Introduction
This section deals with control measures for soil water erosion in response to weather
risks which are likely to affect crop production on sloping lands. About 70% to 80% of
small farmer production in the Caribbean is on sloping lands of varying steepness in a
system called hillside farming. In this system, increased rainfall intensities and
quantities associated with storms and hurricanes can increase soil erosion, landslides
and nutrient leaching on sloping lands, and flood of lowlands. Enhanced soil and water
conservation practices can reduce these risks. This section considers the practical
steps that farmers can take with regards to soil water erosion management.
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Understanding Soil Erosion
Soil erosion is the process by which soil is removed from one place by forces such as
wind, water, waves, and human activities (construction and farming) and eventually
deposited at some new place (Choudhury and Jansen 1997). In the process, soil
aggregates break down into smaller particles and are transported down the land slope.
The water which runs from a slope during a heavy rains is called runoff. This runoff
normally carries soil particles and nutrient elements along with it.
Accelerated soil erosion and landslides
In undisturbed conditions, a hillside will have natural erosion during the times of heavy
rainfall. This natural erosion occurs slowly and is balanced by the formation of new soil
material. In contrast, when the slope is disturbed for agricultural or other purposes, the
risk of erosion is greatly enhanced. The soil is exposed and erosion can happen very
quickly with large amounts of soil being removed. When this occurs, it can be a serious
threat to agricultural production and the environment. Erosion always has on-site
effects, i.e. consequences at the place from which the soil is removed and off-site
effects, i.e. consequences at the places which are affected by the transport of eroded
soil or where the removed soil is deposited.
Even more disastrous are landslides, where large portions of the slope may slip due to
disturbances on the hillside. Land slippages are normally related to specific soil types,
characterised by shrinking and swelling with changes in soil moisture. Landslides not
only pose a risk to agricultural production, but can also affect infrastructure and human
life.
Factors which influence soil erosion
Factors which affect soil erosion caused by water are: rainfall patterns, slope steepness
(gradient), slope length, soil type, existing erosion control structures, cropping practices,
and time. In general, the stronger the rainfall and the steeper the slope, the more
and the faster the rate of water erosion.
The Universal Soil-Loss Equation (USLE)
In order to understand when, and how much, erosion is likely to occur, the factors
mentioned above can be coordinated in such a way as to predict the extent of soil
erosion that can occur in an area on an annual basis. The Universal Soil-Loss Equation
(Wischmeier and Smith, 1962) is the most widely used method of predicting soil loss on
sloping lands. This equation is given by the expression:
E = R*K*L*S*C*P
Where:
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•
•
•
•
•
•
E is mean annual soil loss (T/AC/YR)
R is the rainfall erosivity index
K is the soil erodibility index
LS are the factors of slope length (L) and slope steepness (S) combined in a single
index
C is the crop factor/nature of plant cover
P is the conservation practice factor used to manipulate the LS factor
USLE is important in showing how erosion is related to the factors that cause it. In
effect, the risk of soil erosion occurring is decreased by minimizing of any one of these
factors. Moreover if any term in the equation is zero (0), then there is no erosion. For
example if there is no rain there will be no erosion. Similarly if the plant cover factor can
be reduced by having permanent plant cover as in perennial cropping or forest, the risk
of erosion is similarly diminished.
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Soil Erosion in the Caribbean
Gumbs (2001) points out that the mountainous nature of the topography, the erodibility
of the soils, slope steepness, rainfall, and crop and soil management practices can
cause excessive soil erosion, which is the major factor in the degradation of soils on
Caribbean hillsides . He points out that the major forms of erosion in the Caribbean are
sheet erosion, land slippage, and gully erosion. In Trinidad, Jamaica, and Barbados,
substantial areas of steeply sloping soils are developed on shales; on these soil types,
landslips are common. Other soil types such as the shoal soils of Grenada and some
location of volcanic soils in Dominica and St Lucia are also very erodible.
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Rainfall is another important factor in soil erosion in the Caribbean. Generally rainfall
quantities decrease from the Windward to the Leeward coasts and increase with
elevation. However it is not the total rainfall per se that is critical to soil erosion, but the
intensity of individual storms. Intensities of 25-127 mm per hour have been recorded in
the region. Studies in Trinidad (Lindsay and Gumbs, 1982) have shown that two major
storms (non-hurricane) of 49.7 and 75.5 mm per hour during the year accounted for
between 70% and 90% of the total annual soil loss depending on soil slope.The annual
soil loss in these trials varied from 28 to 55 tons per hectare.
