Background (Encyclopedia of Global Resources)
In order to analyze and assess the impacts of drought, as well as delimit drought areas, the characteristics of “drought” must be defined. Conditions considered a drought by a farmer whose crops have withered during the summer may not be seen as a drought by a city planner. There are many types of drought: agricultural, hydrological, economic, and meteorological. The Palmer Drought Severity Index is the best known of a number of indexes that attempt to standardize the measurement of drought magnitude. Nevertheless, there remains much confusion and uncertainty on what defines a drought.
Roger Graham Barry and Richard J. Chorley, in Atmosphere, Weather, and Climate (1992), noted that drought conditions tend to be associated with one or more of four factors: increases in extent and persistence of subtropical high-pressure cells; changes in the summer monsoonal circulation patterns that can cause a postponement or failure of the incursion of wet maritime tropical air onto the land; lower ocean surface temperatures resulting from changes in ocean currents or increased upwelling of cold waters; and displacement of midlatitude storm tracks by drier air.
(The entire section is 180 words.)
Effects of Drought (Encyclopedia of Global Resources)
Drought can have wide-ranging impacts on the environment, communities, and farmers. Most plants and animals in arid regions have adapted to dealing with drought, either behaviorally or through specialized physical adaptations. Humans, however, are often unprepared or overwhelmed by the consequences of drought. Farmers experience decreased incomes from crop failure. Low rainfall frequently increases a crop’s susceptibility to disease and pests. Drought can particularly hurt small rural communities, especially local business people who are dependent on purchases from farmers and ranchers.
Drought is a natural element of climate, and no region is immune to the drought hazard. Farmers in more humid areas grow crops that are less drought resistant. In developing countries the effects of drought can include malnutrition and famine. A prolonged drought struck the Sahel zone of Africa from 1968 through 1974. Nearly 5 million cattle died during the drought, and more than 100,000 people died from malnutrition-related diseases during just one year of the drought.
Subsistence and traditional societies can be very resilient in the face of drought. American Indians either stored food for poor years or migrated to wetter areas. The !Kung Bushmen of southern Africa learned to change their diet, find alternate water sources, and generally adapt to the fluctuation of seasons and climate in the Kalahari Desert.
More than any...
(The entire section is 289 words.)
Federal Drought Response in the United States (Encyclopedia of Global Resources)
Beginning in the 1930’s, the federal government took an increasing role in drought management and relief. In 1933, the federal government created the Soil erosion Service, known today as the Natural Resources ConservationService. No other single federal program or organization has had a greater impact on farmers’ abilities to manage the drought hazard. President Franklin D. Roosevelt’s Prairie States Forestry Project (1934-1942) planted more than 93,078 hectares of shelterbelts in the plains states for wind erosion control. The federal government purchased approximately 400,000 hectares of marginal farmland for replanting into grass. Federal agencies constructed water resource and irrigation projects.
Post-Dust Bowl droughts still caused hardships, but the brunt of the environmental, economic, and social consequences of drought were considerably lessened. Fewer dust storms ravaged the plains. New crop varieties and better farming practices decreased crop losses during drought years. Government programs and better knowledge have enabled families and communities to better cope with drought.
(The entire section is 152 words.)
Coping with Future Droughts (Encyclopedia of Global Resources)
Numerous attempts have been made to predict droughts, especially in terms of cycles. However, attempts to predict droughts one or more years into the future have generally been unsuccessful. The shorter the prediction interval, the more accurate the prediction. Nevertheless, progress has been made in estimating drought occurrence and timing. For example, the El Niño/Southern Oscillation may be a precursor to drought in some areas. Possibly with time the physical mechanics of climate and drought will be understood adequately for long-term predictions to have value.
Perhaps of greater value is the current capacity to detect and monitor drought in its early stages (usually meaning within one to twelve months). Early recognition of potential drought conditions can give policy makers and resource managers the extra time needed to adjust their management strategies. Information on soil moisture conditions aids farmers with planting and crop selection, seeding, fertilization, irrigation rates, and harvest decisions. Communities that have a few months’ warning of impending drought can increase water storage, implement water conservation measures, and obtain outside sources of water.
The progress made in the world’s developed countries has not always been available to the developing nations. Overpopulation and overuse of agricultural lands have resulted in regional problems of desertification and have impeded...
(The entire section is 239 words.)
Further Reading (Encyclopedia of Global Resources)
Allaby, Michael. Droughts. Illustrations by Richard Garratt. Rev. ed. New York: Facts On File, 2003.
Barry, Roger G., and Richard J. Chorley. Atmosphere, Weather, and Climate. 6th ed. New York: Methuen, 1992.
Brichieri-Colombi, Stephen. The World Water Crisis: The Failures of Resource Management. New York: I. B. Tauris, 2009.
