-day rainfall (R1, R2, R3, RI1, RI2, RI3, respectively) than the other four wells. On the other hand, R4, R5, RI4, RI5 were found to be sensitive in Wells 5 to 7. Although Well 4 was located outside the catchment, it still had a certain degree of sensitivity from past 3-day rainfall (R1, R2, R3, RI1, RI2, RI3, respectively) because the location of its well screen was similar to the one in Well 3. Moreover, R2 was highly sensitive due to the well screen being located above sea level, that is, Wells 1, 2, and 6. It can be concluded that the groundwater table variation in this study area is response-related with the distance from the wells to the river and the ARL, and the rainfall time-lag.
