Prior appropriation is a water rights doctrine used primarily in the western United States, based on the principle of 'first in time, first in right.' Senior rights holders have priority over junior users during water shortages.
The doctrine of prior appropriation is the dominant system of water rights in the 17 western states of the United States, governing the allocation and use of both surface water and, in most jurisdictions, groundwater. Under this doctrine, the first person to divert water from a natural source and put it to beneficial use acquires a senior right that has priority over all later (junior) appropriators. During times of shortage, water is allocated in order of priority date, with the most senior rights satisfied first; junior rights holders may receive no water at all in severe droughts, following the principle of first in time, first in right. To establish and maintain a prior appropriation right, the user must divert water from its natural course, apply it to a recognized beneficial use, and use it without unreasonable waste. Water rights under prior appropriation are considered real property rights that can be bought, sold, leased, or transferred, subject to the no-injury rule that prevents transfers from harming other water rights holders. The doctrine developed during the California Gold Rush era when miners needed to divert water far from riparian lands, making the eastern riparian doctrine impractical. Modern challenges to the prior appropriation system include overallocation of water rights exceeding available supply, recognition of environmental and tribal water rights, and adapting a rigid priority system to the flexibility needed for climate adaptation.