Acre-foot

An acre-foot volume (not drawn to scale)

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,[1] and river flows.

Definitions

As the name suggests, an acre-foot is defined as the volume of one acre of surface area to a depth of one foot.

Since an acre is defined as a chain by a furlong (i.e. 66 ft × 660 ft or 20.12 m × 201.17 m), an acre-foot is 43,560 cubic feet (1,233 m3).

There are two definitions of an acre-foot (differing by about 0.0006%), depending on whether the "foot" used is an "international foot" or a "U.S. survey foot.

1 acre-foot = 43,560 cubic feet = 75,271,680 cu in
1 international acre-foot = 43,560 international cubic feet
≈ 1233.48183754752 m3
≈ 271,328.072596 imp gal
≈ 325,851 3⁄7 US gal
1 U.S. survey acre-foot = 43,560 U.S. survey cubic feet
≈ 1233.4892384681 m3
≈ 271,329.700571 imp gal
≈ 325,853.383688 U.S. gal[nb 1]

Application

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.[2] 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.[3] One acre-foot/year is approximately 893 gallons (3.38 m³) 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 cubic meters.

See also

Notes

  1. ↑ 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.

References

  1. ↑ "NM OSE Glossary". Retrieved 15 March 2014.
  2. ↑ 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 (PDF) from the original on 25 January 2008. Retrieved 2008-01-30.
  3. ↑ 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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