For other uses, see Drought (disambiguation).
Contraction/Desiccation cracks in dry earth (Sonoran desert, Mexico).

A drought is a period of below-average precipitation in a given region, resulting in prolonged shortages in its water supply, whether atmospheric, surface water or ground water. A drought can last for months or years, or may be declared after as few as 15 days.[1] It can have a substantial impact on the ecosystem and agriculture of the affected region[2] and harm to the local economy.[3] Annual dry seasons in the tropics significantly increase the chances of a drought developing and subsequent bush fires. Periods of heat can significantly worsen drought conditions by hastening evaporation of water vapour.

Many plant species, such as those in the family Cactaceae (or cacti), have drought tolerance adaptations like reduced leaf area and waxy cuticles to enhance their ability to tolerate drought. Some others survive dry periods as buried seeds. Semi-permanent drought produces arid biomes such as deserts and grasslands.[4] Prolonged droughts have caused mass migrations and humanitarian crises. Most arid ecosystems have inherently low productivity. The most prolonged drought ever in the world in recorded history occurred in the Atacama Desert in Chile (400 Years).[5]

Causes of drought

Precipitation deficiency

See also: Precipitation
Ancient Meso-American civilizations may have amplified droughts by deforestation.

Mechanisms of producing precipitation include convective, stratiform,[6] and orographic rainfall.[7] Convective processes involve strong vertical motions that can cause the overturning of the atmosphere in that location within an hour and cause heavy precipitation,[8] while stratiform processes involve weaker upward motions and less intense precipitation over a longer duration.[9] Precipitation can be divided into three categories, based on whether it falls as liquid water, liquid water that freezes on contact with the surface, or ice. Drought are mainly course by in low rain areas. If these factors do not support precipitation volumes sufficient to reach the surface over a sufficient time, the result is a drought. Drought can be triggered by a high level of reflected sunlight and above average prevalence of high pressure systems, winds carrying continental, rather than oceanic air masses, and ridges of high pressure areas aloft can prevent or restrict the developing of thunderstorm activity or rainfall over one certain region. Once a region is within drought, feedback mechanisms such as local arid air,[10] hot conditions which can promote warm core ridging,[11] and minimal evapotranspiration can worsen drought conditions.

Dry season

See also: Dry season
Sheep on a drought affected paddock near Uranquinty, New South Wales.

Within the tropics, distinct, wet and dry seasons emerge due to the movement of the Intertropical Convergence Zone or Monsoon trough.[12] The dry season greatly increases drought occurrence,[13] and is characterized by its low humidity, with watering holes and rivers drying up. Because of the lack of these watering holes, many grazing animals are forced to migrate due to the lack of water and feed to more fertile spots. Examples of such animals are zebras, elephants,[14] and wildebeest. Because of the lack of water in the plants, bushfires are common.[15] Since water vapor becomes more energetic with increasing temperature, more water vapor is required to increase relative humidity values to 100% at higher temperatures (or to get the temperature to fall to the dew point).[16] Periods of warmth quicken the pace of fruit and vegetable production,[17] increase evaporation and transpiration from plants,[18] and worsen drought conditions.[19]

El Niño

Regional impacts of warm ENSO episodes (El Niño)
See also: El Niño

Drier and hotter weather occurs in parts of the Amazon River Basin, Colombia, and Central America during El Niño events. Winters during the El Niño are warmer and drier than average conditions in the Northwest, northern Midwest, and northern Mideast United States, so those regions experience reduced snowfalls. Conditions are also drier than normal from December to February in south-central Africa, mainly in Zambia, Zimbabwe, Mozambique, and Botswana. Direct effects of El Niño resulting in drier conditions occur in parts of Southeast Asia and Northern Australia, increasing bush fires, worsening haze, and decreasing air quality dramatically. Drier-than-normal conditions are also in general observed in Queensland, inland Victoria, inland New South Wales, and eastern Tasmania from June to August. As warm water spreads from the west Pacific and the Indian Ocean to the east Pacific, it causes extensive drought in the western Pacific. Singapore experienced the driest February in 2014 since records began in 1869, with only 6.3 mm of rain falling in the month and temperatures hitting as high as 35 °C on 26 February. The years 1968 and 2005 had the next driest Februaries, when 8.4 mm of rain fell.[20]

Erosion and human activities

Fires on Borneo and Sumatra, 2006. People use slash-and-burn deforestation to clear land for agriculture.

