If soil is a dirty word, erosion is downright obscene. It is also misused and misunderstood. Erosion is a natural process. It is the surficial counterpart of the weathering of parent material. If erosion did not occur, soils would get thicker and thicker. Erosion is the natural wearing away. Literally, erosion is the movement or transportation of material which was soil but becomes sediment as soon as it begins to move. There is nothing wrong with or bad about erosion, contrary to popular opinion. It is not caused by human misuse. When most people think of erosion as something bad, they are really thinking of accelerated erosion, or erosion increased at a rate greater than natural due to human agency.
There are three main ways to obtaining information about erosion, be it natural or accelerated.
Qualitative observations involve the simple viewing of the land, looking for various signs or indicators of erosion.
1) Rills [example] or gullies [example] reflect concentrated overland flow where sheetflow might once have existed.
2) Exposed E horizons often appear as white patches on hilltops or convex slopes where dark soils are otherwise prevalent [example]. In the way of an analog, think of what an orange with part of its peel removed looks like [example].
3) Exhumed rocks, either relatively large boulders that were exposed by removal of overlying soil, or rocks lying loose on the surface due to the soil in which they were once firmly embedded, typically indicate soil loss [examples].
4) Exposed roots are similar to exhumed boulders--what should be beneath the ground surface is not [examples] [example]. One must be careful, however, and not confused exposed roots with buttress roots [example].
5) Pedestals are the result of items on the surface, typically rocks but sometimes artifacts of human activities, protecting the underlying soil from the impact of raindrops. They indicate that surrounding, unprotected soils have been eroded, while those directly under the rock have not [examples].
6) Debris dams or traps are comprised of fallen trees and branches. They tend to catch sediment that has eroded from upslope or upstream [examples].
On the whole qualitative observations are not all that precise, but they can be very informative.
Quantitative observations as the name implies, involve collection and measurement.
Direct or Passive measurements are perhaps the most accurate way of measuring the extent to which soil erosion has occurred, but they are also the most laborious and time consuming. In small settings such as where soil might wash down onto a sidewalk, they involve collecting and weighing deposited materials. In larger settings such as gullies, they involve measuring the widths, depths, and lenghts of gullies and calculating the volume of material lost. If deposits can be identified (such as alluvial fans), these can then be measured compared with areas showing signs of erosion. The degree to which the "cut" and "fill" can be correlated, as well as the amount of material moved and the distances involved, say a great deal about soil loss from a particular locale [examples].
A few other examples of measuring how much erosion has occurred include: 1) A-horizon reconstruction, whereby the thickness of the no longer existant A-horizon on eroded lands can be estimated by comparison with surrounding lands of otherwise similar soil that have not been eroded; and 2) Natural benchmarks such as trees or boulders that might have soil marks, not unlike the high water marks on buildings in recently flooded areas. Volumetic remeasurements can be estimated on the basis of the distance between that mark and the current surface.
Indirect or Active measurements center around preselected points, devices set-up to carry-out long term study (erosion, it must be remembered, involves time), and the actual measuring of ongoing soil loss.
The most common way of measuring active soil loss is with Erosion pins, metal rods set into the ground [examples], typically with a portion sticking up above the surface some known and recorded amount (10 cm). Flagging is tied to the stake to warn possible disturbers. The distances between the tops of the pins and the surface are recorded at regular intervals (e.g., once a month) over an extended period of time. A variation on this theme is to use a very long spike driven through a washer to ground level. Over time, the distance the washer drops from the top of the spike to the eroded ground surface can be recorded.
A variation on this theme is to use bottle caps with nails driven through them, thereby becoming tantamount to pedestalling as described above. Determining the height of the underlying pedestal and the time it took to develop will inform of the rate of erosion.
Indirect methods of determining erosion are all contingent on being able to monitor conditions over extended periods of time. They are also more appropriately considered estimates as the removed material is not available for study. If a fieldworker lacks the luxury of time she or he will have to resort to direct or qualitative measures. The former is undoubtly the best measure, but in some cases the sediment is impossible to reach (e.g., under the Gulf of Mexico in the case of the Mississippi River.
Created by William E. Doolittle. Last revised 2 August 2017, wed