## 3 Results and Discussion
### Parameter Sensitivity Calculation and Effect Analysis
At the same time, the slope affects the migration time of runoff on the planted soil surface, and a smaller slope can give runoff and rainwater more time to contact the soil surface and improve the infiltration efficiency of water. It is also revealed that the flood peak time and the total overflow flow mainly depend on the runoff characteristics when entering the overflow wellhead and are closer to the structure of the planting soil. Therefore, the sensitivity of the two runoff control benefits to the structural parameters of the larger planting soil is higher, while the sensitivity to the porosity of the smaller pipeline structure is lower. In addition, the thickness of the planting soil is closely related to the efficiency of the temporary retention of rainwater. Thicker soil means that the time of absorbing pre-runoff water is more \"lasting\" and the time of the delayed peak is longer. Therefore, its sensitivity to flood peak delay is more obvious. On the other hand, the main function of the aquifer exposed to the air above the planted soil is to collect and temporarily store rainwater from the road surface. Under a certain simulated precipitation time, the higher the height of the aquifer, the more water it can retain, and the less water will enter the overflow well and accumulate in the drainage outlet at a certain time. Therefore, its sensitivity to the total flow reduction rate is also high. However, according to the sensitivity analysis of the above parameters, the sensitivity of the concave herbaceous field model to the parameters of total runoff reduction rate, peak runoff reduction rate, and peak time delay rate is quite different. In order to more objectively evaluate the comprehensive sensitivity of the parameters of the concave herbaceous field structure to each benefit simulation project, the coefficient of variation method (information weight method) was used to determine the weights of different simulation results, and then the weights were used to calculate the comprehensive sensitivity coefficients of each parameter. The calculated results are shown in Table 3, and the comprehensive sensitivity coefficients are shown in Figure 3d. The results of comprehensive parameter sensitivity analysis showed that the thickness of the planted soil, slope, and aquifer height were highly sensitive factors regarding the runoff control efficiency of the concave herbaceous field, while the comprehensive sensitivity of planted soil porosity and seepage pipe porosity was low. Combined with the above analysis and the analysis of geographical factors such as soil permeability in Tianjin, the basic soil permeability in the study area is poor, and LID facilities focusing on permeability do not sufficiently fulfil their function of runoff control. In this study, the runoff regulation ability of the concave herbaceous field mainly focuses on the temporary water storage function of aquifers, the infiltration process of the planting soil itself, the retention of rainwater and the overflow drainage process of drainage pipes, and the runoff seepage through the lower gravel layer of the planting soil. In addition, the original soil is less. Therefore, the porosity of the planting soil and the drainage seepage pipe, which are closely related to the infiltration process, can play a limited role, and the contribution and parameter sensitivity to the overall runoff control benefit of the concave herbaceous field are small. Therefore, the parameter optimization and transformation of the concave green space should mainly focus on the planting soil thickness, slope, and aquifer height with the most significant comprehensive parameter sensitivity. According to the sensitivity analysis results of comprehensive parameters, the runoff control benefits of each scheme with different parameters are further ranked by the optimization method, so as to explore the optimal structural parameters of the concave herbaceous field.