IB Geography Arid Deserts

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

Flash Floods

Common in arid areas as ground is baked to be impermeable

High Pressure Belts

Tri-Cellular Model: Hadley, Ferrel, Polar

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

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

Desertification

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

1/3 of land is at risk today

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)

Natural Causes of Desertification

Drought

* vegetation dies
* soil exposed
* soil is blown away

Human Causes of Desertification

* climate change
* deforestation
* overcultivation
* overextraction
* overgrazing
* colonialism

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

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

Deserts

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

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

Benefits of Agriculture in Arid Areas

* 12 month growing season
* massive potential for photosynthesis
* little competition with other land uses

Saline Soil

contains an excessive amount of soluble salts

Sodic Soil

contains high amount of absorbed sodium ions that damage structure

SAS

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