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GEG135 PT2 - Coggle Diagram
GEG135 PT2
desertification
deserts and drylands
extremely arid: at least 12 consecutive months without rainfall, arid: < 250 mm annual rainfall, semi-arid: 250-500mm rainfall
types of desert: Polar, subtropical, coastal, cold
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rainfall is >65% of evaporative demand, precipitation is balanced by evaporation from surfaces and by transpiration by plants (evapotranspiration)
'Dust Bowl', USA 1930's,:
- prolonged drought in 1930s
- loss of topsoil, unstable / infertile land
- approx 3 million ha land damaged - topsoil erosion ~ 1000 tonnes/ha
Sahel drought, 1968-1973, Sahel rainfall gradient, 100mm - 400mm: - precipitation halved in some regions - Desert "moved south" ~150km (<100mm)
UN Conference on Desertificatino (UNCOD, Nairobi 1977): Aim: to
advance global awareness and understanding of desertfication
conclusion:
- drought has only indirect effect on desertificatino
- land management main cause
- can happen anywhere
- First 'Hazard map' - over 35% of land surface threatened by desertification (3970 million ha) - 20% of world's population - 21x10 to the power of 6 ha/p.a. made unproductive - 6x10 to the power of 6 ha/p.a. beyond economic recovery
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drylands ecology, management & sustainability
satellite estimates 1981-2021:
- rapid response to seasonal and annual rainfall
- recovery from drought
- may miss finer detail / species change
causes of desertification (Lamprey 1978): overgrazing -> herb layer destruction -> soil erosion -> desertification
three main landscape components:
- perennial grass
- annual grass
- shrub
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irrigation and salinisation,
- primary salinisation: evaporation of historic areas, near surface water tables transporting salt
- secondary salinisation: deep-rooted vegetation drains soil
- removal of cover may lead to water logging
- depletion of sources of freshwater inflow to inland seas and lakes due to irrigation salinity reduced crop yields in surrounding land by 20-50%
- surface evapotranspiration balances irrig/rainfall -> increased salinity
- excess irrigation raises water table, increases salinity
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land access, social movements and sustainability
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