Leaflet Series: B
Number: 1
Research Institutes:
Foundation for Sustainable
Development, Netherlands
3D-Environmental Change, Spain
Soil erosion
Anton Imeson, Michiel Curfs
It is very easy and simple to prevent erosion and yet at
least 75 per cent of the world's farmed soils have been
degraded or affected by it. In the Northern Mediterranean
region, this percentage is even higher. Because land has
been degraded by erosion in the past this doesn't mean it is
threatened today. The greatest amount of erosion is
occurring now on agricultural and afforested land. When
erosion occurs, it can also cause flooding and off-site
damage on flood plains. Several different types of erosion
occur in the Mediterranean region and these include
splash and sheet erosion, rill, gully, tunnel and channel
erosion wind erosion as well as erosion caused by animals
and land use activities. Soil climate and culture conditions
create circumstances in which erosion is endemic.
Specific factors that influence erosion are the high
energy and amount of rainfall, the often low capacity of
soil to resist erosion, the short growing season and the
limited amount of the ground covered and protected by
vegetation, the slope characteristics and management
practices. In practice erosion is very challenging to study
because the conditions influencing it are dynamic and
always changing. It is both complicated to measure and to
model.
How seriously should we be concerned about soil
erosion in the Mediterranean? What are the consequences
for soceity and what are the predictions for erosion in
relation to climate change? Erosion is rather easy to
control or prevent. The principles of soil conservation and
protection have been understood probably for millennia
so that when erosion problems occur in the Mediterranean, it is a simple matter to take appropriate action to
stop it. In fact, erosion would be less of an issue if
resources and laws would be enacted to create the
institutions needed to manage and monitor it, such as a
soil, land and water conservation Service.
Erosion in the Mediterranean
Almost everywhere “natural” geological processes of
weathering and erosion have been altered by human
activities. These changes may increase or decrease
different types of erosion impacts according to the
circumstances. Effects of erosion processes can reveal
themself in many ways. Sometimes the visible effects are
really small and sometimes very big.
Types of erosion
Distinctions that can be made are in the agents of the
erosion processes which are mainly Wind and Water
erosion. There is also erosion that is induced by animals
and land use activities which can generally be described as
forms of accelerated erosion.
Wind-erosion is related to the strength of the wind
that exerts a force on the soil that affects particles of a
specific size (coarse silt and sand). When these are not
bound together by organic matter, roots or clay or
moistsure, they are easily transported.
There are several distinct erosion processes related to
water-erosion of which the most common are described in
the textbox.
Types of water-erosion
Splash erosion is the detachment and airborne
movement of small soil particles caused by the impact
of raindrops on soils
Sheet erosion is the removal of a relatively uniform
thin layer of soil from the land surface by rainfall and
surface runoff.
Rill erosion is an erosion process on sloping fields in
which numerous and randomly occurring small
channels of only several decimetres in depth are
formed; occurs mainly on recently cultivated soils on
badlands and roadside embankments.
Gully erosion is the erosion process whereby runoff
is concentrated above or below ground and erodes
narrow channels. In a short space of time, the soil may
be lost to a considerable depth and farmers are unable
to work the land e with ordinary farm equipment.
Typically gulleys ranging from 0.5m to as much as 25 to
30m. Gully erosion usually occurs in specific types of soil
or sediment.
Tunnel erosion occurs in soils that are prone to
piping. These are often soils that contain clay-rich layers
that shrink and swell when moistened or layers that
have soils that disperse spontaneously in water during
rainfall.
For the creation of rills and gullys a threshold needs to
be surpassed which is related to the force exerted by
flowing water and the resistance of the soil to erosion. Rills
and gullys often occur in agricultural fields where there is
subsoil compaction and the top soil has a low coherence.
High erosion rates everywhere, coincide with
the advent of modern agricultural practices,
particularly the use of bulldozers for land levelling
They also occur along road embankments and in
badlands. Gullys that are found along river channels, are
often triggered by the downcutting of rivers.
Badlands are areas where erosion processes
dominate the surface. All erosion processes -splash,
sheet, rill, gully and tunnel erosion- as well as landslides
are very prominent. Landslides can occur because the
vegetation on the soils and rocks, has difficulty in
establishing itself.
