The USLE-M erosivity index is based on the concept that the soil lost from an area during a rainfall event is given by the product of the amount of runoff (Q) and sediment concentration.
In the USLE-M, it is assumed that the sediment concentration varies with the level of kinetic energy of the rain and rainfall intensity. The kinetic energy level for a rainfall event is given by the E term in the EI30 index divided by rainfall amount. Also, it is assumed that the impact of event intensity of the sediment concentration for the event can be reasonably well accounted for by I30. This results in
Event erosivity = Q (E / rainfall amount) I30
However, the runoff ratio (QR) is Q divided by rainfall amount. Thus
Event erosivity = QR E I30
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