# Assessing Groundwater Level with a Unified Seasonal Outlook and Hydrological Modeling Projection
## 4 Results and Discussion  
### Model Validation  
Simulation settings for all parameters in the modeling calibration were fixed. To validate the parameters used in the model, a full-year simulation of 2017 was conducted. As shown in Figure 6a, a comparison of groundwater table variation between simulation and observation data was conducted. This shows that the groundwater level hydrograph of simulation agreed with that of observations at Xinpu station, and the general trends were also comparable before 18 August 2017. The results prior to 18 August show that the errors between simulated and observed groundwater levels were \(R^{2}\) of 0.801 and RMSE of 0.228 m; thus, the simulation model yielded a great result. However, following 18 August 2017, there was a sudden decline in groundwater level observations, resulting in a discrepancy between simulated and observed groundwater level. The error indicators of the simulation without considering pumping after 18 August 2017, shown as cross dots, were \(R^{2}\) of 0.052 and RMSE of 0.862 m. In fact, errors gradually increased after this. Anthropogenic pumping activities were expected to be the primary reason. A crucial discharge mechanism or substantial water use could have caused this abnormal drawdown, which is not easily incorporated into the simulation. As the rainfall during this period was not sufficient to replenish the groundwater, and the amount of groundwater pumped increased, a drawdown in the water level inevitably occurred. Therefore, the effect of pumping was further considered and taken as a factor in the following simulation. Note that records of pumping rate were only adopted from the registered well, while the records from the unregistered well were not exploited in this study because the amount of pumping water from the unregistered well is unknown. Moreover, it was considered minor due to our study site not belonging to the land subsidence areas in Taiwan. Thus, there was not a problem of groundwater being over-pumped. Moreover, cases of pumping groundwater were found to be rare after conducting an investigation of unregistered wells and giving well permits in recent years. Thus, the effect of unregistered well pumping is relatively minor. The pumping rate of the registered well at Xinpu Station was added to the simulation with 3000 cmd (cubic meter per day, cmd). The results are shown in Figure 6a. The hydrograph revealed that the simulated groundwater level when considering pumping, shown as green dots, corresponded with the observed hydrograph (\(R^{2}\) of 0.256, RMSE of 0.749 m). The simulation result was clear as the error of RMSE was minimized, and the value of \(R^{2}\) increased after the effect of pumping was adopted in the study. The simulation result was improved by including anthropogenic pumping. Moreover, this also implies that these parameters are suitable for simulating groundwater in the Fengshan Creek basin. In Figure 6b, there was an extreme event showing the flow patterns for both river and groundwater from 2 June to 14 June in 2017. The simulation results reveal that the simulated hydrograph of the river stage and groundwater level corresponded with the observation results in both the raising and recession periods for the water stage and groundwater level. The lag time following a rainfall event was approximately 2 days (51 h), which is consistent with the average lag time of 2013-2015 simulations. The errors between simulated and observed river water levels were \(R^{2}\) of 0.729 and RMSE of 0.418 m, while the errors between simulated and observed groundwater levels were \(R^{2}\) of 0.826 and RMSE of 0.426 m. According to the above data (surficial: \(R^{2}\) of 0.729 and RMSE of 0.418 m; groundwater: \(R^{2}\) of 0.826 and RMSE of 0.426 m), the high correlation between simulated and observed water level, whether stream flow or groundwater, suggests that the rise of surficial rainfall-induced groundwater level can be accurately assessed by our simulation.