Leaflet Series: C
Number: 2
Universities:
Agricultural University of Athens
Tradicional and new soil conservation
and cultivation structures
Costas Kosmas, Nicholas Yassoglou,
Aikaterini Kounalaki, Orestis Kairis
Figure 1
Typical example of terracing of a sloping land
in the Mediterranean Europe
Introduction
Land terracing is one of the main soil conservation and
cultivation techniques for combating land degradation
and desertification. It is a practice applied to prevent
excessive rainfall runoff on sloping land causing serious
problems of soil erosion.
Land terracing is the construction of relatively flat
surfaces of reasonable size to allow cultivation of sloping
areas. It is accomplished by removing strips of soil parallel
to the contour lines and accumulating the removed
material over the soil surface just below the trench,
transforming the natural slope into a stair-like man-made
environment. Terraces, usually allow better management
of soil and water, improve access to land and facilitate
farming and tillage operations.
Agricultural terraces are among the most distinctive
features of hilly and mountainous landscapes. Terraced
hillsides are scattered across Africa, particularly in Ethiopia.
They are very impressive monumentally in the Peruvian
Andes, and climb vertiginous slopes in the Himalayas. In
China, Japan, and south-east Asia irrigated mountain riceterraces are prodigies of hydraulic engineering.
Terraces extend right across Mediterranean Europe.
They extend northwards into Germany, usually for growing
vines. The Douro valley in north Portugal is highly terraced,
but not in southern Portugal. In the Alpujarra region,
terracing is a fine art, but in most of the rest of Spain it is
uncommon. In Sardinia terracing is very local, perhaps
because the rights of ownership discouraged individuals
from putting effort into farming the land. The main
terraced areas in Sardinia are in the densely popu-lated,
olive-growing vicinity of Bosa, and Barbarian. The eastern
Pyrenees, Provence, Liguria and Croatia, Crete, Aegean
islands, and Majorca are also highly terraced areas.
Although terraces are still a “living tradition”, the art
of terracing is nearly as old as farming itself and was
considered a vital part of agriculture for many ancient
civilizations. Terracing of land is a very old technique for
cultivation and protection of hilly areas. Many scholars
have identified terraces from various ancient periods but
sometimes with insufficient evidence. Van Andel visualized soil erosion in the southern Argolis (Greece) as having
been controlled by terracing in the late Bronze Age, but
the Mycenaean terrace walls are not yet dated with
confidence.
Hilly areas under olive groves (with slopes usually
greater than 6%) on the Greek island of Lesvos were
terraced using stones several hundred years ago.
Individual crescent-like terraces for individual trees or
linear terraces along the contour lines have been carefully
constructed (Figure 2). The length of the stone walls of
these terraces is estimated to 45,000 kilometres. Similar
terraces can be found in other parts of Europe, such as in
Tuscany where the same crescent-like terraces are a
common feature.
In the last 6 decades, land leveling and terracing has
taken on an important role in European agriculture. In
many countries such as Italy, Hungary, Portugal, Spain, and
Greece, land leveling and terracing operations have
concentrated on mechanized cultivation of vineyards or
olive groves. The acreage of rainfed crops such as almonds,
vines and olives has been expanded rapidly even in
marginal areas, encouraged by policies stimulated by the
EU Common Agricultural Policy (CAP). This has led to
increase of land leveling and terracing in hilly areas to
reduce slope gradient, and reshaping of land for modern
mechanized plantations.
Figure 2
Individual crescent-like terraces for individual trees
constructed for planting olive trees several hundred
years ago in the island of Lesvos
permits more intensive cropping than would otherwise be
possible. Soil erosion is one of the most important
processes of land degradation and desertification in hilly
Mediterranean areas. Soil erosion includes not only the
removal of soil particles, but also the loss of organic matter
and plant nutrients. It reduces the productivity of the
agricultural land, causes stream pollution and reduces
water storage of surface and ground reservoirs. Terracing
of the land is considered as an important measure for
reduction of soil erosion in hilly areas. The main purposes
of terracing land can be summarized as follows:
– Redistribution of soil material in sloping areas with
shallow or moderate soil depth
– Increasing the depth of soil for plant roots to allow
better absorption of plant nutrients and water
– Making sloping land less steep, improving access
and facilitating farm operations
– Clearing a field of stones which can interfere with
cultivation and damage farm machinery
– Decreasing surface runoff and increasing absorption
of water by the soil in heavy rainfall events
– Controlling soil erosion in sloping areas.
