Implied-Reservation-of-Water Doctrine is a legal principle that permits the federal government to use and control, for public purposes, water appurtenant to federal lands. This doctrine of water rights evolved to ensure that Indian reservations and public lands set aside by the federal government would have sufficient water to fulfill the purposes for which they were established. The doctrine is rooted in a number of judicial decisions, beginning with a U.S. Supreme Court decision in Winters v. United States, 207 U.S. 564 (U.S. 1908) that came to be called the Winters Doctrine. Winters doctrine formed the basis for reserved water rights on federal lands such as national parks, forests, monuments, wildlife refuges and military installations. Subsequent judicial decisions also authorize federal reserved water rights on lands set aside by statute, treaty, or executive order. They are defined by the documents that set the land aside and recognized within individual states by negotiation or litigation. However, unlimited use of water resources on federal land is not permitted. Implied-Reservation-of-Water doctrine reserves only that amount of water necessary to fulfill the purpose of reservation and no more.