The acre-foot is a non-
SI unit of
volume
Volume is a measure of regions in three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch) ...
equal to about commonly used in the
United States
The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
in reference to large-scale water resources, such as
reservoir
A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation.
Reservoirs are created by controlling a watercourse that drains an existing body of wa ...
s,
aqueducts,
canal
Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface ...
s,
sewer flow capacity,
irrigation
Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...
water, and
river flows.
An acre-foot equals approximately an eight-lane
swimming pool
A swimming pool, swimming bath, wading pool, paddling pool, or simply pool, is a structure designed to hold water to enable Human swimming, swimming and associated activities. Pools can be built into the ground (in-ground pools) or built abo ...
, long, wide and deep.
Definitions
As the name suggests, an acre-foot is defined as the volume of one
acre
The acre ( ) is a Unit of measurement, unit of land area used in the Imperial units, British imperial and the United States customary units#Area, United States customary systems. It is traditionally defined as the area of one Chain (unit), ch ...
of surface area to a depth of one
foot
The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up o ...
.
Since an acre is defined as a
chain
A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A ...
by a
furlong
A furlong is a measure of distance in imperial units and United States customary units equal to one-eighth of a mile, equivalent to any of 660 foot (unit), feet, 220 yards, 40 rod (unit), rods, 10 chain (unit), chains, or a ...
(i.e. ), an acre-foot is .
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 meters since 1893). On December 31, 2022, the
National Institute of Standards and Technology
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into Outline of p ...
, the
National Geodetic Survey
The National Geodetic Survey (NGS) is a List of federal agencies in the United States, United States federal agency based in Washington, D.C. that defines and manages a national geographic coordinate system, coordinate system, providing the fou ...
, and the
United States Department of Commerce
The United States Department of Commerce (DOC) is an executive department of the U.S. federal government. It is responsible for gathering data for business and governmental decision making, establishing industrial standards, catalyzing econ ...
deprecated use of the US survey foot and recommended conversion to either the meter or the international foot.
Application
As a
rule of thumb
In English language, English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associat ...
in US water management, one acre-foot is taken to be the planned annual water usage of a suburban family household. In some areas of the desert Southwest, where
water conservation
Water conservation aims to sustainably manage the natural resource of fresh water, protect the hydrosphere, and meet current and future human demand. Water conservation makes it possible to avoid water scarcity. It covers all the policies, strateg ...
is followed and often enforced, a typical family uses only about of water per year.
Santa Fe, New Mexico
Santa Fe ( ; , literal translation, lit. "Holy Faith") is the capital city, capital of the U.S. state of New Mexico, and the county seat of Santa Fe County. With over 89,000 residents, Santa Fe is the List of municipalities in New Mexico, fourt ...
rate averages 0.25 acre-foot per year per household. See One acre-foot/year is approximately .
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 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
The metric system is a system of measurement that standardization, standardizes a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules gover ...
is in common use and water volumes are normally expressed in
liter
The litre ( Commonwealth spelling) or liter ( American spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metres (m3). A cu ...
,
cubic meter
The cubic metre (in Commonwealth English and international spelling as used by the International Bureau of Weights and Measures) or cubic meter (in American English) is the unit of volume in the International System of Units (SI). Its symbol is m ...
or cubic
kilometer
The kilometre ( SI symbol: km; or ), spelt kilometer in American and Philippine English, is a unit of length in the International System of Units (SI), equal to one thousand metres (kilo- being the SI prefix for ). It is the preferred mea ...
. One acre-foot is approximately equivalent to 1.233 megaliters. Large bodies of water may be measured in cubic kilometers (1,000,000,000 m, or 1000 gigaliter), with 1 million acre-feet approximately equalling 1.233 km.
See also
*
Cubic meter per second
*
Cubic foot per second
*
List of unusual units of measurement
An unusual unit of measurement is a unit of measurement that does not form part of a coherent system of measurement, especially because its exact quantity may not be well known or because it may be an inconvenient multiple or fraction of a base ...
*
United States customary units
United States customary units form a system of measurement units commonly used in the United States and most U.S. territories since being standardized and adopted in 1832. The United States customary system developed from English units that ...
*
Unit of measurement
A unit of measurement, or unit of measure, is a definite magnitude (mathematics), magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other qua ...
Explanatory notes
Citations
{{DEFAULTSORT:Acre-foot
Customary units of measurement in the United States
Units of volume