## 2 Study Sites and Materials  
The study area chosen is situated within the confines of Fukuyama City. Due to its geographical location and steep topography, the study basin is susceptible to flood. Meanwhile, the hydrometeorological indexes are well monitored using two meteorological and hydrological stations, and the high quality of the observational data makes the model's simulation results more reliable. The Ashida River ranks as the third longest river within Hiroshima's boundaries. The river flows from the northwest to the southeast, meandering through Fukuyama City before eventually merging with the Seto Inland Sea (as depicted in Figure 1). The large urban areas within the basin form a high proportion of impervious surface, which reduces the soil's infiltration capacity and results in rapid runoff accumulation, leading to more frequent and intense floods. Nestled within the subtropical monsoon zone, the watershed experiences an average annual temperature of approximately 16.0 \({}^{\circ}\)C. The basin has an average annual precipitation of 1814 mm, equivalent to a yearly water volume of 1.39 km\({}^{3}\). Large precipitation amounts and high-intensity rainfall events have the potential to damage the basin's water storage projects, increasing its vulnerability to flood. Notably, the average runoff at the river mouth during the year 2017 stood at approximately 17.5 m\({}^{3}\)/s. Encompassing an extensive expanse of 759.5 km\({}^{2}\) (with coordinates ranging from 34.46\({}^{\circ}\) N to 34.71\({}^{\circ}\) N and 133.02\({}^{\circ}\) E to 133.43\({}^{\circ}\) E), the watershed encompasses an urban area spanning 77.5 km\({}^{2}\), while the forested domain occupies approximately 562.8 km\({}^{2}\). Fukuyama City has an estimated population of approximately 460,000 residents. Topographically, the area features elevated terrain in the west and north, gradually giving way to lower terrain in the east and south. Mountains and hills are mainly distributed in the northwest, and plains are mainly distributed in the east. The elevation in the basin ranges from 3 m to 720 m. Mountains and hills are mainly distributed in the northwest, and plains are mainly located in the east of the basin. The topographic pattern promotes fast runoff flow in the upstream reaches and relatively slow runoff flow in the downstream reaches. The study includes two carefully selected hydrographic stations: the Fuzhong Station, strategically located upstream of the Ashida River's entry into the city, and the Shanshou Station, strategically positioned prior to the river's merging point with the sea. Building the SWAT model database hinges on pivotal datasets, namely Digital Elevation Model (DEM) data, soil data, land use data, and meteorological data (refer to Figure 2 for the depiction of watershed conditions). The study area's DEM data were sourced from the U.S. Geological Survey website. The Terra Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM) Version 3 (ASTGTM) provides a global DEM of land areas on Earth at a spatial resolution of 1 arc second (approximately 30 m horizontal posting at the equator). [PERSON] et al. [22] demonstrated that the accuracy of the 30 m resolution elevation data-driven SWAT model simulation is higher than that of the simulation results with lower resolution data. The ASTGTM data product was created by automating the processing of the entire ASTER Level 1A scenario archive acquired between 1 March 2000 and 30 November 2013. The data product was cleansed of residuals and outliers, and it was presented in a raster format. The land use classification was derived from the secondary grid classification of SPOT and RapidEye remote sensing images by the Japanese Land Agency, and the image year used was 2016, which is very close to our study year. The data format was shape. Soil classification was derived from field surveys of physical and chemical soil properties conducted by the Japanese Land Agency across the country. The data format was shape.