Soil erosion by water is the result of rain detaching and transporting vulnerable soil, either directly by means of rainsplash or indirectly by rill and gully erosion. Soil erosion is a natural process, occurring over geological time, and indeed it is a process that is essential for soil formation in the first place. With respect to soil degradation, most concerns about erosion are related to accelerated erosion, where the natural rate has been significantly increased mostly by human activity.
In order for erosion to occur, three processes must take place: detachment, entrainment and transport. Erosion also requires a medium to move material. Wind, water, and ice are the environmental media primarily responsible for erosion. Finally, the process of erosion stops when the transported particles fall out of the transporting medium and settle on a surface. This process is called deposition.
The processes of soil erosion involve detachment of material by two processes, raindrop impact and flow traction; and transported either by saltation through the air or by overland water flow. Runoff is the most important direct driver of severe soil erosion by water and therefore processes that influence runoff play an important role in any analysis of soil erosion intensity.
Rain may move soil directly: this is known as ‘rainsplash erosion’ (or just ‘splash erosion’). Spash is only effective if the rain falls with sufficient intensity. If it does, then as the raindrops hit bare soil, their kinetic energy is able to detach and move soil particles a short distance.
Water and soil splashed following a single raindrop impact. (From the WEPP 95 CD-ROM)
Although considerable quantities of soil may be moved by rainsplash, it is all merely redistributed back over the surface of the soil (on steep slopes, however, there will be a modest net downslope movement of splashed soil). Thus a more descriptive term might be ‘rainsplash redistribution’.
Rill and gully erosion
Rainfall may also move soil indirectly, by means of runoff in rills (small channels) or gullies (larger channels, too big to be removed by tillage). In many parts of the world, rill and gully erosion is the dominant form of water erosion.
That fraction of the rainfall which does not infiltrate (soak into) the soil will flow downhill under the action of gravity; it is then known as runoff or overland flow. Runoff may occur for two reasons. Firstly, if rain arrives too quickly (i.e. with too high an intensity) for it to infiltrate: the runoff which results is then known as infiltration excess runoff, or Hortonian runoff. Secondly, runoff may occur if the soil has already absorbed all the water it can hold (i.e. because it is fully saturated, or if the soil is frozen). Runoff which results from this situation is known as saturation excess runoff.