Download Deserts and Desert Environments

Deserts and Desert Environments
Introduction
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Aeolian geomorphology is the study of the effect of
wind on Earth surface processes and landforms
Wind: the movement of air from one place to another,
specifically from an area of high pressure to an area
of low pressure in an attempt to balance out the
pressure differences
Though this occurs everywhere, our focus will be on
desert landscapes
What is a Desert?
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Desert: Regions of very low annual rainfall (less than
300mm), meagre vegetation, extensive areas of bare and
rocky mountains and plateaux, and alluvial plains
(Huggett, 2007)
Covers approx. 1/3 of Earth's land surface
Aridity forms the basis for the classifications of deserts
(Huggett, 2007)
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Driest place in the world- Dry Valleys (Antarctica)
Temperature not only indicator
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Some deserts are hot and some are cold
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Highest recorded temperature 58°C (Libyan desert)
Lowest recorded temperature -89.2°C (Antarctica)
What is a Desert?
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Aridity index (Hugget, 2007):
Variables:
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AI = Aridity Index
PE = Potential evapotranspiration
P = Average annual precipitation
Four types of desert
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Hyperarid: AI < 0.05
Arid: 0.05 < AI < 0.2
Semi-arid: 0.2 < AI < 0.5
Dry Subhumid: 0.5 < AI < 0.65
P
AI =
PE
What the Aridity Index Tells Us
Evapotranspiration
Transpiration
(from vegetation)
+
Evaporation
Precipitation
(from clouds)
(from surface)
Surface
The aridity index is therefore an expression of the relationship between the amount of
water reaching the surface (from precipitation) vs. the amount that gets lost back to the
atmosphere (evapotranspiration). If they are equal, the aridity index would be 1
What is a Desert?
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Lush vegetation
Abundant water
High rainfall
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Desert?
Sparse vegetation
Scarce water
Low rainfall
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Desert?
Absence of vegetation
Water in solid state,
therefore unavailable
Very low rainfall
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Desert?
(Some Global Deserts Indicated)
5
6
4
2
11
7
1
12
3
13
8
10
9
14
15
1.
2.
3.
4.
5.
6.
7.
Sahara
Syrian
Arabian
Iranian
Karakum
Gobi
Taklimakan
8. Namib
9. Kalahari
10. Atacama
11. Great Basin
12. Chichuahnan
13. Simpson
14. Great Victoria
15. Antarctica
Distribution of Deserts
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Low-Latitude Deserts
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In the vicinity of the tropics of Cancer and Capricorn
Result due to the global distribution of air pressure
and winds
Heated air at equator (equatorial low) rises and
spreads out sinking between 20° and 30° lat (north and
south) known as the sub-tropical highs
Air that sinks is compressed and warmed
Examples
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Namib desert, Sahara desert, Iranian desert
Distribution of Deserts
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Mid-Latitude Deserts
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Not controlled by the subsiding air masses associated
with high pressure
Exist because the are sheltered in the interiors of
landmasses
Far removed from ocean (source of moisture)
Presence of mountains (rain shadow effect)
Examples
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Gobi desert, Great Basin, Patagonian desert
Southern African Deserts
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(Adapted from Laity, 2008)
Evolution of Deserts
(Laity, 2008)
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Owe their origin to climatic, topographic, and oceanic
factors that prevent the incursion of moisture bearing
weather systems
Most deserts are arid due to a combination of these factors
Global considerations
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Sub-tropical high pressure belts
Continental interiors
Polar deserts
Regional considerations
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Cold-current influences
Rain shadow effect
Edaphic considerations
Evolution of Deserts
(Laity, 2008)
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Global considerations
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Sub-tropical high pressure belts
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World's arid and semi-arid regions mainly sub-tropical
in distribution
Influenced by descending air associated with Hadley
cells (sub-tropical high pressure belt)
Aridity not present in all longitudes due to presence of
anticyclone cells (influenced by the coriolis force)
associated with descending air masses
Example: Sahara (Northern Africa)
Sub-tropical High Pressure Belts
❶ Warm moist air rises from the
equatorial region, cooling as it rises.
❷ It then spreads out at the upper
troposphere where it moves towards
the polar regions.
30°N
0°
30°S
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❶
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❸ As the air moves to higher latitudes
it becomes cooler and more dense
causing it to descend. Theses are
masses descend in the sub-tropics,
causing high pressure systems and
ultimately contributing to arid
conditions.
The descending sub-tropical high
pressure belt is broken up into anticyclonic cells (local high pressure cells)
which results in the subsidence being
discontinuous and consequently
means that the aridity will not be
present in all longitudes
Evolution of Deserts
(Laity, 2008)
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 Global Considerations (cont)
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Continental Interiors
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Arid due to distance from sea and sources of moisture
Greater range in annual temperature than coastal deserts
Example: Karakum Desert (Turkmenistan)
Polar deserts
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Low levels of solar radiation at poles result in very cold
temperatures
Therefore the atmosphere contains little water for
precipitation
Example: Dry Valleys (Antarctica)
Karakum Desert
Dry Valleys
Evolution of Deserts
(Laity, 2008)
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Regional Considerations
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Cold current influences
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Cool coastal deserts form adjacent to cold currents on
the western margin of continents
Often long and narrow and may be bounded in the east
by north-south mountain ranges
Climate moderated by the cold waters (rainlessness, fog
and dew, cold temperatures)
Examples: Atacama (Chile), Namib (Namibia)
Namib
Cold Benguela
Current
Air masses derived from cold
water currents are generally
lower in moisture. There is
therefore little moisture that can
lead to the formation of
precipitation.
(Some Global Cold Currents Indicated)
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❶ Benguela Current
❷ Peru (Humbolt) Current
❸ California Current
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❹ Canaries Current
❺ West Australia Current
❻ West Wind Drift
Evolution of Deserts
(Laity, 2008)
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 Regional Considerations (cont)
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Rainshadow
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When air crosses mountain barriers it rises on the
windward side and descends on the leeward side
Subsidence prevents convection and produces adiabatic
heating which has a drying effect
In addition as the air rises on the windward side it cools
and looses a lot of moisture in the form of precipitation
Examples: Patagonian Desert (Patagonia), Great Basin
(USA)
The Rainshadow Effect
❷ As this moist air
encounters an
obstacle (e.g.
mountain) it is forced
upward. As the air
rises it cools.
❸ Cool air cannot
hold as much moisture
as warm air which
leads to cloud
formation along with
precipitation.
❶Moist air from the
ocean (water body)
moves towards the
interior
❸
❹ At this point the
air has lost the overall
majority of its
moisture. The dry air
then descends on the
leeward side of the
obstacle, heating as it
does so, leading to dry
conditions prevailing.
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Evolution of Deserts
(Laity, 2008)
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Edaphic factors
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Result in large measure due to the influence of soil
Sandy soils and high evapotranspiration makes surface
waters scarce in areas with relatively high rainfall
Example: Kalahari Desert