X. Desertification

A. Deserts: areas of low rainfall…..

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

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.

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.

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 socio-economic forces. For example:

a. brief period of excess rainfall “the rain follows the

plow”

b. over irrigation (salanization)

c. ground-water depletion

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 socio-economic forces. For example:

2. Drought: rainfall significantly below normal for an extended period.

several years

more than 25% less than normal

Dryland regions of the world (yellow)These are sensitive to desertification

D. Desertification:

4. The Sahel as an example

climate

climate change

agricultural practices

political process

population

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.

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

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.

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

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.

D. Desertification:

4. The Sahel as an example

5. Desertification in the US

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.

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

D. Desertification:

5. Desertification in the US: The Dust Bowl of the 1930s.

Precursers

Causes

Consequences: Changed farming practices

Better? Yes, but….

Phoenix, Arizona in the early 1970s