Geoscience Reference
In-Depth Information
1.6.4
Cold ocean currents
many geomorphological respects, not least because the
means by which plants cope with climatic variability can
have marked impacts on the operation of geomorpholog-
ical processes. The low moisture availability in arid areas
has a profound effect on plant growth. As Bloom (1978,
p. 314) has noted:
Cold ocean currents affect the western coastal margins
of South America, southern Africa and Australia, giving
rise to five west coast subtropical deserts (Meigs, 1966;
Lancaster, 1989). These currents reinforce climatic con-
ditions, causing low sea-surface evaporation, high atmo-
spheric humidity, low precipitation (very low rainfall, with
precipitation mainly in the form of fog and dew) and a low
temperature range. Lack of rainfall in the Namib Desert,
western southern Africa, is due both to the impact of the
Benguela current on local climates and the failure of east-
erly rain-bearing winds to penetrate across the continent
(Schulze, 1972).
In the United States, the boundary between humid
and semi-arid climates is approximated by the tran-
sition westward from medium-height grasses with
a continuous turf or sod in the humid regions to
short, shallow rooted bunch grasses on otherwise
bare ground in semi-arid regions. In arid regions,
even the bunch grasses disappear, and the vegetation
is, at best, widely spaced shrubs and salt tolerant
bushes.
1.7
Climate variability
Interannual variability in precipitation is a marked charac-
teristic of arid regions. Temperate regions may have year-
to-year rainfall variability of under 20 %. In the Kalahari
it ranges up to 45 % (Thomas and Shaw, 1991) and in the
Sahara ranges from 80 to 150 % (Goudie, 2002). Were
the areas of individual arid regions to be calculated us-
ing annual climate data, they would vary from year to
year as precipitation varies. This is reflected environmen-
tally in studies that have monitored, using remote sensing,
fluctuations in dryland biomes (e.g. Tucker, Dregne and
Newcomb, 1991). Figure 1.3 illustrates interannual rain-
fall variability and precipitation and temperature trends
in example dryland regions during the twentieth century
(Hulme, 1996). This analysis shows how 'normal' vari-
ability is, particularly in precipitation, and also suggests
trends that may be a consequence of global warming im-
pacts. For the Sahel (in itself a very large region within
which further subtle spatial variations in variability ex-
ist) the long period of rainfall deficit through the 1970s
and 1980s may amount effectively to a climate change
in terms of its impact on environmental (and social) pro-
cesses. Interpretation and explanation of the Sahel Great
Drought is complex and sometimes controversial (Zeng
et al.
, 1999; Agnew and Chappell, 1999), and may in-
clude rainfall changes driven by sea surface temperature
changes, the impacts of phenomena such as El Nino and
land surface change feedbacks on atmospheric tempera-
ture (e.g. Charney, 1975; Zeng
et al.
, 1999).
The limited (or absent) vegetation cover is of consid-
erable importance for the operation of geomorphological
processes and the development of landforms (Thomas,
1988). The wind can take on the role of geomorpholog-
ical agent to a degree that cannot occur in other terres-
trial environments, except in some coastal locations or
places where human activities have interfered with the
plant cover. None the less, even limited vegetation can
be a very important variable in the operation of arid land
geomorphological processes.
Vegetation in arid regions has to cope with the rain-
fall variability described above, as well as highly seasonal
distributions in rainfall and temperature patterns. Differ-
ent classification schemes exist: a simple scheme based
on common plant assemblages associated with decreasing
moisture inputs demonstrates issues of plant cover and of
plant types (Goddall and Perry, 1979). In Africa and South
America, similar schemes exist but with reference to dif-
fering forms of savanna vegetation, primarily the changing
mix of grass and woody species that comes with moisture
availability changes (e.g. Huntley, 1982). Broadly speak-
ing, and unsurprisingly, above-ground biomass tends to
increase with increasing available moisture (Figure 1.4).
How plants in arid areas cope with moisture deficits and
droughts is particularly important in geomorphological
terms. A range of strategies exist (Table 1.5) for coping
with moisture deficits and droughts: most obvious is that
plant cover is usually less dense in arid areas than in
more temperate regions, and an increased plant spacing
results in less competition for moisture (Nobel, 1981). In
some environments, communities also embrace different
rooting depths, allowing water competition to be further
1.8
Dryland ecosystems
