Soil erosion
Erosion is the wearing away of the land surface by water and wind,
primarily due to inappropriate land management, deforestation,
overgrazing, forest fires and construction activities. In simple terms,
erosion involves three basic steps that are common to both water
and wind erosion: the detachment of soil particles by the impact
energy of rain or wind, their subsequent transport and deposition in
a new location. Erosion rates (usually measured in kg/ha/year) are
very sensitive to soil texture and moisture, vegetation cover, land
use, slope and climate (especially rainfall patterns, wind direction
and strength) as well as to soil conservation practices at field level.
Soil erosion is a natural process but can be accelerated by human
activities such as inappropriate cultivation practices, land clearance,
overgrazing, construction on, or undercutting of, steep slopes and
excessive footpath or vehicle use. Erosion has an impact on soil
fertility due to disrupted nutrient cycles and in severe cases can lead
to a total loss of the soil body. Erosion affects land use and land
value and has negative effects on habitats and biodiversity.
Soil erosion has substantial off-site consequences as well. The soil
removed by runoff, for example during a large storm, will create
mudflows and accumulate below the eroded areas, in severe cases
blocking roadways or drainage channels and inundating buildings. By
removing the most fertile topsoil, erosion reduces soil productivity
and, where soils are shallow, may lead to an irreversible loss of
the entire soil body. Where soils are deep, the loss of topsoil may
often not be conspicuous. Nevertheless, continued erosion may
be potentially very damaging if no corrective action is taken (see
Preventing Soil Erosion box on this page).
Soil erosion by water
Soil erosion by water is one of the main factors limiting soil
productivity and impeding agricultural enterprise in Africa,
especially in the humid tropical regions where population pressure,
deforestation and high rainfall, often in the form of torrential
downpours, can lead to annual soil losses in excess of 50 t ha−1.
Above: A stunning photograph of both sheetwash and rill erosion
on a maize field in Kenya. The thin sheet of water flowing over the
soil surface collects particles on the way. This is evident by the
red colour of the water in this picture. Where the flow becomes
concentrated, small channels or rills occur. Fine particles (i.e. clay
or silt ) can be carried a great distance before deposition. As most
organic matter in mineral soils occurs in the upper part of the
soil, the removal of topsoil can have a significant impact on soil
fertility and water holding capacity. (BO)
Below: Gullies and bare ground on a hill slope in Kenya – clear
signs of soil erosion by water. (BO)
Preventing soil erosion requires a combination of political, economic
and technical considerations.
Politicians and decision makers need to address how soil is used on
a national level and to identify areas that are at risk or vulnerable to
erosion. Schemes could be established to offer incentives to farmers
to manage their soil in a sustainable manner and implement practical
measures to control soil erosion.
Technically, such measures could include:
Three types of wind erosion are recognised:
•the use of contour ploughing (i.e. cultivating the land so that the
plough furrows run at right angles to the slope) to restrict the
initiation of rills and gullies;
• Suspension: Fine particles less than 0.1 mm in size are moved
parallel to the surface then upward into the atmosphere by
strong winds, returning to the ground only when the wind
subsides or with precipitation. Suspended particles can travel
thousands of kilometres.
•planting of hedges or tree belts that act as shelter belts and to
reduce ground wind speeds;
•the use of terraces on sloping land;
• Saltation: Particles from 0.1 to 0.5 mm bounce short distances
along the surface, dislodging additional particles with each
impact. The bouncing particles, ranging in sizes, usually remain
within 30 cm of the surface. This process accounts for 50 to
90% of the total movement of soil by wind.
•minimising the removal of natural vegetation, especially on slopes
or even on flat terrain if wind erosion is an issue;
•strip cultivation - leaving unploughed vegetated strips between
tilled land or allowing buffer zones of indigenous plants to grow
along river banks;
• Soil Creep: Larger soil particles roll and slide along the ground
surface, often aided by impacts of saltating particles.
