Drought
Overview
 
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

. It can have a substantial impact on the ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

 and agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

 of the affected region. Although droughts can persist for several years, even a short, intense drought can cause significant damage and harm the local economy
Economy
An economy consists of the economic system of a country or other area; the labor, capital and land resources; and the manufacturing, trade, distribution, and consumption of goods and services of that area...

.

This global phenomenon has a widespread impact on agriculture.
Discussions
Encyclopedia
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

. It can have a substantial impact on the ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

 and agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

 of the affected region. Although droughts can persist for several years, even a short, intense drought can cause significant damage and harm the local economy
Economy
An economy consists of the economic system of a country or other area; the labor, capital and land resources; and the manufacturing, trade, distribution, and consumption of goods and services of that area...

.

This global phenomenon has a widespread impact on agriculture. The United Nations
United Nations
The United Nations is an international organization whose stated aims are facilitating cooperation in international law, international security, economic development, social progress, human rights, and achievement of world peace...

 estimates that an area of fertile soil the size of Ukraine
Ukraine
Ukraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...

 is lost every year because of drought, deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....

, and climate instability. Lengthy periods of drought have long been a key trigger for mass migration and played a key role in a number of ongoing migrations and other humanitarian crises in the Horn of Africa
Horn of Africa
The Horn of Africa is a peninsula in East Africa that juts hundreds of kilometers into the Arabian Sea and lies along the southern side of the Gulf of Aden. It is the easternmost projection of the African continent...

 and the Sahel
Sahel
The Sahel is the ecoclimatic and biogeographic zone of transition between the Sahara desert in the North and the Sudanian Savannas in the south.It stretches across the North African continent between the Atlantic Ocean and the Red Sea....

.

Consequences

Periods of drought can have significant environmental, agricultural, health, economic and social consequences. The effect varies according to vulnerability. For example, subsistence farmers are more likely to migrate during drought because they do not have alternative food sources. Areas with populations that depend on as a major food source are more vulnerable to drought-triggered famine.

Drought can also reduce water quality, because lower water flows reduce dilution of pollutants and increase contamination
Contamination
Contamination is the presence of a minor and unwanted constituent in material, physical body, natural environment, at a workplace, etc.-Specifics:"Contamination" also has more specific meanings in science:...

 of remaining water sources. Common consequences of drought include:
  • Diminished crop growth or yield productions
    Crop yield
    In agriculture, crop yield is not only a measure of the yield of cereal per unit area of land under cultivation, yield is also the seed generation of the plant itself...

     and carrying capacity for livestock
    Livestock
    Livestock refers to one or more domesticated animals raised in an agricultural setting to produce commodities such as food, fiber and labor. The term "livestock" as used in this article does not include poultry or farmed fish; however the inclusion of these, especially poultry, within the meaning...

  • Dust bowl
    Dust Bowl
    The Dust Bowl, or the Dirty Thirties, was a period of severe dust storms causing major ecological and agricultural damage to American and Canadian prairie lands from 1930 to 1936...

    s, themselves a sign of erosion
    Erosion
    Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

    , which further erode the landscape
    Landscape
    Landscape comprises the visible features of an area of land, including the physical elements of landforms such as mountains, hills, water bodies such as rivers, lakes, ponds and the sea, living elements of land cover including indigenous vegetation, human elements including different forms of...

  • Dust storms, when drought hits an area suffering from desertification and erosion
    Erosion
    Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

  • Famine
    Famine
    A famine is a widespread scarcity of food, caused by several factors including crop failure, overpopulation, or government policies. This phenomenon is usually accompanied or followed by regional malnutrition, starvation, epidemic, and increased mortality. Every continent in the world has...

     due to lack of water for irrigation
    Irrigation
    Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...

  • Habitat
    Habitat
    * Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

     damage, affecting both terrestrial and aquatic
    Aquatic ecosystem
    An aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems....

     wildlife
  • Malnutrition
    Malnutrition
    Malnutrition is the condition that results from taking an unbalanced diet in which certain nutrients are lacking, in excess , or in the wrong proportions....

