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, <25mm is extremely 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)