Geoscience Reference
In-Depth Information
heavier aggregated particles. The existence of this process is sometimes called
splash erosion
. The easier the aggregates are disintegrated, the greater is the mass
of fi ne soil particles splattered into the water and transported downslope. The longer
is the slope, the more intensive is soil erosion, because the thickness of the sheet of
fl owing water increases as well as its carrying force and momentum. A similar rela-
tionship exists between the angle or steepness of the slope and intensity of erosion.
When we observe soil profi les along the wall of a trench dug in the downhill direc-
tion of an eroded soil, we see that the humus horizon thickness decreases as we go
down the slope. The top, missing portion of the humus horizon was carried away by
sheet erosion
. Although the eroded soil may not move entirely off the fi eld during
the fi rst signs of water erosion, it is moved from higher to lower elevations, causing
some parts of the fi eld to be less productive. In reality, there does not exist a regular
sheetlike fl ow of the muddy water. The majority of water fl owing downslope readily
forms initial irregularities owing to the roughness of the existing initial topographi-
cal surface. Hence, very small crevices and miniature gutters are dug out by water
streaming across naturally created surfaces. These small ridgelike miniature forma-
tions are smoothed after subsequent rain by both natural factors and man-made
tools. If they survive until the next rain, they will gravitationally attract the excess
water from the rain. This type of migrating surface fl ow is entirely natural and con-
tains no magic. It is caused by the same principles that led to the fi rst formation of
small channels or rills. But since the surface fl ux is now more directed into the con-
necting formation of rills, water readily fl ows into the accessible rills ahead of it and
does not consume its energy searching for an optimal path down the slope. Following
paths already predetermined down and across the fi eld, water's increasing and con-
served energy is released for the intensive transport of soil particles. Hence, the
initially small, fragile rills sooner or later become deeper and broader and survive
over many years as the result of
rill erosion
. The rills may be interconnected into a
sort of a net covering the whole fi eld.
The more frequent surface runoff is conducted by a rill, the higher is the proba-
bility that soil particles erode from the sides and bottom of the rill until the rill is
made so deep that we speak about a gully. It reaches deep below the humus horizon
of the surrounding terrain, and the process is denoted as
gully erosion
. Gully erosion
together with rill erosion destroys the earlier existing soil, and if the erosion contin-
ues for a long time, the landscape is denuded of soil and is no longer capable of
supporting plants. Hence, the soil dies and loses all of its ability to support life -
plants, animals, and humans. Therefore, we have the priority and commitment to
minimize soil erosion to an estimated designated tolerable soil loss level TSL. By
universal global agreement, it has been determined that TSL should at least equal to
the rate of contemporary soil evolution. Hence, its value roughly ranges between 0.1
and 1 ton per acre per year, or equivalent to 0.1-1 mm of topsoil layer in 1 year.
Soil may also be shifted from the fi eld by another transporter, the prevailing
wind. The process, denoted as
wind erosion
, can happen anywhere and at any time
when the wind blows with a suffi cient wind speed (velocity) measured at the height
of 10 m above the ground at a standard meteorological station. In meteorology,
wind is classifi ed with the Beaufort wind force scale. Here we use its simple, more