Collier, Michael, and Robert H. Webb. Floods, Droughts, and Climate Change. Tucson: University of Arizona Press, 2002.
Hewitt, Ken, ed. Interpretations of Calamity from the Viewpoint of Human Ecology. Boston: Allen & Unwin, 1983.
Riebsame, William E., Stanley A. Changnon, Jr., and Thomas R. Karl. Drought and Natural Resources Management in the United States: Impacts and Implications of the 1987-89 Drought. Boulder, Colo.: Westview Press, 1991.
Wilhite, Donald A., ed. Drought: A Global Assessment. New York: Routledge, 2000.
_______. Drought and Water Crises: Science, Technology, and Management Issues. Boca Raton, Fla.: Taylor & Francis, 2005.
_______. Drought Assessment, Management, and Planning: Theory and Case Studies. Boston: Kluwer Academic, 1993.
Worster, Donald. Dust Bowl: The Southern Plains in the 1930’s. 25th anniversary ed. New York: Oxford University Press, 2004.
Agriculture and Agri-Food Canada. Drought Watch....
(The entire section is 213 words.)
Background (Encyclopedia of Global Warming)
A drought is a period when the water supply of a given area is insufficient for the needs of humans or of ecosystems, usually resulting from below-normal precipitation. If defined simply as a precipitation deficit, such an event is called a meteorological drought. If a drought depletes soil moisture and harms crops, it is an agricultural drought. Eventually, lack of precipitation will diminish water levels in lakes, rivers, and aquifers; this is a hydrological drought. If a drought affects human well-being, it can be called a socioeconomic drought. These categories overlap, and there are many other ways to define droughts based on their causes or effects.
(The entire section is 106 words.)
Causes and Effects of Drought (Encyclopedia of Global Warming)
Precipitation patterns result from large-scale atmospheric and oceanic processes, which in turn are affected by solar forcing, atmospheric composition, land-surface characteristics, and other factors. Scientific understanding of these processes is imperfect but improving; for example, in the 1980’s, it was discovered that a cyclical variation in Pacific sea surface temperature called the El Niño-Southern Oscillation (ENSO) has a profound effect on rainfall around the world. Paleoclimate records show that short- and long-term rainfall fluctuations are common and predate human history, though the causes are often unclear.
Another important cause of drought is above-average temperatures, which increase evaporation and hasten loss of soil moisture. Warm temperatures can also cause precipitation to fall as rain instead of snow, causing it to run off rather than being stored and gradually released. Human land use, such as deforestation or poor soil management, can contribute to drought. Droughts may have multiple causes; for example, the 1930’s Dust Bowl event in the U.S. Great Plains was a result of both meteorological drought and erosion-prone farming techniques.
Drought has both direct and indirect effects. The direct effects most relevant to human well-being include reduced water supply (both in quantity and quality); crop failure, especially of rainfed crops; loss of livestock; soil degradation, such...
(The entire section is 294 words.)
Effects of Climate Change on Drought Patterns (Encyclopedia of Global Warming)
There is general agreement that climate change is likely to worsen droughts worldwide, but the location, timing, and magnitude of these effects are highly uncertain. Averaged globally, climate change will increase precipitation by speeding ocean evaporation. However, rising temperatures will affect oceanic and atmospheric circulation patterns, changing the global distribution of rainfall. Some regions will become wetter; others, drier. Even in regions where total precipitation does not decrease, drought risk may increase as a result of faster land-surface evaporation, loss of water stored in snow and ice, and greater variability of precipitation events (causing not only droughts but also floods).
Climate models generally agree that several regions of the world are likely to suffer from decreased precipitation and increased drought risk under future climate. These regions include the Mediterranean (southern Europe and northern Africa); southern Africa, especially its southwest corner; the southwestern United States; parts of Central America; and southern Australia. Semiarid and arid regions will probably be hardest hit by drought, with subsistence farmers in developing countries being especially vulnerable. In addition, climate change may cause water shortages wherever populations depend upon glaciers or snowmelt for their water supply, such as the Andes in South America, the Himalayas in central...
(The entire section is 207 words.)
Drought Monitoring, Response, and Adaptation (Encyclopedia of Global Warming)
Close monitoring of atmospheric conditions can reveal an impending drought before its agricultural or socioeconomic symptoms become severe, allowing protective measures to be taken. This is the basis of drought early-warning systems, for which there are several regional and global networks. Once a drought has begun, relief efforts may be necessary. Such efforts may include allocation of water to communities and farms, cash payments to farmers, and relocation assistance. However, the availability of relief can ultimately increase vulnerability to drought by encouraging inappropriate settlement and farming practices.
Although drought cannot be prevented, its effects can be minimized through adaptation. Examples of drought adaptation include selecting drought-tolerant crops; improving agricultural soil management; reducing the size of livestock herds; controlling fuel loads in forests; building dams and reservoirs; and encouraging water conservation. A society can insulate itself against drought (or any hazard) by reducing poverty, diversifying its economy, and building strong social institutions.
