## 4 Results and Discussion  
### Calibration of MIKE11 NAM-HD Model  
In this study, the calibration period was from 6 July 2023, 1:00, to 10 July, 23:00, in which heavy rainfall occurred in the study area from 6 July 1:00 to 8 July 12:00. The results of the water level calculations during this period were mainly affected by the NAM model. From 8 July 12:00 to 10 July 23:00, rainfall did not occur in the study area, and the results of the water level calculations in this period were mainly affected by the HD model. The measured water level results of Haiantong, Banjing, and Ruhaiyunhe stations were compared with the model simulation results, and the model parameters were repeatedly debugged, which finally made the model's calculated process simulation verification rate more than 85%; the peak simulation error was less than or equal to 0.1 m, the peak present time error was within the permissible range, and the model calibration was completed.  
The major parameter to be calibrated for the hydrodynamic model was riverbed resistance or roughness. Nine major parameters were calibrated in the hydrological model, and the parameters in each sub-catchment represented the average values in the region. During the calibration process, a trial-and-error method was used to adjust the relevant parameters of the NAM, and the roughness of the hydrodynamic model was adjusted appropriately until the water level simulation results met the accuracy requirements [31]. According to the calibration results, the roughness of the study area was 0.025-0.035, and the parameters of the NAM model in the sub-catchments are shown in Table 4 [32; 33; 34]. The evaluation of the calibration effect and comparison of the water levels at some stations are shown in Table 5 and Figure 4, respectively. It can be concluded that the process simulation verification rate of the MIKE11 model in the selected stations all reached the accuracy of grade A (85%) or above; the peak simulation errors were all less than 0.1 m; the peak present time errors were all less than the Allowable Errors; the maximum RMSE is 0.276 at the Shigang station; and the minimum NSE is 0.353 at the Shigang station. All other stations, except for Shigang station, had an RMSE of less than 0.2 and an NSE greater than 0.8. The MIKE11 model can better simulate the water level in the JiaoGang Basin; however, the fluctuation state of the measured water level cannot be captured in the simulation of the water level process in the second half of the time period at Rutaiyunhezhadong, Banjing, Ruhaiyunhe, and Dingyan stations, mainly because of the difference between the hydraulic structure and the actual situation. For Shigang station, the results are poor, mainly because this station is located in sub-catchment 7, and the generalization of the river network is not comprehensive because of the limitations of historical data.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
\begin{tabular}{c} **Sub-** \\ **Catchment** \\ \end{tabular} & **Umax** & **Lmax** & **CQOF** & **CKIF** & **TOF** & **TIF** & **TG** & **CK1,2** & **CKBF** \\ \hline
1 & 7 & 70 & 0.4 & 1000 & 0.4 & 0.2 & 0.3 & 9 & 2000 \\
2 & 10 & 100 & 0.4 & 1000 & 0.4 & 0.2 & 0.3 & 12 & 2000 \\
3 & 7 & 70 & 0.6 & 1000 & 0.4 & 0.2 & 0.3 & 3 & 2000 \\
4 & 7 & 70 & 0.6 & 1000 & 0.4 & 0.2 & 0.3 & 2 & 2000 \\
5 & 12 & 120 & 0.4 & 1000 & 0.4 & 0.2 & 0.3 & 10 & 2000 \\
6 & 12 & 120 & 0.4 & 1000 & 0.4 & 0.2 & 0.3 & 12 & 2000 \\
7 & 8 & 80 & 0.6 & 1000 & 0.4 & 0.2 & 0.3 & 6 & 2000 \\ \hline \hline \end{tabular}
\end{table}
Table 4: NAM model parameters.  
Figure 4: Comparison chart of observed and simulated water level in physical mechanism model: (**a**) Haiantong, (**b**) Rutaiyunhezhadong, (**c**) Banjing, (**d**) Ruhaiyunhe, (**e**) Jiaogangzha, (**f**) Yangkouwaizha, (**g**) Juegang, (**h**) Dingyan, (**i**) Jiuweigangzha, and (**j**) Shigang.