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Principles and Practices of Soil Erosion Control
Principles of Soil Erosion Control
The principles utilized in erosion control can be direct and indirect.
Direct erosion control involves measures which aim at stopping, or slowing down,
erosion processes. In the case of soil erosion caused by water, it is based on the
following principles:
•
•
•
•
Protecting the soil surface from the forces of rain drops
Increasing water infiltration
Decreasing the speed (force) of runoff water
Intercepting the transported soil
Indirect erosion control is aimed at using land in ways that prevent and/or reduce the
risk of erosion occurring, i.e. land use planning. The various factors which affect soil
erosion caused by water are also influenced by one or more of the following
complementary measures:
•
•
•
•
Agronomic
Vegetative
Structural
Management
These must be manipulated in the field to ensure a positive contribution to soil and
water conservation.
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Erosion Control Practices
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Erosion Control Practices
Agronomic Practices
Agronomic measures are undertaken within the cropping area for crop production
purposes, and include practices such as: inter-cropping, contour cultivation, minimum
tillage, mulching, manuring, etc. Many of these practices have been mentioned already
in this training course, but on slope lands they have the added benefit of controlling soil
erosion in the event of high intensity rainfall. These agronomic practices for erosion
control:
•
•
•
•
•
•
Are particularly beneficial for use in annual crops as soil cover is not continuous;
Are repeated routinely each season or in a rotational sequence;
Are of short duration and not permanent;
Do not lead to changes in slope profile;
Are not zoned; and,
Are independent of slope
An example of agronomic control measures is contour cultivation. Carrying out
ploughing, planting, and cultivation on the contour can reduce soil loss from sloping land
by up to 50% compared with cultivation up-and-down the slope. The effectiveness of
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contour farming varies with slope steepness and slope length, and it is inadequate as
the sole conservation measure for lengths greater than 180 metres at 1 degree
steepness. The allowable length declines with increasing steepness to 30m at 5.5
degrees and 20m at 8.5 degrees. Moreover, the technique is only effective during
storms of low rainfall intensity. For higher rainfall intensities, practices such as contour
strip-cropping, contour ridging, etc., are recommended. Contour farming does not
increase the cost of cultivation and basically only requires that farmers know how to
layout contour lines using, for example, the “A” frame.
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Erosion Control Practices
Example: Pegging with the A-Frame
Objective: To learn to peg level contour lines using an A-frame
Materials: A-frame, 20 pegs or stones
Procedure:
Pegging with an A-frame requires at least two people.
1. Go to the centre of the field and mark it with a peg. Place one leg of the A-frame right
next to the peg.
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2. Hold this leg in place while slightly moving the other leg up and down the slope until
the spirit level is dead centre. Mark this point on the ground with a second peg.
3. Pivot the first leg around while holding the other leg at the second peg. Move the first
leg slightly up and down the slope until the spirit level is again dead centre. Again, mark
this point on the ground with a third peg.
4. Continue like this to the field boundary.
5. Return to the centre of the field where the very first peg was placed and move in the
opposite direction to the other end of the field following steps 2-3.
6. Run lines from the first centre peg to all pegs on both sides of the field.
Useful tips:
•
•
Every time, before using the A-frame, check its accuracy on a level floor.
If it is possible, carry out this exercise before any soil disturbance.
Adapted from Simpson (2009)
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Erosion Control Practices
Vegetative Practices
Vegetative measures involve the deliberate planting of trees, shrubs, grasses, etc., or
the retention of areas of natural vegetation (e.g. reforestation, contour hedge rows,
natural vegetative strips). These vegetative control measures:
•
•
•
•
•
Involve the use of perennial grasses/pasture legumes,shrubs or trees;
Are of long duration;
Often lead to a change in slope profile;
Are often zoned on the contour or at right angles to wind direction; and,
Are often spaced according to slope
An example of vegetative control measures is contour hedge rows. Contour hedge rows
are part of erosion measures referred to as living soil barriers. These barriers are
planted along the contour to trap or filter run-off and retain soil. Grass barriers, which
are strips of grass planted on the contour, have been shown to be effective for reducing
soil loss even on steep slopes. Vetiver, or Khus Khus (Vetiveria zizanioides), is the most
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commonly used grass. It remains in the contour strips and does not spread to become a
weed.
