|Special Public Works Programmes - SPWP - Anti-Erosion Ditches - Training Element and Technical Guide for SPWP Workers, Booklet No. 1 (ILO - UNDP, 84 p.)|
Erosion is the loss of topsoil, the part of the soil which we till by means of the daba or plough. It is in this layer of soil that the roots of plants establish themselves and here that they find the water and nutrients essential to their growth (minerals, trace elements). Such soil is said to be FERTILE, as it can be cultivated. Beneath this fertile layer lies a layer of unproductive or stony soil (the opposite of fertile soil).
The type of EROSION we will deal with here is that caused by water. Other types of erosion exist, in particular WIND EROSION, which is the result of the wind carrying away the topsoil.
Water-induced erosion is brought about by rainfall.
Rainwater falls onto the surface of the soil and follows one of three courses:
1. Part of it evaporates, the wind and warmth of the sun turning it into water vapour.
2. A certain amount of rainwater is absorbed by the soil.
3. The rest runs off over the surface of the soil.
It is this third course, the run-off of excess rainwater, which gives rise to erosion.
Drops of rain strike the soil, breaking up the particles of soil. Rivulets of rainwater running off over the soil wash these particles away.
Continuing its course, the water will carry away particles from the soil it runs over. The stronger the flow of run-off, the greater the quantity of soil washed away.
However, water does not run off everywhere in the same way or at the same rate. Therefore, in order to control erosion resulting from the run-off of rainwater over the soil, the exact causes of this erosion must be known.
An increased flow of rainfall run-off over soil may be due to:
the climate: heavy rain falling on soil which is already saturated will result in an increased flow.
the slope of the land: the rate of water run-off is greater over steep land than over gently sloping land.
the absence of vegetation: grass, trees and certain crops protect the soil from the impact of drops of water and slow down the flow of run-off; in addition, their roots help to hold the soil together.
the nature of the soil: clay soils are less permeable and lead to a greater flow of run-off water than that occurring over sandy soil where more water is absorbed.
More often than not, Man himself, without realising it, is responsible for erosion. There are several reasons for this:
- the clearing of forests for firewood;
- the overgrazing of pastureland;
- unsuitable farming techniques, such as ploughing in the same direction as the slope;
- the overworking of land through intensive cropping and reduced fallowing, i.e. the amount of time during which land is left uncultivated.