5.8: Soil Erosion

Worldwide there are approximately five billion hectares of degraded land, which is approximately 43% of the Earth’s land surface. Of those five billion hectares, the most common cause of degraded land (two billion hectares) is soil degradation due to erosion, compaction, surface crusting, acidification, or salt accumulation. Most of that degradation (85%) comes from water and wind erosion, however, other human activities such as overgrazing, industrial activities, unsustainable agricultural activities, overexploitation, and vegetation removal cause many negative side effects on the land ultimately causing degradation of the soil (Figure 4).

Figure 4. The negative side-effects (physical, chemical, and biological) caused by various human impacts that degrade soil. Source: Palm et al., 2007.

Soil erosion includes two separate processes – soil erosion by water, and by wind.

Water erosion… begins with detachment as rain drops bombard soil aggregates, separating some of them from the aggregate. These stand-alone soil particles are much smaller and are more easily transported. The transported particles are eventually deposited in a low-lying area, completing the three-part process of detachment-transport-deposition (Figure 5). Transport can happen due to splashes from the raindrop, or from running water carrying sediment downhill.

Water erosion is initiated by sheet erosion where splashed soil is moved uniformly, but some columns of soil that were protected by pebbles may remain. When the water gathers into small channels due to irregularities in the landscape, those channels incise into the soil surface forming a rill. Rills can be smoothed by tillage equipment. If enough water gathers, a gully can form, which is essentially a large rill that is so deep that it cannot be smoothed by tillage equipment. Interrill erosion is sheet erosion that occurs between rills. The majority of soil erosion is due to sheet and rill erosion.

Figure 5. The effects of water erosion across the soil-landscape. Source: trival.co

Wind erosion… is greatest in arid and semiarid regions, such as Wyoming, though it can occur to some extent in humid regions. Similar to water erosion, wind erosion involves three processes – detachment, transportation, and deposition (Figure 6). Detachment occurs as heavy winds push and bounce heavy particles along the surface. As this happens, silt and clay particles can be broken away from aggregates and can become airborne and transported for great distances. The sediment is transported by three methods – saltation, soil creep, and suspension.

Figure 6. The effects of wind-erosion on soil. Source SeaFriends.

Read through and complete the following activity and questions. All questions should be answered and completed labs are due at the end of the laboratory period. No late work will be accepted.