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< prev - next > Agriculture Irrigation soil erosion (Printable PDF)
The loss of valuable topsoil due to wind and water erosion in the tropical lands of Africa is one of
the greatest problems that the continent is facing, but many people do not realize it. Equally, the
need to conserve and make full use of scarce rainfall in the semi-arid areas is yet to be fully
Erosion is caused by the downward movement of water on sloping land, and can happen on slopes
as little as one half percent, or a 10 cm drop in a 20 metre length. The amount of erosion that
occurs depends on
o The quantity of water moving
o The speed of the water
o The state of the soil surface and the type of soil
The same factors affect the ability of the soil to absorb the water.
Therefore, these are the three factors that we need to influence in order to prevent erosion and
conserve water in the soil.
The quality of water is affected by the amount of rainfall that falls in a given time and by the area
over which it collects. We cannot influence the rainfall but we can reduce the area over which it
collects by making barriers of some sort across the slope.
The speed at which the water travels over the soil is affected by the slope of the land and by the
length of that slope. Again we cannot change the slope except by bench terracing the land, a
process, which takes a long time. We can, however, control the length of the slope by making
barriers at regular intervals.
The third factor is the state of the soil and soil type. We cannot change the soil type, although we
can improve it by composting etc. We can, however, influence the state of the surface by cultivation
and or the amount of vegetative cover that we leave or put on the land, but this can be discussed in
another article. Putting barriers across the slope at intervals, therefore, seems to be the easiest way
to reduce erosion and increase water retention.
The next questions are "what sort of barriers" and how "across the slope?" The barriers can be in the
form of a simple plough line, a line of planted vegetation, a wall called a "bund" or any other
available material.
In arid and semi-arid areas the need is to increase infiltration of water into the soil, which is
being cultivated, often by collecting and distributing water from outside the cultivated areas. In
this case the barriers may be permeable loose stone bunds or vegetation banks or "staggered"
earth bunds.
Whatever method of bunding is used it is very important that the bunds are constructed accurately
on the contour, or to a specific gradient. Depending on the type of bund constructed it is possible
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