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 in reference to large-scale water resources, such as reservoirs, aqueducts, canals, 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 flows.

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==

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.

Since an acre is defined as a 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 international foot (0.3048 m) or a 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 in3 |- | || = {{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}}  |- | || |- | international || = {{val|1233.48183754752}} m3 (using the 0.3048 m foot) |- | U.S. survey || ≈ {{round|{{#expr:43560*((1200/3937)^3)}}|4}} m3 (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}} 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 liters, cubic meters or cubic kilometers. 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. To 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.

== Explanatory notes == {{Notelist}}

== Citations == {{Reflist}}

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