Prior Appropriation Doctrine

A system for allocating water rights used in the western United States under which the first person (or entity) to divert water from a source has a priority to that water right, and so on. Under the system of prior appropriation, water rights that are junior are not allowed to prevent senior water rights holders from obtaining their allocation of water. Thus, in times of drought, a junior water rights holder may not be entitled to its share of the resource. However, even senior water rights holders are not allowed to change the time of use, place of use, purpose of use, or point of diversion of the right, if it would injure other water rights holders within a basin.