Sizing of constructed wetlands depends upon the contaminants you are interested in removing. The amount of time it takes for water to travel through a wetland (from inflow to outflow) is called the Hydraulic Retention Time (HRT), which should range from 3 to 10+ days, depending upon the volumes of water treated and the concentration/load of contaminants within the water.
A 3 to 5 day retention time should be adequate to manage most contaminants in warmer (temperate / tropical) climates, whereas in cooler climates (Zone 6 and below) longer retention times may be needed.
If your average daily runoff volume ranges from 20,000 to 30,000 gal, and you estimated you needed a 4 day HRT, that would mean you want to size your wetland to hold 4 times the volume of your average daily runoff. It would need to be sized to hold 80,000 to 120,000 gallons of water. You would use that volume for calculating the required surface area and depth.
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