Acre-foot
Non-SI unit of volume
- 2 min read
- 2 chapters
Related on Lably
Through International System of Units, Volume and Reservoir
An acre-foot is approximately the volume of water needed to fill 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 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. 66 ft × 660 ft or 20.12 m × 201.17 m), an acre-foot is 43,560 cubic feet (1,233.5 m3).
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 1200/3937 meters since 1893). On 31 December 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.
Sign in to keep reading
One more chapter is free with a Lably account.
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. In some areas of the desert Southwest, where water conservation is followed and often enforced, a typical family uses only about 0.25 acre-feet (310 m3) of water per year. One acre-foot per year is approximately 119.26 cubic feet per day (3.38 cubic meters per day).
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 kilometers 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 liters, cubic meters or cubic kilometers. One acre-foot is approximately equivalent to 1.233 megaliters. Large bodies of water may be measured in cubic kilometers (1 km3 equals 1 billion cubic meters or 1,000 gigaliters); 1 million acre-feet approximately equals 1.233 km3.
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 1.01 ft3/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.