An extended period of dry weather, generally associated with a blocking anticyclone in which evapotranspiration exceeds precipitation, causing soil moisture deficits. Some regions, especially arid and semi-arid areas, are particularly prone to droughts, which can result in food shortages and human suffering. In the Sahel region on the S edge of the Sahara Desert, rainfall 1968–1972 was only 50% of the 1931–60 average, and was accompanied by major famine. Other areas prone to drought include the W and mid-W USA, parts of Australia and S Africa, but it can occur in any place where there is a low rainfall, including Britain.

See also

anticyclone

arid zone

desertification

Dust Bowl

evapotranspiration

famine

sahel

Crystal Reference Encyclopedia, © Crystal Reference Systems Limited 2006

Drought

Wikipedia, the free encyclopedia – Cite This Source

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. However, the effects can also be worsened by man. 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 .

For most regions, drought is a normal, recurrent feature of the climate, and 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.

Influences

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 drought.

Factors include:

* Above average prevalence of high pressure systems

* Winds carrying continental, rather than oceanic air masses (ie. reduced water content)

* El Nino (and other oceanic temperature cycles)

* Deforestation

* Many speculate that global warming will have a substantial impact on agriculture throughout the world, and especially in developing nations.

Consequences

Periods of drought can have significant environmental, economic and social consequences. The most common consequences 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.

* Desertification

* 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.

* 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

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

* Land use – Carefully planned crop rotation can help to minimize erosion and allow farmers to plant less water-dependant 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).

from encarta:

Drought, condition of abnormally dry weather within a geographic region where some rain might usually be expected. A drought is thus quite different from a dry climate (see Climate), which designates a region that is normally, or at least seasonally, dry.

The term drought is applied to a period in which an unusual scarcity of rain causes a serious hydrological imbalance: Water-supply reservoirs empty, wells dry up, and crop damage ensues. The severity of the drought is gauged by the degree of moisture deficiency, its duration, and the size of the area affected. If the drought is brief, it is known as a dry spell, or partial drought. A partial drought is usually defined as more than 14 days without appreciable precipitation, whereas a drought may last for years.

Droughts tend to be more severe in some areas than in others. Catastrophic droughts generally occur at latitudes of about 15°-20°, in areas bordering the permanently arid regions of the world. Permanent aridity is a characteristic of those areas where warm, tropical air masses, in descending to earth, become hotter and drier (see Meteorology). When a poleward shift in the prevailing westerlies occurs (see Wind), the high-pressure, anticyclonic conditions of the permanently arid regions impinge on areas that are normally subject to seasonally wet low-pressure weather and a drought ensues. A southward shift in the westerlies caused the most severe drought of the 20th century, the one that afflicted the African region called the Sahel for a dozen years, beginning in 1968.

In North America, archaeological studies of Native Americans and statistics derived from long-term agricultural records show that six or seven centuries ago whole areas of the Southwest were abandoned by the indigenous agriculturists because of repeated droughts and were never reoccupied. The statistics indicate that roughly every 22 years—with a precision of three to four years—a major drought occurs in the United States, most seriously affecting the Prairie and midwestern states. The disastrous drought of the 1930s, during which large areas of the Great Plains became known as the Dust Bowl, is one example. The effect of the drought was aggravated by overcropping, overpopulation, and lack of timely relief measures. In Africa, the Sahel drought was also aggravated by nonclimatic determinants such as overcropping, as well as by problems between nations and peoples unfriendly with one another.

Although drought cannot be reliably predicted, certain precautions can be taken in drought-risk areas. These include construction of reservoirs to hold emergency water supplies, education to avoid overcropping and overgrazing, and programs to limit settlement in drought-prone areas.