For a unit mass of water, larger raindrops and faster flowing runoff have greater energies for causing erosion. Vegetation is especially important in that it intercepts rainfall, restrains soil movement, improves infiltration, decreases runoff velocities, and improves soil aggregation. Without question, the model with the greatest acceptance and use, especially within the U.S., is the Universal Soil Loss Equation, USLE, developed by ARS scientists W. Wischmeier and D. Smith. As expected, it varies geographically, but its seasonal distribution is also important to calculate C-values. We are able to estimate K knowing certain soil properties.
Longer slopes accumulate runoff from larger areas and also result in higher flow velocities.
Thus, both result in increased erosion potential, but in a non-linear manner. For convenience L and S are frequently lumped into a single term.