7.2.1

Causes of Drought

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Physical Causes of Drought

Drought happens when an area suffers from a water deficit. If rainfall falls well below average for a specific place then a deficit can happen.

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Types of drought

  • Meteorological - this type of drought presents a degree of dryness compared to what is considered to be normal precipitation levels for that area, climate and season.
  • Agricultural - when there is insufficient water for crops, leading to wilting or loss of crops without irrigation.
  • Hydrological - when the drainage basin suffers shortfalls such as reduced streamflow or an increase of river flow into the reservoirs.
  • Socio-economic - when there is a demand for water to be used for social and economic purposes such as crop irrigation or when HEP exceeds water availability.
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ENSO: El Nino

  • ENSO cycles (El Niño Southern Oscillation) refer to the El Niño and La Niña changes in the Pacific Ocean.
  • El Niño events happen every 3-5 years, where winds weaken across the South Pacific Ocean and reverse direction. Warm water moves to the South American coastline, where lower air pressure causes large increases in rainfall, promoting the potential for flooding.
  • During El Niño, Australasia has cooler than normal ocean temperatures and is subject to high air pressure, resulting in little rainfall and an increased risk of drought.
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ENSO: La Niña

  • ENSO cycles (El Niño Southern Oscillation) refer to the El Niño and La Niña changes in the Pacific Ocean.
  • During La Niña years, low air pressure descends on Southeast Asia and Australia leading to increased rainfall and risk of flooding.
  • South America receives high air pressure, which leads to reduced rainfall and increased likelihood of drought.
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Short-term precipitation deficit

  • There can be a short-term precipitation deficit in places when the cycle that creates clouds is interrupted.
  • A water deficit may be experienced in areas that are used to low air pressure systems and have a change of weather front to experience a high air pressure system. The cool sinking air in high air pressure systems does not have the ability to form clouds and produce precipitation.

Human Causes of Drought

In addition to the weather, human activity can contribute to the risk of drought. The main human activities associated with drought risk are over-abstraction of surface water and aquifers.

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Case study: Brazil

  • In 2014-15 industries, farmers and domestic users in Brazil increasingly used groundwater, until the rivers began to run low.
  • Poor and rural communities suffered because they only had access to groundwater because of rivers running low.
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Causes of the over-abstraction

  • Brazil’s government introduced a charge for drilling wells of around US$35,000- US$100,000.
  • Residents drilled illegally because they could not afford the charge, meaning the water abstracted was not monitored.
  • Hydrologists believe that the government's official records of wells drilled in 2014 only represent a 1/3 of the total, as 70% of wells were illegal.
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Problems with over-abstraction

  • The government’s well charge resulted in illegal drilling.
  • Illegal wells are shallower and so have not percolated deep into the bedrock. Water from illegal wells is less filtered by bedrock and may contain industrial and agricultural pollutants with higher levels of bacteria.
  • An increase in poor sanitation and water-borne diseases happens if the water is not filtered further.

Jump to other topics

1Tectonic Processes & Hazards

2Option 2A: Glaciated Landscapes & Change

3Option 2B: Coastal Landscapes & Change

4Globalisation

5Option 4A: Regenerating Places

6Option 4B: Diverse Places

7The Water Cycle & Water Insecurity (A2 only)

8The Carbon Cycle & Energy Security (A2 only)

9Superpowers (A2 only)

10Option 8A: Health & Human Rights (A2 only)

11Option 8B: Migration & Identity (A2 only)

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