Impacts of Climate Change and Land Use  on the Southwestern United States

Impacts of climate change on society

Changing Water Use and Demand in the Southwest

Jon Unruh and Diana Liverman
The University of Arizona


As the end of the 20th century approaches water supply and demand in the Southwest is entering a critical juncture. Management of changing water supplies must contend with many traditional demands as well as new values that are not explicitly recognized in the current approach to water resource management. This brief paper outlines some of the different aspects of changing water demand and availability in the Southwest, condensing a few more extensive reports on these issues (Morrison et al 1996; Gleick et al 1995; Pearson 1994)

The Challenge

It has become evident in recent years that established water policies, under the current laws and management regimes that have allowed a flourishing agriculture and economy in the Southwest, are not appropriate to the challenges facing water demand in the 21st century. Yet only limited institutional and policy steps have been taken to develop new tools and approaches addressing these challenges. The circumstances listed below illustrate some of the more important unsustainable aspects of water use in the Southwest.

Water demand exceeds supply

Considerably more water has been committed to users in the Southwest than water sources can adequately supply--even without considering water needs of aquatic ecosystems. In 1990, for the first time, the lower Colorado river basin (Arizona, California, Nevada) utilized its full 7.5 million acre-foot legal allotment. As well, long term groundwater pumping exceeds replenishment in many locations. It has been estimated that average annual groundwater over-pumping in the lower Colorado basin (including Mexico) totals 1.24 million acre-feet, with about 80 percent of that occurring in Arizona alone (Table 1).

Table 1: Groundwater Overdraft in the Lower Colorado Basin 1990 and 2020
Region 1990 2020
Arizona 1,000,000 621,000(a)
Southern Nevada 51,000 NA
Southern California 97,000 70,000
Mexico Valley, Mexico 96,000 NA
Lower Basin Total 1,244,000 691,000
(a) ADWR estimate for 2025 in Active Management Areas only.
Source: Arizona Department of Water Resources 1994

As in most of the Southwest, the largest current water use in Arizona is agriculture. However, agricultural water use is projected to decline, whilst municipal use will double over the next 50 years (Table 2).

  1990 2015 2040 1990-2040 1990-2040
Municipal and Industrial 1,332,000 1,922,000 2,605,000 1,273,000 96%
Agriculture 5,339,000 5,220,000 5,037,000 302,000 -6%
STATE TOTAL 6,671,000 7,142,000 7,642,000 971,000 15%
Source: Arizona Department of Water Resources 1994

In New Mexico, the largest water user is also agriculture, but groundwater withdrawals for all uses are seriously depleting available groundwater (Table 3).

Table 3: New Mexico Water Use Percent of total water withdrawals Percent of withdrawal that is groundwater depletion
Municipal 7.24 53.95
Domestic (self supplied) 0.63 49.73
Irrigated agriculture 79.85 76.83
Livestock 0.57 96.14
Commercial 0.46 60.36
Industrial 0.17 74.21
Mining 2.10 60.15
Power 1.34 95.26
Reservoir 7.66 0.00
Total   72.76
Source: New Mexico Water Resources Research Institute

Threats to ecological and human communities

Prior to major dam construction and large water withdrawals, natural flows watered many important riparian and other wetland areas of the Southwest. Presently much of what were natural flows a re now captured and used long before reaching these habitats. The Colorado river delta is presently desiccating and shrinking, cutting off nutrients to the sea, thereby reducing critical habitat for gulf fisheries, and bringing ruin to the economic, social, and cultural life of local human populations. In addition, the Cienega de Santa Clara, the largest wetland bird habitat remaining in the delta, is sustained primarily by the agricultural drainage water from the Wellton-Mohawk Irrigation District in Arizona, an extremely insecure water supply (see the Pacific Institute report on the Lower Colorado River).

Dam construction and water withdrawl, along with the presence of many non-native fish species, have profoundly affected many native fish species. Five big river fish of the Colorado river are now close to extinction, along with 24 species of fauna listed as endangered, and 67 species listed as at risk.

The standard of living and quality of life of those living in the southwest is also affected by the imblances between supply and demand for water. The cost of water has increased in many areas because of inadequate supplies and increasing demand. Some regions, such as central Tucson, are subsiding due to groundwater withdrawals and this is affecting property values. In other areas, recreational activities are diminished when river flows are low or riparian ecosystems are damaged.

