## 4 Results and Discussion
### Model Calibration and Validation
Note that the calculations of hydraulic head differences are based on the cell-by-cell comparison of means of hydraulic heads for wet and dry seasons from 2015 to 2017. The results demonstrate that the significant differences of heads in wet and dry seasons are in the upstream areas of rivers in the PPGB. Figure 7b shows that the hydraulic head differences are similar along the rivers because the landforms and stream conditions in the model have significantly constrained the hydraulic head variations near the rivers. These areas with large hydraulic head differences are in the upstream zones of Kaoping, Laonung, and Ailiao Rivers. However, the highest variation of hydraulic heads in wet and dry seasons is in the upstream of the Linien River. The averaged head differences can vary over 4 m, indicating the great environmental impact of wet and dry seasons on the groundwater variations. Additionally, the upstream areas of the Ailiao and Laonung River also show relatively high hydraulic head differences in wet and dry seasons. Such high groundwater variations are mainly induced by the considerable variations of topography in these upstream areas. These areas have been defined to be the recharge sensitivity zones for groundwater resource protection in the PPGB [26]. With the significant observation from previous investigations, an artificial recharge lake is therefore determined in the upstream area of the Linien River to enhance the recharge and mitigate the impact of the  
Figure 6: Comparison of observed (solid symbols) and simulated (dashed and solid lines) groundwater levels for the six selected wells. The monthly average rainfall data used the weather stations that are closest to the observation wells in the PPGB: (**a**) variations of groundwater levels at Jiyang station, (**b**) groundwater levels at Chiuju station, (**c**) groundwater levels at Jianxing station, (**d**) groundwater levels at Yongfang station, (**e**) groundwater levels at Kanding, and (**f**) groundwater levels at Fangliao station. Note that the dashed and solid lines represent the calibrated and validated results obtained from the groundwater model. See Figure 1 for the locations of the observation wells. over-pumping on the aquifer system near the coastal area. Assessing the effect of the anthropogenic construction on the interactions of groundwater and surface water will be presented later in the study. Figures 8a and 9a show the results of observed and simulated streamflow at the Li-lin Bridge and Xing She Da Qiao stations. Similar to the groundwater model, the calibration in the study considered the data obtained in the year of 2015. However, the validation process used the data obtained in 2016 and 2017. The comparison shows good agreement between the field observations and the solution from the GSFLOW model. The GSFLOW model resolves clearly the peak values and the small variations in wet and dry seasons through the simulation period. With a relatively larger drainage basin at the Li-lin Bridge station, the discharge rates of the Kaoping River at the Li-lin station are two orders of magnitude higher than those at the Xing She Da Qiao station. High variations of precipitation are the key factors that lead to significant variations in the streamflow. Figures 8b and 9b further present the scatterplot for the data obtained from the field and the solution from model simulations. Based on the selected criteria to evaluate the accuracy of the simulations, we obtained high correlations for the surface water flows at the Li-lin Bridge and Xing She Da Qiao stations. The coefficient of determination (R\({}^{2}\)) for the Li-lin Bridge station is 0.99 and a slightly small value of 0.95 is obtained for the Xing She Da Qiao station. Coefficients of correlation (RE) values are 0.99 for the Li-lin Bridge station and 0.98 for the Xing She Da Qiao station.