    , dehydration
    Dehydration
    In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

     and related diseases
  • Mass migration, resulting in internal displacement
    Internally displaced person
    An internally displaced person is someone who is forced to flee his or her home but who remains within his or her country's borders. They are often referred to as refugees, although they do not fall within the current legal definition of a refugee. At the end of 2006 it was estimated there were...

     and international refugee
    Refugee
    A refugee is a person who outside her country of origin or habitual residence because she has suffered persecution on account of race, religion, nationality, political opinion, or because she is a member of a persecuted 'social group'. Such a person may be referred to as an 'asylum seeker' until...

    s
  • Reduced electricity production
    Electricity generation
    Electricity generation is the process of generating electric energy from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...

     due to reduced water flow through hydroelectric dam
    Dam
    A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

    s
  • Shortages of water for industrial
    Industry
    Industry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...

     users
  • Snake
    Snake
    Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

     migration and increases in snakebites
  • Social unrest
    Unrest
    Unrest is a sociological phenomenon, for instance:* Industrial unrest* Labor unrest* Rebellion* Riot-Notable historical unrests:* 19th century Luddites* 1978–79 Winter of Discontent...

  • War
    War
    War is a state of organized, armed, and often prolonged conflict carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political...

     over natural resources, including water and food
  • Wildfires, such as Australia
    Australia
    Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

    n bushfires, are more common during times of drought

Globally

Drought is a normal, recurring feature of the climate in most parts of the world. It is among the earliest documented climatic events, present in the Epic of Gilgamesh
Epic of Gilgamesh
Epic of Gilgamesh is an epic poem from Mesopotamia and is among the earliest known works of literature. Scholars believe that it originated as a series of Sumerian legends and poems about the protagonist of the story, Gilgamesh king of Uruk, which were fashioned into a longer Akkadian epic much...

 and tied to the biblical
Bible
The Bible refers to any one of the collections of the primary religious texts of Judaism and Christianity. There is no common version of the Bible, as the individual books , their contents and their order vary among denominations...

 story of Joseph
Joseph (Hebrew Bible)
Joseph is an important character in the Hebrew bible, where he connects the story of Abraham, Isaac and Jacob in Canaan to the subsequent story of the liberation of the Israelites from slavery in Egypt....

's arrival in and the later Exodus
The Exodus
The Exodus is the story of the departure of the Israelites from ancient Egypt described in the Hebrew Bible.Narrowly defined, the term refers only to the departure from Egypt described in the Book of Exodus; more widely, it takes in the subsequent law-givings and wanderings in the wilderness...

 from Ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...

. Hunter-gatherer migrations in 9,500 BC Chile have been linked to the phenomenon, as has the exodus of early man out of Africa
Recent African origin of modern humans
In paleoanthropology, the recent African origin of modern humans is the most widely accepted model describing the origin and early dispersal of anatomically modern humans...

 and into the rest of the world around 135,000 years ago.

Modern peoples can effectively mitigate much of the impact of drought through irrigation and crop rotation. Failure to develop adequate drought mitigation strategies carries a grave human cost in the modern era, exacerbated by ever-increasing
Population growth
Population growth is the change in a population over time, and can be quantified as the change in the number of individuals of any species in a population using "per unit time" for measurement....

 population densities.

Regions

Recurring droughts leading to desertification
Desertification
Desertification is the degradation of land in drylands. Caused by a variety of factors, such as climate change and human activities, desertification is one of the most significant global environmental problems.-Definitions:...

 in the Horn of Africa
Horn of Africa
The Horn of Africa is a peninsula in East Africa that juts hundreds of kilometers into the Arabian Sea and lies along the southern side of the Gulf of Aden. It is the easternmost projection of the African continent...

 have created grave ecological catastrophes
Disaster
A disaster is a natural or man-made hazard that has come to fruition, resulting in an event of substantial extent causing significant physical damage or destruction, loss of life, or drastic change to the environment...

, prompting massive food shortages
1984 - 1985 famine in Ethiopia
A widespread famine affected the inhabitants of today's Eritrea and Ethiopia from 1983 to 1985. In northern Ethiopia, famine led to more than 400,000 deaths; over half this mortality can be attributed to human rights abuses that caused the famine to come earlier, strike harder, and extend further...

