Geoscience Reference
In-Depth Information
Soil surface crusting influences both run-off rate and
soil erodibility. It also alters soil properties such as shear
strength and surface roughness, which in turn affect
sediment detachment and transport. A commonly used
classification of soil surface crusts recognizes three stages
in crusting. The
initial fragmentary stage
has all particles
clearly distinguishable. Next the
structural crust
is formed
by an
in situ
reorganization of particles without sorting
or sedimentation. The
sedimentary crust
results from
particle displacement and sorting in puddles.
There are several additional influences of the Mediter-
ranean vegetation on erosion rates. It is common to see
shrubs growing on low mounds of soil. It was previously
argued that, in addition to protection from rain splash,
the roots have a binding effect, holding the soil against
erosive forces. Whilst this will happen, there are additional
processes at work. Any soil grains detached by splash may
become 'caught' by the leaves of the plant, and will thus
contribute to the mound. Plant litter also adds to the soil
organic matter around plants, improving infiltration by
producing a better formed and more stable soil structure.
More infiltration leads to less overland flow and less
erosion; there also comes a time when the mound can
divert any flowing water around it. A clear positive
feedback system thus exists between the plants and the
mounds. Litter production and lower erosion rates create
better soil structures and soil moisture conditions (more
infiltration) as well as providing more plant nutrients
(especially nitrogen). They also provide more attractive,
shady habitats for soil fauna (worms, isopods), whose
burrowing causes further mounding. The pattern of
erosion at the micro-scale is one of low-energy minimum
erosion beneath plants and high-energy maximum
erosion on bare ground between vegetation. This simple
model of the interaction between vegetation cover and
erosion is complicated in the real world by other factors
such as rock type, aspect, the nature of grazing by animals
and the extent of burnt areas.
Soils in Mediterranean areas often have a high stone
content, whether due to stony colluvial and alluvial
deposits or to their shallowness over bedrock. Whilst the
bare surfaces of fine-grained soils develop crusts with low
infiltration rates, the presence of stones improves
infiltration, reduces erosion and protects the surface of
stony soils from direct raindrop impact. The process of
stones being concentrated at the surface by the erosion of
finer particles is called
armouring
. Once armouring has
developed, it reduces soil erosion by increasing infiltration
and making the surface material harder to transport.
The process of armouring is an important subsystem
in the complex system which governs Mediterranean
desertification.
Figure 25.13
shows the Medalus deserti-
fication model, developed as a process-based model to
simulate erosion in Mediterranean regions. Three loops
provide feedback in the system. The organic loop controls
vegetation cover;
reduced vegetation cover leads to
+
Vegetation
Cover
Soil Organic
Matter
Organic
loop
+
—
+
—
Run-off and
Erosion Rates
Soil Water
Capacity
—
Erosion
loop
Armour
loop
+
+/—
Soil
Depth
+/—
—
Soil
Stoniness
Surface
Armouring
+
+
Positive relation
—
Negative relation
+/—
Positive at low values, negative at high values
