Arid environments - Environmental Sedimentology - page 182

ABCDE F ABCDE F

until it hits a sink. Runoff will be continuous
only where the source cells dominate the sinks
(Fig. 5.18). As rainfall inputs vary then so will
the number of cells acting as sources or sinks.
The amount of sediment carried by this run-
off will be dependent on the 'erodibility' of the
sediment and the 'erosivity' of the rainfall. The
latter will be determined mainly by the rain-
storm characteristics (rainfall intensity and drop
size), but may be modified by the intervention
of vegetation. The erodibility of the sediments
will be determined by the aggregate stability,
which in turn is affected by the soil structure
(amount of binding organic matter or presence
of dispersing agents such as sodium salts). Thus,
the nature of the sediment characteristics across
and into the slope, combined with the slope
morphology and cover characteristics, added
to the spatial variability of an individual storm
event will generate a spatially varied movement
of sediment and water across a slope. This can
be further complicated by the intervention of
humans, modifying the local hydrological
characteristics (Case Study 5.3). All factors
considered it is not difficult to see why the geo-
logical concept of a 'sheetflood' rarely, if ever,
occurs in the natural environment!

G GH H

Slope
top

Slope
base

(a)

Flow in channel at slope base

High

Zero

ABCDE F ABCDE F G GH H

(b)

Fig. 5.18 Conceptual model of the mosaic of surface runoff
pathways on a hypothetical slope (a). Each cell represents a
source (dark) or sink (light). If it is assumed that (i) rainfall is
even across the slope and (ii) that source and sink capabilities per
cell (pixel) are equal, the contribution of runoff from the slope
mosaic to the channel is displayed in (b).

5.5

ANTHROPOGENIC IMPACTS

Recent research has highlighted the signific-
ance of heterogeneities in slopes, which generate
a 'mosaic' of hydrological characteristics. These
mosaics can be considered as 'pixels' in a digital
picture, where each pixel has a different indi-
vidual response to a rainstorm event, even if
the intensity and duration of the event were con-
stant across the entire area (which is unlikely).
The hydrological response of each pixel will be
determined by its ability to store water. This will
be determined through a range of factors such as
porosity, permeability, organic matter content,
slope angle, etc. As a function of these charac-
teristics some pixels will act as 'sources' and
some will act as 'sinks' (Fig. 5.18). The sources
will respond to the rainstorm event and gener-
ate runoff. This runoff will continue downslope

Humans tend to affect the landscape on the
same time-scales as climate. Thus, the impact
of humans is often difficult to unravel from the
natural controls discussed above. This section
will illustrate the significance that these impacts
exert on controlling the modern landscape,
which in turn has implications for how environ-
ments can be managed sustainably. The world's
arid environments have long been occupied
by humans. In more recent times, however,
improvements in technologies of water manage-
ment, improved healthcare and hygiene have
meant an increase in population of these regions.
In many cases this may be exacerbated by
the availability of mineral deposits. The section
examines the impact of humans on the erosion,
transport and deposition of sediment in deserts.