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An acre-foot is a unit of volume commonly used in the United States in reference to large-scale water resources, such as reservoirs, aqueducts, canals, sewer flow capacity, irrigation water and river flows.
It is defined by the volume of one acre of surface area to a depth of one foot. Since the acre is defined as a chain by a furlong (66 ft × 660 ft) the acre-foot is exactly 43,560 cubic feet. Thus, the two definitions of the acre foot (differing by about 0.0006%) depend on whether the international or U.S. survey foot is used.
|1 international acre foot||≡ 43,560 international cubic feet (by definition)|
|= 1233.48183754752 m3 (exactly)|
|= 325,851 3⁄7 U.S. gal (exactly)|
|≈ 271,328.072596 imp gal|
|1 U.S. survey acre foot||≡ 43,560 U.S. survey cubic feet (by definition)|
|≈ 1233.4892384681 m3|
|≈ 325,853.383688 U.S. gal[nb 1]|
|≈ 271,329.700571 imp gal|
As a rule of thumb in U.S. water management, one acre-foot is taken to be the planned water usage of a suburban family household, annually. In some areas of the desert Southwest, where water conservation is followed and often enforced, a typical family uses only about 0.25 acre-feet of water per year. One acre-foot/year is approximately 893 gallons per day.
The acre-foot (or more specifically the time rate unit of acre-foot per year) has been used historically in the U.S. in many water-management agreements, for example the Colorado River Compact, which divides 15 million acre-feet (MAF) per year or (586 m³/s) among seven western U.S. states.
Elsewhere in the world, where the metric system is in common use, water volumes can be expressed in either cubic metres (as in flow rates of cubic metres/second, or "cumecs") or, for water usage, storage or irrigation volumes, in kilolitres (kL, = 1 cubic metre), megalitres (ML, = 1,000 cubic metres), or gigalitres (GL, = 1,000,000 cubic metres). One acre foot is approximately equivalent to 1.233 megalitres.
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