Desertification and
the Sahel
https://youtu.be/tDWS6AzEkE0 desert in the Sahel
Pg.134
https://youtu.be/w9RxnuBiFbg desert [American]
https://youtu.be/gRJBuM7qjQ0 desert: causes, impacts and solutions
Learning objectives:
L4-All will be able to identify which physical process operates in arid regions.
L5-Most to describe when a high pressure system occurs.
L6-Few to recognise and describe an anticyclone on a weather chart.
L7-Extension: to draw isobars at specific intervals.
A high pressure system, anticyclone, occurs when the
weather is dominated by stable conditions.
Air is descending forming an area of high pressure on the
surface.
Hot air lifts
and cools
Cool air descends
Surface
Stable conditions means that the weather is settled with
only small amount of cloud cover.
Winds blow in a clockwise direction
Isobars are widely spaced around an anticyclone so the
winds are quite light.
Learning objectives:
L4-All will be able to identify which physical process operates in arid regions.
L5-Most to describe when a high pressure system occurs.
L6-Few to recognise and describe an anticyclone on a weather chart.
L7-Extension: to draw isobars at specific intervals.
Look at the weather chart and use the following words
to annotate it:
Winter
Warm and sunny weather
High centre point
High pressure
Cold and foggy weather
wind
Widely spaced isobars
Clockwise direction
summer
Highest pressure
Extension: complete the diagram by drawing isobars at intervals of 4 mb
Learning Objectives:
All will be able to describe where the Sahel is and what is happening in that region
- application
Most will be able to list the human causes of drought and desertification and offer an opinion
on possible management of the problem.- analysis
Some will be able to understand how the ITCZ affects the rain in the Sahel -evaluation of
physical causes (high pressure)
Drought –
A period of dry weather that affects crop
growth.
DesertificationThe process of turning productive land into
desert.
https://youtu.be/H2szHrMSXpU stone lines
https://youtu.be/x28NpUZjmN8 Zai
Variability of Rainfall:
Rainfall is not only relatively low in the Sahel, or very seasonal (summer) - it is also
very variable.
The total amount can vary enormously from one year to another.
Also the amount received can vary markedly over a relatively short distance.
High pressure system formation and associated hazards of drought.
High Pressure Systems:
Low Rainfall, High Evaporation Rates, Drought and sometimes High Winds.
Causes:
These hazards are associated with anticyclonic conditions.
Global warming is recognised to contribute to drought.
Triggering secondary hazards: a falling water table, loss of vegetation, wild fires,
soil erosion – desertification.
Areas dominated by high pressures are always dry.
A long term high pressure in a region which is not expecting it
will lead to a drought i.e. a lack of the expected rainfall.
Drought varies from place to place.
In drier places drought are often longer term climatic hazards
(rather than a short term weather hazard) as they may last several
years.
The expected rainy season may fail to arrive for several years
consecutively, so the hazard to people gradually intensifies.
http://www.geogonline.org.uk/G3a_ki4.4.htm diagram for desertif process
In the Sahel there is usually rainfall in summer.
ITCZ=
Intertropical Convergence Zone:
is a belt of low pressure which
circles the Earth generally near the
equator where the trade winds of
the Northern and Southern
Hemispheres come together.
The High Pressure over the
Azores in some years spreads
across North Africa. This is a
'blocking high' preventing the
usual northward spread of the
ITCZ over west Africa in the
(northern hemisphere) summer.
Azores
Azores High Pressure
Schematic of surface wind
(arrows) and pressure (mb) over
North Africa during winter and
at the peak of the summer
monsoon.
http://people.cas.sc.edu/carbon
e/modules/mods4car/africaitcz/index.html rain moving
and hydrogr
In some years a 'Blocking' super big High Pressure spreads out from the usual
Azores 'High' over the Sahel too.