Contour hedge rows are also an agroforestry system where leguminous, fast-growing
species are planted along the contour to trap soil washed downhill. In some systems,
two rows of trees are planted one meter apart and the space between may be filled with
stones. As the trees grow, they provide a natural barrier to soil movement. The double
row hedges are planted four to six metres apart and the intervening land is stabilized for
cropping.
Between the hedge rows, the recommendation is to plant a combination of permanent,
semi-permanent, and annual crops. Crop combinations will vary according to local
conditions and preferences, but they should be chosen to enhance soil fertility and
maximize yields. The leguminous legume trees are pruned periodically to provide mulch
and green manure to the cropping area.
In the Blue Mountains of Jamaica, contour hedge rows of Calliandra calothyrsus have
been shown to reduce water runoff and soil erosion.
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Erosion Control Practices
Example: Planting Vetiver Contour Hedges
Objective: To learn how to use Vetiver grass for soil and water conservation and learn
correct planting techniques.
Materials: Vetiver planting material, hoe.
Procedure:
First decide where to establish a Vetiver hedge. If it is in a field with a contour ridge, the
hedge should be established in front of the drain, so that run-off water from the field will
meet the Vetiver barrier before entering the drain.
1. Plant the Vetiver slips very closely together at 8-10 cm spacing (less than one fist).
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2. After 3-4 weeks, check the newly established hedge for plants that have not started
shooting. They should be replaced immediately with new slips from the nursery because
a Vetiver hedge must be continuous with no gaps; otherwise it does not work very well.
3. Three to four years later, the hedge will be big and strong and all the soil trapped by
the hedge will be forming a strong and permanent terrace of fertile soil.
4. The terrace is formed by all the soil carried away from the field by run-off water,
which is now trapped by the Vetiver hedge.
Useful tips:
•
•
For checking rills and small gullies, plant small but dense Vetiver hedges across
where the water flows.
Try planting fruit trees in front of a Vetiver hedge, the moisture and soil trapped
there will provide an excellent environment for fruit trees.
Adapted from Simpson (2009)
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Erosion Control Practices
Structural Practices
Structural measures involve the construction of physical structures (e.g. graded banks
or bunds, contour stone lines, level bench terraces, artificial waterways, and drop
structures) which:
•
•
•
•
•
•
Lead to a change in slope profile
Are of long duration or permanent
Are carried out primarily to control runoff and erosion
Require substantial inputs of labour or money when first installed
Are zoned on the contour
Are spaced according to slope
An example of a structural control measure is the terrace. Terraces are earth
embankments or a ridge and channel constructed across the slope at a suitable spacing
and with an acceptable grade. They are built to: reduce erosion, reduce sediment
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content in runoff water, intercept and conduct surface runoff of a non-erosive velocity to
a stable outlet, prevent gully development, improve farmability, reduce flooding, and
increase soil moisture.
Terraces are classified into 3 main types:
•
•
•
Diversion Terraces are used to intercept overland flow on a hillside and channel
it across slope to a suitable outlet, e.g. a grass waterway or soak away to tile
drain, built at slight down-slope grade from contour. Diversion terraces are
classified in the following types:
o Magnum type: formed by taking soil from both sides of the embankment.
o Nicholas type: Formed by taking soil from upslope side of the
embankment only.
o Broad-based type: Bank and channel occupy width of 15 metres.
o Narrow-based type: Bank and channel occupy width of 3-4 metres.
Retention Terraces are level terraces used where water must be conserved by
storage on the hillside. Ground slope must be 4.5 degrees or less.
Bench Terraces are alternating series of shelves and risers used to cultivate
steep slopes. Risers are often faced with stones or concrete. Various
modifications are possible to permit inward-sloping shelves for greater water
storage, or protection on very steep slopes, or to allow cultivation of tree crops
and market -garden crops.
Other structural control measures that may be applicable to smallholder agricultural
producers in the Caribbean are wood barrier and stone lines. These methods depend
on the availability of the barrier material. In the interior of Guyana, land clearing
normally includes the felling of trees and the small logs from these trees are effective as
wooden barriers. Rocky soils are not commonplace in the Caribbean, but in specific
locations such as reclaimed bauxite lands in Jamaica and seriously eroded hillsides in
Belize, rocks are becoming a part of the landscape. Under these conditions the use of
rock lines as a means of erosion control can be effective.
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Erosion Control Practices
Example: Making Physical Contour Barriers with
Wood/Stone
Objective: To construct physical contour barriers with wood/stone.
