What is soil erosion? Soil erosion is a natural process, occurring over geological time, and indeed it is a process that is essential for soil formation in the first place. With respect to soil degradation, most concerns about erosion are related to accelerated erosion, where the natural rate has been significantly increased mostly by human activity. Soil erosion by water is a widespread problem throughout Europe. Soil is naturally removed by the action of water or wind: such 'background' (or 'geological') soil erosion has been occurring for some 450 million years, since the first land plants formed the first soil. Even before this, natural processes moved loose rock, or regolith, off the Earth's surface, just as has happened
Where it does so, the soil’s surface will be lowered slightly. Lowered areas form preferential flow paths for subsequent flow, and these flow paths are in turn eroded further. Eventually, this positive feedback results in small, well-defined linear concentrations of overland flow (‘microrills’ or ‘traces’).
In many cases, individual microrills become ineffective over time due to sedimentation. A subset, however, grow further to become rills; and a smaller subset may go on to develop into gullies. This process of ‘competition’ between microrills and rills leads to the self-organized formation of networks of erosional channels (dendritic on natural soil surfaces; constrained by the direction of tillage on agricultural soils), which form efficient pathways for the removal of water from hillslopes. It is in such erosional channels that water erosion also operates most effectively to detach and remove soil by its kinetic energy. In most situations erosion by concentrated flow is the main agent of erosion by water.
The flow-dominated erosional channels are separated by interrill areas where the dominant processes are rainsplash and diffuse overland flow; however, boundaries between rill and interrill areas are both ill-defined and constantly shifting.

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Large rills (possibly big enough to be called gullies?) on an eroding hillslope.

In some circumstances subsurface flow may be important in determining where