## 3 Results and Discussion
### Parameter Sensitivity Calculation and Effect Analysis  
In order to reflect the sensitivity of structural parameters under different types of benefits of runoff control, the precipitation scenario with a return period of 1 year, as set in Section 2.4, was selected to simulate the runoff characteristics. The initial values of each structural parameter and disturbance values were compared, as shown in Table 2. In the 1a precipitation scenario, the sensitivity analysis results of each parameter based on the three benefits of total runoff reduction rate, peak runoff reduction rate, and peak time delay rate of the overflow pipe outlet are shown in Figure 3.  
Based on the analysis of Formula (11), it can be seen that the sensitivity types of parameters of the concave herbaceous field can be divided into highly sensitive parameters, sensitive parameters, medium sensitive parameters, and insensitive parameters. In this study, the \(|SN|\) of the sensitivity analysis results of each parameter is basically \(>\)0.05, that is, all of them have a certain sensitivity and are not insensitive parameters without analytical significance. Different kinds of benefits of runoff control have different effects on the sensitivity of the structural parameters of the concave herbaceous field. For example, when the flood peak function is reduced, the sensitivity of the pipeline porosity is much higher than the flood peak time delay and total runoff reduction (Figure 3a), while the sensitivity of the planted soil slope is completely opposite. Under the scenario aimed at flood peak delay and total overflow reduction, the parameters of planted soil porosity and slope are \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Disturbance Ratio (\%)** & \(\mathbf{-30}\) & \(\mathbf{-20}\) & \(\mathbf{-10}\) & \(\mathbf{0}\) (Initial) & \(\mathbf{10}\) & \(\mathbf{20}\) & \(\mathbf{30}\) \\ \hline
**Indicator** & & & & & & & \\ \hline A & 140 & 160 & 180 & 200 & 220 & 240 & 260 \\ B & 450 & 480 & 540 & 600 & 660 & 720 & 780 \\ C & 1.05 & 1.20 & 1.35 & 1.50 & 1.65 & 1.80 & 1.95 \\ D & 31.50 & 36 & 40.50 & 45 & 49.50 & 54 & 58.50 \\ E & 21 & 24 & 27 & 30 & 33 & 36 & 39 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Initial value and disturbance value of structural parameters. Figure 3: Sensitivity analysis diagram of structural parameters to three benefits under different benefits of runoff control. (**a**) Flood peak reduction rate; (**b**) Peak time delay rate; (**c**) Overall overflow reduction rate; (**d**) Integrated runoff control benefits. highly sensitive, while planted soil thickness is highly sensitive under the flood peak delay function (Figure 3b); meanwhile, aquifer height is highly sensitive under the total overflow reduction benefit (Figure 3c). It is not difficult to understand that the increase in the peak discharge value of the drainage outlet often means that the area of the runoff and the drainage seepage pipe increases, or the water pressure value in the pipeline increases. Obviously, the reduction degree at the outlet of the pipeline mainly depends on the seepage amount of water in the pipeline, that is, the size of the porosity. Therefore, the benefit of flood peak reduction is closely related to the nearest drainage seepage pipe. However, the structure that is far away from the drainage outlet of planting soil and the aquifer has less of a relationship, so the parameter sensitivity is lower. In terms of flood peak delay and total overflow reduction, the rainwater retention function of the concave herbaceous field mainly depends on the water storage capacity of the planting soil, while the storage space of seepage water depends mainly on the space between soil particles. When the porosity is sufficient, the infiltrated runoff and rainwater can be trapped more effectively in the planting soil. As a result, the runoff water generated in the early precipitation period is mainly held and stored by the soil, delaying the time of flood peak generation and reducing the total overflow flow.