Acre-foot

The acre-foot is a non-SI 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.

Acre-foot
An acre-foot volume (not drawn to scale)
General information
Unit systemUS Customary units
Unit ofVolume
Symbolac⋅ft 
Conversions
1 ac⋅ft in ...... is equal to ...
   SI units   ≈ 1,233.5 m3
   US customary units   = 43,560 cu ft
   US customary units   ≈ 325,850 US gal
   Imperial units   ≈ 271,330 imp gal

An acre-foot equals approximately an 8 lane swimming pool, 25 meters long, 16 m wide and 3 m deep.

Government policy in the United States, as stated by law, is to phase out imperial and US customary units and replace them with metric equivalents.[2] As of 2018, the metric system is mandatory to use in trade and commerce in all countries except the United States, Myanmar and Liberia.[3]

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 (1,233.5 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
= 1,233.48183754752 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 US water management, one acre-foot is taken to be the planned water usage of a suburban family household, annually.[4] In some areas of the desert Southwest, where water conservation is followed and often enforced, a typical family uses only about 0.25 acre-foot of water per year.[5] 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 US in many water-management agreements, for example the Colorado River Compact, which divides 15 million acre feet per year (590 cubic metres per second) among seven western US states.

Water reservoir capacities in the US are commonly given in thousands of acre-feet, abbreviated TAF.

In most other countries except the US, the metric system is in common use and water volumes are normally expressed in litre, cubic metre or cubic kilometre. One acre-foot is approximately equivalent to 1.233 megalitres. Large bodies of water may be measured in cubic kilometres (1,000,000,000 m3, or 1000 gigaliter), with 1 million acre-feet approximately equalling 1.233 km3. As of 2018, the metric system is mandatory to use in trade and commerce in all countries except US, Myanmar and Liberia.[6]

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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". Archived from the original on November 14, 2005. Retrieved March 15, 2014.
  2. "US 1988 law on metrification". Retrieved September 21, 2019.
  3. "Countries not using SI". Retrieved September 21, 2019.
  4. 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 (PDF) on January 25, 2008. Retrieved January 30, 2008.
  5. Santa Fe, New Mexico rate averages 0.25 acre-foot 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 January 30, 2008.
  6. "Countries not using SI". Retrieved September 21, 2019.


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