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Great Basin's Truckee- Carson system, and Utah's Salt Lake valley. Throw the possibility of
<br />climatic change and variability into the mix and things could get truly interesting.
<br />THE INSTITUTIONAL SETTING
<br />The risk of future water shortages in the West is distributed unevenly, in part due to geographic and
<br />demographic factors, but also due to laws, policies, and other institutional factors. The most
<br />important of these considerations is the region's primary legal mechanism for water allocation: prior
<br />appropriation. The prior appropriation doctrine is not only the dominant water allocation mechanism
<br />in the region, but it is the West's de facto water policy, recognized in several state constitutions.
<br />The hallmark of the prior appropriation system is the concept of "first -in -time, first -in- right." This
<br />notion allows for the establishment of a priority system to determine the allocation of water amongst
<br />users on a stream when supplies are insufficient to satisfy all demands. Priority is based on
<br />seniority; "senior" rightsholders are those who first established a pattern of beneficial water use—as
<br />recognized in an administrative permit or judicial decree— as compared to more "junior" users.
<br />Seniority is important since it determines a water user's vulnerability to climatic events and other
<br />possible sources of shortage. In a water short year, senior water rightsholders receive all of their
<br />water before any water is made available to junior rightsholders. When necessary, a senior water
<br />rightsholder may place a "call on the river" requiring upstream junior rightsholders to cease
<br />diversions until more senior users receive their full entitlements.
<br />Water rights acquired through appropriation and officially recognized by permit or decree generally
<br />specify the type, timing and place of use, carry a seniority date corresponding to the date of first
<br />diversion and use, and are quantified based on the historic level of use calculated in either volumetric
<br />amounts (e.g., acre - feet), rate of flow (e.g., cubic - feet -per- second), or described more generally in
<br />terms of crop needs. Rights can also be obtained for water storage, with the understanding that water
<br />collected during wet periods will be released and consumed in dry seasons. The quantity of water in
<br />an appropriation right is the amount of water that is put to a beneficial use in a reasonable time with
<br />reasonable diligence. In this respect, diverting more water than reasonably necessary is considered
<br />wasteful and inefficient, and is thus not considered part of the water right.
<br />Prior appropriation allows the movement of water within basins and between basins within states,
<br />but across state lines, different rules apply. At this larger scale, allocation decisions are typically
<br />either determined by litigation before the U.S. Supreme Court, or more commonly, in state -to -state
<br />negotiations leading to interstate water allocation compacts. Both approaches generally reject the
<br />priority system and instead reserve fixed quantities (or fixed percentages) of water for each state
<br />based on a wider variety of considerations. Interstate compacts exist for the Arkansas, Bear, Belle
<br />Fourche, Big Blue, Canadian, Colorado, Klamath, La Plata, Pecos, Red, Republican, Rio Grande,
<br />Sabine, Snake, South Platte, Upper Colorado, Upper Niobrara, and Yellowstone Rivers, and on
<br />Costilla Creek. None of these compacts explicitly address the possibility of climate change, and only
<br />one —the Upper Colorado River Compact —even mentions the word drought.
<br />Several different types of governmental and quasi - governmental organizations play important roles
<br />in managing western water resources. Prior appropriation is state law devised by state legislators
<br />and administered by state agencies, with the overriding management objective simply being to
<br />record and enforce the priority system. This role, along with the oversight of interstate agreements,
<br />is the extent of major state governmental roles in many western states. The actual planning,
<br />development and operation of water systems are typically conducted by other entities, including
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