Erosion by Water: Erosivity and Erodibility

Conceptually, rainfall erosivity is the capacity of rain to produce erosion, whereas soil erodibility is the susceptibility of the soil to be eroded. However, no absolute numerical values can be determined for them because rainfall erosion results from various forms of erosion (splash erosion, sheet erosion, rill erosion, interrill erosion) that are driven by different forces. Originally, the terms erosivity and erodibility were associated with the rainfall factor (R) and the soil factor (K) in the Universal Soil Loss Equation where the event erosivity factor was given by the product of the kinetic energy of the storm (E) and the maximum 30 min rainfall intensity (I30). Soil loss at the scale at which the USLE applies results from the discharge of sediment with runoff, but because the USLE does not include any direct consideration of runoff in the event erosivity factor, it is not good at accounting for event soil loss at some geographic locations. Erodibility values have units of soil loss per unit of the erosivity index and cannot be used with erosivity indices other than the one that was associated in their determination. Some models ignore this fact and so ignore certain fundamental rules that should apply to the modelling of water erosion. So-called process-based models attempt to account for the effects of the various forms of erosion and do consider the effect of runoff on erosivity associated with each form. Particles travel across the soil surface at virtual velocities that vary from the velocity of the flow to near zero. Little regard is given to this fact in the determination of soil erodibility values.