Drought is an extended period when a region receives a deficiency in its water supply, whether atmospheric, surface or ground water. A drought can last for months or years, or may be declared after as few as 15 days. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region. Although droughts can persist for several years, even a short, intense drought can cause significant damage and harm to the local economy. Prolonged droughts have caused mass migrations and humanitarian crises.
Many plant species, such as those in the family Cactaceae or cacti, have adaptations such as 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. Most arid ecosystems have inherently low productivity.
Periods of droughts 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 famine.
Drought can also reduce water quality, because lower water flows reduce dilution of pollutants and increase contamination of remaining water sources. Common consequences of drought include:
- Diminished crop growth or yield productions and carrying capacity for livestock
- Dust bowls, themselves a sign of erosion, which further erode the landscape
- Dust storms, when drought hits an area suffering from desertification and erosion
- Famine due to lack of water for irrigation
- Habitat damage, affecting both terrestrial and aquatic wildlife
- Hunger, drought provides too little water to support food crops.
- Malnutrition, dehydration and related diseases
- Mass migration, resulting in internal displacement and international refugees
- Reduced electricity production due to reduced water flow through hydroelectric dams
- Shortages of water for industrial users
- Snake migration, which results in snakebites
- Social unrest
- War over natural resources, including water and food
- Wildfires, such as Australian bushfires, are more common during times of drought and even death of people.
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. Hunter-gatherer migrations in 9,500 BC Chile have been linked to the phenomenon, as has the exodus of early humans out of Africa and into the rest of the world around 135,000 years ago.
Modern 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.
Well-known historical droughts include:
- 1900 India killing between 250,000 to 3.25 million.
- 1921-22 Soviet Union in which over 5 million perished from starvation due to drought
- 1928-30 Northwest China resulting in over 3 million deaths by famine.
- 1936 and 1941 Sichuan Province China resulting in 5 million and 2.5 million deaths respectively.
- The 1997-2009 Millenium Drought in Australia led to a water supply crisis across much of the country. As a result many desalination plants were built for the first time (see list).
- In 2006, Sichuan Province China experienced its worst drought in modern times with nearly 8 million people and over 7 million cattle facing water shortages.
- 12-year drought that was devastating southwest Western Australia, southeast South Australia, Victoria and northern Tasmania was "very severe and without historical precedent".
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.
Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers. 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. 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.
In 2005, parts of the Amazon basin experienced the worst drought in 100 years. 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. 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.
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. 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. Australia could experience more severe droughts and they could become more frequent in the future, a government-commissioned report said on July 6, 2008. 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. 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 1984-1985, 2006 and 2011. During the 2011 drought, an estimated 50,000 to 150,000 people were reported to have died, though these figures and the extent of the crisis are disputed. In February 2012, the UN announced that the crisis was over due to a scaling up of relief efforts and a bumper harvest. Aid agencies subsequently shifted their emphasis to recovery efforts, including digging irrigation canals and distributing plant seeds.
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.
Generally, rainfall is related to the amount (determined by air temperature) of water vapour carried by regional atmosphere, combined with the upward forcing of the air mass containing that water vapour. If these combined factors do not support precipitation volumes sufficient to reach the surface, the result is a drought. This can be triggered by 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 from behaviors which prevent or restrict the developing of thunderstorm activity or rainfall over one certain region. Oceanic and atmospheric weather cycles such as the make drought a regular recurring feature of the Americas along the Midwest and Australia.
Human activity can directly trigger exacerbating factors such as over farming, excessive irrigation, deforestation, and erosion 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 are expected to trigger droughts with a substantial impact on agriculture 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 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.
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:
- Meteorological drought is brought about when there is a prolonged period with less than average precipitation. Meteorological drought usually precedes the other kinds of drought.
- 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.
- 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. Similar circumstances also place their largest lake, Balkhash, at risk of completely drying out.
Protection and relief
Strategies for drought protection, mitigation or relief include:
- Dams - many dams and their associated reservoirs supply additional water in times of drought.
- Cloud seeding - a form of intentional weather modification to induce rainfall.
- Desalination - 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 has revealed that their water table (underground water level) is put at grave risk by over-use to fertilize their Khat crop. Careful monitoring of moisture levels can also help predict increased risk for wildfires, using such metrics as the Keetch-Byram Drought Index or Palmer Drought Index.
- Land use - Carefully planned crop rotation can help to minimize erosion and allow farmers to plant less water-dependent crops in drier years.
