Photograph by: Vincent Laforet, National Geographic March 2008
Simply put, a drought is a period of protracted dryness caused by reduced precipitation. Drought is usually considered to have several key properties, including intensity, duration, and location.
There are three main ways of defining drought: meteorological, hydrological, and agricultural. Meteorological drought concerns the amount of dryness and the duration of the dry period for a specific location. Atmospheric conditions that result in deficiencies of precipitation differ from area to area, and this type of drought is usually measured by how much the precipitation shortfall differs from the amount a specific region normally receives.
Hydrological drought encompasses the effects periods of rain and snowfall shortages have on water supply. Water in rivers, reservoirs, and aquifers is frequently used for many different purposes, such as irrigation, flood control, hydropower, navigation, recreation, and wildlife habitat. Competition for water rises during a drought, and conflicts between water users usually increase substantially.
Agricultural drought involves mainly farming and food production. Inadequate precipitation leads to soil water deficits and reduced groundwater or reservoir levels. If the topsoil is too dry when seeds are planted, it may prevent germination, leading to low plant yields.
Streamflow measures the velocity and volume of water flowing through a channel such as a river or stream. Researchers are interested in reconstructing streamflows from the past to determine the historic climate conditions for a given area, including periods of heavy precipitation, flooding, and drought.
Records of the Colorado River streamflow go back only to the end of the 19th century, when gauges were first installed along the river. To delve deeper into the past, scientists must first generate a tree-ring chronology from sample trees.
Scientists use tree rings to determine streamflow because many of the factors that affect tree growth--like precipitation and evapotranspiration--also affect annual streamflow. Reconstructions are created by correlating tree-ring data and gauge records where they overlap, and then applying that model to the tree-ring data for a period prior to when gauge records began.
Streamflow reconstructions have been made for rivers all over the world, including New York's Hudson River, the White River in Arkansas, China's Yellow River, and the Selenge River in Mongolia. The hope is that by understanding the past conditions of a river, we can determine what the future holds and plan accordingly.
Running, Steven W. “Is Global Warming Causing More, Larger Wildfires?”
Science (August 18, 2006), 927.
Seager, Richard, and others. “Model Projections of an Imminent Transition to a More Arid Climate in Southwestern North America.” Science (May 25, 2007), 1181-1184.
Swetnam, Thomas. “Fire History and Climate Change in Giant Sequoia Groves.” Science (November 5, 1993), 885.
Westerling, A. L., and others. “Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity.” Science (August 18, 2006), 940-943.
Whitlock, Cathy. “Forests, Fires, and Climate.” Nature (November 4, 2004), 28.
Brown, Timothy J., Beth L. Hall, and Anthony L. Westerling. “The Impact of Twenty-First Century Climate Change on Wildland Fire Danger in the Western United States: An Applications Perspective.” Climatic Change (January 2004), 365.
Meko, David M., and others. “Medieval Drought in the Upper Colorado River Basin.” Geophysical Research Letters (May 24, 2007).
Westerling, Anthony L. and Thomas W. Swetam. “Interannual to Decadal Drought and Wildfire in the Western United States.” EOS (December 2003).
Mote, Philip W., and others. “Declining Mountain Snowpack in Western North America.” Bulletin of the American Meterological Society (January 2005).
Breshears, David D., and others. “Regional Vegetation Die-Off in Response to Global-Change-Type Drought.” PNAS (October 18, 2005).
Semeniuk, Ivan. “Stand by for a Parched America.” New Scientist (June 9, 2007).
Swetnam, Thomas W. “Fire and Climate History in the Western Americas From Tree Rings.” Science Highlights (April 2002).
Street-Perrott, F. Alayne. “Drowned Trees Record Dry Spells.” Nature (June 16, 1994).
Walsh, Bryan. “The Fire This Time.” Time (November 5, 2007).
Seager, Richard, Celine Herweijer, and Ed Cook. “The Characteristics and Likely Causes of the Medieval Megadroughts in North America.” Lamont-Doherty Earth Observatory, 2003.
Jones, Terry L., and others. “Environmental Imperatives Reconsidered: Demographic Crises in Western North America During the Medieval Climatic Anomaly.” Current Anthropology (April 1999).
Swetnam, Thomas W., and Christopher H. Baisan. “Tree-Ring Reconstructions of Fire and Climate History in the Sierra Nevada and Southwestern United States.” Fire and Climate Change in Temperate Ecosystems of the Western Americas. Ecological Studies (No. 160, 2003).
Childs, Craig. “Phoenix Falling?” High Country News (April 16, 2007).
Last updated: January 16, 2008
Keywords: drought, American West, Colorado River, climate, evapotranspiration, precipitation, streamflow, tree rings, water supply, rain shortages, snowfall shortages