## 4 Results and Discussion
### Surface Water and Groundwater Interactions
#### 4.2.1 Water Cycle for the Entire PPG  
Table 2 summarizes the cell-by-cell water budgets of the groundwater model for the entire PPG. The results in Table 2 also present the statistics of the groundwater budgets obtained from the calibrated GSFLOW model during the three-year simulation period. The numerical model shows the mass conservation for the groundwater system because of the small value of the percent discrepancy. An order of magnitude difference was obtained for the cell-by-cell \"storage in\" and \"storage out\" for the simulation domain. In the study, we specified the constant hydraulic head boundaries at the upstream valleys of the rivers and along the coastal line. Results in Table 2 have indicated that the average flow rate along the coastal line is slightly larger than the flow rates along the inlets near the upstream valleys. The flow rate variations (i.e., the standard deviation (STD) of the flow rates) along the coastal line are also slightly higher than those along the inlets near the upstream valleys. The GSFLOW model enables the calculation of detailed processes involved in the water cycle. Specifically, the surface dynamics have been resolved with more water budget components in the PRMS model. Figure 11 summarizes the fluxes of different water cycle components in the PPG. Note that the wet season was defined from May to October for each year, and the dry season was collected from November to April in the next year. We have focused on the interaction dynamics between the surface water and the first layer of the groundwater system. To verify the flow from the unsaturated zone to the saturated zone from the output of the GSFLOW model, the calculation of groundwater recharge relies on the available results of water budget components in the PRMS. These components include the precipitation, evapotranspiration, surface runoff, soil infiltration, interflow to streams, stream leakage to saturated and unsaturated zones, groundwater discharge to streams, gravity drainage from soil zones to unsaturated zones, recharge from unsaturated zones to saturated zones, and groundwater discharge from the saturated zones to soil zones. In the study, the groundwater recharge in the GSFLOW was manually calculated by subtracting the flow rates of evapotranspiration, surface runoff, and interflow from the flow rate of the total precipitation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sources/Sinks** & **In (m\({}^{3}\)/d)** & **Out (m\({}^{3}\)/d)** & **STD\({}_{\text{in}}\) (m\({}^{3}\)/d)** & **STD\({}_{\text{out}}\) (m\({}^{3}\)/d)** \\ \hline Storage & \(3.96\times 10^{6}\) & \(2.00\times 10^{5}\) & \(1.86\times 10^{6}\) & \(2.19\times 10^{5}\) \\ Constant head & \(1.12\times 10^{5}\) & \(1.74\times 10^{5}\) & \(9.68\times 10^{4}\) & \(2.09\times 10^{5}\) \\ Wells & 0 & \(3.43\times 10^{6}\) & 0 & \(1.38\times 10^{6}\) \\ UZF recharge & \(2.13\times 10^{5}\) & 0 & \(2.88\times 10^{5}\) & 0 \\ GW Evapotranspiration & 0 & \(2.32\times 10^{4}\) & 0 & \(3.42\times 10^{4}\) \\ Surface leakage & 0 & \(5.44\times 10^{5}\) & 0 & \(6.30\times 10^{5}\) \\ Stream leakage & \(3.53\times 10^{5}\) & \(2.75\times 10^{5}\) & \(1.19\times 10^{5}\) & \(3.61\times 10^{5}\) \\ Total & \(4.64\times 10^{6}\) & \(4.64\times 10^{6}\) & \(1.79\times 10^{6}\) & \(1.79\times 10^{6}\) \\ In-Out & \(-8.77\times 10^{2}\) & & & \\ Percent Discrepancy & \(-1.97\times 10^{-2}\) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The statistics of the groundwater budgets that were obtained from the MODFLOW in the GSFLOW model. The results of the water budgets show that the main input (i.e., precipitation) in the water cycle is \(9.94\times 10^{6}\) m\({}^{3}\)/d, which is accounted for \(3.6\times 10^{9}\) m\({}^{3}\) on average in a year (see Figure 11). The discrepancy of the precipitation in wet and dry seasons is extremely high. Over 80% of the precipitation is from the wet season.