, still recurring
2006 Horn of Africa food crisis
In 2006, an acute shortage of food affected the countries in the Horn of Africa , as well as northeastern Kenya. The United Nations's Food and Agriculture Organization estimated on January 6, 2006, that more than 11 million people in these countries may be affected by an impending widespread...

. To the north-west of the Horn, the Darfur conflict in neighboring Sudan
Sudan
Sudan , officially the Republic of the Sudan , is a country in North Africa, sometimes considered part of the Middle East politically. It is bordered by Egypt to the north, the Red Sea to the northeast, Eritrea and Ethiopia to the east, South Sudan to the south, the Central African Republic to the...

, also affecting Chad
Chad
Chad , officially known as the Republic of Chad, is a landlocked country in Central Africa. It is bordered by Libya to the north, Sudan to the east, the Central African Republic to the south, Cameroon and Nigeria to the southwest, and Niger to the west...

, was fueled by decades of drought; combination of drought, desertification
Desertification
Desertification is the degradation of land in drylands. Caused by a variety of factors, such as climate change and human activities, desertification is one of the most significant global environmental problems.-Definitions:...

 and overpopulation
Overpopulation
Overpopulation is a condition where an organism's numbers exceed the carrying capacity of its habitat. The term often refers to the relationship between the human population and its environment, the Earth...

 are among the causes of the Darfur conflict, because the Arab
Arab
Arab people, also known as Arabs , are a panethnicity primarily living in the Arab world, which is located in Western Asia and North Africa. They are identified as such on one or more of genealogical, linguistic, or cultural grounds, with tribal affiliations, and intra-tribal relationships playing...

 Baggara
Baggara
The Baggāra Arabs are a set of communities inhabiting the portion of Africa's Sahel between Lake Chad and southern Kordofan, numbering over one million. They have a common language which is one of the regional colloquial Arabic languages...

 nomads searching for water have to take their livestock further south, to land mainly occupied by non-Arab farming peoples.

Approximately 2.4 billion people live in the drainage basin
Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...

 of the Himalayan rivers. India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, Pakistan
Pakistan
Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...

, Bangladesh
Bangladesh
Bangladesh , officially the People's Republic of Bangladesh is a sovereign state located in South Asia. It is bordered by India on all sides except for a small border with Burma to the far southeast and by the Bay of Bengal to the south...

, Nepal
Nepal
Nepal , officially the Federal Democratic Republic of Nepal, is a landlocked sovereign state located in South Asia. It is located in the Himalayas and bordered to the north by the People's Republic of China, and to the south, east, and west by the Republic of India...

 and Myanmar
Myanmar
Burma , officially the Republic of the Union of Myanmar , is a country in Southeast Asia. Burma is bordered by China on the northeast, Laos on the east, Thailand on the southeast, Bangladesh on the west, India on the northwest, the Bay of Bengal to the southwest, and the Andaman Sea on the south....

 could experience floods followed by droughts in coming decades. Drought in India
Drought in India
Drought in India has resulted in tens of millions of deaths over the course of the 18th, 19th, and 20th centuries. Indian agriculture is heavily dependent on the climate of India: a favorable southwest summer monsoon is critical in securing water for irrigating Indian crops...

 affecting the Ganges is of particular concern, as it provides drinking water
Drinking water
Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...

 and agricultural irrigation
Irrigation
Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...

 for more than 500 million people. The west coast of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, which gets much of its water from glaciers in mountain ranges such as the Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...

 and Sierra Nevada, also would be affected.