The Sirocco (or ghibli) is strong wind which causes dusty conditions
along the north African coast during the autumn and the spring. It
causes storms in the Mediterranean and cool wet weather in Europe.
Warm, dry, tropical air is pulled northward by low-pressure cells
that move eastwards across the Mediterranean Sea. Many people
attribute health problems to the Sirocco either because of the heat
and dust in Africa or because of the dampness in Europe. The fine
dust carried by the Sirocco winds can cause abrasion in mechanical
devices and enter buildings.
• Sheet Erosion: Removal of a thin layer of soil from a large area
which occurs when soil is saturated. Usually the first sign that
erosion is occurring. Also known as sheetwash.
Preventing soil erosion
Aeolian erosion occurs when strong winds blow across dry soil on
unprotected surfaces. As in water erosion, the wind detaches soil
particles from the surface. Once detached the particles are transported
to a new location. Wind erosion is particularly evident where annual
rainfall is below 600 mm and the dry season lasts more than six
months. The Sahel, the Mediterranean and parts of southern Africa
are particularly affected. It is a major source of land degradation, can
cause crop failure and has significant human health issues.
The Khamsin (or khamaseen) is a dry, hot and dusty local wind that
blows across North Africa from February to June. The Khamsin carries
great quantities of sediment from the deserts, with wind speeds in
excess of 100 km/h. Temperatures can rise by up to 20°C in a few
hours. The wind is triggered by depressions moving eastwards along
the southern parts of the Mediterranean.
• Splash Erosion: Direct movement of soil by the impact of
raindrops. Soil particles can be displaced by up to 1 m.
• Gully Erosion: Over time, rills can develop into large, wide
channels. As a rule of thumb, a gully is so large that it cannot be
smoothed out by conventional tillage.
Soil erosion by wind (aeolian erosion)
From November to March, a dry, southwesterly wind known as the
Harmattan, blows large amounts of dust from the Sahara. In some
West African countries, the amount of dust in the air can severely limit
visibility, lower air quality and disrupt air travel for several days. However,
the Harmattan can also be a significant source of soil nutrients.
Four types of water erosion are recognised:
• Rill Erosion: Increased speed of sheet erosion leads to the
formation of small channels or rills, often interconnected.
While there are no systematic assessments for Africa as a whole,
soil erosion by water is pronounced in Northern Africa, Madagascar
and South Africa.
Saharan winds
When raindrops hit the soil surface, their impact can shatter
aggregates into smaller pieces or displace individual particles
laterally by up to 1 m. If the rainfall rate exceeds the rate at
which water can infiltrate the soil, the excess water flows down
the slope (i.e. as run off ), carrying detached soil particles with it.
Some particles float into holes or gaps in the soil surface which
then become clogged and further reduce infiltration into the soil.
As surface water velocity reduces, the energy needed to continue
carrying the suspended sediment is lost causing the soil particles to
be deposited in a new location.
•ensuring adequate plant cover in rainy, dry or windy periods,
especially in the period immediately after ploughing;
Above: A satellite image of a dust storm over the coast of west Africa
carrying particles from the Sahara, out over the Atlantic then northwards
towards the Canary Islands. Dust from the Sahara is often deposited over
Europe and can affect water turbidity in the Caribbean. (NASA)
•making sure that organic matter levels are maintained. Organic
matter binds soil particles together and plays an important part in
preventing erosion;
•avoiding overgrazing and the over-cultivation of crop lands.
Below: A wall of dust indicates an approaching sandstorm - a relatively
common meteorological phenomenon in arid and semi-arid regions.
Strong winds blow loose sand and dust from dry surfaces. Soil organic
matter and nutrient-rich clay and silt particles are particularly susceptible.
The storm front in this striking image, taken near Niamey in Niger in June
2010, is several hundred metres high. (BH/IRD)
Erosion control on steep land in East Africa through the constrution
of terraces. In addition to providing land for cultivation, terraces
reduce surface runoff. (VL)
Issues affecting the soils of Africa | Soil Atlas of Africa
155