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

Climatic variability

Coping With Severe and Sustained Drought in the Southwest

Michael D. Dettinger
U.S. Geological Survey

A Summary of Recent Studies of the Susceptibility of a Major Southwestern Water Resource to Long-term Climate Variations

Abstracted from the October 1995 Special Issue of the Water Resources Bulletin of the American Water Resources Association


The Colorado River is the lifeblood of much of the Desert Southwest. It has its major sources in the Rocky Mountains and has average flows of roughly 13.5 million acre-feet per year (one acre-foot is the volume of water required to fill a one-acre swimming pool to a depth of one foot). Its watershed includes parts of Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming. Before development, the Colorado River drained into Mexico and the Gulf of California (Sea of Cortez). The Colorado River is now fully utilized for offstream purposes within and beyond its water-short drainage basin, and its waters reach the Gulf of California only during the highest flow periods.

Map of the Colorado River Basin

Figure 1. Map of the Colorado River Basin (modified from Harding and others, 1995)

The U.S. Bureau of Reclamation has constructed storage facilities with a capacity of roughly four times the average annual flows. This is a large capacity in comparison to most rivers and has rendered drought impacts unimportant during "normal" climate fluctuations. Tree-ring studies, however, provide evidence of very severe and prolonged droughts of magnitudes not seen during the last century, during the period when flow measurements have been made.

Hoover Dam

Figure 2. Hoover Dam (Photo courtesy of the United States Bureau of Reclamation - Lower Colorado Region)

In May 1990, a workshop was held in Denver, Colorado, sponsored by the Pacific Institute for Studies in Development, Environment, and Security, which posed questions regarding the possible impacts of global climate change on the water resources and management of the Colorado River. Studies conducted in preparation for the workshop indicated that streamflow variations of 30% (plus or minus) might develop in response to climate change. The workshop also concluded that, despite a guarded confidence in the considerable flexibility of the existing water-management systems, the potential impacts of prolonged droughts or intense flood events were uncertain. Water marketing offered much hope for even greater flexibility, but its impacts and the future of ecosystems along the river were causes of much concern.

More recently, an interdisciplinary team of engineers, hydrologists, tree-ring scientists, attorneys, economists, sociologists, public administrators, and others were funded by the U.S. Department of the Interior and U.S. Army Corps of Engineers to evaluate, in great detail, the potential impacts of a Severe Sustained Dought in the Colorado River basin. Their general objectives were to:

Management of the Colorado River's resources during severe droughts of magnitude and duration studied in the reports summarized here provides one of the best available examples of the extent to which the Southwest is ready and able to cope with extremes of climate variability and, even, change. The findings of the Department of the Interior/Army Corps of Engineers-sponsored studies were detailed and extensive, and although not specifically directed towards the issue of global change, they have much to offer as a basis for discussing the resiliency of the Colorado River system to long-term deficits of the magnitude that could accompany global climate change. Their findings were summarized in a special issue of the Water Resources Bulletin of the American Water Resources Bulletin (October 1995). A brief overview of their results is presented here.

Other Reference Cited

Gleick, P.H., ed., 1990. The Colorado River Basin and the greenhouse effect: Water Resources and water management. Proceedings of a workshop on the implications of climatic change for the Colorado River Basin, Denver, Colorado, May 17-18, 1990, 72 p.

How severe might a "really bad" drought in the Colorado River basin be?

What kinds of hydrologic, environmental, and economic impacts would result?

What management alternatives might help?

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