Lesson 6 - Drought causes and impacts Flashcards
What is a drought?
A drought is an extended period, a season, year or several years of deficient rainfall relative to the statistical multi-year average for a region.
State 6 natural causes of drought?
- ENSO cycles
- Blocking anti-cyclones
- Global atmospheric circulation
- Climate change
- Hydrological
- Reduced soil moisture
Give two short term natural causes of drought?
- Blocking anticyclones
- Reduced soil moisture
Give two medium term natural causes of drought?
- ENSO cycles
- Hydrological
Give two long term natural causes of drought?
- Global atmospheric circulation
- Climate change
How do ENSO cycles cause drought?
ENSO cycles occur in the Pacific Ocean on average every seven years. During El Nino, warm water moves closer to the Americas, reducing rainfall in the Western Pacific. During La Nina, warm water moves to the extreme west of the tropical Pacific, reducing rainfall in western parts of the Americas. These cycles are known to have global effects (teleconnections) such as La Nina causing drought in East Africa.
How does blocking anticyclones cause drought?
In mid-latitude areas, the Polar Front Jet Stream may move northwards, allowing subtropical high-pressure areas to move northwards. These bring stable, calm hot weather with little or no rainfall, e.g. the UK in 2018.
How does global atmospheric circulation cause drought?
Descending air between the Hadley and Ferrel cells and between Polar cells creates hot and cold deserts respectively. Areas adjacent to these are also influenced, creating semi-arid climates where droughts are common.
How does climate change cause drought?
Evidence shows that higher temperatures are changing atmospheric systems. Subtropical high-pressure areas are getting stronger which stops the seasonal movement of the ITCZ. Rainfall events have become more intensive rather than being spread over time. Higher sea temperatures have changed evaporation and wind patterns, which may cause rain-bearing winds to fail, and have changed ocean currents; where cold currents become stronger evaporation is reduced and drier conditions result.
How do hydrological changes cause drought?
Reduced amounts of water in rivers or the ground may occur if precipitation is reduced in mountainous areas over several years, especially reducing the amount of snowmelt.
How does reduced soil moisture cause drought?
There may not be enough precipitation to soak into soils because plants use it quickly during the growing season or there is high evaporation because of drier air.
Give some human causes of drought?
- Irrigation of crops
- Abstracting water from rivers and the ground
- Deforestation
- Overuse of water for leisure purposes
- Removal of vegetation
- Dam construction
