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Twentieth-century water policies relied on the construction of massive infrastructure in the form of dams, aqueducts, pipelines, and complex centralized treatment plants to meet human demands. These facilities brought tremendous benefits to billions of people, but they also had serious and often unanticipated social, economical, and ecological costs. Many unsolved water problems remain, and past approaches no longer seem sufficient. A transition is under way to a "soft path" that complements centralized physical infrastructure with lower cost community-scale systems, decentralized and open decision-making, water markets and equitable pricing, application of efficient technology, and environmental protection.
The world is in the midst of a major transition in water resource development, management, and use. This transition is long overdue. The construction of massive infrastructure in the form of dams, aqueducts, pipelines, and complex centralized treatment plants dominated the 20th-century water agenda. This "hard path" approach brought tremendous benefits to billions of people, reduced the incidence of water-related diseases, expanded the generation of hydropower and irrigated agriculture, and moderated the risks of devastating floods and droughts.
But the hard path also had substantial, often unanticipated social, economic, and environmental costs. Tens of millions of people have been displaced from their homes by water projects over the past century, including more than 1 million whose villages are now being flooded by the reservoir behind the Three Gorges Dam in China (1). Twenty-seven percent of all North American freshwater fauna populations are now considered threatened with extinction (2), a trend mirrored elsewhere around the world. Adequate flows no longer reach the deltas of many rivers in average years, including the Nile, Huang He (Yellow), Amu Darya and Syr Darya, and Colorado, leading to nutrient depletion, loss of habitat for native fisheries, plummeting populations of birds, shoreline erosion, and adverse effects on local communities (3-5).
In arid regions of North America, the hard path for water was pursued especially aggressively. Massive dams and thousands of kilometers of aqueducts were built, permitting human withdrawal of much of the water formerly flowing to wetlands, deltas, and inland sinks, and hydrologic mastery over many watersheds. Since 1905, flows in the Colorado River have decreased markedly because seven states and Mexico withdraw the river's entire flow for agricultural and urban uses (Fig. 1). In...