Soil erosion

Rill erosion in maize field. Photograph: Liesbeth Wilschut  Figure 1: Rills in maize field, Upper Tana basin, Kenya. Photograph: Liesbeth Wilschut
Rills in maize field, Upper Tana basin (L.Wilschut)

Soil erosion is defined as the detachment and movement of the topsoil by wind and flowing water. Soil erosion is a natural process, but is often accelerated by human influence.

Consequences of soil erosion

Soils contain minerals and micro-organisms that are of vital importance to the growth of crops. Due to soil erosion, the quality and quantity of the soil decreases. This leads to lower agricultural yields. Another important function of the soil is the filtering effect it exhibits on water. Without soil, agricultural pollutants will move faster into the groundwater, or will flow downstream.

Soil erosion also affects the hydrological cycle (Ward and Robinson, 2000). A non-eroded soil has a larger infiltration capacity than an eroded soil. The lower infiltration capacity of eroded soils induces more runoff, resulting in an increase in the number and severity of flooding events. Sediment deposited on roads and the silting up of reservoirs is yet another consequence of soil erosion.

Erosion processes

Soil erosion by water consists of two processes: flow erosion and splash erosion. The severity of the erosion depends on the amount and energy of the water that reaches the soil. This is determined by the intensity of the rain, the amount of intercepted water by vegetation and the infiltration rate. When raindrops fall on bare soil splash erosion occurs. The raindrops impel a certain momentum on soil particles. This has three different effects: crusting, detachment of soil and transport of soil (Evans, 1980, Troeh et al., 1999). If rainfall intensity exceeds the infiltration capacity of the soil, or when the soil moisture storage capacity is exceeded (that is the amount of water that can enter the pores in the soil), overland flow occurs (Morgan, 2005). Overland flow both detaches and transports sediment. When the erosive energy of a flow is greater than the ability of the soil to resist detachment, soil particles are picked up and can be transported downstream, a process called entrainment (van Roode, 2000).

Preventing soil erosion

Excessive soil erosion can be prevented by implementing green water management practices. Examples are mulching, minimum tillage, and terracing. More information on green water management practices can be found here.

References:

Evans, R., 1980. Mechanics of water erosion and their spatial and temporal controls: an empirical viewpoint. In: Kirkby, M.J. and Morgan, R.P.C. (ed.). Soil Erosion. p. 109-128. John Wiley & Sons. New York.

Morgan, R.P.C., 2005. Soil erosion and conservation. Third edition, Blackwell publishing, Oxford, UK. 303 p.

Troeh, F.R., Hobbs, J.B. and Donahue, R.L., 1999. Soil and water conservation, productivity and environmental protection. Third edition, Prentice Hall, Englewood Cliffs, N.J., USA, 610 p.

Van Roode, M. 2000. The effects of vegetative barrier strips on surface runoff and soil erosion in Machakos, Kenya, A statistical versus a spatial modelling approach. Nederlands Geografische Studies 278, University Utrecht, Utrecht. 283 p.

Ward, R.C. and Robinson, M. 2000. Principles of Hydrology. Bell and Bain Ltd, Glasgow, 449 pp.