The surface of the Earth is subject to decay and deterioration due to geological pressures and frictions though they occur steadily and slowly.
This natural erosion of soil is accompanied by formation of new soils, as the nature always maintains the universal equilibrium and harmony. It is, thus, not a load, in true sense of the term, but a process of replacement.
Erosion caused by incessant rainfall is the result of the application of energy from two different sources, viz., heavy downpour and surface flow.
When showers fall heavily upon the Earth with high velocity, they disintegrate and detach soil particles and subsequently the surface flow of rainwater transports them into the high areas.
The high infiltration:
Of downpour into the upper crust of soil makes the major contribution to the transportation of fertile soils particularly on unprotected slopes during the period of storms.
Four forms of water erosion are described below:
1. Sheet erosion. This signifies an elimination of thin fertile film of soil from the large area. It usually occurs on the landscapes of gentle slopes.
The top fertile soil is ripped off every year and the loss is quite excessive yet it is imperceptible.
2. Rill erosion. Subsequent stage of sheet erosion is known as rill erosion in which fungi like rills begin to appear on the landscape.
The rills are usually smoothened out by the working of the farm implements.
But year after year, the rills slowly increase not only in numbers but also in their shape and size.
The growth of rills hampers the movement of farm implements, reduces the actual areas under cultivation and results in minimising the crop yields.
3. Gully erosion. It is the developed stage of rill erosion.
When rill erosion is overlooked, the rills get deepened and widened every year and begin to form gullies.
The tiny grooves develop into wider and deeper channels and grow in huge size.
They further get deepened and widened in every rainfall and if they are not controlled they destroy soils to a considerable extent.
4. Slip erosion. This is usually caused by hydraulic pressure exerted by moisture penetrating into the soils during the heavy rains.
Due to low permeability of soil they could percolate below the impermeable strata. Thus, a great mass of overlying soil on steep land comes down bodily. This cuts down the arable soils to a great extent.