EVSC10001 Lecture Notes - Lecture 31: Great Sandy Desert, Coast, Cool Air
10 Jun 2018
School
Department
Course
Professor
Deserts
30% of Earth surface are drylands
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Inhospitable, barren, useless, unvegetated, devoid of water
Imply climatic deliniation
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MOISTURE DEFICIT
Aridity < 300m/a
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Precipitation < evaporation
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Aridity index - precipitation / potential evapotranspiration
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Dryland climate
Inter-annual variability
Temperate regions may have year-year rainfall variability under 20%
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In Kalahari ranges up to 45%
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In Sahara ranges from 80-150%
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Temp variations affect seasonal availability of moisture (influences evapo-
transpiration rates + form of precipitation)
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Hot: central sahara, Great sandy desert
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Mild: southern sahara, kalahari
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Cool: northern sahara, turkish steppes
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Cold: gobi, chinese deserts
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WHY DRY?
Atmospheric stability
1.
Descending air masses in hadley cell
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Tropical and subtropical deserts
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Cold ocean currents
2.
Affect western coastal margins of many continents
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Cold water cools air above -> high pressure and cool air cants store
moisture -> transport of moisture from ocean to land is limited
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Continentality
3.
Distance from ocean prevents penetration of rain-bearing winds into centre
of large continents
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Topography
4.
Mountain barriers -> Rain shadowing
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Namib = oldest and most persistent of current arid zones on Eaeth - dating
back to Cretaceous
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Aridity since Precambrian times
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Water and Plants
Temporal variability1.
Spatial variability (convective rainfall) 2.
Rainfall intensities 3.
Strategies developed to cope with moisture deficits and drought
Escape (dormant during dry seasons), evade (tap deep water), resist ,
endure (reduce transpiration)
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Weathering
Slow and physical weathering (lack of moisture) + efficient wash processes
(lack of vegetation) -> dominance of slopes without soil / regolith
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Hillslope form depends on slope resistance to erosion and/or failure (rock
strength)
Strong structural and lithological control on morphology
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Dryland hydrology
70-80% of dryland soil surface devoid of vegetation
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Bedrock landscapes - soil cover thin, patchy or absent
Loose sediment -> no stabilisation
Highest sediment yields
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Flash flooding!
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Rivers
Ephemeral (temporary) channels with high variability in flow
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High infiltration + evaporation: sheet flow and flash-floods
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Abundance of coarse sediments + channel bank instability
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Discharge declines downstream (doesn’t always make it to the ocean
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30% of world's drylands are closed (endorheic) basins
Abundant sediment1.
Wind transport (dune field)2.
Evaporative base level lake systems - leaves behind evaporites
(minerals)
3.
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Wind in drylands
Abrasion + deflation
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More particles erode lower down
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Wind -> Deflation + dust storm source areas
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Desert pavements are coarse on top + dust is trapped below
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Dunes and dunefields
Dunes: windblown accumulation of sand
Determined by wind - velocity and consistency of direction
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Types controlled by
Sand supply
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Wind
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Vegetation
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Gigantic oceans of sand = ergs
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Playa: (salt lakes) arid zone basin, dry for over 75% of time with evaporation
causing water to discharge -> evaporates
Accumulation of evaporative minerals
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Base-level lakes, salt lakes, ephemeral lakes
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Expansion and contraction of drylands due to climate changes
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Deserts
Thursday, 7 June 2018
9:20 pm
Document Summary
Temperate regions may have year-year rainfall variability under 20% Temp variations affect seasonal availability of moisture (influences evapo- transpiration rates + form of precipitation) Cold water cools air above -> high pressure and cool air cants store moisture -> transport of moisture from ocean to land is limited. Distance from ocean prevents penetration of rain-bearing winds into centre of large continents. Namib = oldest and most persistent of current arid zones on eaeth - dating back to cretaceous. Strategies developed to cope with moisture deficits and drought. Escape (dormant during dry seasons), evade (tap deep water), resist , endure (reduce transpiration) Slow and physical weathering (lack of moisture) + efficient wash processes (lack of vegetation) -> dominance of slopes without soil / regolith. Hillslope form depends on slope resistance to erosion and/or failure (rock strength) 70-80% of dryland soil surface devoid of vegetation. Bedrock landscapes - soil cover thin, patchy or absent. Ephemeral (temporary) channels with high variability in flow.
