## 3 Results and Discussion  
Response Relationship between Volume Capture Ratio of Annual Rainfall and Runoff Volume Capture Ratio  
The key indicator of the total annual runoff control rate of sponge city construction is to control runoff pollution and restore the natural hydrological state before development by controlling the volume of runoff [32]. In the construction of sponge cities, the total annual runoff control is used to calculate and set the scale of the source facilities. As analyzed in the previous Section 2.3.2, the more accurate expression of the total annual runoff control rate should be \"the total annual rainfall control rate\". The influence of runoff pollution reduction should be concentrated on the runoff volume control. The proposed total annual runoff control rate index may be used to calculate design rainfall and infer the type and scale of rainwater control facilities. Under the conditions set in this study, the relationship  
Figure 4: Method for designing models for connected facilities: (**a**) terminal storage tank, (**b**) rain barrel. between the total annual runoff control rate and the runoff volume control rate is shown in Figure 5.  
The four facilities of bioretention, permeable pavement, rainwater barrel, and storage tank all increased with the expansion of scale, as shown in Figure 5, and the runoff volume control rate steadily increased. The runoff volume control rate gradually decreased as the rainfall return period increased at the same design scale, that is, the same annual total runoff control rate. Under the same rainfall return period, the runoff volume control rate differs for various facilities. For example, for rain barrels, when the total runoff control rate of the year is 90%, the runoff volume control rate is close to 100%. Under the same conditions, the runoff volume control rate of the other three facilities is about 70%. For the design rainfall with a 10-year return period, the total runoff control rate of the year is 90%, the maximum runoff volume controlled by the terminal storage tank is 43%, and the minimum volume controlled by the bioretention tank is 24%. As a consequence, depending on the circumstances, it is required to make a rational decision on whether to employ source or terminal facilities. In addition, for the storage tank, under the design rainfall return period of 0.5 years, the value continues to increase after the total annual runoff control rate reaches 80%, but the corresponding runoff volume control rate remains at 70%. This is because when the design scale reaches 80% (and possibly 75\(-\)80%), the generated runoff rainwater is fully collected. Therefore, in the actual design stage, the design scale of the facility cannot be blindly increased, and the scale of the facility must be set scientifically and reasonably according to the target. Overall, rainfall and runoff have a strong nonlinear relationship, which verifies the results of [PERSON]'s research [33]. Rainfall is the direct source of runoff, and runoff control can be achieved through rainfall control. The runoff coefficient, which is strongly connected to soil moisture, precipitation intensity, underlying surface combination, and facility type, affects the runoff volume control rate. Since the runoff coefficient varies depending on the circumstances, the runoff volume control rate varies as well. What we need to concentrate on is the pollution transported in the runoff volume. As a consequence, in the follow-up  
Figure 5: Correspondence between the total annual runoff control rate and the runoff volume control rate. research, the dimensionless constant \(\mathrm{Dc}\) is used as the independent variable, and the runoff volume control rate is used as the dependent variable.