IB Geography Arid Deserts

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Causes

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High pressure belts → descending air warms and expands → warm air has lower humidity, same amount of water vapour relative to parcel of air

Rain Shadow → air is forced up mountains → cools and condenses → rain falls on windward side → leeward side is high pressure and dry

Continentality → air that travels over large land masses becomes dry

Cold Ocean Currents → no precipitation from evaporation, e.g. Humbolt, Benguela, California

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Flash Floods

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Common in arid areas as ground is baked to be impermeable

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16 Terms

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Causes

High pressure belts → descending air warms and expands → warm air has lower humidity, same amount of water vapour relative to parcel of air

Rain Shadow → air is forced up mountains → cools and condenses → rain falls on windward side → leeward side is high pressure and dry

Continentality → air that travels over large land masses becomes dry

Cold Ocean Currents → no precipitation from evaporation, e.g. Humbolt, Benguela, California

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Flash Floods

Common in arid areas as ground is baked to be impermeable

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High Pressure Belts

Tri-Cellular Model: Hadley, Ferrel, Polar

Hadley Cell: Low pressure at equator, high pressure at 30° north and south(subtropical high)

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Human Challenges

Heat:

  • >35° intolerable in longterm
  • Sandstorms and dust → respiratory issues, can carry diseases/pollutants/spores

Resource Development:

  • inavailability of water, food, fuel
  • infrastructure → roads and rail lines buckle from heat
  • topography → steepness, unstable ground
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Desertification

results from large-scale, longterm land degradation, land becoming desert

1/3 of land is at risk today

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Land/Soil Degradation

Process whereby soil becomes less productive as a result of physical factors(e.g. drought) or human factors(e.g. overgrazing, bad land management)

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Natural Causes of Desertification

Drought

  • vegetation dies
  • soil exposed
  • soil is blown away
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Human Causes of Desertification

  • climate change
  • deforestation
  • overcultivation
  • overextraction
  • overgrazing
  • colonialism
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Changing Distribution: Sahar & Kalahari

  • Tufu deposits → petrified waterfalls in Namibia
  • Cave and rock art in Tassilli Mountains
  • Kroll-Milankovitch cycles → Northern Hemisphere warmer, more rain
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Changing Distribution: Fertile Crescent

  • Syria, Iraq, Iran, Turkey, Lebanon, Palestine, Israel, Jordan, Egypt
  • first society, developed agriculture 10-12,000 years ago, Tigris+Euphrates(+Nile+Jordan)

Why is it no longer fertile?

  • slightly wetter
  • irrigation ~6000 ypb, salinisation
  • population grew, deforestation
  • Mesopotamian marshes in Iraq → once largest wetlands in West Asia
  • from 1950s → dams in Turkey, Syria, etc
  • 1990s → Saddam Hussein built large canals to drain marshes to punish the Shiite Muslims who lived there
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Deserts

Climatic region that receives less than 250mm a year of rain, 1/3 of the world is arid, <25mm is extremely arid

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Challenges of Agriculture in Arid Areas(8)

  • soil has low levels of organic matter/mineral content, poor soil structure
  • low soil biodiversity
  • lack of clay(binds to nutrients like Ca, Mg, K)
  • salt in soils not removed by precipitation
  • precipitation levels often too low to support crops
  • negative water balance(pEVT > pPT)
  • flash floods mean that rain runs off fast
  • frequent loss of topsoil due to erosion
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Benefits of Agriculture in Arid Areas

  • 12 month growing season
  • massive potential for photosynthesis
  • little competition with other land uses
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Saline Soil

contains an excessive amount of soluble salts

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Sodic Soil

contains high amount of absorbed sodium ions that damage structure

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SAS

Soil-affected soil, 8.7% of land(833 million hectares)