Geoscience Reference
In-Depth Information
latitudes suggests that the extent and location of
arid and semi-arid regions have changed (Fig.
5.4). Generally greater aridity occurred in tropical
latitudes during glacial periods in high latitudes.
In mid-latitudes altitude played an important
role, with greater aridity in non-glaciated low-
land areas such as the Mediterranean during
glacials, and relatively greater humidity during
associated interglacial periods. Thus changes in
relative aridity in these regions would have led
to an expansion of areas susceptible to aeolian
activity. It has also been suggested that increased
wind speeds would have affected some areas dur-
ing glacial periods, further exacerbating aeolian
sediment movement. This in part is reflected in
the extent of relict ergs to be found in areas such
as Australia and the Sahara.
5.4.3 Short term time-scales: storminess
For shorter time-scales, such as the individual
storm event, it becomes apparent that the sim-
pler models of (net) water and sediment move-
ment on slopes considered for longer time-scales
are too simple. The problems are in monitoring
the more complicated detail of storm events.
Much work has been undertaken using rainfall
simulation in the field, but there is much debate
on what is a suitable scale for these plots to be
of use. There is also much debate about the pre-
cise controls of sediment and water movement,
and it would appear that the control is scale-
dependent. In order to consider overall sediment
transfer this section will concentrate on the hill-
slope scale (Case Study 5.3).
Case study 5.3 Runoff and sediment movement on a hillslope scale, results of the MEDALUS and
IBERLIM European Union projects
Partly as a response to growing concerns over desertification in the drier (arid to semi-arid)
regions of Europe, the European Union supported research into sediment and water transfer
across the region, at the catchment and hillslope scale, in the 1990s. Examples of two of these
projects are MEDALUS (Mediterranean Desertification and Land Use) and IBERLIM (Erosion
Limitation in the Iberian Peninsula). The MEDALUS project was primarily involved in the
examination of uncultivated sites, whereas IBERLIM was focused on cultivated (afforested) sites.
The field results and field experimentation at the plot scale will form the case study into short
term (individual storm) significance in sediment transfer and runoff in managed (IBERLIM,
central Spain) and non-managed (MEDALUS, south-eastern Spain) semi-arid catchments.
The MEDALUS project developed one of its erosion monitoring sites in the Rambla Honda
in the early 1990s. The experimental catchment is a first-order catchment developed on mica-
schists and consists of bare hillslope components in the upper parts and alluvial fan material
derived from the mica-schist in the lower part. In general the hillslope element was acting as a
source of hydrological pathways (surface runoff ) and sediment movement, and the alluvial
material at the base as a sink. At the individual storm-event scale, however, it was found that
widespread transfers of water and sediment were unusual and of short duration, even in extreme
rainstorm events. In the latter events although local runoff generation may be high on slope
lengths of 10 m, they decrease dramatically on longer, 50 m length hillslopes (Puigdefabregas
et al. 1998). On the slope surface runoff occurred by infiltration excess in the early part of
the individual events, but was dominated by local subsurface saturation of upper layers of the
soil. Across the hillslope surface mosaics of plant clumps and bare earth became important.
Runoff was generated from bare earth areas but lost to vegetation clumps, which acted as sinks.
Runoff was thus laterally discontinuous, even in the larger observed areas in storm events. On
the hillslope scale the only time connectivity between hillslope and channel elements can occur
is when the saturation of the shallow surface layer occurs across the entire slope to enable the
