## 4 Results and Discussion
### Surface Water and Groundwater Interactions
#### 4.2.2 Seasonal Variations of the SGIs and Interactions Near the Soil Zones  
The precipitation is the primary input for a regional water cycle. The budgets of the water cycle components are useful for evaluations of watershed sustainability. However, the percentages of water budgets might not be sufficient for water resources management for most practical problems. Previous investigations have shown that the temporal variations of the water budget are crucial for efficient water resources management. The temporal variations are essential for areas with highly nonuniform precipitation, complex land-use, and anthropogenic activities. In the study, the model outputs allow the assessment of detailed variations near the interface of surface water and groundwater. We are interested in the temporal variations of SGIs. Specifically, two interfaces, including the saturated zones near stream beds and unsaturated zones near the soil layer, were considered to quantify the dynamics of the interactions. The main supplies of the rivers in the PPGB include surface runoff, interflow, and groundwater discharge to the stream. Figure 12 shows the temporal variations of three main components that contribute to streams. In general, the surface runoff in the PPGB provides 70% of the streamflow. The interflow offers approximately 20% volumetric flow to the streamflow. Only 10% of water is from net groundwater discharge to streams. The map of the water cycle shows that rainfall is the main factor that influences the streamflow. The temporal variations of interflow are similar to those of the surface runoff and precipitation. However relatively small values of net groundwater discharge to streams were obtained for the PPGB. The differences between the surface runoff and net groundwater discharge to streams vary significantly through a year. Note that the net groundwater discharge to streams is calculated by subtraction of the surface runoff and interflow from the streamflow. Investigations have recognized that the net groundwater discharge to streams is an important source of stream base flow. In the dry seasons, the volumetric water in streams is critical to fit the ecological conditions. The daily values of the net groundwater discharge to streams are relatively small and highly variable in recent years. Human activities such as groundwater pumping might be the main factors that directly influence the variations of net groundwater discharge to streams. To improve the groundwater environment, the increase of the groundwater levels might be required to enhance the net groundwater discharge to streams in the PPGB. In the PPGB, we have obtained a relatively high potential of the water resource from interflow. However, the spatial variation of the interflow along streams is not available because of the model limitations. Figure 12: The time series of the volumetric flow rates that contribute to the streamflow. On the interface near the soil and unsaturated zones, there are three components considered in the GSFLOW model. These three components, including soil zone, unsaturated zone, and saturated zone, might have interactions between two of them. The fluxes near the interface are gravity drainage from the soil zone to the unsaturated zone, recharge from the unsaturated zone to the saturated zone, and groundwater discharge from the saturated zone to the soil zone, respectively. Note that the condition that the groundwater discharge into the soil zone is that the groundwater head in a connected finite-difference cell is higher than the soil zone base. In Figure 13, we have extracted the temporal variations of different total fluxes from January 2015 to December 2017. The volumetric flow rates of the groundwater discharge to the soil zone are relatively high in the PPGB. Nearly identical fluxes were obtained for the recharge from the unsaturated zones to the saturated zones and the gravity drainage from the soil zones to the unsaturated zones.