View source for Acre-foot

{{Short description|Non-SI unit of volume}}
{{Use dmy dates|date=August 2020}}
{{Infobox unit
| bgcolor       = 
| name          = acre-foot
| image         = Acre foot.svg
| caption       = An acre-foot volume (not drawn to scale)
| standard      = [[US Customary units]]
| quantity      = Volume
| symbol        = ac⋅ft
| symbol2       = 
| extralabel    = 
| extradata     = 
| units1        = SI units
| inunits1      = ≈ {{cvt|1|acre ft|m3|sigfig=7|disp=out|lk=on}}
| units2        = US customary units
| inunits2      = {{cvt|1|acre ft|ft3|sigfig=5|disp=out|lk=on}}
| units3        = US customary units
| inunits3      = ≈ {{cvt|1|acre ft|USgal|sigfig=7|disp=out|lk=on}}
| units4        = Imperial units
| inunits4      = ≈ {{cvt|1|acre ft|impgal|sigfig=7|disp=out|lk=on}}
}}

The '''acre-foot''' is a non-[[SI]] unit of [[volume]] equal to about {{cvt|1|acre ft|m3|sigfig=4|disp=out}} commonly used in the western [[United States customary units#Volume|United States]] in reference to large-scale water resources, such as [[reservoir]]s, [[aqueduct (water supply)|aqueduct]]s, [[canal]]s, [[sanitary sewer|sewer]] flow capacity, [[irrigation]] water,<ref>{{cite web|title=NM OSE Glossary|url=http://www.ose.state.nm.us/water_info_glossary.html#A|access-date=15 March 2014|url-status=dead|archive-url=https://web.archive.org/web/20051114202301/http://www.ose.state.nm.us/water_info_glossary.html|archive-date=14 November 2005}}</ref> and [[river flow]]s.

An acre-foot equals the volume of water needed to fill approximately an eight-lane [[swimming pool]], {{cvt|25|m|order=flip|||}} long, {{cvt|16|m|order=flip|||}} wide and {{cvt|3|m|order=flip|||}} deep.

==Definitions==
<!-- Don't convert anything in this paragraph. The relevant conversions are hereafter. -->

As the name suggests, an acre-foot is defined as the volume of water that would cover one [[acre]] of surface area to a depth of one [[Foot (unit)|foot]].

Since an acre is defined as a [[chain (unit)|chain]] by a [[furlong]] (i.e. {{convert|66|×|660|ft|m|2|abbr=on|disp=or}}), an acre-foot is {{convert|43,560|ft3|m3|1|lk=on}}.

There have been two definitions of the acre-foot (differing by about 0.0006%), using either the [[Foot (unit)#International foot|international foot]] (0.3048 m) or a [[Foot (unit)#Survey foot|U.S. survey foot]] (exactly {{sfrac|1200|3937}}{{nbsp}}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.<ref>{{Cite web| title = U.S. Survey Foot| work = National Institute of Standards and Technology| access-date = April 4, 2024| date = January 4, 2023| url = https://www.nist.gov/pml/us-surveyfoot}}</ref>

{| style="padding-left: 3em"
| 1 acre-foot               || = 43,560 cubic feet 
|-
|                                   || = 75,271,680 in<sup>3</sup>
|-
|                           || =  {{convert|1|acre.foot|USgal|frac=7|disp=output only}}{{efn|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}}
<!-- 1 US gal = 231 in<sup>3</sup> 
     75,271,680 in<sup>3</sup> / 231 in<sup>3</sup>  = 325,851 3/7 US gal - because 231 is multiple of, the definition is not decimal but fraction of 7  -->
|-
|                                   || 
|-
| international   || ≈ {{round|{{#expr:43560*((0.3048)^3)}}|4}} m<sup>3</sup>  (using the 0.3048 m foot)
|-
| U.S. survey  || ≈ {{round|{{#expr:43560*((1200/3937)^3)}}|4}} m<sup>3</sup> (using the {{sfrac|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.{{efn|The state of Montana assumes {{convert|1.0|acre-foot/year|m3/year|lk=on}} for a family of five.<ref>{{cite web|url=http://dnrc.mt.gov/wrd/water_rts/wr_general_info/wrforms/627.pdf|title=Form No. 627 R8/03 Notice of Water Right|author1=Water Rights Bureau|author2=state of Montana|date=13 April 2004|access-date=30 January 2008|archive-url=https://web.archive.org/web/20080125225729/http://dnrc.mt.gov/wrd/water_rts/wr_general_info/wrforms/627.pdf|archive-date=25 January 2008|url-status=dead}}</ref>}} In some areas of the desert Southwest, where [[water conservation]] is followed and often enforced, a typical family uses only about {{convert|0.25|acre-ft|m3|lk=on}} of water per year.<ref>[[Santa Fe, New Mexico]] rate averages 0.25 acre-foot per year per household. See {{cite web|url=http://www.santafenm.gov/DocumentView.asp?DID=1427|title=Water Use in Santa Fe: A survey of residential and commercial water use in the Santa Fe urban area|format=[[PDF]]|author=((Planning Division, Planning & Land Use Department, City of Santa Fe, New Mexico))|date=February 2001|access-date=30 January 2008
|archive-url=https://web.archive.org/web/20120722005241/http://www.santafenm.gov/DocumentView.aspx?DID=1427|archivedate=2012-07-22}}</ref> One acre-foot per year is approximately {{convert|1|acre-ft/year|ft3/d m3/d|2|disp=(or)|order=out|abbr=off|sp=us}}.

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 {{convert|15|e6acre-ft/year|km3/year|abbr=off|sp=us}} 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 [[litre|liter]], [[cubic metre|cubic meter]] or cubic [[kilometre|kilometer]]. One acre-foot is approximately equivalent to {{convert|1.000|acre-ft|ML|disp=out|abbr=off|sp=us}}. Large bodies of water may be measured in cubic kilometers ({{val|1|u=km3}} equals {{convert|1|km3|e9m3 GL|0|disp=or|order=out|spell=on|sp=us|abbr=off}}); {{convert|1|e6acre-ft|km3|3|disp=x| approximately equals }}.

A volumetric change of 10 acre-feet per hour is equivalent to exactly 121 cubic feet per second. Dividing by 120, this reduces to 1 '''acre-inch''' per hour is approximately {{val|1.01|u=ft3|up=s}}. This can be visualized as such: suppose a 1-acre field is receiving a sustained rainfall of 1 inch per hour. In order prevent the field from flooding or overflowing, the drainage system must be able to handle a discharge of at least 1 cubic foot per second.

==See also==
{{Portal|Science|Water}}
* [[Cubic metre per second|Cubic meter per second]]
* [[Cubic foot per second]]
* [[List of unusual units of measurement]]
* [[United States customary units]]
* [[Unit of measurement]]

== Explanatory notes ==
{{Notelist}}

== Citations ==
{{Reflist}}

{{United States Customary Units}}
{{Authority control}}

{{DEFAULTSORT:Acre-foot}}
[[Category:Customary units of measurement in the United States]]
[[Category:Units of volume]]