Acre-foot

acre-foot
An acre-foot volume (not drawn to scale)
General information
Unit system	US Customary units
Unit of	Volume
Symbol	ac⋅ft
Conversions
1 ac⋅ft in ...	... is equal to ...
SI units	≈ 1,233.5 m^3
US customary units	43,560 cu ft
US customary units	≈ 325,850 US gal
Imperial units	≈ 271,330 imp gal

The acre-foot is a non-SI unit of volume equal to about 1,233 m³ 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.

An acre-foot equals approximately an eight-lane swimming pool, 82 ft (25 m) long, 52 ft (16 m) wide and 9.8 ft (3 m) deep.

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 has been two definitions of the acre-foot (differing by about 0.0006%), using either the international foot (0.3048 m) or a U.S. survey foot (exactly ⁠1200/3937⁠ meters since 1893). On December 31, 2022, the National Institute of Standards and Technology, the National Geodetic Survey, and the United States Department of Commerce deprecated use of the US survey foot and recommended conversion to either the meter or the international foot.^[2]

1 acre-foot	= 43,560 cubic feet
	= 75,271,680 cu in
	= 325,851+3⁄7 US gal [a]
international	≈ 1,233.4818 m^3 (using the 0.3048 m foot)
U.S. survey	≈ 1,233.4892 m^3 (using the ⁠1200/3937⁠ m foot)

Application

As a rule of thumb in US water management, one acre-foot is taken to be the planned annual water usage of a suburban family household.^[b] 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 per year (310 m³/a) of water per year.^[4] One acre-foot/year is approximately 119 cu ft/d (3.38 m³/d).

The 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 (19 cubic kilometres per year) among seven western US states.

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

In most other countries except the US, the metric system is in common use and water volumes are normally expressed in liter, cubic meter or cubic kilometer. One acre-foot is approximately equivalent to 1.233 megaliters. Large bodies of water may be measured in cubic kilometers (1,000,000,000 m³, or 1000 gigaliter), with 1 million acre-feet approximately equalling 1.233 km³.

See also

• Cubic meter per second
• Cubic foot per second
• List of unusual units of measurement
• United States customary units
• Unit of measurement

Explanatory notes

1. As the foot is used via the inch to define the U.S. gallon, the number of US gallons in an acre feet is the same no matter which foot is used
2. The state of Montana assumes 1.0 acre-foot per year (1,200 m³/a) for a family of five.^[3]

Citations

1. "NM OSE Glossary". Archived from the original on 14 November 2005. Retrieved 15 March 2014.
2. "U.S. Survey Foot". National Institute of Standards and Technology. 4 January 2023. Retrieved 4 April 2024.
3. Water Rights Bureau; state of Montana (13 April 2004). "Form No. 627 R8/03 Notice of Water Right" (PDF). Archived from the original (PDF) on 25 January 2008. Retrieved 30 January 2008.
4. 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". Archived from the original (PDF) on 22 July 2012. Retrieved 30 January 2008.