Climate change and future water supply and demand

The frequency and severity of climatic events in the southwest can potentially be altered by global climate change; with water supply, hydroelectric generation, reservoir levels, and salinity all being sensitive to both the expected changes in climate, and to the policy approaches to selected to respond to these.

Changing Water Supply: Underlying Issues

Uncertain Supply

Part of the problem involving water supply in the Southwest is that the average annual water supply in rivers is the subject of considerable debate. Estimates for the average annual natural flow of the Colorado river for example range from 18 to 13.5 million acre-feet. However even the higher estimates of average annual flow for the Colorado are not sufficient to meet the total legal entitlement to river water, and thus the river has been deemed over-apportioned. With considerably more water legally allocated than can be reliably delivered, water allocations to Mexico under treaty obligations become problematic. For three out of the last eight years the lower basin of the Colorado has exceeded its basic entitlement of 7.5 million acre-feet. The upper Colorado river however presently uses just over half of its basic entitlement, thus the lower basin's use of more than its apportionment is not yet critical. However, continued diversion of the upper Colorado's water for lower basin uses hastens degradation of the river's ecological systems due to disruptions of the quantity, timing, and quality of natural flows.


Increasing salinity levels are a water supply concern for both the Southwest U.S., and northwestern Mexico. Higher salinity levels can make existing water supply unusable or less usable, requiring greater access to alternative water, thus increasing competition for such supplies, which may very well already be allocated. Salinity levels in the lower Colorado river was such that the U.S. was involved in a dispute with Mexico over the deterioration in the quality of the water crossing the border. The dispute was brought about largely by the development of the Wellton-Mohawk Irrigation district in Arizona and the subsequent drainage of irrigation water into the Colorado river. Salinity levels of water entering Mexico rose from approximately 800 parts per million (ppm) to about 1500 ppm, with levels going as high as 2700 ppm in late 1961. The dispute with Mexico resulted in the U.S. Congress passing The Colorado River Basin Salinity Control Act to deal with the problem. This act stipulated that salinity levels should be no greater than 115 ppm higher than river water arriving at the Imperial Dam, and included a 10,000 acre reduction in irrigable acreage in the Wellton-Mohawk Irrigation District either by purchase or eminent domain, and authorized construction of a desalting plant at Yuma, Arizona.

New values for water

As well changing values for water will affect supply to different uses and users. For example the recent Grand Canyon Protection Act of 1992 requires that the Glen Canyon Dam be operated in such a way as to protect and improve values in accordance with the reasons that the Grand Canyon National Park and Glen Canyon National Recreation Area were established. The Act ensures that releases from the Glen Canyon Dam stay within a range that protects the sand beaches along the river used by river boaters and rafters. The Act thus effectively raises the priority of recreational values and lowers the priority of hydroelectric power values in operation of the dam. Other federal statutes may similarly alter values placed on how water resources are managed, thus affecting water supply for certain uses. Some of the more important statues are: the Clean Water Act, the Endangered Species Act, and the National Environmental Policy Act, each of which may impose mandates on habitat maintenance and/or environmental protection onto current water rights allocations, dam operations and river management. Environmental interest groups are presently attempting to use this powerful federal environmental legislation as a tool to modify dam operations, and forestall water development projects.

Changing Water Demand: Underlying Issues

Population Increase

Population growth rates in the Southwest are among the highest in the nation. Supplying water to growing numbers of people will continue to have priority in any future management approach to water resources in the Southwest. In the lower Colorado basin alone, some 23 million people are at least partially depend on the river, with 74 percent residing in the greater Los Angeles and San Diego areas. While residing outside the basin, this 74 percent is served by aqueducts and dams transporting and storing Colorado river water. Demand on specific water resources varies however across the southwest, with some communities in the Coachella and Imperial Valleys relying on one water source (also the Colorado) for 95 percent of their supply, while other areas within the Southwest are able to obtain water from various sources.

For example, Arizona's population is projected to more than double in the next 50 years, with the populations of Phoenix and Tucson increasing dramatically (Table 3).