In 2005, parts of the Amazon basin
Amazon Basin
The Amazon Basin is the part of South America drained by the Amazon River and its tributaries that drains an area of about , or roughly 40 percent of South America. The basin is located in the countries of Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, and Venezuela...

 experienced the worst drought in 100 years. A 23 July 2006 article reported Woods Hole Research Center
Woods Hole Research Center
The Woods Hole Research Center addresses pressing environmental issues, including climate change, through scientific and policy initiatives. The Center has projects in the Amazon, the Arctic, Africa, Russia, Alaska, Canada, New England, and the Mid-Atlantic, working in collaboration with a wide...

 results showing that the forest in its present form could survive only three years of drought. Scientists at the Brazilian National Institute of Amazonian Research
National Institute of Amazonian Research
The National Institute of Amazonian Research is a public educational and research institution in Manaus, Brazil. It was founded in 1952, with the purpose of providing a further knowledge about the Brazilian Amazon Region...

 argue in the article that this drought response, coupled with the effects of deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....

 on regional climate, are pushing the rainforest towards a "tipping point
Tipping point
In sociology, a tipping point is the event of a previously rare phenomenon becoming rapidly and dramatically more common. The phrase was coined in its sociological use by Morton Grodzins, by analogy with the fact in physics that adding a small amount of weight to a balanced object can cause it to...

" where it would irreversibly start to die. It concludes that the rainforest
Rainforest
Rainforests are forests characterized by high rainfall, with definitions based on a minimum normal annual rainfall of 1750-2000 mm...

 is on the brink of being turned into savanna
Savanna
A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses.Some...

 or desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...

, with catastrophic consequences for the world's climate. According to the WWF
World Wide Fund for Nature
The World Wide Fund for Nature is an international non-governmental organization working on issues regarding the conservation, research and restoration of the environment, formerly named the World Wildlife Fund, which remains its official name in Canada and the United States...

, the combination of climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

 and deforestation increases the drying effect of dead trees that fuels forest fires.

By far the largest part of Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 is desert
Deserts of Australia
Deserts cover a large portion of the land in Australia. Most of the deserts lie in the central and north-western part of the country. The largest part of Australia is desert or semi-arid...

 or semi-arid lands commonly known as the outback
Outback
The Outback is the vast, remote, arid area of Australia, term colloquially can refer to any lands outside the main urban areas. The term "the outback" is generally used to refer to locations that are comparatively more remote than those areas named "the bush".-Overview:The outback is home to a...

. A 2005 study by Australian and American researchers investigated the desertification of the interior, and suggested that one explanation was related to human
Human
Humans are the only living species in the Homo genus...

 settlers who arrived about 50,000 years ago. Regular burning by these settlers could have prevented monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...

s from reaching interior Australia. In June 2008 it became known that an expert panel had warned of long term, maybe irreversible, severe ecological damage for the whole Murray-Darling basin
Murray-Darling Basin
The Murray-Darling basin is a large geographical area in the interior of southeastern Australia, whose name is derived from its two major rivers, the Murray River and the Darling River. It drains one-seventh of the Australian land mass, and is currently by far the most significant agricultural...

 if it does not receive sufficient water by October. Australia could experience more severe droughts and they could become more frequent in the future, a government-commissioned report said on July 6, 2008. The Australian of the year
Australian of the Year
Since 1960 the Australian of the Year Award has been part of the celebrations surrounding Australia Day , during which time the award has grown steadily in significance to become Australia’s pre-eminent award. The Australian of the Year announcement has become a very prominent part of the annual...

 2007, environmentalist Tim Flannery
Tim Flannery
Timothy Fridtjof Flannery is an Australian mammalogist, palaeontologist, environmentalist and global warming activist....

, predicted that unless it made drastic changes, Perth
Perth, Western Australia
Perth is the capital and largest city of the Australian state of Western Australia and the fourth most populous city in Australia. The Perth metropolitan area has an estimated population of almost 1,700,000....

 in Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...

 could become the world’s first ghost metropolis
Ghost town
A ghost town is an abandoned town or city. A town often becomes a ghost town because the economic activity that supported it has failed, or due to natural or human-caused disasters such as floods, government actions, uncontrolled lawlessness, war, or nuclear disasters...

, an abandoned city with no more water to sustain its population.

East Africa
East Africa
East Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...

 currently faces its worst drought in decades, with crops and livestock destroyed. The U.N. World Food Programme
World Food Programme
The World Food Programme is the food aid branch of the United Nations, and the world's largest humanitarian organization addressing hunger worldwide. WFP provides food, on average, to 90 million people per year, 58 million of whom are children...

 recently said that nearly four million Kenya
Kenya
Kenya , officially known as the Republic of Kenya, is a country in East Africa that lies on the equator, with the Indian Ocean to its south-east...

ns urgently needed food.

