## 2 Materials and Methods
### Calibration and Validation of the Model  
A 2D depth-averaged hydrodynamic model based on MIKE 21 FM was applied to investigate the flooding process in the Dongting Lake area, which includes a part of the Yangtze River (i.e., Jingjiang section), Dongting Lake, and its main tributaries. The topography data of the Dongting area and the Yangtze River was based on the surveyed data, with a resolution from 30-200 m updated in 2003 and 2016, respectively. Unstructured triangular grids that can be adapted to the complex shape of the Dongting Lake area and vary in different grid sizes in any region were used to construct the model. In order to improve the computational efficiency and accuracy, a higher mesh resolution (30 m) was used in the channel and a coarse mesh (700 m) covered the Dongting Lake. Therefore, a total of 118,542 nodes and 210,341 triangular elements were included in the grid system with a minimum and maximum element size of 30m and 700m, respectively. Daily catchment inflows from Yangtze River and four rivers were specified as the upstream boundary conditions in the model. Luoshan gauging station on the Yangtze River was set as the downstream boundary of the model while using the rating curve that was observed at Luoshan gauging station. The model has been calibrated to obtain the proper [PERSON]'s roughness coefficient (M), which is the most important parameter in hydrodynamic modeling of rivers and lakes, while using the daily measured data during the flood period in 1996 (i.e., from 11 July to 2 August). In the current model, the value of M was initially set at 32 m\({}^{1/3}\)/s and then modified during the calibration. The calibrated M ranges from 32 m\({}^{1/3}\)/s to 60 m\({}^{1/3}\)/s in Yangtze River; 45 m\({}^{1/3}\)/s to 50 m\({}^{1/3}\)/s in the three channels, and 32 m\({}^{1/3}\)/s to 60 m\({}^{1/3}\)/s in Dongting Lake, as well as in the four rivers, which agree well with the previous studies [18]. A simulation was conducted to validate the current model while using the observation data from the flood period in 1998 (i.e., from 6 June to 17 September). The minimum time step was set to 2 s and the run time is about seven days using a computer with i7-4790 CPU (3.6 GHz, Intel Core, LENOVO, Beijing, China). The Nash-Sutcliffe efficiency coefficients (NSE) [19] of water level and the difference of the maximum water level between the measured and simulated data are given in Table 1 to estimate the accuracy of the current numerical model. Table 1 shows that all of the NSE values are over 0.87, the R\({}^{2}\) values are greater than 0.928, and the RMSE values are less than 0.484, which indicate that the calculated results agree well with  
Figure 2: Location of Dongting Lake. The lake collects water from the Yangtze River through ‘three channels’ (Songzikou, Taipingkou, and Ouchikou) and ‘four rivers’ (Xiang, Zi, Yuan, and Li) and it discharges to the Yangtze River at Chenglingji. the observations. Generally, the NSE values are close to previous studies [20]. Table 1 also shows that the difference of the maximum water level between the measured and simulated data is less than 0.16 m at all selected stations in Dongting Lake, which indicated that the current model could represent the flood well. Figure 3 shows the time series of water level at the main hydrologic stations in Dongting Lake. The results show that the model appears to simulate the noticeable water level fluctuation of the lake successfully, since the calculated results include the flood rising and falling process and the flood peak value agrees well with the measured data. Although there are some differences between the simulated data and measured data, the numerical simulations reasonably agree with the observations overall, which indicated that the current model can dynamically simulate the flooding process in Dongting Lake. Figure 3: Comparison of water level process during the flood season (June to September) in 1998. **(a)** Nanzui; **(b)** Xiaohezui; **(c)** Xiangyin; **(d)** Yingtian; **(e)** Luijiao; and, **(f)** Chenglingji.