Western water rights reflect a short and stable climate history, but that period of stability is ending. Looming climate change of 4°C will produce not only higher temperatures, but decreased snowpack, shifts in runoff patterns, and the dramatic shrinkage of giant reservoirs. The climatic changes that have already traumatized the West will only intensify and cross even more dangerous thresholds, necessitating the deliberate adaptation of water rights systems.

Hydrology in a Changing Climate

A climate-changed future is inherently uncertain, but a general consensus predicts a dire future for water supply in the arid West.  Precipitation patterns are the biggest source of uncertainty due to the potential increase in extreme weather events.  This could both increase and decrease water supplies as larger snowstorms could dump more precipitation in some years, while other impacts on the snowpack would lead to declines.  However, higher temperatures in the summer and fall are expected to offset potential increases in snowpack in most years, leading to an overall trend of less water supply in a warming future.  The Colorado River, which supplies vital water to seven states and 23 tribal nations, may experience flow losses due to temperature increases by more than 20 percent midcentury and 35 percent by 2100.  Intra- and inter-annual variability will continue as a hallmark of western water systems. In the Colorado River basin, for example, the impacts of a changing climate are already apparent as the current megadrought fueled by climate change recently led to the first ever federal water shortage declaration.  Experts urge us to plan for even worse impacts to come.

Looking beyond the Colorado River basin, a 4°C world leads to large declines in snowpack in the western United States, perhaps in the range of a 40 percent decrease due to generally less precipitation and shifts from snow to rain.  The snowpack in the Sierra Nevada mountains recently hit its lowest point going back at least 500 years. Decreasing snowpack reduces water availability throughout hot, dry summers, resulting in significant seasonal water shortages.

Warmer temperatures and less frequent precipitation also mean that even normal snowpacks do not necessarily bring relief from droughts.  For example, Colorado’s 2021 snowpack was almost normal, but because soils in many western watersheds were unusually dry, most of the water went into the soils and not into streams, rivers, and reservoirs for human uses.  Another driver of drought in a warming world is the increase in evapotranspiration caused by higher temperatures as plants need more water and evaporation from rivers and reservoirs increases. These factors point towards a drying and warming future in the southwestern United States, particularly in the Colorado River basin.

Water Law Historically Adapted to Hydrology

The changes in the West’s hydrology are very likely to produce changes in water law, which has historically evolved in response to differing climactic conditions. Early U.S. water law decisions drew heavily on English water law, establishing a system based on riparian water rights. Riparian rights come from ownership of land that abuts a watercourse, and they are generally limited to reasonable use of the water on the riparian land. Riparian rights have many other limits; they do not allow storage or long distance transportation of water, for example, and they are not absolute, leading to some uncertainty about the quantity and reliability of water. These limitations meant that riparian rights were poorly suited to western hydrologic regimes where seasonal (and total) water availability patterns require storage and transportation of water to maximize the benefits of available water.

California developed a system of appropriative rights based on the use of water, not on land ownership. Appropriative rights allow water storage and transportation, and these advantages led all of the states west of the Mississippi to adopt some version of appropriative rights. Coastal states like California and Washington, and midwestern states like Kansas and Nebraska, tend to blend riparian and appropriative doctrines, while drier western states like Colorado embrace a purer appropriative rights approach that does not recognize riparian rights at all. Appropriative water rights systems give priority to the first user of the water, an approach often styled “first in time, first in right,” so that later users may not get their full allocation of water in dry years. Maintaining appropriative water rights requires constant vigilance; these rights can be lost through disuse or to other users who take the water. Water rights are tied to the land and to its use; transferring the right to someone else or changing the place or use of the water generally requires permission of a state level water agency.

The appropriative rights and the blended appropriative/riparian rights approaches are both inherently based on historic hydrology and patterns of land use and ownership. This can make them a barrier to climate adaptation. For example, the first-in-time approach gives priority to the earliest water users, regardless of how well suited these uses are to a changing climate or changing societal needs.  Historical aspects of water rights thus sometimes allow lower value agricultural uses to take priority over domestic and industrial uses. Formally, water use is generally required to be reasonable and beneficial, but as a practical matter, courts and water boards rarely rein in inefficiency. Recent droughts have shown many existing uses of water to be even more anachronistic. Because most water in the West is already appropriated, in a drier, climate-changed future, a reordering of priorities seems necessary.

Consider California’s agricultural sector. Though productive and valuable, California’s agricultural sector constitutes about 80 percent of the state’s human water use while accounting for less than 3 percent of the State’s GDP. The ag sector has become significantly more efficient in the last 30 years, using less total water to produce more agricultural value. Nevertheless, in some cases and in some years, California water currently used in agriculture would be more valuable as drinking water or as water to support ecosystems. Growing fewer almonds or making less milk and cheese will likely be necessary in a 4°C world. A successful water rights system should encourage and accommodate these shifts.

Further, use-it-or-lose-it requirements disincentivize water conservation or land use changes that could free up water for other users.  Riparian rights give strong rights to riparian landowners, a system that tends to maintain existing land uses and perpetuate distributive justice concerns. Use-based appropriative rights inherently value use over conservation or other “passive” uses, such as fish and wildlife habitat.

Finally, although existing water rights systems have succeeded in spurring economic development, they have done so at great environmental and social cost. The extensive, massive water diversions that have made California the most productive agricultural state in the United States also completely transformed the entire Central Valley and altered the ecology of much of the state, driving many native species to extinction.

Can Water Law Adapt to a Warmer Future?

How is the western United States to cope with a world that is warmer by as much as 4°C and chronically short of water? We suggest three steps, emphasizing that these represent just a few out of many constructive actions that might be taken to prepare for a much warmer, much drier American West.

New diversions must be evaluated under a conception of reasonableness that includes climate change. Western water law has always embedded notions of reasonableness and almost always been, at least formally if not in practice, predicated on some consideration of public interest. These terms have been either ill-defined or defined in a way that gives short shrift to considerations of conservation and passive uses. The phrase public interest has been infrequently deployed to protect passive or instream uses and has in some cases simply been ignored in water permit applications. Notions of reasonableness or beneficial use must take account of future scarcity of water and an increased need for conservation and domestic uses.

Existing reasonableness requirements must be enforced as a limit on current water rights. With climate change already well underway, many existing allocations of water are economically inefficient, with too little regard for non-agricultural uses. Because most water in the West is already allocated, current water uses must be re-examined and curtailed if they fail to meet a realistic reasonableness test that accounts for the drier, hotter realities of a climate-changed future.

States must actively secure water rights in preparation for severe and prolonged water shortages in the future. States must create new institutions to collect water rights as an effective stockpile against future scarcity, which may present more serious threats than the loss of crops or livestock. States must migrate some water and water rights into a governance mechanism that operates outside of traditional water law. A state-chartered trust instrument, such as a “Resources Trust,” might be legislatively charged with gathering up water and water rights in order to act as a water supplier of last resort should the dire need arise. In hedging against severe and prolonged water shortages, such a Resources Trust might employ a range of legal instruments, such as options, to secure future supply.

Authored by:

  • Kevin Lynch, Associate Professor of Law with Tenure at the Sturm College of Law.
  • Shi-Ling Hsu, D’Alemberte Professor at Florida State University College of Law.
  • Karrigan Bork, Acting Professor of Law at UC Davis School of Law.

*** This blog post is part of a series of essays by the Environmental Law Collaborative (ELC) on the adaptation challenges of the worst-case climate scenario: a world that warms to 4°C by 2100.
Pace | Haub Environmental Law’s GreenLawBlog is co-hosting this series, along with Environmental Law Prof Blog, over the course of this month. ***