Reasons for terracing LAND
The major reason and benefit of terracing is the
conservation of soil and water. Terraces reduce both the
amount and velocity of water moving across the soil
surface, which greatly reduces soil erosion. Terracing thus
Land terracing is usually integrated with dam building
as the terraces are designed to improve the flow of water
into the dam catchment area. Terracing also prevent soil
silting behind the dam, which would reduce the effectiveness of the dam. Appropriate design of terraces promote
Figure 3
Example of terracing land for soil and water
conservation in sloping areas
water conservation by retarding runoff water flow and
allowing it to infiltrate into the soil (figure 3). Modern
practice in terracing, however, consists of the construction
of low-graded channels or levees to: (a) allow more time
for infiltration into the soil, and (b) carry the excess rainfall
from the land at non-erosive velocities.
Land terracing is usually integrated with dam building
as the terraces are designed to improve the flow of water
into the dam catchment area. Terracing also prevent soil
silting behind the dam, which would reduce the
effectiveness of the dam. Appropriate design of terraces
promote water conservation by retarding runoff water
flow and allowing it to infiltrate into the soil (figure 3).
Modern practice in terracing, however, consists of the
construction of low-graded channels or levees to: (a) allow
more time for infiltration into the soil, and (b) carry the
excess rainfall from the land at non-erosive velocities.
Description of the ancient and modern terrace
structures
Techniques of terrace construction have changed
through the centuries. In the earliest examples, the land
was shaped into a series of nearly level benches or step-like
formations bounded on the lower side by an almost
vertical bank and usually protected by a stone wall. These
structures were narrow and step-sided so that cultivation
with conventional farm implements was difficult or
impossible. Modern practice in terracing, however, consists of the construction of low-graded channels or levees
to carry the excess rainfall from the land at non-erosive
velocities. These structures are wide enough to be cultivated, seeded, and harvested with ordinary machinery.
Terraces can be distinguished in the following forms:
– Bench or step terraces, which are parallel, either
straight or more usually curving round the contours.
– Braided terraces, which zigzag up the slope, being
joined by switchbacks at the ends, so that animals
and ploughs can get up without climbing.
– Pocket terraces, which are crescent-shaped, walls
providing roothold for individual olives, chestnuts
or fruit-trees.
– Broad-base terraces or terraced fields, which are
square fields in which one end is built up above the
hillside and the other end sunk in.
– Alluvial terraces on the floors of valleys or in the river
beds are much used as proxy evidence for erosion.
The modern bench terraces are constructed by moving
the soil laterally or both laterally and longitudinally using
heavy earth movement machinery such as a bulldozer.
Pocket or braided terraces are mainly traditional structures
which can be constructed in areas with shallow soil
overlying a consolidated bedrock. Broad-base terraces are
primarily constructed for conserving water, where rainfall
is limited and/or where the soil has sufficient rate of
infiltration so that runoff water will not overtop the terrace
ridge. Alluvial terraces are constructed on the floors of
valleys or in the river beds for reducing erosion and recording the accumulation or removal of sediments by the river.
Figure 4
Bench terrace construction characteristics
The vertical interval between two adjacent terraces
(Figure 4) is related mainly to rainfall, slope gradient and
type of crop. The best vertical interval (VI in meters) can be
estimated by the formula given by the U.S. Soil Conservation Service:
VI = xS + y
Where x is rainfall factor, S is slope gradient (%), and y
is soil and cropping factor. The U.S. Soil conservation
Service recommends values for x and y 0.12-0.24, and 0.31.2, respectively. The horizontal interval (HI in meters) can
be calculated from the equation:
HI = (VI/S)*100
Modern terrace design And Construction
The main types of terrace constructed at present are
bench or broad-base terraces. The field to be terraced is
cleared of vegetation or boulders, the dead furrows filled
in, and small ridges leveled before construction is started.
