Erodability (or erodibility) is the inherent yielding or nonresistance of soils and rocks to erosion. A high erodability implies that the same amount of work exerted by the erosion processes leads to a larger removal of material. Because the mechanics behind erosion depend upon the competence and coherence of the material, erodability is treated in different ways depending on the type of surface that eroded.
Soil erodibility is a lumped parameter that represents an integrated annual value of the soil profile reaction to the process of soil detachment and transport by raindrops and surface flow. The most commonly used model for predicting soil loss from water erosion is the Universal Soil Loss Equation (USLE) (also known as the K-factor technique), which estimates the average annual soil loss as:
Other factors such as the stone content (referred as stoniness), which acts as protection against soil erosion, are very significant in Mediterranean countries.
Shear stress model
Geological and experimental studies have shown that the erosion of bedrock by rivers follows in first approach the following expression known as the shear stress model of stream power erosion:
where z is the riverbed elevation, t is time, K_\tau is the erodability, is the basal shear stress of the water flow, and a is an exponent. For a river channel with a slope S and a water depth D, can be expressed as:
Note that embeds not only mechanical properties inherent to the rock but also other factors unaccounted in the previous two equations, such as the availability of river tools (pebbles being dragged by the current) that actually produce the abrasion of the riverbed.
can be measured in the lab for weak rocks, but river erosion rates in natural geological scenarios are often slower than 0.1 mm/yr, and therefore the river incision must be dated over periods longer than a few thousand years to make accurate measurements. Ke values range between 10−6 to 10+2 m yr−1 Pa−1.5 for a=1.5 and 10−4 to 10+4 m yr−1 Pa−1 for a=1. However, the hydrological conditions in these time scales are usually poorly constrained, impeding a good the quantification of D.
Unit stream power model
An alternative model for bedrock erosion is the unit stream power, which assumes that erosion rates are proportional to the potential energy loss of the water per unit area:
where is the erodability, and is the unit stream power, which is easily calculated as:
where Q is the water discharge of the river [m3/s], and W is the width of the river channel [m].
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- Soil Erodability (K-Factor) in Europe, European Commission.
- Panagos, Panos; Meusburger, Katrin; Ballabio, Cristiano; Borrelli, Pasqualle; Alewell, Christine (2014). "Soil erodibility in Europe: A high-resolution dataset based on LUCAS". Science of the Total Environment. 479-480: 189–200. doi:10.1016/j.scitotenv.2014.02.010. PMID 24561925.
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