Geoscience Reference
In-Depth Information
1.3.1
Terminology
have been no more immune from the impacts of Quater-
nary timescale climate changes than any other parts of the
Earth's surface, so today's drylands commonly contain
landscape and landform expressions inherited from past,
different, climatic regimes (Chapter 3). Third, as Parsons
and Abrahams (1994, p. 10) succinctly note:
'Desert', 'arid zone', 'dryland' and sometimes other terms
such as 'thirstland' are all used somewhat interchangeably
and imprecisely in both popular and scientific literature.
This can lead to confusion regarding differences in lev-
els of moisture availability from place to place. Consider
the example of the Kalahari Desert (Thomas and Shaw,
1991) (Figure 1.1), a place that by name is familiar to
many people and that at first glance conjures up images
of extreme dryness. Yet the Kalahari is widely regarded
not as a 'true' desert, and few parts of it today achieve
extremes of moisture deficit, particularly when compared
with southern Africa's other desert, the Namib. In fact
the Kalahari, which spans over ten degrees of latitude
from northern South Africa to northern Namibia, em-
braces environments that range from arid to dry-subhumid
and mean annual rainfalls from around 200 mm to over
600 mm (the driest areas of England receive c.500 mm
p.a.). Ecosystems range from sparse savanna grassland to
subtropical woodland.
Collectively, the Kalahari as described here is undoubt-
edly 'dryland', given that seasonality of rainfall and po-
tential evaporation give rise to annual moisture deficits.
The two uniting factors across the region are structural
(the Kalahari occupies an internal structural basin) and
sedimentological (the Kalahari is predominantly covered
by unconsolidated sands). These sands in fact extend even
further north, over a further 15 degrees of latitude, into
wet tropical environments. In part the characteristics of
the modern Kalahari are a consequence of major climatic
and environmental changes in the Late Quaternary period
that resulted at times to major expansions of the arid zone
(Thomas and Shaw, 2002).
Overall, arid zone and dryland are perhaps more appo-
site terms than desert to use collectively in the description
of moisture deficit regions, and this is reflected in the title
and subtitle of this topic. However, within the literature,
consistency of use is absent, and it is necessary to con-
sider carefully, rather than assume, the environmental and
climatic characteristics of areas referred to by any of the
terms used in this explanation.
...
the emphasis of geomorphology has shifted away
from morphogenesis within specific areas towards
the study of processes
per se
. This shift
in large
measure undermines the distinctiveness of desert ge-
omorphology.
...
Nonetheless, the relative importance of individual pro-
cesses and the magnitude and frequency of their opera-
tion may differ in arid environments compared to other
areas. This, together with growing human populations in
drylands and the common treatment of them as environ-
mentally distinguishable, is reason enough to pursue arid
zone geomorphology in its own right.
The last three decades or so have seen a new rigour en-
ter geomorphological research in arid environments. New
techniques have been employed and new methodologies
pursued. Landform description for its own sake has largely
been eschewed, though it does, of course, still have a valid
role in geomorphological research, and has been replaced
by studies of process and form, measurement, explana-
tion and application. It is these that this topic focuses
upon.
1.3
Arid zones: terminology and
definitions
Definitions and delimitations of arid environments and
deserts abound, varying according to the purpose of the
enquiry or the location of the area under consideration.
Literary definitions, thoroughly reviewed by Heathcote
(1983), commonly employ terms such as 'inhospitable',
'barren', 'useless', 'unvegetated' and 'devoid of water'.
Scientific definitions have been based on a number of cri-
teria, including erosion processes (Penck, 1894), drainage
patterns (de Martonne and Aufrere, 1927), climatic crite-
ria based on plant growth (Koppen, 1931) and vegeta-
tion types (Shantz, 1956). Whatever criteria are used, all
schemes involve a consideration of moisture availability,
at least indirectly, through moisture balance: the relation-
ship between precipitation and evapotranspiration.
1.3.2
Definition
The origins of direct delimitation of arid environments
by consideration of moisture balance date back to 1953
and the growth in concern at the United Nations (specif-
ically UNESCO) with global food production and living
in dry regions of the world. The classification scheme,