Materials: Fork, spade, shovel
Procedure:
Having already marked out your contour line using an A-Frame:
Wood barriers
1. Collect all logs and brambles that may be available from land clearing or tree
trimming.
2. Lay this material along the contour lines and secure them in place by driving small
wooden pegs into the ground.
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Stone lines
Where there are plenty of stones in the field, they can be moved and arranged in lines
along the contour. Like wood barriers, stone lines are permeable.
Procedure:
1. Prepare a shallow drain or trench, 10-15cm deep and about 30-40cm wide, by
digging along the contour line.
2. Place larger stones at the bottom of the trench as a foundation.
3. Place smaller stones on top to a height of 25-30cm.
Useful tips:
Wood barriers and stone lines can be further improved by placing trash or planting
grasses just in front of them.
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Erosion Control Practices
Management Practices
Management
These measures arise from deliberate managerial decisions taken with the intention of
protecting land from erosion and improving production, or other issues (e.g. land use
changes, area closures, rotational grazing) which:
•
•
•
•
Involve a fundamental change in land use
Involve no agronomic and structural measures
Often result in improved vegetative cover
Often reduce the intensity of use
An example of a management control measure is the change in land uses, for example,
the re-forestation of parts of the Blue Mountains of Jamaica after coffee cultivation. The
soils of the Blue Mountains are developed from shale parent matter. The resultant soil is
very vulnerable to the risks of erosion when denuded. Abandoned coffee plantations
must therefore be re-planted as forest to ensure there in no widespread erosion on the
hillsides. One such project is the Mona Lions Club project, which coordinates
professional expertise to rescue and replant over 40 hectares of denuded coffee lands
in Jamaica's Blue Mountain region. This on-going multi-million dollar project is ensuring
stabilisation of fragile lands and restoring conditions conducive to regular weather and
rainfall patterns, a more balanced ecosystem, and vital water recovery, storage and
availability to the Kingston and St. Andrew regions. This project is being spearheaded
by members of the Club, in collaboration with the Environmental Foundation of Jamaica,
Forest Fund, and the Forestry Department of Jamaica.
Smallholder agricultural producers in the Caribbean have found combining trees with
livestock, or trees with livestock, and annual crops economically and sociologically
beneficial. Systems utilizing coconut (or citrus), pasture for cattle and/or sheep are
common (Gumbs, 2001). While farmers are reluctant to grow grass as barriers or as
pastures for soil conservation, they are more receptive to growing grass if livestock is
included in the system.
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Erosion Control Practices
Other Approaches
Measures that combine two or more agronomic, vegetative, structural, and
management measures (in conditions where one measure does not work effectively
without the other), e.g.:
•
•
•
Structural: an outward sloping terrace with
Vegetative: grass and trees planted on the riser with
Agronomic: crops grown on contour soil ridges
In effect, in most cases a combination of measures will be undertaken to control soil
water erosion on a farm.
Waterways
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On sloping lands, drainage should be part of the conservation practices of the farm. The
purpose of waterways in a conservation system is to convey runoff at a non-erosive
velocity to a suitable disposal point. A waterway must therefore be carefully designed.
Normally its dimensions must provide sufficient capacity to confine the peak runoff from
a storm with a ten-year return period.
Three types of waterways are used with conservation systems:
•
Diversion channels or cutoff drains
These are placed up-slope of farmland to intercept water running off the slope above
and divert it across the slope to a grass waterway or lined channel for safe transport
down the hillside. These drains may be placed at 10m to 30m intervals along the slope,
depending on the degree of slope and intensity of rainfall expected. These drains are
normally 50cm to 60cm wide and 40cm deep.
•
Terrace channels
These are similar to diversion channels and collect runoff from the inter-terraced areas
and convey it across the slope to a suitable waterway
•
Down slope waterways
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These are designed to transport the runoff from diversion and terrace channels down
slope to empty into the natural river system. These are natural waterways either lined or
unlined. The lining may be stone, concrete or grassed to ensure there is no erosion of
the waterway.
Stabilization structures
Stabilization structures play an important role in gully reclamation and gully erosion
control. Small dams, usually 0.4m to 2.0m in height, made from locally available
materials such as earth, wooden planks, brushwood, or loose rock, are built across
gullies to trap sediment and thereby reduce channel depth and slope. Stabilization
structures are also used to control erosion on steep slopes. They provide stability for a
short time until a dense vegetation cover has had a chance to grow.
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Table 1: Recommended Soil Conservation Practices in
Relation to Soil Slope in the Caribbean
(from Gumbs, 2001)
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