Geoscience Reference
In-Depth Information
recorded (but see McGee, 1897, and Rahn, 1967). More
important is that even low-intensity rainfall events can
generate runoff (Cooke
et al.
, 1982; Goudie, 1985) be-
cause of the nature of desert surface conditions. However,
the 'spottiness' of desert rainfall events can result in con-
siderable spatial and temporal inequalities in its hydrolog-
ical and geomorphological effects. The interval between
individual rainfall events may also have significant impli-
cations for both surface conditions and the operation of
specific geomorphic processes.
Many, perhaps all, landforming processes and their
morphological expressions are not unique to arid envi-
ronments. Some processes may operate more favourably
or assume a greater relative importance than in other en-
vironments, and some landforms may be better developed
or better exposed, the latter at least in part due to the
limited vegetation cover. On the other hand, for many fea-
tures, arid conditions may set the possibilities for their
development, but their ultimate formation is dependent
on suitable materials, lithologies or topographic settings
being available.
8000
A
B
6000
D
4000
C
2000
0
0
200
400
600
800
Precipitation (mm)
Figure 1.4
Suggested relationships between dryland precipi-
tation and plant activity, as measured by biomass (A, B, C) or
primary production (D) (adapted from Bullard, 1997, using data
sourced from Desmukh, 1984, Le Houerou and Hoste, 1977, and
Sims and Singh, 1978). A is from east and southern Africa, B
from Mediterranean Africa, C from the Sahel-Sudan zone and D
from the SW USA.
1.9
Arid zone geomorphology and people
wet to dry seasons in Mtera, Tanzania (Thomas, 1988).
During extended dry periods this can result in a marked
and prolonged increase in bare ground (Smith
et al.
, 1993;
Thomas and Leason, 2005, Figure 1.5), which leads to an
expansion in areas that are susceptible to wind and water
erosion (e.g. Yeaton, 1988).
The human population of the global arid zone increased by
63.5 % between 1960 and 1974. By 1979, 15 % (651 mil-
lion) of the world's population lived in arid lands (Heath-
cote, 1983). It is now estimated that drylands, as de-
fined in UNEP (1992), support over 2 billion people
(UNDP/UNCCD 2010). In some continents, arid areas
are central to human occupation. In Africa, for example,
49.5 % of the total population live in arid areas (UNEP,
1992). Arid areas present a wide range of environmental
hazards for their occupants, many of which are geomor-
phological (Table 1.6), which has prompted a strong and
growing correlation (Goudie, 1985) between arid zone
geomorphology and applied research (see, for example,
Cooke
et al.
, 1982). To carry out applied geomorphologi-
cal research requires a strong underpinning in the salient
aspects of geomorphology and allied disciplines. It is to
1.8.1
Arid zone geomorphology
The role of moisture is often underrated in the assess-
ment of geomorphological activity in arid environments.
Surface runoff, whether occurring ephemerally or episod-
ically, is of considerable importance. Even in the driest ar-
eas, high-magnitude sheet floods can have significant geo-
morphic effects, though they have rarely been observed or
Table 1.5
Plant coping strategies for arid zone moisture shortages.
Escape
Evade
Resist
Endure
Plant type
Annuals and ephemerals
Perennials
Perennials
Perennials
Strategy
Dormant/die during dry
season; survive as seeds
Tap deep water
via extensive
roots
Stored water in stems and
roots
Strategies for reducing
transpiration (inc night
photosynthesis)
Example
Grasses, some herbs
Trees
Cacti and other succulents
Shrubs
