Measuring Erosion

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.

 1) Rills or gullies reflect concentrated overland flow where sheetflow might once have existed.

 2) Exposed B horizons often appear as white patches on hilltops or convex slopes where dark soils are otherwise prevalent.[example][example] In the way of an analog, think of what an orange with part of its peel removed looks like.

 3) Exhumed rocks scattered loosely on the surface in areas surrounded by rock-free areas typically indicate soil loss.[example]

 4) Exposed roots are similar to exhumed boulders.[example]

 5) Pedestals are the result of items on the surface (e.g., rocks) protecting the underlying soil from the impact of raindrops. They indicate that surrounding, unprotected soils have been eroded.[example]

 6) Debris dams are comprised of organic matter such as branches, twigs, grass, and animal feces. They are the result, or deposition, of water and wind erosion. [example] On the whole qualitative observations are not all that precise, but they can be very informative.

Direct measurements are perhaps the most accurate way of measuring soil erosion, but also the most laborious and time consuming. They involve collecting deposited materials and taking volumetric and weight measurements. These are then 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.

Indirect 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 soil loss.

 1) A-horizon reconstruction is the comparison of A-horizon thicknesses between lands suspected of being eroded and surrounding tracks of otherwise similar soil but that are known not to have been affected by human action.

 2) Natural benchmarks such as trees or boulders 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 the surface and the mark.

 3) Erosion pins are metal rods set into the ground, 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 distance between the top of the pin and the surface are recorded over 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.

 4) Erosion pipes are similar to pins except that soil remains undisturbed within the pipe while it erodes away on the outside. Differences between soil height inside and outside of the pipe can be compared over time.

 5) Bottle caps with nails driven through them are tantamount to pedestalling as described above. The height of the underlying pedestal should reveal something about 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. 

Suggested Additional Readings

Job 10

 Created by William E. Doolittle. Last revised 20 July 2004, wed