Causes

Generally, rainfall is related to the amount of water vapor in the atmosphere, combined with the upward forcing of the air mass containing that water vapor. If either of these are reduced, the result is a drought. This can be triggered by an above average prevalence of high pressure system
Pressure system
A pressure system is a region of the Earth's atmosphere where air pressure is a relative peak or lull in the sea level pressure distribution. The surface pressure at sea level varies minimally, with the lowest value measured and the highest recorded...

s, wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

s carrying continental, rather than oceanic air masses (i.e. reduced water content), and ridges of high pressure area
High pressure area
A high-pressure area is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward due to the higher density air near their center and friction with land...

s form with behaviors which prevent or restrict the developing of thunderstorm activity or rainfall over one certain region. Oceanic and atmospheric weather cycles such as the El Niño-Southern Oscillation
El Niño-Southern Oscillation
El Niño/La Niña-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years...

 (ENSO) make drought a regular recurring feature of the Americas along the Midwest and Australia. Guns, Germs, and Steel
Guns, Germs, and Steel
Guns, Germs, and Steel: The Fates of Human Societies is a 1997 book by Jared Diamond, professor of geography and physiology at the University of California, Los Angeles . In 1998 it won the Pulitzer Prize for general non-fiction and the Aventis Prize for Best Science Book...

author Jared Diamond
Jared Diamond
Jared Mason Diamond is an American scientist and author whose work draws from a variety of fields. He is currently Professor of Geography and Physiology at UCLA...

 sees the stark impact of the multi-year ENSO cycles on Australian weather patterns as a key reason that Australian aborigines
Australian Aborigines
Australian Aborigines , also called Aboriginal Australians, from the latin ab originem , are people who are indigenous to most of the Australian continentthat is, to mainland Australia and the island of Tasmania...

 remained a hunter-gatherer society rather than adopting agriculture. Another climate oscillation known as the North Atlantic Oscillation
North Atlantic oscillation
The North Atlantic oscillation is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high. Through east-west oscillation motions of the Icelandic low and the Azores high, it controls the...

 has been tied to droughts in northeast Spain.

Human activity can directly trigger exacerbating factors such as over farming, excessive irrigation, deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....

, and erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 adversely impact the ability of the land to capture and hold water. While these tend to be relatively isolated in their scope, activities resulting in global climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

 are expected to trigger droughts with a substantial impact on agriculture
Climate change and agriculture
Climate change and agriculture are interrelated processes, both of which take place on a global scale. Global warming is projected to have significant impacts on conditions affecting agriculture, including temperature, carbon dioxide, glacial run-off, precipitation and the interaction of these...

 throughout the world, and especially in developing nations. Overall, global warming will result in increased world rainfall. Along with drought in some areas, flooding and erosion will increase in others. Paradoxically, some proposed solutions to global warming
Mitigation of global warming
Climate change mitigation is action to decrease the intensity of radiative forcing in order to reduce the potential effects of global warming. Mitigation is distinguished from adaptation to global warming, which involves acting to tolerate the effects of global warming...

 that focus on more active techniques, solar radiation management
Solar radiation management
Solar radiation management projects are a type of geoengineering which seek to reflect sunlight and thus reduce global warming. Examples include the creation of stratospheric sulfur aerosols. They do not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems...

 through the use of a space sunshade for one, may also carry with them increased chances of drought.

Types of drought

As a drought persists, the conditions surrounding it gradually worsen and its impact on the local population gradually increases. People tend to define droughts in three main ways:
  1. Meteorological
    Meteorology
    Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

     drought is brought about when there is a prolonged period with less than average precipitation. Meteorological drought usually precedes the other kinds of drought.
  2. Agricultural
    Agriculture
    Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

     droughts are droughts that affect crop production or the ecology of the range
    Range (biology)
    In biology, the range or distribution of a species is the geographical area within which that species can be found. Within that range, dispersion is variation in local density.The term is often qualified:...