Table 3 1990 2015 2040
State of Arizona 3,664,900 6,373,600 9,405,000
Phoenix 2,145,100 3,908,400 5,941,000
Tucson 681,800 1,126,200 1,595,500
Source: Arizona Department of Water Resources (1994)

The population demand for water is not only dependent on the actual numbers of people but also on their per capita use of water. Per capita water use depends on a variety of factors including house type, garden style, attitudes to conservation, and the cost of water. Variations in southwestern per capita water use are illustrated in graphics from the City of Alburqueque which show per capita water consumption in different cities and within the city (Figures 1 and 2 from City of Albuquerque)

Figure 1: Per capita Water Consumption

Figure 2 : Water consumption within the city of Albuquerque

Institutional Structure

Much of the Southwest has no organization or entity responsible for long-term planning, making it nearly impossible to develop a comprehensive plan of action for the region. Many state, federal, and international agencies have only specific management roles, resulting in fragmented attempts to plan for future water use and management; making recovery of important watersheds and ecosystems very problematic.

Water rights of Indian tribes

Indian water rights claims have become one of the more complex and difficult water management issues in the Southwest in recent years. Unsettled tribal water claims involve potentially large amounts of water (estimated at 3.1 million acre-feet per year for Arizona alone), and are senior to most non-Indian water rights (Table 4). As a result they have the potential to seriously affect existing water use and access. The unsettled nature of these claims present a serous hindrance to long-term water planning. The problem begins with the existence of two systems of law that govern water rights in the west--federal and state. While state water rights have been based on the prior appropriation doctrine (first in time first in right), Indian water rights on the other hand have been established primarily through the federal system, and are based on federal courts' interpretations of historic treaties, Executive Orders, and other agreements between the federal government and Indian tribes. These rights in most cases are senior to most non-Indian rights. And the increasing probability that tribes will exercise these rights in the near future, whereas in the past they have not, lend considerable uncertainty to most present uses and users established and operational for some time under state law. In Arizona, by some estimations, Indian claims could exceed the average annual surface flow of the state. Currently most of this water is being used by non-Indian users who will have to reduce water use once Indian tribes begin full utilization.

Current and Future Water Use by Hopi and Navajo
  Claimed Present Use (Acre feet) Future Additional Use (Acre feet) Total future use (Acre feet)

Type of Use





































Not Listed


Not Listed





Not Listed














Not Listed


Not Listed

















Stock Ponds














Other Uses







Source: Arizona Department of Water Resources 1994

Legislation affecting demand

Changes in water demand can affect supply via legislation. An example is the settlement of eleven years of litigation between Arizona and California over Arizona's allocation from the mainstem of the Colorado river. California maintained that Arizona's 2.8 million acre feet entitlement included tributary flows from within Arizona, while Arizona contended that these flows were not part of the entitlement. The Supreme Court decided in favor of Arizona thereby complicating California's water supply arrangements.

Climate Change and Future Water Supply and Demand

Global climate changes have the potential to alter both water supply and demand in the Southwest. All present hydrologic means for evaluating and planning for the frequency and magnitude of extreme climatic events as well as for allocating available water supplies, rely on the assumption that future climate will resemble past conditions. Thus the design, construction, and management of structures to store, convey, and drain large quantities of water are based on analyses of past hydrologic records. While there is general scientific agreement that global climate change is a genuine problem and will alter hydrologic systems in a variety of ways, there is considerably less certainty about what these changes will look like. Thus past hydrologic records may not be a reliable indicator of future climatic conditions. Among the changes that are likely to affect both water supply and demand, are: rising temperatures, increasing evaporation and evapotranspiration, early melting of snowpacks, altered seasonal cycles of runoff, altered frequency and severity of extreme events, increases in river and stream flow and variability.

Especially problematic in the southwest, is that relatively small changes in temperature and precipitation, with the resulting changes in soil moisture and evapotranspiration, can result in large changes in runoff. Thus not only will water managers in the Southwest need to contend with increased uncertainly in water supply, but also changes in population growth, technology, and economic, social and legislative conditions which will very likely increase water demand. In the Southwest, water supply, hydroelectricity generation, reservoir levels, and river salinity are all very sensitive to both the changes that are expected to occur, and to the policy approaches selected to respond to climate changes.


Gleick PH, Loh P, Gomez SV, Morrison J (1995) California Water 2020: A Sustainable Vision. Pacific Institute for Studies in Development Environment and Security, Oakland CA

Morrison JI, Postel SL, Gleick PH (1996) The Sustainable Use of Water in the Lower Colorado River Basin. Pacific Institute for Studies in Development Environment and Security, Oakland CA

New Mexico. Water Resources Research Institute. New Mexico State University.

Arizona Department of Water Resources RP (1994) Arizona Water Resources Assessment: vol I, Inventory and Analysis. State of Arizona, Department of Water Resources.

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