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 3380 litres/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.
- This conversion assumes the international foot is used to define the U.S. gallon and the U.S. survey foot to define the acre foot. If the same foot is used for both, the result is the 325,851 3⁄7 U.S. gal figure as obtained previously.
- The state of Montana assumes 1.0 acre-foot per year for a family of five. See Water Rights Bureau, state of Montana (April 13, 2004). "Form No. 627 R8/03 Notice of Water Right" (PDF). Archived from the original on 25 January 2008. Retrieved 2008-01-30.
- Santa Fe, New Mexico rate averages 0.25 acre-feet per year per household. See Planning Division, Planning & Land Use Department, City of Santa Fe, New Mexico (February 2001). "Water Use in Santa Fe: A survey of residential and commercial water use in the Santa Fe urban area" (PDF). Retrieved 2008-01-30.
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