Acre-foot

The acre-foot is a non-SI unit of volume equal to about 1 acre ft 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.

An acre-foot equals approximately an eight-lane swimming pool, 25 m long, 16 m wide and 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 ×), an acre-foot is 43,560 cuft.

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".

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-foot of water per year. One acre-foot/year is approximately 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 e6acre-ft/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 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 m$325,851 3/7$, or 1000 gigaliter), with 1 million acre-feet approximately equalling 1.233 km$3$.