Sahel Flashcards
What is land degradation?
It is linked to soil deterioration (when the soil gets poorer). cannot be used in the way it was previously used, or it cannot support the vegetation
What is desertification?
If erosion happens faster than soil formation then the soil begins to degrade. If a lot of soil is eroded the land cannot be farmed and it becomes a desert.
Where/what is the Sahel?
The Sahel runs parallel to the southern edge of the Sahara Desert from the west coast to the east coast of Africa. The average width of the Sahel is 500km. Its proximity to the Sahara is one of the principal reasons it is prone to desertification.
Arable Farming
A wide variety of crops(barley, sorghum) are grown with drought resistant crops. Trees are planted amongst the crops to provide protection from wind erosion and to provide a source of fuel for cooking. The reason this method of of subsistence is successful is due to the fact that farmers allow sections of their land fallow (empty) for up to 20 years which allows the soil to rejuvenate. Also, farmers grow a variety of crops which means not all the same nutrients are being leached (taken) from the soil. If nutrients are continually removed from the soil the structure of the soil will be destroyed and will leave the soil more susceptible to wind and water erosion.
Pastoral Farming
Livestock rearing of cattle, goats and sheep is also practised in the Sahel. The majority of farmers are nomadic which means they move from one area to another following the rains. The success of this farming depends on:
Avoiding overgrazing in an area, which leaves the soil vulnerable to erosion, as the vegetation no longer offers any protection
Finding sources of water for animals to drink (wells, rivers, lakes)
The ITCZ providing sufficient rainfall for vegetation to grow in the area and so maintaining herd sizes at a level that is sustainable
Rain splash
Rain splash is the impact of raindrops on the surface of a soil which is usually the first stage of erosion. The bigger the raindrop, the faster it hits the soil and the bigger the splash causes the soil to be moved further. The sediment blocks soil pores which results in surface crusting and lower infiltration. Gravity causes more down rather than Up.
Sheet wash
The removal of a thin layer of surface of a soil that has already been disturbed by rain splash. The tiny soil particles which are displaced during rainsplash find their way into soil pore spaces located between soil crumbs and clog them up. The result is the formation of a surface crust (1mm thick), but the rate at which rainwater can now infiltrate the soil is now reduced. It only flows for a few metres before concentrating into rills. Usually it results in loss of the finest soil particles (containing nutrients and organic).
Rill erosion
is the creation of very small (up to ~30cm) eroded channels across a soil surface. These are the most serious forms of soil erosion by running water. Most land surfaces are irregular, with natural depressions, slopes and channels. A sudden burst of rainwater, which is unable to soak into the soil, will flow over the surface, finding its way into natural channels. Here it will gather in volume, power and speed as it makes its way down slope, dislodging and moving soil particles down slope as it goes. These can develop into gullies or might be wiped out by the next rainstorm.
Gully erosion
is the creation of large gullies (metres deep) by large quantities of water flowing over the soil surface. Gullies are steep sided water channels which carry water only during rainstorms and, unlike rills, these are usually permanent. Water flowing through gullies further erodes the soil by undercutting soil and then slumping.
These factors would make land vulnerable to wind erosion:
Soil which is loose, dry and finely grained
A relatively flat land surface
Little or no vegetation
Large fields
Strong winds
The wind erodes the land by:
Surface creep – the slow movement of larger and heavier particles across the surface.
Saltation – the bouncing along of lighter particles.
Suspension – the lightest particles (dust) blown off ground for up to several hundred kilometres, (can produce dust storms).
Wind erosion of soil therefore has the following effects:
Loss of topsoil in one area
Deposition of material in another area where it can:
form topsoil
clog waterways
cause air quality problems (dust)
Stunted growth of trees and plants -wind “pruning”
Physical damage - especially if carrying sand particles (tall plants may be flattened)
Transporting pests
All of these essentially lead to a reduction of crop yields and/or quality.
Physical Causes
Regions such as Sahel have unpredictable climates – they can experience flash floods and drought conditions.
Overgrazing destroys the vegetation cover, leading to soil erosion and poor irrigation methods lead to evaporation of stagnant water, leaving a salty infertile crust.
With little rain, plants can’t grow so there are no roots to hold the soil together.
The soil dries out and is washed or blown away easily.
This leads to desertification as roots can no longer anchor the soil which in turn is easily blown away.
Flash floods also wash away the exposed topsoil.
Water shortages caused by major drought can lead to a drop in the water table meaning many plants die as their roots cannot reach the water.
The concentration of livestock in these areas results in vegetation being stripped down to its roots leaving the soil exposed.
Deforestation removes the binding effect of tree roots and prevents trees from reducing wind speed.
Human Causes
Many farmers moved away from traditional farming techniques and grew cash crops, such as cotton and rice. Monoculture has led to the soil becoming infertile in some areas.
Increased population has led farmers to use more of the land for arable farming, but this is not viable for sustained crop production and increases the rate of desertification.
In many regions of Africa the birth rate is high and there is significant population growth. This has placed increased pressure on countries to increase food production.
In some countries like Burkina Faso and Mali, the population growth is twice the rate of increase in food production.
The increased need for timber in construction and for firewood has led to widespread deforestation.
The shortage of firewood means families have to use more animal dung and crop residue for their domestic energy requirements. In the long term this results in lower crop yields.
Frequent water shortages lead to the need for more wells, lowering the water table further.
The increased demand for food results in farming on marginal land and also farmers decreasing the fallow period. This means the land does not have time to recover or regenerate.
Environmental Consequences of Land Degradation
The soil structure deteriorates as the wind blows it away, causing the advance of the Sahara Desert through the process of desertification.
Loss of fertile topsoil leading to the inability of the land to support vegetation.
In the last 50 years, 65 million hectares of the Sahel have turned into desert.