- Outdoor water-use restriction - Regulating the use of sprinklers, hoses or buckets on outdoor plants, filling pools, and other water-intensive home maintenance tasks.
- Rainwater harvesting - 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 in drought-prone areas.
- Aridity index
- Climate change
- Collapse: How Societies Choose to Fail or Succeed
- Drought refuge
- Food security
- Permanent wilting point
- United Nations Convention to Combat Desertification
- Water conflict
- Water crisis
- World Water Day
- World Water Forum
- It's a scorcher - and Ireland is officially 'in drought' Irish Independent, 2013-07-18.
- Living With Drought
- Australian Drought and Climate Change, retrieved on June 7th 2007.
- Keddy, P.A. 2007. Plants and Vegetation: Origins, Processes, Consequences. Cambridge University Press, Cambridge, UK. 666 p.
- C.Michael Hogan. 2010. Abiotic factor. Ed. Emily Monosson. Encyclopedia of Earth. National Council for Science and the Environment, Washington DC
- Drought affecting US hydroelectric production | Daily Estimate
- Parched village sues to shut tap at Coke March 6, 2005
- Greenpeace reports on a Swedish drought and its potential impact on their nuclear industry. 4 August 2006
- Australians Face Snake Invasion.
- Texas Forest Service description of the Keetch-Byram Drought Index (KBDI) from 27 December 2002
- BBC - Weather Centre - Features - History and Religion - Weather in the Bible - Drought and Famine
- Ancient Chile Migration Mystery Tied to Drought
- Drought pushed ancient African immigration
- Looking to water to find peace in Darfur
- Big melt threatens millions, says UN
- Ganges, Indus may not survive: climatologists
- Glaciers melting at alarming speed
- Himalaya glaciers melt unnoticed
- Glaciers Are Melting Faster Than Expected, UN Reports
- Water shortage worst in decades, official says, Los Angeles Times
- Environmental News Service - Amazon Drought Worst in 100 Years
- Drought Threatens Amazon Basin - Extreme conditions felt for second year running
- Amazon rainforest 'could become a desert' , The Independent, July 23, 2006. Retrieved September 28, 2006.
- Dying Forest: One year to save the Amazon, The Independent, July 23, 2006. Retrieved September 28, 2006.
- Climate change a threat to Amazon rainforest, warns WWF, World Wide Fund for Nature, March 9, 2996. Retrieved September 28, 2006.
- Sensitivity of the Australian Monsoon to insolation and vegetation: Implications for human impact on continental moisture balance, Geological Society of America
- Australian rivers 'face disaster', BBC News
- Australia faces worse, more frequent droughts: study, Reuters
- Metropolis strives to meet its thirst, BBC News
- Sara Pantuliano and Sara Pavanello (2004) Taking drought into account Addressing chronic vulnerability among pastoralists in the Horn of Africa Overseas Development Institute
- "Fatal Failure: Did Aid Agencies Let Up To 100,000 Somalis Die in 2011?". Time. January 18, 2012.
- Warah, Rasna (2 October 2011). "Manufacturing a famine: How Somalia crisis became a fund-raising opportunity". The East African. Retrieved 16 March 2013.
- U.N. Says Famine in Somalia Is Over, but Risks Remain
- A biblical tragedy as Sea of Galilee faces drought Belfast Telegraph
- Kenya: Deforestation exacerbates droughts, floods
- NOAA Drought and climate change: implications for the West December 2002
- Record rise in wheat price prompts UN official to warn that surge in food prices may trigger social unrest in developing countries
- Fuel costs, drought influence price increase
- Nigerian Scholar Links Drought, Climate Change to Conflict Africa Oct, 2005
- Is Water the New Oil?
- Sunshade' for global warming could cause drought 2 August 2007 New Scientist, Catherine Brahic
- NOAA factsheet, retrieved April 10, 2007
- BBC article on the World Bank loan to save the Aral Sea
- BBC article from 2004 concerning the risk of Kazakhstan losing the lake
- Cloud seeding helps alleviate drought
- BBC's From Our Own Correspondent on khat water usage
- Disappearing Lakes, Shrinking Seas
- Shrinking African Lake Offers Lesson on Finite Resources
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- GIDMaPS Global Integrated Drought Monitoring and Prediction System, University of California, Irvine
- Water scarcity from FAO Water (Food and Agriculture Organization of the United Nations)
- Drought: Hearing before the Committee on Energy and Natural Resources, United States Senate, One Hundred Thirteenth Congress, First Session, on Exploring the Effects of Drought on Energy And Water Management, April 25, 2013