Geoscience Reference
In-Depth Information
Dryland system variability
David S. G. Thomas
This short chapter explores the geomorphological vari-
ability within drylands. We have already noted in Chap-
ter 1 that specific landforming processes are not unique
to arid environments. Some processes are, however, more
readily facilitated, or occur on a greater scale, in dry-
lands than elsewhere. For example, beyond coastal con-
texts and those of poorly managed agricultural land, aeo-
lian processes have their greatest landform expressions in
drylands than in any other environment. Similarly, salt-
weathering processes have their greatest propensity to
operate in certain dryland situations, because of moisture
deficits.
For many landforms, arid conditions may set the possi-
bilities for development, but their ultimate formation is de-
pendent on suitable materials, lithologies or topographic
settings being available. As Abrahams and Parsons (1994)
noted, the assumption that existed of a 'distinct' desert
geomorphology was based on a belief that 'similarity
of climate throughout desert areas outweigh differences
that may arise from other influences' (pp. 9-10). Arid re-
gions occupy a variety of structural and tectonic settings
(Murphy, 1968), discussed in Chapter 2, which range from
a tectonically active montane setting to stable continen-
tal cratons (Heathcote, 1983; Thomas, 1988). Drylands of
course embrace a range of climatic contexts, with differ-
ences in 'effective moisture' between the 'zones' along the
hyper-arid-dry-subhumid continuum (Chapter 1), as well
as differences in the nature and duration of seasons (af-
fecting precipitation receipt) and in temperature (affecting
moisture loss). A third layer that contributes to difference
and diversity is environmental and climatic history, es-
pecially during the Late Quaternary Period (Chapter 3),
which has left a legacy of palimpsest landforms indicative
of different geomorphic regimes in the past. The climatic
and structural variations present within and between arid
regions, together with the impact of climatic changes dur-
ing the Quaternary Period, are therefore important vari-
ables contributing to the present appearance of arid zone
landscapes.
4.1
A framework for dryland diversity
Clements
et al.
(1957) and Mabbutt (1976) provided early
attempts to classify the landforms present in some arid ar-
eas (Table 4.1). Notwithstanding the limitations of such
spatially extensive generalisations, the data they present
serves to illustrate some important issues, e.g. the im-
portance of erosional and water-worked features in arid
areas with high and frequent changes in relative relief,
such as in much of the southwestern United States, and
the greater (but not necessarily predominant) importance
of the wind as a geomorphological agent in regions with
extensive areas of more limited relative relief, such as in
the Australian arid zone. Even the Sahara, the ultimate
'sandy desert' in the eyes of many nonspecialists, is not
especially sandy, with less than a third of its area being
sand seas (including dunefields). The Sahara is actually
dominated by mountainous zones, with sand seas sitting
either in intermontaine depressions (see Chapter 17) or in
areas to the south of the Sahara where, in the Sahelian
zone, they represent legacies from more extensive aridity
during periods of the Late Quaternary.
There is, in fact, no simple classification of dryland
environments, nor really is there a need for one, save
as a basis for seeking explanations for understanding the
drivers of processes, as well as the controls on their occur-
rence and resultant environmental expressions. However,
illustrating the geographical diversity and distribution
of different desert/dryland 'types' does provide a useful