(The entire section is 151 words.)
Context (Encyclopedia of Global Warming)
Humans have always needed to cope with drought; it has been a cause of hardship, conflict, and migration throughout recorded history. Megadroughts may have led to the disappearance of some civilizations, such as the Maya. More than 10 million people are thought to have been killed by drought during the twentieth century. Climate change will likely not only increase the incidence of drought but add other stresses to human society as well.
Drought is a natural phenomenon, and a given drought cannot be attributed definitively to climate change. Furthermore, there is much uncertainty about when and where climate change will increase drought incidence. This does not, however, imply that adaptation efforts should be delayed. Many drought-adaptation actions will have benefits in the present as well as the future.
(The entire section is 127 words.)
Further Reading (Encyclopedia of Global Warming)
Boken, Vijendra K., Arthur P. Cracknell, and Ronald L. Heathcote, eds. Monitoring and Predicting Agricultural Drought: A Global Study. New York: Oxford University Press, 2005. Technical volume discussing drought monitoring and its use in different world regions. Includes a chapter on climate change. Figures, tables, maps, index.
Botterill, Linda C., and Melanie Fisher, eds. Beyond Drought: People, Policy, and Perspectives. Collingwood, Vic.: Commonwealth Scientific and Industrial Research Organisation, 2003. Evaluates the social, cultural, political, and economic aspects of drought and drought relief. Focuses on Australia. Figures, tables, maps, index.
Cooley, Heather. “Floods and Droughts.” Chapter 4 in The World’s Water, 2006-2007: The Biennial Report on Freshwater Resources, edited by Peter Gleick. Washington, D.C.: Island Press, 2006. Discusses causes and effects of drought; provides statistics on historical droughts; distinguishes between drought management, mitigation, and response. Figures, tables.
Intergovernmental Panel on Climate Change. Climate Change, 2007—The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Susan Solomon et al. New York: Cambridge University Press, 2007. Forecasts weather and climate, including precipitation and drought, for each region of...
(The entire section is 284 words.)
Drought (Encyclopedia of Science)
Drought is an extended period of exceptionally low precipitation. A drought can feature additional weather characteristics, including high temperatures and high winds.
Although low precipitation (rain, snow, or sleet) marks both droughts and deserts, the two are different. A desert is a region that experiences low precipitation as an everyday occurrence. A drought, on the other hand, is a temporary condition in which precipitation is abnormally low for a particular region. Droughts may occur at any time in any part of the world and last anywhere from days to weeks to decades.
The U.S. National Weather Service recognizes three categories of drought. A dry spell occurs when there is less than .03 inch (.08 centimeter) of rainfall during a minimum of 15 consecutive days. A partial drought occurs when the average daily rainfall does not exceed .008 inch (.02 centimeter) during a 29-day period. An absolute drought occurs when there is no measurable rainfall over a period of at least 15 days.
The intensity of a drought may be measured by the ability of living things in the affected area to tolerate the dry conditions. Some plants quickly fall prey to droughts while others, such as cacti and mesquite trees, survive dry conditions by either storing water in their tissues or by going dormant (a state in which growth activity stops). Although a drought may end...
(The entire section is 783 words.)
Drought (World of Earth Science)
Drought is a temporary hazard of nature occurring from a lack of precipitation over an extended period of time. Drought differs from aridity, a permanent feature of climate restricted to regions of low rainfall. Rainfall deficiencies caused by a drought create a severe hydrologic imbalance resulting in considerable water shortages.
The beginning of a drought is typically determined by comparing the current meteorological situation to an average based on a 30-year period of record. This "operational" definition of drought allows meteorologists to analyze the frequency, severity, and duration of the aberration for any given historical period and aides in the development of response and mitigation strategies.
Characteristics of drought are highly variable from region to region, depending on atmospheric factors such as temperature, wind, relative humidity, and amount of sunshine and cloud cover. High temperatures and lots of sunshine can increase evaporation and transpiration to such an extreme that frequent rainfall is incapable of restoring the loss. Meteorological definitions of drought, therefore, may deviate from operational definitions and are usually based on the length of the dry period and the degree of dryness in comparison to the daily average.
Drought is more than a physical phenomena; an extended period of dryness can have a significant socioeconomic impact. Drought presents the most serious physical hazard to crops in nearly all regions of the world. The agricultural sector is usually the first to be affected by dryness, since crops are heavily dependent on stored soil water. In addition to a decline in agricultural products, a shortfall in the water supply can disrupt availability of other economic goods such as hydroelectric power. The 19889 Uruguay drought resulted in a significant decline of hydroelectric power because the dryness disrupted the streamflows needed for production.
See also Hydrologic cycle