## 2 Methods
### Assessment and Analysis Method
#### 2.3.2 Evaluation Indicators  
The runoff pollution reduction rate, or the ratio between the pollutant removal amount and the total quantity of runoff pollutants, is usually used to characterize the effect of sponge facilities on runoff pollution reduction [30]. However, this strategy does not account for variations in rainfall circumstances or the impact of various facilities on runoff management. The approach is unable to effectively define the control impact of source facilities or to objectively assess the control effect of various facilities on runoff pollution. For the construction of sponge cities, the total annual runoff control rate is one of the most important design parameters. Its significance refers to the percentage of annual controlled rainfall in a certain site through natural and artificial infiltration, accumulation, evaporation, transpiration, and other methods to the total annual rainfall [29]. Statistical methods are used to calculate the control rate of the total annual runoff. According to the observation data of daily precipitation in a certain area for many years (generally longer than 30 years to reflect the precipitation law), the effective precipitation events are screened. Then, count the ratio (\(\alpha\)) of the total rainfall less than a certain day's rainfall (H) to the total rainfall. The corresponding rainfall is the design rainfall H when the total annual runoff control rate is \(\alpha\). From the above description, we can see that the more accurate expression of \"annual total runoff control rate\" should be \"annual rainfall total control rate\". In fact, although \"rainfall\" and \"runoff\" are closely related, they are not equivalent. Only when the rainfall is converted into runoff, the pollutants carried in it will pollute the water environment, and the goal of sponge city construction is to remove the pollutants in the runoff. The effect of sponge city construction is to control runoff pollution by controlling the volume of runoff, but this effect has a limit because the source facilities are designed according to the design rainfall, and when the rainfall is less than the design rainfall, it will be completely absorbed and dealt with. If the rainfall exceeds the design rainfall, however, the surplus will be released through overflow if it is not appropriately handled. For example, if a source facility is designed to control 25 mm of rainfall, then a 40 mm daily rainfall will result in 15 mm of rainfall overflow discharge [22]. However, due to the overflow of 15 mm, the pollution load of the 25 mm part could not be absorbed and dealt with and was brought into the natural water body due to mixing and overflow, causing pollution. This brings difficulties to the scientific evaluation of the control effect of source facilities on runoff pollution. The relationship between runoff volume control and runoff pollution control in sponge city development construction is not evident from the above study, and the present research and evaluation indicators are difficult to describe. In order to scientifically evaluate the control effect of source facilities on runoff pollution, this study proposes dimensionless constant of runoff pollution control (Dc). The formula for calculating the overall pollutant mass reduction rate of a single rainfall simulation result by the total runoff volume reduction rate simulated by the field is presented in Formula (5). The Dc value is 1 when reducing 1% of the runoff volume can control 1% of the pollutant load.