drought, abnormally long period of insufficient rainfall. Drought cannot be defined in terms of inches of rainfall or number of days without rain, since it is determined by such variable factors as the distribution in time and area of precipitation during and before the dry period. Since ancient times droughts have had far-reaching effects on humankind by causing the failure of crops, decreasing natural vegetation, and depleting water supplies. Livestock and wildlife, as well as humans, die of thirst and famine; large land areas often suffer damage from dust storms or fire. Drought is thought by some to have caused migrations of early humans. In India and China drought has periodically brought widespread privation and death. In 1930 lack of rainfall devastated the Great Plains of the United States; called the Dust Bowl, its area spread to alarming dimensions (about 50 million acres). During 1962 much of the eastern part of the U.S. experienced the worst drought in more than 50 years; more recent severe droughts have afflicted countries in many parts of Africa. Clearcutting of trees for firewood, overgrazing, and overcultivation, which lead to land degredation, contribute to this drought cycle.

Lack or insufficiency of rain for an extended period that severely disturbs the hydrologic cycle in an area. Droughts involve water shortages, crop damage, streamflow reduction, and depletion of groundwater and soil moisture. They occur when evaporation and transpiration exceed precipitation for a considerable period. Drought is the most serious hazard to agriculture in nearly every part of the world. Efforts have been made to control it by seeding clouds to induce rainfall, but these experiments have had only limited success.

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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. 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 the local economy According to the UN, an area of fertile soil the size of Ukraine is lost every year because of drought, deforestation and climate instability.

Implications

Drought is a normal, recurring feature of the climate in most parts of the world. Having adequate drought mitigation strategies in place can greatly reduce the impact. Recurring or long-term drought can bring about desertification. Recurring droughts in the Horn of Africa have created grave ecological catastrophes, prompting massive food shortages, still recurring. To the north-west of the Horn, the Darfur conflict in neighboring 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 peoples.Himalayan glaciers that are the sources of Asia's biggest rivers - Ganges, Indus, Brahmaputra, Yangtze, Mekong, Salween and Yellow - could disappear by 2035 as temperatures rise. 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. Paradoxically, some proposed short-term solutions to global warming also carry with them increased chances of drought.

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 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 2007, 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.

Causes

Generally, rainfall is related to the amount of water vapour in the atmosphere, combined with the upward forcing of the air mass containing that water vapour. If either of these are reduced,the result is a drought. Factors include:

• Above average prevalence of high pressure systems;
• Winds carrying continental, rather than oceanic air masses (ie. reduced water content);
• Ridges of high pressure areas form with behaviors which prevent or restrict the developing of thunderstorm activity or rainfall over one certain region;
• El Nino, La Nina (and other oceanic and atmospheric temperature cycles) and global warming;
• Deforestation and erosion adversly impacting the ability of the land to capture water;
• Climate change has a substantial impact on agriculture throughout the world, and especially in developing nations.

Consequences

Periods of drought can have significant environmental, agricultural, health, economic and social consequences. Examples include:

• Death of livestock.
• Reduced crop yields.
• Wildfires, such as Australian bushfires, are more common during times of drought.
• Shortages of water for industrial users.
• Dust storms, when drought hits an area suffering from desertification and erosion
• Malnutrition, dehydration and related diseases.
• Famine due to lack of water for irrigation.
• Social unrest.
• Substandard or highly limited crop growth or yield productions.
• Mass migration, resulting in internal displacement and international refugees.
• War over natural resources, including water and food.
• Reduced electricity production due to insufficient available coolant for power stations and reduced water flow through hydroelectric dams.
• Snakes have been known to emerge and snakebites become more common.
• Creates windblown dust bowls which erodes the landscape, damages terrestrial and aquatic wildlife habitat

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 subsistence farming as a major food source are more vulnerable to drought-triggered famine. Drought is rarely if ever the sole cause of famine; socio-political factors such as extreme widespread poverty play a major role. Drought can also reduce water quality, because lower water flows reduce dilution of pollutants and increase contamination of remaining water sources.

Stages of drought

As a drought persists, the conditions surrounding it gradually worsen and its impact on the local population gradually increases. Droughts go through three stages before their ultimate cessation:

1. 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.
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.
3. Hydrological drought is brought about when the water reserves available in sources such as aquifers, lakes and reservoirs falls below the statistical average. 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.

Drought mitigation strategies

• 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.
• 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.
• Water restrictions - Water use may be regulated (particularly outdoors). This may involve regulating the use of sprinklers, hoses or buckets on outdoor plants, the washing of motor vehicles or other outdoor hard surfaces (including roofs and paths), topping up of swimming pools, and also the fitting of water conservation devices inside the home (including shower heads, taps and dual flush toilets).
• Cloud seeding - an artificial technique to induce rainfall.

See also

• Maya collapse
• Climate change
• Drought in Australia
• Droughts and famines in Russia and USSR
• Drought in the United States
• Food security
• United Nations Convention to Combat Desertification
• Water crisis
• Dust Bowl
• Sahel drought
• Collapse: How Societies Choose to Fail or Succeed, a 2005 book by Jared Diamond

References

External links

• Water scarcity from FAO Water (Food and Agriculture Organization of the United Nations)