ITCZ= Intertropical Convergence Zone
Descending Limb of the Hadley Cell creates a zone of permanent High Pressure over
the Sahara Desert:
Here is a map of the most affected
areas of the Sahel
Human causes of the drought hazard - desertification
Diagram to explain the desertification
process:
A natural ecosystem with a
seasonally dry climate able to
support some drought resistant
trees like Park Savanna with
baobab trees. Perhaps a longer
period of wetter years has
'greened' the environment.
Exercise – Dustbin Game: High Pressure Hazards
Natural Causes
Man - Made Causes
Direct Impacts
Indirect Impacts
Extra strong high pressure
Lack of shade
Vegetation degraded
El Nino oscillations
Soil erosion
Variable rainfall
Global Warming
Water shortages
Overcultivation
Falling water table
Population growth
Lack of expected seasonal
rainfall
Changing albedo of surface
Descending limb of Hadley
Cell
Using groundwater reserves to
irrigate crops
Crops fail = refugee problem
Loss of livestock
Increased risk of sun burn /
skin cancer
Falling water table
Heatwaves
Desertification
Loss of livestock
Wild fires
Increased poverty
Deforestation
Crops fail = famine
Extra strong high pressure
Overgrazing
Long term drought
Soil erosion
Lack of clouds
• South of the Sahra Desert, extending across most of Africa
• A narrow belt of semi-arid land.
• Little rainfall a year, very unreliable
• The climate is a mixture of the hot desert and the tropical continental
climate
• Climate Change & Conditions
Global warming is making the earth hotter in
general. For africa, this means that water is
evaporating quickly and that the amount of
rainfall decreases. With no water, the soil looses
its nutrients and nothing is able to grow on the
land. This leads to soil erosion which ends up
turning the area into a desert (desertification).
https://youtu.be/VC7rsWVI7so (up to 3:30)
A chart on the amount of rainfall in the
Sahel
• Population Pressure
More and more people are having to move into one
area due to the high birth rates around Africa.
This means that the demand for more crops and
meat is higher on the farmers. The farmers try to
grow as many crops as possible on one stretch of
land. This causes soil erosion as all the nutrients
are used really quickly but none are replaced. The
soil becomes useless, meaning nothing is able to
grow any more and the area becomes deserted.
– Deforestation
Deforestation is the loss of forests due to
overcutting trees. This can cause soil erosion
in two main ways. The roots of the trees bind
the soil together. Without the trees, the soil is
lose and therefore more prone to soil erosion.
Also, the leaves of the trees stops the water
droplets form hitting the ground at such a
high speed. This prevents the soil from being
picked up from the water and carried away.
– Overgrazing
Animals are needed to then provide humans
with meat. However, in the sahel, the animals
are eating all the grass and shrubs, meaning
there is nothing left. This is similar to
deforestation in that there is now nothing to
bind the soil or to prevent the water droplets
form hitting so forcefully. Also, it means that
there are no nutrients for the animals to eat
and no nutrients available to be put in to the
soil
• Over cultivation
Farmers in the sahel are growing too many crops on
their land. In the short term, this is good as it
provides everyone with food. However, it slowly
uses up all the nutrients from the soil, making it
useless. Also, there is nothing binding the soil and
nothing preventing soil erosion. The fact that in
the Sahel it’s mainly the same crops being grown
in one area year after year doesn’t help either, as
it is always the same nutrients being used up.
The soil becomes useless and results in
desertification.
Management Techniques
Strip Cultivation
Strip cultivation is having alternative
crops in the same area. By doing
this, different nutrients are being
used as well as replaced back in to
the soil. This will prevent soil
erosion and will also keep the soil
healthy.
Terracing
Terracing is where artificial steps are
created to catch any water or soil in
the ‘bunds’. This will definitely
prevent soil erosion as it will stop
the water from logging the soil up
and taking the nutrients.
Contour farming
Contour farming is when farmers
plant crops around a hil, instead of
up and down a hill. This will stop
soil erosion as firstly it will prevent
water from running the nutrients
down the hill and secondly it will
stop crops from growing crookedly
and competing for nutrients.
https://youtu.be/jI_nRHg-0l4
• Great green wall in Senegal, reasons for it etc
and issues.