Geoscience Reference
In-Depth Information
conditions. Saline soils are characteristic of coastal salt
marshes, where halophytic vegetation (saltwort, glasswort)
will be found. As soil leaching is at low levels, geological
substrates which contain salt and/or gypsum give rise to
distinctive tolerant plants.
extreme and low erosion rates (Kirkby 2001). Cover seems
to be more important than biomass, so that bushes and
low clumps of plants can be just as effective as trees. The
main effect of the vegetation, of course, is to break the fall
of raindrops, thus preventing both rain splash and surface
sealing.
Our knowledge of Mediterranean desertification has
increased markedly since the 1990s, largely through the
researches of EU Medalus projects. Soil erosion models
have been developed which cover a range of time and
spatial scales. The use of different time scales in these
models follows the Schumm and Lichty (1965) model
discussed in
Chapter 1.
Table 25.1
illustrates the parame-
ters at the steady, graded and cyclic timescales.
EROSION
Processes leading to the erosion and degradation of the
land surfaces of arid, semi-arid and sub-humid areas are
collectively known as
desertification
. The term refers to
adverse impacts on all ecosystem components (soil, water,
vegetation, wildlife) and the processes are considered to
be due to human impacts on land use. Although normally
associated with desert fringes like the Sahel on the
southern margins of the Sahara, Mediterranean regions
are highly susceptible to desertification processes because
of the properties of their climate, soils and vegetation.
Against this backcloth of an inherently sensitive physical
environment, there has been a long history of drastic
human modification for agriculture, water supplies, urban
and residential development, and tourism. Most of these
impacts have gone on for millennia in the Mediter-
ranean region itself, and for several centuries in other
Mediterranean areas.
Slopes in Mediterranean landscapes are rarely covered
100 per cent by plants; thus bare soil is open to erosion
processes and the production of loose sediment, which is
then carried downslope. Plant cover is therefore a major
control of run-off and sediment yield, although another
important variable is the intensity of rainfall, with low-
intensity rain producing much smaller amounts of
sediment and run-off. A vegetation cover of about 50 per
cent seems to mark an approximate threshold between
Soil surface conditions
Soil surface conditions, i.e. surface crust development,
roughness and vegetation cover have an important
influence on infiltration rates, run-off generation and
erosion. Surface conditions depend on soil characteristics
like texture and organic matter, and on factors related to
land use, vegetation cover, biological activity and factors
related to climate. On bare soils, whether cultivated or
non-cultivated, crusting has the prime effect on surface
structure. Surface crusts and seals are a common feature
of soils in Mediterranean, arid and semi-arid regions.
The force of the raindrop breaks the aggregates at the soil
surface into loose mineral particles of sand, silt and clay,
which become dispersed. Subsequent drying causes
repacking into dense and impermeable surface 'skins'. If
there are free chemicals such as salts, lime or gypsum in
the soil, as is common in these regions, these will in
addition provide chemical cements which on drying are
extremely tough. Thus surface crusts are usually partly
physical and partly chemical.
Steady time
Graded time
Cyclic time
(short periods, within one
(decades)
(longer periods of landscape
storm)
evolution)
Soil hydrology (infiltration and
Profile truncation
Soil evolution
overland flow)
Armouring of the soil surface by stones
Physical and chemical denudation
Aggregate stability
Cation dispersion
Surface crusting and roughness
Vegetation growth
Litter fall
