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Drought is commonly defined as a deficiency of precipitation over an extended period of time, usually a season or more, relative to some long-term average condition. Droughts have affected the United States, particularly the American West, for centuries. Drought affects societies because of the combination of reduced supply (e.g., less precipitation, reduced reservoir levels, a lower groundwater table) and competing demand (e.g., for irrigation, municipal and industrial supply, energy production, species protection). This report focuses on the physical causes of drought, its history in the United States, and what may be expected in the near future. Although currently drought can be predicted for a particular region for at best a few months in advance, past history suggests that severe and extended droughts are inevitable and part of natural climate cycles, particularly in the West.
Some studies suggest that the American West may be in transition to a more arid climate, raising concerns that the region may become more prone to extreme drought than was the norm during most of the 20th century. While drought is most common in the West, drought can also provoke water resource conflicts in other parts of the country. For example, the 2007-2008 drought in the Southeast has heightened a long-standing dispute over water in the Apalachicola-Chattahoochee- Flint River basin, even though the three states at odds with each other—Georgia, Alabama, and Florida—receive more rainfall in dry years than many western states receive in average years.
The physical conditions causing drought in the United States are increasingly understood to be linked to sea surface temperatures (SSTs) in the tropical Pacific Ocean. Studies indicate that cooler-than-average SSTs have been connected to the recent severe western drought, severe droughts of the late 19th century, and precolonial North American “megadroughts.” Some climate model projections suggest that warming temperatures resulting from increased greenhouse gases in the atmosphere could return the western United States within decades to more arid baseline conditions similar to those during earlier times.
The prospect of extended droughts and more arid baseline conditions in parts of the United States could suggest new challenges to federal water projects, the construction of which was based largely on 20th century climate conditions. In responding to competing demands for water, such as deliveries to serve agricultural demands, municipal needs, endangered species, and others, federal water delivery systems may have to be re-tuned to match a drier average climate in the West. As a further complication, federal, state, and local authorities make water resource decisions within the context of multiple and often conflicting laws and objectives, competing legal decisions, and entrenched institutional mechanisms.
The evolving nature of drought, split federal and non-federal responsibilities, and a patchwork of federal programs and congressional committee jurisdictions make development of a comprehensive national drought policy difficult. Although Congress has considered some of the recommendations issued by the National Drought Policy Commission in 2000, comprehensive drought legislation has not been enacted. Congress may move to review how federal agencies such as the U.S. Army Corps of Engineers and the Bureau of Reclamation have responded to recent droughts in the Southeast, West, and Northwest to help assess whether the National Drought Policy Commission’s recommendations are still relevant.