Soil Erosion

Additional scientific description

Soil erosion is a slow-onset process that occurs naturally under all climatic conditions and on all continents but is significantly increased and accelerated by unsustainable or inappropriate human activities through intensive agriculture, deforestation, overgrazing and land use changes. The Status of the World's Soil Resources report identified soil erosion as one of the major global soil threats (FAO, 2015). Soil erosion rates are generally much higher than soil formation rates; as soil is a finite resource, its loss and degradation through erosion is not recoverable within a human lifespan (FAO, 2020).

Soil erosion decreases soil quality and functions, agricultural productivity, degrades ecosystem functions, amplifies hydrogeological risk such as landslides or floods, causes significant losses in biodiversity, damage to human infrastructure and, in severe cases, leads to displacement of human populations (FAO, 2020).

Soil erosion poses a major threat to global food security and to the achievement of the Sustainable Development Goals (SDGs) (UN, 2015) on ensuring the well-being of at least 3.2 billion people around the world (FAO, 2020).

Generally, four types of soil erosion are recognized: (1) water; consisting of rain drop or splash, sheet, rill, and gully; (2) wind; (3) harvest; and (4) tillage.

• Splash erosion is caused by raindrops falling onto the soil surfaces causing small particles to move. The detached particles may form a crust that is easily removed by rain or winds.
• Sheet erosion occurs when the thin upper portion of the soil is displaced due to surface runoff after heavy rain. During this process, nutrients and organic matter are lost. This type of soil erosion is gradual and often goes unnoticed until larger portions of soil are displaced through the development of rills or the evidence of exposed roots.
• Rills are shallow drainage channels that form as rainwater flows into depressions in the soil surface. Over time, depressions can widen during subsequent precipitation events, which, in turn, increase susceptibility to erosion.
• Gully erosion occurs when runoff flows strongly and is concentrated in one location. In many cases, gullies begin as a rill that transitions into small channels. As water picks up energy, gully slopes become steeper. This type of soil erosion is very visible and damaging. It often has a drastic effect on the productivity of the soil and can make the land unusable. Gullies can range from 2m to 10-15m in depth.
• Tillage erosion occurs in cultivated fields due to the movement of soil by cultivating the soil (e.g. ploughing). Tillage erosion is often denoted by thinner soils on the upper slopes of cultivated fields or a loss of soil downslope of field boundaries,
• Soil erosion due to crop harvesting generally occurs during harvesting of root crops such as potato, sugar beet, carrot or chicory roots. During the harvest, soil particles stick to the crops and are removed from the field.

Metrics and numeric limits

Approximately 7.5 million km2 (6.1% of the global land surface) exceeds a generic tolerable soil erosion threshold of 10 Mg ha/yr (Borrelli et al., 2022). South America shows the highest prediction of average soil erosion rate (22.0 Mg ha/yr) in 2019, followed by Africa (17.1 Mg ha/yr) and Asia (12.8 Mg  ha/yr) (Borrelli et al., 2022). North America, Europe and Oceania show considerably lower predicted values, totalling 11.0, 2.6 and 2.7 Mg  ha/yr, respectively (Borrelli et al., 2022). A total of 75 billion tons of fertile soil is removed every year from the global soilscape by erosion. As a result, precious soil resources, which should be preserved for future generations, are continuously reduced. Every year approximately 12 million ha of land are lost (FAO and IAEA, 2017).

Climate projections, for all global dynamics scenarios, indicate a trend moving towards a more vigorous hydrological cycle, which could increase global water erosion (+30 to +66%; Borrelli et al., 2020).