How is the size of the initial rainwater tank determined?

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How is the size of the initial rainwater tank determined? The size of the initial rainwater pond is related to the amount of initial rainfall. There are two methods for determining this amount: one is the rainfall over the 15–30 minutes prior to the start of precipitation, and the other is the rainfall equivalent to a layer 15–30 mm thick (in some standards, it is 20–30 mm thick). Therefore, there are also two methods for calculating the size of initial rainwater ponds: the rainfall duration method and the rainfall depth method. The rainfall depth method is simple to calculate; its basic principle is that volume = area × height. Therefore, to calculate the size of an initial rainwater pond, the first step is to determine the area of the polluted area; the second step is to select an appropriate rainfall depth, with a value between 15–30 mm being suitable depending on actual needs. The product of these two values gives the effective volume of the initial rainwater pond. The rainfall duration method is relatively complex; the formula is initial rainfall volume = storm intensity × runoff coefficient × catchment area. When using this method for calculations, the first step is to consult the local formula for storm intensity; using this formula, the rainfall amount per unit area per unit of time is determined. The most important parameter in this formula is the return period – different design return periods result in different storm intensities ； In the second step, the runoff coefficient is determined based on the type of surface over which water flows. Generally, the runoff coefficient for concrete or asphalt surfaces ranges from 0.85 to 0.95. The better the permeability of a surface, the lower its net runoff coefficient; specific values can be found in GB50014 ； The third step is to determine the area of the polluted zone. The difference between these two methods is that one directly gives the rainfall depth, while the other calculates the accumulated rainfall depth based on local conditions and the design return period.