Figure 1(a) describes the surrounding river and the Yodo River Basin, with Lake Biwa as the upper reaches of the main river, covering an area of 8,240 km². Table 1 shows the flow rate of the main rivers which inflow into Lake Biwa. Figure 1(b) shows the calculation domain with the topography and water depth of Lake Biwa. The horizontal calculated domain is 36 km × 65.5 km with a grid size of 500 m. The vertical domain is 107.5 m, and the vertical grid size is 0.5 m from the surface to the 20 m depth of water and varies from 0.5 to 2.5 m as the water depth increases each 0.1 m in the vertical direction. This is to reproduce the change of water temperature in the surface layer more accurately.