Acre-foot
The acre-foot is a unit of volume commonly used in the United States in reference to large-scale water resources, such as reservoirs, aqueducts, canals, sewer flow capacity, irrigation water, and river flows.
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 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".
As a rule of thumb in U.S. water management, one acre-foot is taken to be the planned water usage of a suburban family household, annually. 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 of water per year. One acre-foot/year is approximately 893 gallons (3.38 m3) per day.
The acre-foot (or more specifically the time rate unit of acre-foot per year) has been used historically in the U.S. in many water-management agreements, for example the Colorado River Compact, which divides 15 million acre-feet (MAF) per year (586 m3/s) among seven western U.S. states.
Elsewhere in the world, where the metric system is in common use, water volumes can be expressed in either cubic metres (as in flow rates of cubic metres/second, or "cumecs") or, for water usage, storage or irrigation volumes, in kilolitres (kL = 1 cubic metre), megalitres (ML = 1,000 cubic metres), or gigalitres (GL = 1,000,000 cubic metres). One acre-foot is approximately equivalent to 1.233 megalitres.
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