The National Soil Erosion Research Laboratory
Surface runoff occurs when rainfall exceeds a soil's maximum saturation level and all surface depressional storage is filled to capacity. The rate of runoff flow depends on the ratio of rain intensity to the infiltration rate. If the infiltration rate is relatively low, such as when a soil is crusted or compacted, and the intensity is high, then the runoff rate will also be high.
Crusted or compacted Soil
High runoff rates can both detach and transport large amounts of soil, as well as transport the associated nutrients and pesticides.