CHAPTER 12

Chapter 12 Arid Regions & Desertification

Desert: DRYLAND

In last 50 - 100 years, deserts have been advancing over millions of acres of PRODUCTIVE LAND

Croplands and villages are destroyed

Almost 70,000 km2 per year of land converted to desert

Advancement caused millions of people to migrate to other areas searching for food and water -> lots of human SUFFERINGS

--Migration produced “ENVIRONMENTAL REFUGEES”

Distribution of Deserts

Found in areas of dry climate

Mainly in Low to Middle Latitudes “HORSE LATITUDES”.

Figure here.

Dry climates cover about 30 % of earth’s surface

Evaporation exceeds precipitation, e.g. Southwest US, northern Mexico, N. Africa (Sahara), Chile and Peru (Atacama Desert), Middle East (Arabian Desert)???, Pakistan and Western India, 40 % of Australia (Desert Continent)

These areas are:

Far from influence of moisture (marine) air
Rain shadow (Lee side) of Mountain Ranges

Characteristics of Deserts:

Temp:

Hot and Dry

Average summer T: 32 - 38 o Celcius (90 - 108 o F)

High Temperature 46 - 50 o C ( 124 - 132o F)

Highest Temperature ever recorded: 58 o C ( 148o F) at El Azizia, Lybia, September 13, 1922.

Winter Temperature: 10 - 18o C (52 - 68oF)

Precipitation:

Less than 25 cm

Unpredictable, unreliable

All precipitation can occur in “Cloud burst”, and very little rain in several years

Vegetation:

Absent or very sparse

Widely spaced trees, slow growth rate

Weathering and Soils:

Mainly MECHANICAL (physical)

Thin or patchy soils, due to little rainfall

Streams:

Many are dry due to:

lack of water
high evaporation

Water table is usually deep??

Desertification:

Expansion of desert on land

Occurs mostly around existing deserts

>>>>>Human Effect very important in desertification

cleaning of natural vegetation
grazing of livestock with scanty vegetation
drilling of water wells, causing stripping of vegetation at well sites
water for irrigation results in salts, i.e., SALINIZATION, dangerous to plants, in N. Africa, Middle East, S. W. Asia, western US
Collecting FIREWOOD for heating & cooking

--especially in developing countries

Geologic Agents in Desertification:

Wind:

Most important geologic agent in deserts

Transport sediments like H2O, but more turbulent than H2O

Less dense than H2O, therefore, FINE GRAINED mainly, CLAY and SILT sized.

Fine grains transported as SUSPENDED LOAD

Few large grained materials are also transported as BED LOAD along the ground

Suspended load:

Silt or clay-sized particles easily picked and carried in air Figure here.

Bed Load:

Particles too large and heavy to be carried in air

Carried in different ways

>>>>>SALTATION, ROLLING, SLIDING

Saltation: Particles transported by bouncing intermittently along the bed.

Figure here.

Wind Erosion:

Erosion by wind is less than by WATER and GLACIERS

By 2 processes:

Abrasion: Saltation grains rub on one another (Sand Blasting)

Occurs at a small scale, because sand can only be carried about 1 m above surface

Therefore, abrasion creates NO NEW FEATURES, but modifies existing features

by etching, pitting, smoothing, polishing

Deflation : Removal of loose surface sediments

Forms DEFLATION HOLLOWS OR BLOW OUTS e.g., in Great Plains Regions of US , Egypt, Australia

Removal of fine materials exposes coarse particles and rocks, called “DESERT PAVEMENT”

Once formed, desert pavements protect underlying material from further erosion

Wind Deposits

2 main types - Dunes and Loess

A. Dunes:

Mound or ridge of wind deposited sand

Formed when wind blows over obstruction

First, 2 wind shadows are formed around obstacle:

Figure here.

The size, shape and arrangement of dunes result form various factors:

sand supply
direction of wind
velocity of wind
amount of vegetation

---With these factors, DUNES often migrate

Different types of Dunes:

4 major types:
BARCHAN, LONGITUDINAL, TRANSVERSE, PARABOLIC DUNES

Barcham Dunes:

Crescent shaped dunes, with tips pointing WINDWARD

Figure here.

Longitudinal Dunes (Self Dunes)

Long parallel ridges of sand
Aligned parallel to wind direction
Formed when winds from different directions converge to produce a PREVAILING DIRECTION

Figure here

Transverse Dunes:

Long ridges perpendicular to wind direction

Parabolic Dunes:

Like barchan dunes BUT TIPS point upwind

Figure here.

B. LOESS

Wind blown silt and clay deposits
Originate in deserts but NOT deposited there

You need VEGETATION and MOISTURE to stabilize loess

Loess is one of world’s most fertile soils

Cover up to 10 % of earth’s land surface

Also found in glacial outwash deposits and flood plains of rivers

Some Effects of Desertification:

1.Operation Restore Hope in Somalia:

ORH is a UN program to help Somalians from famine thought to be a result of politics:

However, it is known today that famine in Somalia resulted from causes relating to desertification
Britain and Italy changed Somalia’s economy from the Natural Nomdic pastord society to EXPORT of Livestock
Somalians now used the scanty resources to sustain the production and export of goats, sheep, cattle etc.

These led to:

overgrazing
excessive soil loss from erosion
degradation of forest (scanty)
(all factors of desertification)

ORH may not succeed until actual causes are known and treated as such (P 302 - Old Book)

2.Deflation Basin and World War II in N. Africa:

A deflation basin in Egypt (Qattara Depression)
Covers about 18,000 km2
Dangerous and impassable
Block German Soldiers during WW II
Germans were forced to change route of attack to face British Army, therefore, defeated

Mitigation of Desertification mainly by:

Building fences

Planting wind breakers of wind and drought- tolerant trees

Paving and oiling areas of migrating sand, etc.

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