Soil erosion - Soil Essentials: Managing Your Farm's Primary Asset - page 102

Variables affecting water erosion
Water velocity
As the velocity and therefore the energy of moving water increases, its power to
cause erosion increases markedly. This is especially so if the water is moving over
bare unprotected ground or is channelled into drainage lines. Fast moving water
can move large boulders and carry other large material such as trees, which can
cause great damage to any structures (such as fences, buildings and bridges) the
water plus debris impacts upon.
Water depth
As the depth of water increases, so does its ability to move soil particles, as it can
cut into harder ground than shallow water and move larger objects such as stones,
boulders and so on.
Distance
If water is able to travel long distances down even a gentle slope, in a heavy rain
event the volume of moving water especially over the lower reaches of the slope can
be very considerable. In this situation severe sheet and/or rill erosion can occur in
a very short space of time.
Soil type
Light sandy soils are more prone to erosion than heavier soils that have better
developed crumb structure and are better able to resist the erosive power of the
water. Regardless of soil type, a soil with good plant cover will be better able to
resist erosion, but even so, the lighter soil is still more at risk.
The other variable with soil type is the effect of soil chemistry, and a highly
dispersive soil such as a sodic soil has a high erosion risk compared to a chemically
well-balanced soil.
The structure of the soil is also important, and a soil with well-balanced
chemistry can also have good physical properties such as well-developed crumb
structure, good aggregation, good porosity and low bulk density. Such a soil will
allow rapid water infiltration, support strong healthy plant growth and soil
microbiology, and the organic acids and polysaccharides present bind the soil
particles strongly together. Another soil problem is that of nonwetting sands, or
hydrophobic soils. In this case, water is unable to penetrate into the soil because of
waxy coatings formed on the soil particles by fungi. This repels water, causing it to
run off the soil surface causing erosion.