Prior appropriation water rights, sometimes known as the "Colorado Doctrine", is a system of allocating water rights from a water source that is markedly different from riparian water rights. Water law in the Western United States generally follows the appropriation doctrine which developed due to the scarcity of water in that area.

The legal details vary from state to state, however the general principal is that water rights are unconnected to land ownership, and these rights can be sold or mortgaged like other property. The first person to use a quantity of water from a water source for a beneficial use has the right to continue to use that quantity of water for that purpose. Subsequent users from the same source can use the remaining water for their own beneficial purposes provided that they do not impinge on the rights of previous users.

Beneficial use is commonly defined as agricultural, industrial or domestic use. Ecological purposes, such as maintaining a natural body of water and the wildlife that depends on it was not historically held to be beneficial uses but has recently been accepted in some jurisdictions.

Each water right has a yearly quantity and an appropriation date. Each year, the user with the earliest appropriation date (known as the "senior appropriator") may use up to their full allocation (provided the water source can supply it). Then the user with the next earliest appropriation date may use their full allocation and so on. In times of drought, users with junior appropriation dates might not receive their full allocation or even any water at all. Shortages do not result in sharing of the resource or any diminishment of the amount the senior appropriator can take, provided there is enough water for that.

When a water right is sold, it retains its original appropriation date. Only the amount of water historically consumed can be transferred if a water right is sold. For example, if alfalfa is grown, using flood irrigation, the amount of the return flow may not be transfered, only the amount that would be necessary to irrigate the amount of alfalfa historically grown. If a water right is not used for a beneficial purpose for a period of time it may lapse under the doctrine of abandonment. Abandonment of a water right is rare, but occured in Colorado in a case involving the South Fork of San Isabel Creek in Saguache County, Colorado. Unused irrigation wells are especially vulnerable to being considered abandoned. Good records of electricity or fuel used and crops grown and harvested can be very important.

For water sources with many users, a government or quasi-government agency is usually charged with overseeing allocations. Allocations involving water sources that cross state borders or international borders can be quite contentious, and are generally governed by federal court rulings, interstate compacts and international treaties.

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