## 3 Materials and Methods
### Parameter Determination and Model Verification
#### 3.3.2 Parameter Calibration and Model Verification  
SWMM parameters are classified into accessible parameters and those requiring calibration. Accessible parameter values can be established using model statistics, while parameters requiring calibration include the Manning coefficient for impermeable areas,the Manning coefficient for permeable areas, the storage volume of permeable areas, and other infiltration model parameters. Following guidance from the SWMM manual, we assigned initial values to the parameters to be determined (see Table 1) and subsequently calibrated them. It is worth noting that the permeability coefficient, a key parameter in the SWMM hydrological model, plays a crucial role in the SWMM model. This coefficient serves as an indicator of soil permeability, directly influencing the simulation of surface runoff and groundwater recharge. In SWMM, sub-catchments are divided into pervious and impervious areas, with the permeability coefficient governing the infiltration behavior of rainfall in these zones. Within the LID controls in SWMM, the permeability coefficient is closely linked to the treatment capacity of LID facilities. These facilities manage runoff from impervious areas, and the permeability coefficient significantly affects their treatment efficiency. Model validation was conducted by selecting four waterlogging-prone points: HJD95, HJD87, HJD121, and HJD80 (refer to Figure 3). Two rainfall events, dated 10 July 2020 and 12 May 2021, were used to simulate waterlogging at these locations. The rainfall intensity and key characteristics of these events are detailed in Table 2. Catchment parameters listed in Table 1 were calibrated based on the observed ponding depth at the four field research sites, leading to the final set of model parameters. The parameters were iteratively adjusted around their initial values until the simulated ponding depths closely matched the observed field conditions. The validation results, along with error analysis comparing the simulated and observed ponding depths, are presented in Table 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Unit** & **Explanation** & **Value Range** & **Initial Values** \\ \hline \(N\)-imperv & s/m\({}^{1/3}\) & Manning coefficient for impermeable area & 0.011–0.015 & 0.015 \\ \(N\)-perv & s/m\({}^{1/3}\) & Manning coefficient for permeable area & 0.05–0.8 & 0.18 \\ Dstore-imperv & mm & Depth of storage of impermeable area & 0–3 & 2 \\ Dstore-perv & mm & Depth of storage of permeable area & 3–10 & 5 \\ MaxRate & mm/h & Maximum permeability coefficient & 10–100 & 50 \\ MinRate & mm/h & Minimum permeability coefficient & 0–10 & 1 \\ Decay & 1/h & Decay coefficient & 0–7 & 3 \\ \hline \hline \end{tabular} Note: Zero-imperv refers to “the percentage of impervious area without depression storage”. This parameter accounts for the immediate runoff that occurs at the start of rainfall before the depression storage is satisfied. The default value is 25%.  
\end{table} 
Table 1: Initial values of sub-catchment parameters. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Rainfall Event** & **Peak Rainfall Intensity** & **Time of Duration** & **Accumulated Rainfall** \\ \hline
20200718 & 62.4 mm/h & 24 h & 97.5 mm \\
20210510 & 29.2 mm/h & 2.5 h & 121.7 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Rainfall characteristics of normalized rainstorm events. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Rainfall Event** & **Ponding Point** & \multicolumn{2}{c}{**Ponding Depth \(h\) (m)**} \\  & & **Simulation Result** & **Actual Situation** & **Error Rate** \\ \hline  & HJD95 & 0.22 & 0.20 & \(-\)10.00\% \\
20200718 & HJD87 & 0.19 & 0.18 & \(-\)5.56\% \\  & HJD121 & 0.11 & 0.12 & 8.33\% \\  & HJD80 & 0.16 & 0.15 & \(-\)6.67\% \\  & HJD95 & 0.31 & 0.29 & \(-\)6.90\% \\
20210512 & HJD87 & 0.26 & 0.24 & \(-\)8.33\% \\  & HJD121 & 0.15 & 0.15 & 0 \\  & HJD80 & 0.19 & 0.20 & 5.56\% \\ \hline \hline \end{tabular}
\end{table}