Geoscience Reference
In-Depth Information
and 430 g m
-2
, representing a surface lowering of 0.08 mm yr
-1
and 0.35 mm yr
-1
respectively. The results indicate
that these semi-arid Mediterranean badlands are eroding only slowly. Rainfall events
80 mm d
-1
are rare but these
are the events that cause 36 per cent of the sediment export. Most rainfall events are much smaller, and these cause
little erosion except when the soil is near saturation, and a few millimetres of rain are sufficient to produce Hortonian
run-off. In the experiments it is difficult to separate this effect from the effect of surface crusts being formed in the
first few minutes of rainfall. Smaller events are important in preparing the surface through weathering.
Plate 25.7
Badland scenery of the Tabernas 'desert', AlmerÃa province, south-east Spain. Stream incision has exposed
hundreds of metres of silty clay marls. Despite the dramatic vista, erosion rates are less than might be expected, for reasons
discussed in the text. Badland landscapes make good sites for making 'westerns', as seen in the 'mini-Hollywood' centre left.
Photo: Ken Atkinson
reduced soil organic matter, which leads to reduced soil
water storage, which leads to reduced productivity. In the
erosion loop an increase in stoniness due to an increase
in erosion leads to increased soil moisture and increased
vegetation. However, above a threshold of 50-70 per cent
stones, soils have reduced water storage owing to the
decrease in fines, and an increase in erosion thereafter
completely erodes the soil, entailing irreversible degrada-
tion. The armour loop has positive feedback at low
armour levels, owing to increased overload flow and
roughness, but above 30-50 per cent armouring the
armour reduces erosion rates.
QUANTITY AND QUALITY
Since the 1960s three separate economic revolutions have
been impacting upon the Mediterranean region. First,
mass tourism increases the population by several orders
of magnitude during the critical summer months. Second,
urbanization causes an increase in the size of urban
centres around the entire basin. Third, an agricultural
revolution, caused partly by tourism and partly by the EU
Common Agricultural Policy, has produced a drastic
transformation of
the rural landscape, with greatly