    . This condition can also arise independently from any change in precipitation levels when soil
    Soil
    Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

     conditions and erosion triggered by poorly planned agricultural endeavors cause a shortfall in water available to the crops. However, in a traditional drought, it is caused by an extended period of below average precipitation.
  3. Hydrological
    Hydrology
    Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...

     drought
    Meteorology
    Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

     is brought about when the water reserves available in sources such as aquifer
    Aquifer
    An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...

    s, lake
    Lake
    A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

    s and reservoir
    Reservoir
    A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...

    s fall below the statistical average
    Average
    In mathematics, an average, or central tendency of a data set is a measure of the "middle" value of the data set. Average is one form of central tendency. Not all central tendencies should be considered definitions of average....

    . Hydrological drought tends to show up more slowly because it involves stored water that is used but not replenished. Like an agricultural drought, this can be triggered by more than just a loss of rainfall. For instance, Kazakhstan
    Kazakhstan
    Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...

     was recently awarded a large amount of money by the World Bank
    World Bank
    The World Bank is an international financial institution that provides loans to developing countries for capital programmes.The World Bank's official goal is the reduction of poverty...

     to restore water that had been diverted to other nations from the Aral Sea
    Aral Sea
    The Aral Sea was a lake that lay between Kazakhstan in the north and Karakalpakstan, an autonomous region of Uzbekistan, in the south...

     under Soviet rule. Similar circumstances also place their largest lake, Balkhash
    Lake Balkhash
    Lake Balkhash is one of the largest lakes in Asia and 12th largest continental lake in the world. It is located in southeastern Kazakhstan, in Central Asia, and belongs to an endorheic basin shared by Kazakhstan and China, with a small part in Kyrgyzstan. The basin drains into the lake via seven...

    , at risk of completely drying out.

Drought protection and relief

Strategies for drought protection, mitigation or relief include:
  • Dam
    Dam
    A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

    s - many dams and their associated reservoirs supply additional water in times of drought.
  • Cloud seeding
    Cloud seeding
    Cloud seeding, a form of intentional weather modification, is the attempt to change the amount or type of precipitation that falls from clouds, by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud...

     - an artificial technique to induce rainfall.
  • Desalination
    Desalination
    Desalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from saline water...

     of sea water for irrigation or consumption.
  • Drought monitoring - Continuous observation of rainfall levels and comparisons with current usage levels can help prevent man-made drought. For instance, analysis of water usage in Yemen
    Yemen
    The Republic of Yemen , commonly known as Yemen , is a country located in the Middle East, occupying the southwestern to southern end of the Arabian Peninsula. It is bordered by Saudi Arabia to the north, the Red Sea to the west, and Oman to the east....

     has revealed that their water table
    Water table
    The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...

     (underground water level) is put at grave risk by over-use to fertilize their Khat
    Khat
    Khat, qat, gat or Waquish Spoken from true Yemeni, is a flowering plant native to tropical East Africa and the Arabian Peninsula....

     crop. Careful monitoring of moisture levels can also help predict increased risk for wildfires, using such metrics as the Keetch-Byram Drought Index
    Keetch-Byram Drought Index
    The Keetch-Byram Drought Index , created by John Keetch and George Byram in 1968 for the United States Department of Agriculture's Forest Service, outlines mathematical models for predicting the likelihood of wildfire based on soil moisture and other conditions related to drought.KBDI is a...

     or Palmer Drought Index
    Palmer Drought Index
    The Palmer Drought Index, sometimes called the Palmer Drought Severity Index and often abbreviated PDSI, is a measurement of dryness based on recent precipitation and temperature. It was developed by meteorologist Wayne Palmer, who first published his method in the 1965 paper Meteorological Drought...

    .
  • Land use - Carefully planned crop rotation
    Crop rotation
    Crop rotation is the practice of growing a series of dissimilar types of crops in the same area in sequential seasons.Crop rotation confers various benefits to the soil. A traditional element of crop rotation is the replenishment of nitrogen through the use of green manure in sequence with cereals...

     can help to minimize erosion
    Erosion
    Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

     and allow farmers to plant less water-dependent crops in drier years.
  • Outdoor water-use restriction
    Outdoor water-use restriction
    An outdoor water-use restriction is a ban or other lesser restrictions put into effect that restricts the outdoor use of water supplies. Often called a watering ban or hosepipe ban, it can affect:*irrigation of lawns*car washing...

