## 2 Materials and Methods
### Numerical Models
#### 2.2.2 Conditions and Parameters  
Conditions and parameters were set default, and the observational data are as follows:  
(1) Boundary conditions:  
Rivers feeding and draining the wetland respectively represent sources and sinks of water and pollution. To simplify and shorten the computational time, 19 main rivers (Table 1, Figure 2) were selected for use in the model with reference to the continuity equation and mass conservation equation of pollutants. The location of these rivers with reference to the study's monitoring and analysis points is shown in Figure 2.  
(2) Initial conditions:  
The initial water level, discharge, and water quality were determined from 2016 monitoring data. The initial velocity of the flow field was set to zero, the initial water level was set to 4.7 m, and the initial concentrations of ammonia nitrogen (NH\({}_{3}\)-N) and TP were set to 5.2 and 0.31 mg/L, respectively.  
(3) Hydraulic parameters:  
The critical Courant-Friedrichs-Lewy condition was set as 0.8 to ensure stable operation of the model. The Manning number in the rivers, determined by the sediment particle size and water depth of the river bed, was set to 38 m\({}^{1/3}\)/s. The eddy viscosity coefficient was calculated using the Smagorinsky formula, where the Smagorinsky factor, Cs, was set as 0.28.  
The Smagorinsky formula:  
\[\tau_{ij}-\frac{1}{3}\tau_{kk}\delta_{ij}=-2\mu_{t}\overline{S_{ij}} \tag{5}\]  
\[\mu_{t}=(C_{s}\Delta)^{2}|\overline{S}| \tag{6}\]  
\[\overline{S_{ij}}=\frac{1}{2}\bigg{(}\frac{\partial\overline{u_{i}}}{\partial \overline{x}_{j}}+\frac{\partial\overline{u_{j}}}{\partial\overline{x}_{i}} \bigg{)},|\overline{S}|=\sqrt{2\overline{S_{ij}S_{ij}}}\,\Delta=\big{(}\Delta _{x}\Delta_{y}\Delta_{z}\big{)} \tag{7}\]  
\[C_{s}=\frac{1}{\pi}\bigg{(}\frac{2}{3}C_{k}\bigg{)}^{\frac{3}{2}} \tag{8}\]  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Number** & **River Name** & **Number** & **River Name** \\ \hline
1 & Sanyangtaio & 11 & Shanqianzhi \\
2 & Wangtanganyang & 12 & Shangongdian \\
3 & Qianhediananhe & 13 & Dongfengtou \\
4 & Henggangtou & 14 & Zhongxingqiao \\
5 & Laodianhou & 15 & caozhaiyang \\
6 & Dongyang & 16 & Shuliangyangnan \\
7 & Dianqian & 17 & Shuliangyangxi \\
8 & Lincun & 18 & Shangcaihou \\
9 & Shengmenhou & 19 & Haipai \\
10 & Yongfengqiao & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Primary rivers entering and exiting the Sanyang wetland.  
Figure 2: Location of rivers and distribution of monitoring points in the Sanyang wetland.  
where \(\mu_{t}\) is the turbulent viscosity at sublattice scale; \(\Delta_{i}\) is the mesh dimensions along axis i; \(C_{s}\) is the Smagorinsky factor; \(C_{k}\) is Kolmogorov constant.  
Water levels between the water and land boundaries can be determined by the difference between dry and flood conditions. If the calculated water level had a good fit with the observed level, it was used in the calculation; otherwise, it was discarded. The dry, flood, and wetting depths were set to 0.005, 0.05, and 0.1 m, respectively.  
The wind friction coefficient is a weak function of wind speed. For medium and strong winds in open seas, a value of 0.0026 produces good results, but, for gentler breezes, a smaller coefficient is needed. If wind speed changes are included in a model, the friction coefficient must be set as a change coefficient. In this case, as the average wind speed in the area is 2.3 m/s, the friction coefficient was set as 0.0026.  
(4) Water quality parameters:  
The degradation coefficients K\({}_{\text{NH3-N}}\) and K\({}_{\text{TP}}\) of NH\({}_{3}\)-N and TP were drawn from results for similar wetlands and set to 0.0069/d and 0.001/d, respectively, according to previous studies in similar wetlands [20; 21; 22; 23].