In order to provide more flexibility in their on-site retention rules, cities may create “stormwater credit trading” programs, which allow developers to meet their stormwater retention requirements on their own sites or elect to purchase “credits” for stormwater retention from others who have voluntarily retrofitted their properties through a stormwater credit-trading program. “Demand” for credits will arise as construction projects trigger the on-site retention requirements and developers seek to comply in part through buying credits generated by stormwater management practices (SMPs) located on other properties. Credits would be “supplied” by property owners with relatively low-cost, on-site retention options who voluntarily implement SMPs on their property with the intention of selling retention credits.