Human activity can directly trigger exacerbating factors such as over farming, excessive irrigation,[21] deforestation, and erosion adversely impact the ability of the land to capture and hold water.[22] In arid climates, the main source of erosion is wind.[23] Erosion can be the result of material movement by the wind. The wind can cause small particles to be lifted and therefore moved to another region (deflation). Suspended particles within the wind may impact on solid objects causing erosion by abrasion (ecological succession). Wind erosion generally occurs in areas with little or no vegetation, often in areas where there is insufficient rainfall to support vegetation.[24]

Fields outside Benambra, Victoria, Australia suffering from drought conditions.

Loess is a homogeneous, typically nonstratified, porous, friable, slightly coherent, often calcareous, fine-grained, silty, pale yellow or buff, windblown (Aeolian) sediment.[25] It generally occurs as a widespread blanket deposit that covers areas of hundreds of square kilometers and tens of meters thick. Loess often stands in either steep or vertical faces.[26] Loess tends to develop into highly rich soils. Under appropriate climatic conditions, areas with loess are among the most agriculturally productive in the world.[27] Loess deposits are geologically unstable by nature, and will erode very readily. Therefore, windbreaks (such as big trees and bushes) are often planted by farmers to reduce the wind erosion of loess.[23] Wind erosion is much more severe in arid areas and during times of drought. For example, in the Great Plains, it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years.[28]

Climate change

See also: Climate change

Activities resulting in global climate change are expected to trigger droughts with a substantial impact on agriculture[29][30] throughout the world, and especially in developing nations.[31][32][33] Overall, global warming will result in increased world rainfall.[34] Along with drought in some areas, flooding and erosion will increase in others. Paradoxically, some proposed solutions to global warming that focus on more active techniques, solar radiation management through the use of a space sunshade for one, may also carry with them increased chances of drought.[35]


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: [36]

  1. Meteorological drought is brought about when there is a prolonged time with less than average precipitation. Meteorological drought usually precedes the other kinds of drought.[37]
  2. Agricultural droughts are droughts that affect crop production or the ecology of the range. This condition can also arise independently from any change in precipitation levels when soil 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.[38]
  3. Hydrological drought is brought about when the water reserves available in sources such as aquifers, lakes and reservoirs fall below the statistical 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 was recently awarded a large amount of money by the World Bank to restore water that had been diverted to other nations from the Aral Sea under Soviet rule.[39] Similar circumstances also place their largest lake, Balkhash, at risk of completely drying out.[40]

Consequences of drought

A Mongolian gazelle dead due to drought.

The effects of droughts and water shortages can be divided into three groups: environmental, economic and social consequences. In the case of environmental effects: lower surface and subterranean water levels, lower flow levels (with a decrease below the minimum leading to direct danger for amphibian life), increased pollution of surface water, the drying out of wetlands, more and larger fires, higher deflation intensity, losing biodiversity, worse health of trees and the appearance of pests and den droid diseases. Economic losses include lower agricultural, forest, game and fishing output, higher food production costs, lower energy production levels in hydro plants, losses caused by depleted water tourism and transport revenue, problems with water supply for the energy sector and technological processes in metallurgy, mining, the chemical, paper, wood, foodstuff industries etc., disruption of water supplies for municipal economies. Meanwhile, social costs include the negative effect on the health of people directly exposed to this phenomenon (excessive heat waves), possible limitation of water supplies and its increased pollution levels, high food costs, stress caused by failed harvests, etc. This is why droughts and fresh water shortages may be considered as a factor which increases the gap between developed and developing countries.[41]

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 water sources as a major food source are more vulnerable to famine.

Drought can also reduce water quality,[42][43] because lower water flows reduce dilution of pollutants and increase contamination of remaining water sources. Common consequences of drought include:


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 and tied to the biblical story of Joseph's arrival in and the later Exodus from Ancient Egypt.[54] Hunter-gatherer migrations in 9,500 BC Chile have been linked to the phenomenon,[55] as has the exodus of early humans out of Africa and into the rest of the world around 135,000 years ago.[56]

A South Dakota farm during the Dust Bowl, 1936


Main article: List of droughts

Well-known historical droughts include:

Affected areas in the western Sahel belt during the 2012 drought.

The Darfur conflict in Sudan, also affecting Chad, was fueled by decades of drought; combination of drought, desertification and overpopulation are among the causes of the Darfur conflict, because the Arab Baggara nomads searching for water have to take their livestock further south, to land mainly occupied by non-Arab farming people.[57]

Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers.[58] India, China, Pakistan, Bangladesh, Nepal and Myanmar could experience floods followed by droughts in coming decades. Drought in India affecting the Ganges is of particular concern, as it provides drinking water and agricultural irrigation for more than 500 million people.[59][60][61] The west coast of North America, which gets much of its water from glaciers in mountain ranges such as the Rocky Mountains and Sierra Nevada, also would be affected.[62][63]

Drought affected area in Karnataka, India in 2012.