Table 1 shows the different types of gullys found in the
Mediterranean area and how they are related to different
processes.
Soil erosion scientists consider water erosion to be a
consequence of the following factors:
a) the energy of the rain (which increases with rainfall
intensity),
b) the strength of the soil (which is influenced by organic matter, soil chemistry, and the size of soil particles;
c) the slope gradient and length;
d) the vegetation and ground cover;
e) management practices.
Evaluating how these factors influence erosion has
been the approach of the models that scientists developed
to predict erosion. These show that by far the quantitatively most important factor is the ground and surface
cover of vegetation and stones.
Soilmoisture management in the form of Mulching
in orchards where the soil is kept free of weeds. Changes in
climate and the use of pesticides mean that in soils there
are less organic substances that hold soil particles together.
These would help the soil to retain water which infiltrates
deep into the soil, stimulating plant growth. Higher
evaporation rates, fertilizers and irrigation mean that there
tends to be more salt in the top soil, which increases its
sensitivity to water erosion. In such cases even low intensity
rainfall causes erosion. Research identified a threshold of
below about 350 mm below which salt dominated soil
behaviour and erosion rates could be very high. In Spain
the highest areas along climatological gradients were
vulnerable because of the short growing season and the
negative effect of this on soil structure stability.
Key findings on erosion
Fire and erosion
The most serious effects of erosion are downstream
on flood planes where runoff transports eroded sediment
into river channels which reduces the water carrying
capacity of rivers. So, runoff increases the occurence and
magnitude of floods in riverplanes, leaving bigger areas
under the threat of flood risk.
The fertility of the soil is affected by erosion.
The transport and deposition of soil particles can
create a major hazard, because during deposition, fine and
coarse particles are separated by a process of sorting. This
means that the organic matter and clay become
concentrated at the locations where they accumulate.
These are the particles that contain nearly all nutrients and
toxic substances. They therefore become hotspots and
sometimes cause contamination.
Maximum erosion is related to human pressure. High
erosion rates everywhere, coincide with the advent of
modern agricultural practices, particularly the use of bulldozers for land levelling. Today erosion rates on bulldozed
land can be as high as 20-30 cm in a single storm lasting a
few hours.
The relationships between erosion, fire and
desertification are paradoxical in the Mediterranean. In
1994 a workshop was organised that reviewed all of
the then current research findings. The unexpected
conclusion, which still holds today, was that, except under
special circumstances, fire tends to improve the ability
of soil to retain water and increases its fertility. Only
where soil temperatures exceeded a critical value of about
400-600°C did soil erosion increase.
If fires themselves do not result in erosion, why then
does erosion often increases following a fire?
Forests fires and wildfires generally do not cause
erosion in themselves, so there is no need to panic. The
main problems are the post fire activities of the
community, who are extracting a living from the land
and exploiting the situation. Preventing erosion could
largely be achieved by keeping people off the land and
allowing nature to recover, which generally takes three
to four years.
Grazing and erosion
Climate change, land management and erosion
The frequency of soil erosion events has increased in
the Mediterranean region, because there are more heavy
prolonged storms associated with the higher temperatures. The greatest risk to erosion is on arable farmland or
Animals can make the soil compact, trample and
displace rocks and stones downslope and consume or
destroy vegetation that is protecting the soil. But animals
can also have positive impacts that reduce runoff, for
example by adding nutrients that promote plant growth.
Cows in the Alentejo causing erosion
Precise understanding of the impacts is very difficult
because the interactions and feedbacks between plants,
animals and erosion occur over tens or even hundreds of
years; they involve the use of fire as a means of improving
plant palatability and changes in management practice.
The concept of overgrazing means that there is more
livestock than the carrying capacity of the land warrants
so eventually the land will be damaged. Research has
shown that, where cattle range freely, the cow is perhaps
the most important agent of erosion in the world today.
Soil, land and water Conservation
The soil and its properties can not be isolated from the
landscape in which it developed nor from the people and
lifeforms that are and were responsible for it. To succeed,
therefore soil conservation approaches must have
concepts that include both space and time and which
honestly addresses the main drivers of erosion which are
both physical and cultural. It requires policies that treat the
land, soil and water and its use by people as an entity for
which society is ethically responsible.
