Acre-foot

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.

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 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 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 = .mw-parser-output .frac{white-space:nowrap}.mw-parser-output .frac .num,.mw-parser-output .frac .den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output .frac .den{vertical-align:sub}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}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[a]

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 of water per year.[3] 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 15×106 acre⋅ft/a (590 m3/s) 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.