## 3 Results and Discussion
Simulation Analysis of the Control Effect of Initial Runoff Volume Capture Facilities on Runoff Pollution Reduction  
Simulate the regional models of different control facilities using 0.5-year, 1-year, 3-year, 5-year, and 10-year return period 2 h rainfall data for different design scales (according to 60%, 70%, 75%, 80%, 85%, and 90% of the total annual runoff control rate), and draw a curve for the \(\mathrm{Dc}\) value obtained through the simulation results, as shown in Figure 6.  
The \(\mathrm{Dc}\) value of the source volume control facility is much higher than that of the terminal volume control facility under different return period rainfall conditions under the same rainwater runoff volume control rate, as shown in Figure 6. The \(\mathrm{Dc}\) value of source facilities with osmotic function (bioretention, permeable pavement) is higher than that of terminal facilities, while the \(\mathrm{Dc}\) value of source facilities with regulation and storage function (rain barrel) and terminal storage tank is relatively low. For the three source facilities, including bioretention, permeable pavement, and rainwater bucket, under the same design rainfall return period, with the increase in the runoff volume control rate, the \(\mathrm{Dc}\) value gradually decreased. In this study, when the bioretention facility has a runoff volume of about 10% under the 10-year rainfall return period, the \(\mathrm{Dc}\) value is the highest at 1.7 and then gradually decreases until it approaches 1. Under short-duration and high-return period rainfall, the source control facilities for the two initial runoff volume control implementations of bioretention and permeable pavement cannot provide normal rainwater storage performance. As a result, the curvature of the high return period curve gradually becomes consistent with the curvature of the low return period curve as the facility's design scale grows larger. For the facilities with storage functions such as rainwater barrels and terminal storage tanks, it shows a trend of first decreasing and then increasing until it approaches one. For example, with a return period of 0.5 years, the \(\mathrm{Dc}\) value remains around 0.95, and when the scale of the facility continues to expand, the \(\mathrm{Dc}\) value approaches 1. Overall, as the scale of the facility increases (the runoff volume control rate is increasing), the final \(\mathrm{Dc}\) value approaches one for both the source facility and the terminal facility. The formula shows that as the facility scale is increased, such as in extreme circumstances, all runoff rainwater is managed, suggesting that all contaminants in the rainfall runoff are also controlled. From Formula (5), the denominator value is one, and the value of the numerator is also one, so the \(\mathrm{Dc}\) value is one. Although the pollutant control effect of terminal facilities is not as good as that of source facilities, the terminal facilities can obtain more volume control of rainwater runoff under the large return period in a single rainfall simulation. If we just examine the simulation results in the rainfall simulation, it appears that the terminal facilities can  
Figure 6: Variation curve of \(\mathrm{Dc}\) value of source facilities and terminal facilities under different volume control rates. achieve a superior runoff control rate, pollution control rate, and peak shaving impact in a single rainfall under the same design scale. The source facilities have not had time to play their role due to the design rainfall circumstances in the high return period, and the runoff will flow to the end. The terminal facilities appear to be more effective in terms of reducing total pollutants, which also misleads the runoff pollution control measures adopted by engineering designers during the planning and design stage. In fact, numerous environmental parameters such as rainfall period, rainfall intensity, rainfall interval, facility type, type of underlying surface of the catchment area, and pollutant control facility will all impact the runoff pollution control rate.