## 4 Results and Discussion
### Surface Water and Groundwater Interactions
#### 4.2.1 Water Cycle for the Entire PPG
With the relatively open area and gravel deposit on the land surface, the average infiltration is approximately 40% of the average precipitation and the average surface runoff is about 57% of the average precipitation. In the PPG, the volumetric flux of the evapotranspiration is \(3.22\times 10^{6}\) m\({}^{3}\)/d. This evapotranspiration value obtained from the GSFLOW model comprises the evaporation of intercepted precipitation, the evapotranspiration from the unsaturated and saturated zones, and the evapotranspiration from HRUs. Note that the GSFLOW model has modified the features of the UZF package in the MODFLOW model. The volume is accounted for \(1.2\times 10^{9}\) m\({}^{3}\) on average in a year. The water from the soil zone to the unsaturated zone is involved in the overall behavior of the infiltration. In the PPG, the volumetric flux from the soil zone to the unsaturated zone (i.e., \(0.16\times 10^{6}\) m\({}^{3}\)/d) is 4% of the total average infiltration (i.e., \(4.33\times 10^{6}\) m\({}^{3}\)/d). In the PRMS, calculating infiltration depends on the precipitation rate, surface runoff, and soil material. In the study, we obtained similar ratios of water fluxes from wet and dry seasons for the precipitation, surface runoff, and infiltration. With the available results from the GSFLOW, it is possible to calculate the detailed sources of different components in the water cycle. However, the post-processing is required to extract the solution from the GSFLOW model. The surface runoff contributes approximately 77% and 64% of the streamflow for wet and dry seasons. The interflow provides 21% and 25% of the streamflow in the wet and dry seasons. The groundwater discharge has relatively small contributions to the streamflow. The percentages are 2% and 11% in the wet and dry seasons. Based on the observations from streamflow stations in the PPG, the variations of streamflow in wet and dry seasons can vary up to three orders of magnitude differences. The small percentage of groundwater discharge to the streamflow can be significant for the dry seasons. The components \"stream leakage to unsaturated and saturated zones\" and \"groundwater discharge to streams\" are the indicators that show the local dynamics of streams and groundwater interactions. Simulation results show that the flux of stream leakage is relatively higher than the flux of groundwater discharge. We also found that the average discharge of groundwater to streams is noticeable in wet (\(0.22\times 10^{6}\) m\({}^{3}\)/d) and dry (\(0.37\times 10^{6}\) m\({}^{3}\)/d) seasons. However, the differences between groundwater discharge to stream and stream leakage to groundwater are similar in wet and dry seasons. We found that the interflow in the water cycle plays an important role in the river system in the entire PPG. The interflow rate is approximately \(1.65\times 10^{6}\) m\({}^{3}\)/d on average. In the wet seasons, the flux of the interflow rate is 200% more than the dry seasons. A large amount of interflow had motivated a different type of water usage in many areas near the rivers. In the PPG, many underground channels were built along or partially cross the streams to collect the interflow water. Much of the irrigation and  
Figure 11: The water budgets for the water cycle components in the PPG. Note that the blue and red colors represent the wet and dry seasons. consumption water is from the interflow. Practical operations have shown that the interflow water resource is relatively stable as compared to the water directly obtained from the streams. The quality of the interflow water is also better because the natural underground deposits can be the low-level filters that reduce the water turbidity. This feature supports the important water resources in flooding seasons because of the high turbidity of the river water in storm events.