acre-foot

Water managers think in depth over area because that is how water arrives and how it is used. Rain falls as a depth, a field is irrigated to a depth, and a reservoir's surface is an area. The acre-foot collapses those two ideas into one number: an acre of land under one foot of water, 43,560 cubic feet, about 1,233 cubic metres. A rancher can convert an entitlement directly into an irrigation schedule without ever leaving the units the land is described in.

It is the backbone unit of western American water law. The Colorado River Compact of 1922 divided the river's flow between upper and lower basin states in millions of acre-feet per year, a figure now widely understood to have been set during an unusually wet period, and the shortfall has driven litigation and negotiation ever since. Lake Mead and Lake Powell storage, state allocations and groundwater overdraft are all reported in acre-feet or thousand-acre-feet (TAF). The metric equivalent is the megalitre, which is convenient: one acre-foot is 1.233 ML, so a million acre-feet is 1.233 cubic kilometres of water.