An acre-foot is a unit of volume commonly used in water resources, hydrology, and stormwater management to describe large quantities of water. It represents the volume of water required to cover one acre of surface area to a depth of one foot.
In practical terms, one acre-foot is equal to 43,560 cubic feet of water, which corresponds to approximately 325,851 gallons or about 1,233 cubic meters. This unit is especially useful because it directly ties a measurable land area, an acre, to a depth of water, making it intuitive for evaluating storage, runoff, and water supply in land-based systems.
Within stormwater management, the acre-foot is frequently used to quantify detention and retention basin capacities, watershed runoff volumes, and regulatory storage requirements. For example, a stormwater pond designed to capture one acre-foot of runoff can theoretically store the equivalent of one foot of water spread evenly across an acre of contributing drainage area. Engineers and regulators use this unit to size infrastructure, assess flood mitigation capacity, and compare water volumes across different sites or storm events.
The acre-foot is also widely used in broader water resource planning, including reservoir storage, irrigation demand, and municipal water supply, particularly in the United States. Because stormwater systems often deal with large-scale volumes over land surfaces, the acre-foot provides a practical and standardized way to express and communicate those quantities.