Adaptive management considers adaptive cycles of
accumulation and release. These cycles describe how
things accumulate, reach a critical threshold that may then
collapse, freeing materials to be exploited by other
processes. This happens at different scales. Erosion and
conservation can be considered at scales ranging from
thousands of years to scales of just a few hours and all of
these can be managed appropriately.
Many soil erosion phenomena show characteristics of
adaptive cycles. In hillsope hollows and in channels, soil or
sediment can accumulate, reach a critical thickness and
then erosion can suddenly occur, leaving a gully.
Soil erosion occurs mainly because the soil loses its
ability to absorb and store water. The soil structure should
have the capacity to resist erosion and transport by wind
and water.
Development of soil structure
In the top soil, biological, chemical and
climatological processes result in the transformation
of bedrock into a material that is composed of
agglomerated particles, separated by voids. Soil
structure develops over time. Agglomerated particles
become water stable as a result of organic substances,
fungi and roots binding them together. When soil
particles are moistened they can break down into finer
particles by slaking and dispersion, which can lead to
the formation of a soil surface crust. These processes
involve the chemistry of the rainfall and soil moisture.
Small amounts of water-soluble salts can trigger
swelling and dispersion. This results in soils that are very
hard when dry but soft and weak when wet. Such soils
are prone to piping, rill and gully erosion.
Accelerated soil erosion is a consequence of present
and past land management which reduces the capacity to
provide ecosystem services. This is not always obvious
because erosion processes are not always identified by
decision makers as being the cause of the impacts that
they cause
Soil erosion is a major factor in food security, climate
change and flooding
Soil, water and land conservation and its management would not only prevent erosion, it would also go a
long way to solving climate change and poverty reduction.
Addressing erosion problems, just as desertification
requires an interdisciplinary approach and the involvement of society. The adaptive management framework is
recommended for this.
Soil erosion should be managed within an adaptive
management framework of soil, water and land
conservation that looks at long, medium and short time
scales. It should assess how culture and its impacts are
affecting the regulation of the hydrological and
production services.
As mentioned, most of the erosion processes are easy
to prevent or control if the processes are understood.
What you can do:
Conclusions
European research during the last ten years has
greatly increased the understanding of erosion processes.
The human resources and knowledge are available that
society could use to manage erosion issues if laws were
enacted and institutions built with a political good will
and mandate.
The nature and extent of erosion in Europe is well
known. The sensitivity of landscapes to erosion is a
question of geology, geomorphology and climate.
Whether accelerated erosion occurs or not is a question of
land use and land management.
The main erosion processes are sheet erosion, rill ,
tunnel; gully, tillage and wind erosion. Preventing these
requires different strategies and methods.
Climate change is affecting erosion in many ways. The
maximum erosion rate occurs in areas independent of the
amount of rainfall. The sensitivity, to erosion is influenced
by rock type, growing season and human pressure.
At the local scale, it is simple to monitor erosion
susceptibility, using soil stability and other indicators. A
soil stability kit can be used to identify protential erosion
risk and to manage the impact of actions.
Let the rain stay where it falls. Keep runoff on the slope
by e.g. placing stones downslope of your plants,
increasing surface roughness
Avoid bare soil, cover soil with vegetation, mulch or
stones, and shade. Promote life in the soil.
Maintain a good healthy soil structure and conserve
soil moisture. Avoid poisoning soil organisms, the habitats
and chemicals that they create in the soil regulate the
hydrology and prevent erosion.
Avoid unnecessary compaction and disturbance of the
soil structure
Consult your soil, water and land conservation officer
or ask the responsible authority to provide you with one
Contributing projects:
The material for this leaflet was mostly drawn from DESERTLINKS
Combating desertification in Mediterranean Europe: linking science
with stakeholders (EVK2-CT2001-00109)
UNIVERSIDADE LUSÓFONA
de Humanidades e Tecnologias
Humani nihil alienum
However, it also builds on work from MEDALUS Mediterranean Desertification and Land Use
(MEDALUS I – EPOC-CT90-0114; MEDALUS II – EV5V-CT92-0128/0164/0165/0166;
MEDALUS III – ENV4-CT95-0115/0118/0119/0121)