     - Regulating the use of sprinklers, hoses or buckets on outdoor plants, filling pools, and other water-intensive home maintenance tasks.
  • Rainwater harvesting
    Rainwater harvesting
    Rainwater harvesting is the accumulating and storing of rainwater for reuse before it reaches the aquifer. It has been used to provide drinking water, water for livestock, water for irrigation, as well as other typical uses. Rainwater collected from the roofs of houses and local institutions can...

     - Collection and storage of rainwater from roofs or other suitable catchments.
  • Recycled water - Former wastewater (sewage) that has been treated and purified for reuse.
  • Transvasement - Building canals or redirecting rivers as massive attempts at irrigation
    Irrigation
    Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...

     in drought-prone areas.

See also

  • Climate change
    Climate change
    Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

  • Collapse: How Societies Choose to Fail or Succeed
    Collapse (book)
    Collapse: How Societies Choose to Fail or Succeed is a 2005 book by Jared M. Diamond, professor of geography and physiology at University of California, Los Angeles...

  • FEMA
    Federal Emergency Management Agency
    The Federal Emergency Management Agency is an agency of the United States Department of Homeland Security, initially created by Presidential Reorganization Plan No. 1 of 1978 and implemented by two Executive Orders...

  • Food security
    Food security
    Food security refers to the availability of food and one's access to it. A household is considered food-secure when its occupants do not live in hunger or fear of starvation. According to the World Resources Institute, global per capita food production has been increasing substantially for the past...

  • Permanent wilting point
    Permanent wilting point
    Permanent wilting point or wilting point is defined as the minimal point of soil moisture the plant requires not to wilt. If moisture decreases to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours...

  • Topsoil
    Topsoil
    Topsoil is the upper, outermost layer of soil, usually the top to . It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs.-Importance:...

  • United Nations Convention to Combat Desertification
    United Nations Convention to Combat Desertification
    The United Nations Convention to Combat Desertification in Those Countries Experiencing Serious Drought and/or Desertification, Particularly in Africa is a Convention to combat desertification and mitigate the effects of drought through national action programs that incorporate long-term strategies...

  • Water conflict
    Water conflict
    Water conflict is a term describing a conflict between countries, states, or groups over an access to water resources. The United Nations recognizes that water disputes result from opposing interests of water users, public or private....

  • Water crisis
    Water crisis
    Water crisis is a general term used to describe a situation where the available water within a region is less than the region's demand. The term has been used to describe the availability of potable water in a variety of regions by the United Nations and other world organizations...



Regional:
  • Amazon Rainforest drought
  • Droughts and famines in Russia and USSR
  • Drought in Australia
    Drought in Australia
    Drought in Australia is defined as rainfall over a three month period being in the lowest decile of what has been recorded for that region in the past. This definition takes into account that drought is a relative term and rainfall deficiencies need to be compared to typical rainfall patterns...

    , Exceptional Circumstances relief payments
    Exceptional circumstances
    Exceptional circumstances is a phrase or descriptor most often used to denote the conditions required to grant additional powers to a government or government leader so as to alleviate, or mitigate, unforeseen or unconventional hardship....

  • Drought in the United Kingdom
    Drought in the United Kingdom
    Droughts in the United Kingdom are a relatively common feature of the weather in the UK, with one around every 5–10 years on average. These droughts are usually confined to summer, when a blocking high causes hot, dry weather for an extended period. However droughts can vary in their characteristics...

  • Drought in the United States
    Drought in the United States
    Drought in the United States is similar to that of other portions of the globe. Below normal precipitation leads to drought, which is caused by an above average persistence of high pressure over the drought area...

  • Maya collapse
  • Sahel drought
    Sahel drought
    [[File:Greening Sahel 1982-1999.jpg|thumb|300px|Recent "Greening" of the Sahel: The results of trend analyses of time series over the Sahel region of seasonally integrated NDVI using NOAA AVHRR NDVI-data from 1982 to 1999...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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