The best interval between terraces is that which provides
farmable land as well as control of erosion. The more
permeable the soil, the less intense the rainfall, and the
more erosion-resisting the crops raised are, the wider the
safe terrace interval on a particular slope gradient can be.
Usually, the recommended space interval is not narrower
than 30 meter.
The horizontal interval on hand cultivated land can be
considerably narrower than would be used for mechanized
agriculture.
Managing terracED land
Throughout the Mediterranean Europe terraced land
is mainly cultivated with cereals, vegetables, vines and
olives. Olives, vines and cereals can be mixed on the same
terrace. Pocket terraces are associated with olives and
other orchard trees. Well-built, levelled terraces are often
cultivated with vines or olives. Poorly built, braided
terraces, especially in remote places and on shallow soils,
are often cultivated with annual crops.
Terrace cultivation was adapted for the power of men
and beasts, or for small machines, mechanical donkeys
that served farmers during the mid twentieth century.
Today many of the traditional constructed bench or pocket
terraces are not cultivated. It is nearly impossible to use
agricultural machinery on these lands, not due to lack of
productivity and quality of products, but due to the
difficulties facing mechanized cultivation.
In modern terraced fields crop cultivation is fully
mechanized. In such terraced fields all farm operations
should carried out as nearly as parallel to the terrace as
possible to minimize water and soil movement between
terraces and to reduce damage to the terrace ridges. The
most evident effect of tillage operations, after several
years, is the increase in the base width of the terrace. The
best method of maintaining the shape of the terrace cross
section and counteracting erosion from the inter-terraced
area is by ploughing with a reversible mouldboard.
Many of the traditional terraced areas around the
Mediterranean have been abandoned, especially those
cultivated with cereals, and in some cases with olives and
vines. In the last few decades, agricultural production has
been concentrated on gentle terrain, where large farm
machinery can be used, and moderately high yields can be
obtained by irrigation and application of fertilizers and
pesticides, resulting in abandonment of terraced land in
hilly areas. The value of traditional terraces has markedly
declined due to:
– difficulties associated with accessibility and use of
machinery,
– decreasing price of agricultural products and increasing labour costs,
– high input agriculture developed on plain areas,
– high cost of maintenance,
– extensive migration of people from rural to urban
areas.
Future perspectives and recommendations
Land terracing is a human intervention in sloping
semi-natural landscapes, which have suffered losses, to
some degree, in their sustainability and resilience. If
sloping areas are not protected by erosion measures,
continuing exploitation of these landscapes will inevitably
result in high degradation and desertification. The most
effective measure would be their sufficient or suitable revegetation. This method, however, does not always secure
satisfactory economic returns to the users today. In
contrast, land terracing proved to be more profitable on
many occasions in the past.
Constructing terraces on cultivated sloping land
creates unstable landscapes, which require energy and
financial inputs to maintain their productivity. Under the
present socio-economic conditions these inputs may or
may not satisfy the expectations of the investors. In cases
where the results are not satisfactory, the maintenance of
old and new terraces may be interrupted while the
exploitation continues, resulting in severe land
degradation and desertification.
Under present
circumstances someone must decide on maintaining or
constructing terraces based on serious feasibility studies.
Among the parameters and conditions that might be
considered are the following:
– The range of the slope gradient suitable for land
terracing
– The existing soil erosion risk
– The land terrain characteristics
– The minimum soil depth and the presence of
limiting subsurface soil horizons bedrock, other
irreversibly impermeable formations
– The land capability and soil suitability for specific
land uses
– The magnitude of environmental impacts and
benefits
– The damage caused to the landscape by farm
machinery
– The compatibility with existing policies (e.g. CAP)
– The impact on water resources
– The impact on the lowlands from soil erosion and
flooding
– The socio-economic characteristics and infrastructural conditions
– The expected cost benefit ratio
– The agreement and the commitment of the
stakeholders.
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)