# Assessing Groundwater Level with a Unified Seasonal Outlook and Hydrological Modeling Projection
## 3 Study Site and Modeling Configurations  
### Basin Information  
This study selected the Fengshan Creek basin in northern Taiwan as the study site to test our proposed approach (Figure 3). The Fengshan Creek basin is located in a subtropical zone with parts in the Taoyuan, Hsinchu, and Miaoli areas, where the Hsinchu Science Park is located. Therefore, water resources are particularly crucial because many semiconductor industries are situated in the Hsinchu Science Park. Fengshan Creek originates from Najie Mountain at an altitude of 1320 m. Its trunk stream is approximately 45.45 km long, the longitudinal profile of the river is 1/250, and the catchment area covers 250.1 km\({}^{2}\). The basin's meteorological characteristics include 1608 mm of mean annual rainfall and an average of 167 raining days. The rainfall is mainly concentrated in July, August, and September due to typhoons and monsoons. The mean monthly rainfall is 96-270 mm, and the annual rainfall in the area ranges from 1700 to 2700 mm. The maximum monthly rainfall occurred in June 1997. According to the flow gauge records, the annual flow rate ranges from 1.93-17.60 cms (cubic meter per second), with an average of 9.81 cms over 50 years. The maximum flow rate was3860 cms during Typhoon Toraji in 2001, and the annual runoff from the basin is 376 million m\({}^{3}\). The area has a temperate climate, with an average temperature of around 23 \({}^{\circ}\)C.  
In the simulations, the HEC-HMS hydrological model was applied to generate the discharge rates for the Fengshan Creek basin (zones A and B in Figure 3a), and a physical model, WASH123D, was used for the remaining watershed simulations. As shown in Figure 3b, two rain gauges, one flow gauge, and five hydrogeological drilling wells are located in the Fengshan Creek basin. Measurements were obtained with more than 30 years of records from the Water Resource Agency, Ministry of Economic Affairs, Taiwan (WRA). The land-use types, namely, agricultural (57.3 km\({}^{2}\)), forestry (133.8 km\({}^{2}\)), traffic (8.8 km\({}^{2}\)), water (7.8 km\({}^{2}\)), buildings (15.9 km\({}^{2}\)), and other (27.6 km\({}^{2}\)), were used in this study from field investigations conducted by the National Land Surveying and Mapping Center, Ministry of Interior, Taiwan (NLSC). Hydro-stratigraphic investigations of drilling wells were conducted by the Central Geological Survey (CGS), Ministry of Economic Affairs, Taiwan. The groundwater levels used in this study were from Xinpu well, which is located in the simulated area.  
The WASH123D model was applied to simulate surface water and groundwater in the Fengshan Creek basin. The 1D river routing model was constructed using cross-sections taken from field geometry measured by the Second River Management Office, WRA. The setting of 2D surface grids utilizes unstructured meshes, which are constructed using triangular surface elements. The 2D grid is used as the surface layer for the triangular prism meshes for the 3D groundwater modeling. Figure 4a illustrates the hydrogeological classifications of the Fengshan Creek basin. The hydrogeological drilling data from different well sites were applied to build the 3D groundwater layers. As shown in Figure 4a, the upstream part of the Fengshan Creek basin is composed of Tamaopu conglomerate. Part of the upstream and the downstream of the basin is characterized by the Chaochin member of the Yangmei formation, consisting of less permeable sandstone and shale. Other materials (e.g., a general alluvial layer, channel fill deposits, the Tientzhu formation, and Terrsoe formation) are also interspersed  
Figure 3: (a) Surface elevation of the Fengshan Creek basin, (b) gauge stations, (c) degree of slope, and (d) land use.  
throughout the watershed. In total, 61,340 grids, 106,371 elements, and nine layers made up the underground grids (Figure 4b).