One of the things which makes soil erosion difficult to understand and so to predict and control is that it is affected by both common and rare events, and so must be studied over both short and long timespans. Erosion is also affected by factors on very small and very large spatial scales, and has its impacts over a similarly wide range of spatial scales.
Soil erosion occurs both incrementally, as a result of many small rainfall or wind-blow events, and more dramatically, as a result of large but relatively rare storms. It is the large storms which produce the big hard-to-miss erosional featues such as deep gullies. But while erosion due to small common events may appear insignificant on the field, its cumulative impact (both on the eroding field, and elsewhere) may, over a long timescale, be severe.
Water erosions complex hierarchy of processes mean that erosion by water operates (and is studied) over a wide range of spatial scales. Rainsplash redistribution and the initiation of microrills and rills occur at a scale of millimeters. Rill erosion on agricultural hillslopes operates at a scale of meters to tens of meters, while gully erosion can occur on a scale of hundreds of meters, or even kilometers. The offsite impacts of erosion can affect very large areas, sometimes hundreds or even thousands of square kilometers.
At every spatial scale, however, soil erosion by water is highly patchy. Even in areas of severe erosion, rates of soil loss can vary greatly from point to point on the landscape as the vagaries of topography and land use concentrate erosive flows on a wide range of spatial scales. Obvious erosion in one field can be found side-by-side with virtually untouched areas; and within an eroded field, the severity of erosion can vary markedly.
Dr David Favis-Mortlock, April 2017