In 2005, parts of the Amazon basin experienced the worst drought in 100 years.[64][65] A 23 July 2006 article reported Woods Hole Research Center results showing that the forest in its present form could survive only three years of drought.[66][67] Scientists at the Brazilian National Institute of Amazonian Research argue in the article that this drought response, coupled with the effects of deforestation on regional climate, are pushing the rainforest towards a "tipping point" where it would irreversibly start to die. It concludes that the rainforest is on the brink of being turned into savanna or desert, with catastrophic consequences for the world's climate. According to the WWF, the combination of climate change and deforestation increases the drying effect of dead trees that fuels forest fires.[68]

Lake Chad in a 2001 satellite image. The lake has shrunk by 95% since the 1960s.[69][70]

By far the largest part of Australia is desert or semi-arid lands commonly known as the outback. A 2005 study by Australian and American researchers investigated the desertification of the interior, and suggested that one explanation was related to human settlers who arrived about 50,000 years ago. Regular burning by these settlers could have prevented monsoons from reaching interior Australia.[71] 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 if it did not receive sufficient water by October 2008.[72] Australia could experience more severe droughts and they could become more frequent in the future, a government-commissioned report said on July 6, 2008.[73] Australian environmentalist Tim Flannery, predicted that unless it made drastic changes, Perth in Western Australia could become the world’s first ghost metropolis, an abandoned city with no more water to sustain its population.[74] The long Australian Millennial drought broke in 2010.

Recurring droughts leading to desertification in East Africa have created grave ecological catastrophes, prompting food shortages in 1984–85, 2006 and 2011.[75] During the 2011 drought, an estimated 50,000 to 150,000 people were reported to have died,[76] though these figures and the extent of the crisis are disputed.[77] In February 2012, the UN announced that the crisis was over due to a scaling up of relief efforts and a bumper harvest.[78] Aid agencies subsequently shifted their emphasis to recovery efforts, including digging irrigation canals and distributing plant seeds.[78]

In 2012, a severe drought struck the western Sahel. The Methodist Relief & Development Fund (MRDF) reported that more than 10 million people in the region were at risk of famine due to a month-long heat wave that was hovering over Niger, Mali, Mauritania and Burkina Faso. A fund of about £20,000 was distributed to the drought-hit countries.[79]

Protection, mitigation and relief

Succulent plants are well-adapted to survive long periods of drought.
Water distribution on Marshall Islands during El Niño.

Agriculturally, people 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 densities. President Roosevelt on April 27, 1935, signed documents creating the Soil Conservation Service (SCS)—now the Natural Resources Conservation Service (NRCS). Models of the law were sent to each state where they were enacted. These were the first enduring practical programs to curtail future susceptibility to drought, creating agencies that first began to stress soil conservation measures to protect farm lands today. It was not until the 1950s that there was an importance placed on water conservation was put into the existing laws (NRCS 2014).[80]

Aerosols over the Amazon each September for four burning seasons (2005 through 2008) during the Amazon basin drought. The aerosol scale (yellow to dark reddish-brown) indicates the relative amount of particles that absorb sunlight.

Strategies for drought protection, mitigation or relief include:

