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DESERTIFICATION
E d i t e d by J o e N a u m a n n
X. Desertification
A. Deserts: areas of low rainfall…..
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Arid:
less than 10 cm (4 in)
Semi-arid:
less than 25 cm (10 inches)
X. Desertification
A. Deserts
B. Drylands: What life cares about.
Effective Moisture: the ratio of precipitation
(PPT) to potential evapo-transpiration (PET).
Hyperarid: PPT/PET <0.05
Arid:
PPT/PET 0.05 to 0.20
Semi-arid: PPT/PET 0.20 to 0.5
Dry sub-humid:
0.5 to 0.65
Humid:
PPT/PET >1
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X. Desertification
A. Deserts
B. Drylands
C. Distribution
1. Sunbelt
2. Rainshadow
3. Continental interior
4. Coastal
Adjacent to these real deserts are the
sensitive drylands.
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X. Desertification
A. Deserts
B. Drylands
C. Distribution
D. Desertification:the degradation of land
in arid, semi-arid, and dry sub-humid
areas.
Can be caused by human activity or
climate change.
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X. Desertification
D. Desertification:the degradation of land in
arid, semi-arid, and dry sub-humid areas.
1. Causes of Desertification: Complex
interplay of physical and ecological
processes, and developmental and socioeconomic forces. For example:
a. brief period of excess rainfall
“the rain follows the plow”
b. over irrigation (salanization)
c. ground-water depletion
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X. Desertification
D. Desertification: the degradation of land
in arid, semi-arid, and dry sub-humid areas.
1. Causes of Desertification: Complex
interplay of physical and ecological
processes, and developmental and socioeconomic forces. For example:
2. Drought: rainfall significantly below
normal for an extended period.
several years
more than 25% less than normal
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Dryland regions of the world (yellow)
These are sensitive to desertification
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D. Desertification:
4. The Sahel as an example
climate
climate change
agricultural practices
political process
population
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D. Desertification:
3. The Sahel as an example
a. Sub-saharan rainfall index shows
drought from 1970 to 2000.
b. Impact on human activity in the
Sahel.
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D. Desertification:
3. The Sahel as an example
a. Sub-saharan rainfall index shows
drought from 1970 to 2000.
b. Impact on human activity in the
Sahel.
c. Possible explanations for the Sahel
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Remember what drives the monsoon circulation.
• Strong seasonal pressure gradient between land
and sea (hot land mass)
• Warm oceans to supply moisture.
• Sufficient recycling of rainfall by plants
(evapotranspiration) to allow moisture to
reach deep into the interior.
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D. Desertification:
c. Possible explanations for the Sahel
• Natural climate variability:
Monsoon activity controled by sea surface
temperatures, related to ENSO variability.
• Inappropriate technology/misguided foreign
aid.
• Aerosols produced by industrialization in
Europe cool Sahel, reducing strength of the
African Monsoon.
d. Positive feedbacks
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D. Desertification:
c. Possible explanations for the Sahel
d. Positive feedbacks
Fuel wood gathering and stock overgrazing
reduces vegetation cover. Two impacts:
Albedo increases: cooling the region in summer
and weakening monsoon circulation.
Less vegetation = less evapotranspiration. So less
water vapor in the atmosphere to fuel the monsoon
and deliver precipitation over the Sahel region.
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D. Desertification:
4. The Sahel as an example
5. Desertification in the US
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Precursers
10-fold increase in population between
1860 and 1920.
Deep plowing and monoculture destroyed
soil structure and increased sensitvity to
erosion.
Additional factors:
Great Depression: no $ for prairie farmers
Out migration “Okies”
True drought.
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D. Desertification:
5. Desertification in the US: The Dust Bowl of the
1930s.
Precursers
Causes
Consequences: Changed farming practices
Plowing techniques
Shallow, along contours
Established windbreaks
Crop rotation
Irrigation
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D. Desertification:
5. Desertification in the US: The Dust Bowl of
the 1930s.
Precursers
Causes
Consequences: Changed farming practices
Better? Yes, but….
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Phoenix, Arizona in the early 1970s
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