Acre-Foot

One foot of water spread over one acre, being twelve acre-inches, and the standard unit of reservoir capacity and water allocation.

Twelve acre-inches, and the unit in which water is bought, sold and litigated over. An acre-foot is conventionally reckoned as roughly a year’s supply for one or two households, which is why it became the working unit of water rights in the American west, where it appears in statutes, court judgments and interstate compacts. The figure of 43,560 cubic feet is not arbitrary: it is simply the square footage of an acre, since multiplying by a depth of one foot changes nothing but the dimension. British usage favours the acre-inch for field irrigation and megalitres for reservoirs, so the acre-foot is more often met in American contexts, but it descends directly from the English acre and belongs to the same family of units built by multiplying an area by a depth.