In the United States, legal rights to the use of irrigation water are called water rights, or water decrees. They are governed by water law—a complicated interweaving of statutes, court decisions, contracts (usually called compacts), and international treaties. Water law may decide how much water a farmer is entitled to apply to a cornfield on a given date, or the law may specify the rights of one state to the waters of a river that has its headwaters in another state.

Rights to surface waters are based on the doctrine of appropriation or the doctrine of riparian (streambank) rights, or a combination of these doctrines. Rights to the use of ground water for irrigation are not as clear-cut as surface rights; various modifications of surface-right doctrines usually are invoked.

All Western states recognize the doctrine of appropriation: beneficial use of the water creates the right. The right is lost when beneficial use ceases.

are acquired by ownership of land along a stream or other body of water, in states that recognize this ancient doctrine. Originally the doctrine meant that the landowner was entitled to any or all of the water flowing past or through his property. Hardships and quarrels resulting from the doctrine led the courts to declare that riparian rights must be limited by reasonable use. What constitutes “reasonable use” was left to the courts to decide in particular cases.