Surface runoff occurs when the topsoil is fully saturated with water. Water, no longer able to soak into the soil, flows by gravity to lower areas of the site. In areas with a larger area of depressions in the ground or with a steeper slope, surface runoff can accumulate, which can lead to the appearance of water erosion in the form of rills and even gullies.
The formation of erosion gullies during a single, larger rainstorm is common. The ratio of the amount of water that has not soaked into the soil and run off on the soil surface to the amount of water from a given rainfall is called the run-off coefficient. The greater the soil is, the wetter it is (the soil can hold less water from rainfall) and the heavier its composition, i.e. the more clay content, i.e. fine dust grains (0.002mm-0.05mm) and clay (<0.002mm) and less sand (0.05mm-2mm), through which water percolates much more slowly than through the free spaces between the larger mineral grains. The higher the organic matter content of the soil and the lower the compaction of the subsoil, the higher its retention rate and the easier it is for water to percolate into deeper layers. Sometimes high surface run-off does not mean that high erosion occurs. This is determined by a property of soils called erosion susceptibility, i.e. the ease with which soil grains can be pulled away from the soil by water flow. The most susceptible soils to water erosion are loess soils, and the least susceptible are heavy clay and loam soils, whose grains are bound together very strongly. Less susceptible to erosion are soils with good structure, with soil aggregates formed from mineral and organic soil components. Areas prone to surface runoff and water erosion should be left as bare as possible, especially during periods of heavy rainfall and snow melt. The use of catch crops, catch crops and intercrops is recommended so that the soil is protected from the damaging effects of precipitation and surface runoff. For the protection of the soils of a farm located in an area at risk of water erosion, a number of possible principles for anti-erosion measures should be consulted, published here.