Causes of Drought (Edexcel GCSE Geography A): Revision Note

Arid Environments vs Drought

• Drought is a temporary but lengthy period of very low rates of precipitation

• Drought can happen anywhere in the world

• In the UK, drought is 15 consecutive days without rain

• A drought can last from weeks to months and years

• A drought is considered an extreme weather event

• An arid region has a climate that is normally dry

• Prevailing winds often come from over land, which means they contain little moisture

• High-pressure conditions all year are normal, which lead to low rates of precipitation

• Arid regions usually have less than 250 mm of rainfall each year

• Some arid regions are in a rain shadow so there is little rainfall

• Semi-arid regions usually receive between 250 and 500 mm of rainfall per year

• Arid regions are natural environments

• Two factors determine if an area is likely to suffer drought

  • When rainfall happens: time of year, winter or summer 

  • How: any time or in a rainy season, such as the monsoon

Comparison of Arid Regions to Drought Situation

Causes of Drought

• There are different causes of a drought

• Meteorological 

  • This is where an area receives less than usual amounts of rainfall

  • High-pressure systems block low-pressure rain-bearing systems 

  • This reduces or prevents rain from falling

• Hydrological

  • This is the impact of reduced rainfall on the water (hydrological) cycle, such as

    • Groundwater supplies and reservoirs do not refill

    • Stream flow is reduced or dries up

    • Reservoir and lake levels drop

• Human

  • Dam building

    • This restricts the flow of water in a river

    • This lowers water levels and volume downstream

  • Agricultural use

    • Some crops need more water than others, such as cotton, which removes soil water

    • Large amounts of water are used to irrigate crops

    • This removes water from lakes, rivers and groundwater

  • Deforestation

    • Trees reduce evaporation and add moisture to the atmosphere through transpiration

    • Removing trees can reduce the amount of water stored in the soil

Global Circulation & Drought

• The global circulation can reduce the amount of precipitation to an area, making the region more vulnerable to drought

  • For instance, where the Hadley and Ferrel circulation cells meet, the descending air is dry with little precipitation

  • This creates arid and semi-arid regions with seasonal rainfall

  • Any changes in this usual atmospheric circulation mean that:

    • The rains do not arrive

    • The area has a drought condition

• Any changes in wind patterns also change moisture levels in various regions

  • A lack of water vapour in the atmosphere means there is less precipitation and a higher chance of drought

  • High-pressure systems reduce evaporation and moisture in the atmosphere

• Scientists have found a link between an El Niño and La Niña weather event and drought

  • This is where the surface water in the Pacific Ocean along the central South American coast rises in temperature

  • The warmer water changes storm patterns and causes droughts in Indonesia, Australia, and north-eastern South America

  • La Niña: the surface temperature of the Pacific Ocean around the central South American coast decreases

  • Storms are disrupted and North and South America become prone to droughts again

• Most areas affected by drought are already arid regions and are at risk of longer drought situations

  • At-risk areas include Australia, the Middle East, central and southern Africa, parts of North America and eastern South America

• Droughts may become more frequent and severe in the future with global warming

Location of arid regions

Arid regions are normal environments due to descending high pressure of the Hadley cell