The urbanization of the southwestern United States is a modern legacy of Manifest Destiny, that nation-shaping vision of an America settled from sea to shining sea. But there was one problem: Westward expansion was predicated on adequate water supplies. That water was secured through a century of massive government dam-building and irrigation projects, diverting every major and minor water flow in the west. Those days are over. The greatest engineering project in human history has run its course. "The systems we have built are unsustainable without fundamental change," wrote Peter Gleick, president of the Pacific Institute for Studies in Development, Environment, and Security, in a Dec. 14 Proceedings of the National Academy of Sciences article. "The 20th century approaches used to deal with water challenges are now failing, and new thinking and management approaches are needed." Gleick's is one of eight new PNAS papers on the future of water in the southwestern United States. The region has become a giant laboratory for arid regions around the world, where limited water resources and growing populations are on a collision course. The analysis contained in the papers is sobering, but they reinforce what experts have said for years: If people want to live in the desert, they can't avoid its environmental realities. Image: Grand Coulee Dam./Bureau of the Interior.

Cadillac Desert The landmark analysis of western water use was actually written almost a quarter-century ago by journalist Marc Reisner. In Cadillac Desert, he anticipated regionwide dysfunction, with business and farmers and cities fighting over dwindling water supplies. In PNAS, researchers led by Arizona State University ecologist John Sabo compared Reisner's predictions to available data. They found that Reisner's journalism "led him to the same conclusions as those rendered by copious data, modern scientific tools, and the application of a more genuine scientific method." In the Cadillac desert itself, a microcosm of the American west, the researchers estimate that humans now appropriate 76 percent of all fresh water flows. As that number rises, so does the likelihood of Reisner's ultimate argument: "That impaired function of dams, reservoirs, and crop lands, coupled with rapidly growing western cities, would eventually pit municipal water users against farms and catalyze an apocalyptic collapse of western US society." Image: Crop damages due to increasing soil salinity./PNAS Citation: "Reclaiming freshwater sustainability in the Cadillac Desert." By John L. Sabo, Tushar Sinha, Laura C. Bowling, Gerrit H. W. Schoups, Wesley W. Wallender, Michael E. Campana, Keith A. Cherkauer, Pam L. Fuller, William L. Graf, Jan W. Hopmans, John S. Kominoski, Carissa Taylor, Stanley W. Trimble, Robert H. Webb, Ellen E. Wohl. Proceedings of the National Academy of Sciences, Vol. 107 No. 50, December 14, 2010.

Evaporation of Lake Mead During the beginning of the 21st century, the southwestern United States has experienced severe drought. But two PNAS studies, drawing on sedimentary records of historical regional water flows and fossilized tree ring records, show that drought is a regular, cyclical feature of the southwest. The current drought is bad, but it's also not unusual. And as psychologically difficult as it may be, it's time to stop thinking of it as an anomaly. If people are going to live in a desert, they need to be realistic about it. Image: Evaporation rings in Lake Mead./Evelyn Proimos, Flickr. Citation: "A 1,200-year perspective of 21st century drought in southwestern North America." By Connie A. Woodhouse, David M. Meko, Glen M. MacDonald, Dave W. Stahle, Edward R. Cook Proceedings of the National Academy of Sciences, Vol. 107 No. 50, December 14, 2010.

Drought Patterns Multiple models of near-future climates predict the southwestern United States will become even hotter and dryer over the next century. This inevitably raises the question of whether the current drought is caused by global warming. Two PNAS studies consider this question, and say that it's not possible to make a definitive attribution. The current drought fits historical patterns. Past droughts also seem linked to cooling patterns in Pacific Ocean surface temperatures, and the eastern Pacific has indeed been cooling since the late 1970s. Image: Historical records of southwestern water flow./PNAS Citation: "Future dryness in the southwest US and the hydrology of the early 21st century drought." By Daniel R. Cayan, Tapash Das, David W. Pierce, Tim P. Barnett, Mary Tyree, Alexander Gershunov. Proceedings of the National Academy of Sciences, Vol. 107 No. 50, December 14, 2010. Citation: "Greenhouse warming and the twenty-first-century hydroclimate of southwestern North America." By Richard Seager, Gabriel A. Vecchi. Proceedings of the National Academy of Sciences, Vol. 107 No. 50, December 14, 2010.

Subtropical Dry Zones Even if the current drought can't be linked to climate change, the results of 24 different climate models all point to the same thing: As the planet warms, subtropical dry zones will spread. Climate change might not have caused the current drought, but it will almost certainly make future droughts worse. Image: Predictions of the difference between precipitation and evaporation, 2021-2040./PNAS. Citation: "Future dryness in the southwest US and the hydrology of the early 21st century drought." By Daniel R. Cayan, Tapash Das, David W. Pierce, Tim P. Barnett, Mary Tyree, Alexander Gershunov. Proceedings of the National Academy of Sciences, Vol. 107 No. 50, December 14, 2010.

Tree Rings Tell the Story Using tree-ring data from more than 1,000 forest populations, researchers developed a model of the relationship between aridity and tree growth. They found that southwestern trees are extremely sensitive to drought and warmth, and are primed for outbreaks of disease and wildfires. Image: Dan Griffin, University of Arizona. Citation: "Forest responses to increasing aridity and warmth in the southwestern United States." By A. Park Williams, Craig D. Allen, Constance I. Millar, Thomas W. Swetnam, Joel Michaelsen, Christopher J. Still, Steven W. Leavitt. Proceedings of the National Academy of Sciences, Vol. 107 No. 50, December 14, 2010.

Designing for Smarter Water Use The situation seems bleak, but it's far from hopeless. Tools like WaterSim, a regional hydrology simulation developed by Arizona State University sustainability researchers Patricia Gober and Craig Kirkwood, can guide the design of water use policies and translate science into adaptation. In PNAS, they run WaterSim for Phoenix, Arizona, where planting native rather than temperate foliage, giving up on private outdoor swimming pools and reducing urban sprawl could attain water balance under all but the worst-case scenarios. Such changes are "challenging but feasible," wrote Gober and Kirkwood. As University of California, Los Angeles geoscientist Glen MacDonald noted, residents of Tucson use half as much water as residents of Phoenix. "Decreasing per-capita demand does not have to mean fundamental hardships in terms of drinking water and cleanliness," he wrote. Image: A WaterSim graph of water availability./PNAS Citation: "Vulnerability assessment of climate-induced water shortage in Phoenix." By Patricia Gober, Craig W. KirkwoodProceedings of the National Academy of Sciences, Vol. 107 No. 50, December 14, 2010. Citation: "Water, climate change, and sustainability in the southwest." By Glen M. MacDonald. Proceedings of the National Academy of Sciences, Vol. 107 No. 50, December 14, 2010.