See also


  1. It's a scorcher - and Ireland is officially 'in drought' Irish Independent, 2013-07-18.
  2. Living With Drought
  3. Australian Drought and Climate Change, retrieved on June 7th 2007.
  4. Keddy, P.A. 2007. (http://www.amazon.com/Plants-Vegetation-Origins-Processes-Consequences/dp/0521864801 Plants and Vegetation: Origins, Processes, Consequences). Cambridge University Press, Cambridge, UK. 666 p.
  5. "Driest Place: Atacama Desert, Chile". Extreme Science. Retrieved September 25, 2016..
  6. Emmanouil N. Anagnostou (2004). "A convective/stratiform precipitation classification algorithm for volume scanning weather radar observations". Meteorological Applications. Cambridge University Press. 11 (4): 291–300. Bibcode:2004MeApp..11..291A. doi:10.1017/S1350482704001409.
  7. A.J. Dore; M. Mousavi-Baygi; R.I. Smith; J. Hall; D. Fowler; T.W. Choularton (June 2006). "A model of annual orographic precipitation and acid deposition and its application to Snowdonia". Atmospheric Environment. 40 (18): 3316–3326. Bibcode:2006AtmEn..40.3316D. doi:10.1016/j.atmosenv.2006.01.043.
  8. Robert Penrose Pearce (2002). Meteorology at the Millennium. Academic Press. p. 66. ISBN 978-0-12-548035-2. Retrieved 2009-01-02.
  9. Robert A. Houze, Jr. (1994-06-28). Cloud Dynamics. Academic Press. p. 348. ISBN 0080502105. Retrieved 2015-02-18.
  10. Roland Paepe; Rhodes Whitmore Fairbridge; Saskia Jelgersma (1990). Greenhouse Effect, Sea Level and Drought. Springer Science & Business Media. p. 22. ISBN 0792310179. Retrieved 2015-02-18.
  11. Joseph S. D'Aleo; Pamela G. Grube (2002). The Oryx Resource Guide to El Niño and La Niña. Greenwood Publishing Group. pp. 48–49. ISBN 1573563781. Retrieved 2015-02-18.
  12. Bin Wang (2006-01-13). The Asian Monsoon. Springer Science & Business Media. p. 206. ISBN 3540406107. Retrieved 2008-05-03.
  13. Vijendra K. Boken; Arthur P. Cracknell; Ronald L. Heathcote (2005-03-24). Monitoring and Predicting Agricultural Drought : A Global Study: A Global Study. Oxford University Press. p. 349. ISBN 0198036787. Retrieved 2015-02-18.
  14. TONY RENNELL (June 29, 2007). "It's dry season and elephants are desperately seeking water - but poachers lie in wait". London: Daily Mail.
  15. "Wet & Dry Seasons".
  16. Alistair B. Fraser (1994-11-27). "Bad Meteorology: The reason clouds form when air cools is because cold air cannot hold as much water vapor as warm air". Retrieved 2015-02-17.
  17. Cooperative Extension Service (January 2014). Home Vegetable Gardening in Kentucky (PDF). University of Kentucky. p. 19. Retrieved 2015-02-18.
  18. North Carolina State University (2013-08-09). "Evapotranspiration". Retrieved 2015-02-18.
  19. National Oceanic and Atmospheric Administration (2002-05-16). "Warm Temperatures and Severe Drought Continued in April Throughout Parts of the United States; Global Temperature For April Second Warmest on Record". Retrieved 2015-02-18.
  20. "A biblical tragedy as Sea of Galilee faces drought". BelfastTelegraph.co.uk.
  21. "Kenya: Deforestation exacerbates droughts, floods". forests.org.
  22. 1 2 Vern Hofman; Dave Franzen (1997). "Emergency Tillage to Control Wind Erosion". North Dakota State University Extension Service. Retrieved 2009-03-21.
  23. United States Geological Survey (2004). "Dunes – Getting Started". Retrieved 2009-03-21.
  24. F. von Richthofen (1882). "On the mode of origin of the loess". Geological Magazine (Decade II). 9 (7): 293–305. doi:10.1017/S001675680017164X.
  25. K.E.K. Neuendorf; J.P. Mehl, Jr.; J.A. Jackson (2005). Glossary of Geology. Springer-Verlag, New York. p. 779. ISBN 3-540-27951-2.
  26. Arthur Getis; Judith Getis and Jerome D. Fellmann (2000). Introduction to Geography, Seventh Edition. McGraw-Hill. p. 99. ISBN 0-697-38506-X.
  27. Wiggs, Giles F.S. (2011). "Geomorphological hazards in drylands". In Thomas, David S.G. Arid Zone Geomorphology: Process, Form and Change in Drylands. John Wiley & Sons. p. 588. ISBN 978-0-470-71076-0. The distribution of all the water on the earth’s surface is not even. Some places have lots of fresh water (rivers, lakes, lagoons, ponds etc.) and are continuously replenished by rainfall, runoffs and water from underground. Others places too are known to have very little water. Therefore, if a region that has lots of rainfall, goes for a couple of weeks without rains, and people, animals and plants begin to experience a bit of dryness, it can be called drought. At the same time, that condition may be very normal for places with no water, and can go for months without any rains with little problems.
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  52. http://news.ucsc.edu/2016/10/microcystin-toxin.html
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  62. Water shortage worst in decades, official says, Los Angeles Times
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  64. Drought Threatens Amazon Basin - Extreme conditions felt for second year running
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  70. Sensitivity of the Australian Monsoon to insolation and vegetation: Implications for human impact on continental moisture balance, Geological Society of America
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  72. Australia faces worse, more frequent droughts: study, Reuters
  73. Metropolis strives to meet its thirst, BBC News
  74. Sara Pantuliano and Sara Pavanello (2004) Taking drought into account Addressing chronic vulnerability among pastoralists in the Horn of Africa Overseas Development Institute
  75. "Fatal Failure: Did Aid Agencies Let Up To 100,000 Somalis Die in 2011?". Time. January 18, 2012.
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  84. BBC's From Our Own Correspondent on khat water usage
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