Drought is the unusual dryness of soil caused by levels of rainfall significantly below average over a prolonged period. Hot dry winds, shortage of water, high temperatures and consequent evaporation of moisture from the ground can also contribute to conditions of drought. Droughts result in crop failure and shortages of water.


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. Prolonged droughts have caused mass migrations and humanitarian crisis. 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).

Well-known historical droughts include the 1997–2009 Millennium 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 . In 2011, the State of Texas lived under a drought emergency declaration for the entire calendar year and severe economic losses. The drought caused the Bastrop fires.

Causes of drought

Precipitation deficiency

Mechanisms of producing precipitation include convective, stratiform, and orographic rainfall.Convective processes involve strong vertical motions that can cause the overturning of the atmosphere in that location within an hour and cause heavy precipitation,while stratiform processes involve weaker upward motions and less intense precipitation over a longer duration.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. Droughts occur mainly in areas where normal levels of rainfall are, in themselves, low. If these factors do not support precipitation volumes sufficiently 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,hot conditions which can promote warm core ridging,] and minimal evapotranspiration can worsen drought conditions.

Dry season

Within the tropics, distinct, wet and dry seasons emerge due to the movement of the Intertropical Convergence Zone or Monsoon trough.The dry season greatly increases drought occurrence,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, and wildebeest. Because of the lack of water in the plants, bushfires are common.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). Periods of warmth quicken the pace of fruit and vegetable production, increase evaporation and transpiration from plants, and worsen drought conditions.

El Niño


Erosion and human activities

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.In arid climates, the main source of erosion is wind. 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.

Loess is a homogeneous, typically nonstratified, porous, friable, slightly coherent, often calcareous, fine-grained, silty, pale yellow or buff, windblown (Aeolian) sediment. 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. Loess tends to develop into highly rich soils. Under appropriate climatic conditions, areas with loess are among the most agriculturally productive in the world. 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. 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.

Climatic changes




  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.

Consequences of drought

One can divide the effects of droughts and water shortages into three groups: environmental, economic and social.

  • 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, loss of biodiversity, worse health of trees and the appearance of pests and dendroid diseases.
  • Social costs include the negative effect on the health of people directly exposed to this phenomenon (excessive heat waves), possible limitation of water supplies, increased pollution levels, high food-costs, stress caused by failed harvests, etc. This explains why droughts and fresh water shortages operate as a factor which increases the gap between developed and developing countries.

Effects vary 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, 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
  • 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 snake-bites
  • Social unrest
  • War over natural resources, including water and food
  • Wildfires, such as Australian bushfires, become more common during times of drought and may cause human deaths.
  • Exposure and oxidation of acid sulfate soils due to falling surface- and ground-water levels.
  • Cyanotoxin accumulation within food chains and water supply (some of which are among the most potent toxins known to science) can cause cancer with low exposure over the long term. High levels of microcystin appeared in San Francisco Bay Area salt-water shellfish and fresh-water supplies throughout the state of California in 2016.


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.

Well-known historical droughts include:

  • 1540 Central Europe, said to be the “worst draught of the millennium” with eleven months without rain and temperatures of five to seven °C above the average of the 20th century
  • 1900 India killing between 250,000 and 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.
  • 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.The lake Chad has shrunk by 95% since the 1960s.


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.


Recurring droughts leading to desertification in East Africa have created grave ecological catastrophes, prompting food shortages in 1984–85, 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.


Protection, mitigation and relief

Succulent plants are well-adapted to survive long periods of drought.They have adopted to the condition by modifying their shapes and sizes.



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.This remains a hotly debated topic, as the United States National Research Council released a report in 2004 stating that to date, there is still no convincing scientific proof of the efficacy of intentional weather modification.
  • Desalination – use 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 Indexor 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. Xeriscaping yards can significantly reduce unnecessary water use by residents of towns and cities.
  • 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


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