Geoscience Reference
In-Depth Information
Tabl e 1 . 1 .
Aridity zones defined by P/E
pot
Ratios
(After UNEP,
1992a
,UNEP,
1992b
)
%oftheworld
covered
Climate zone
P/E
pot
ratio
Hyper-arid
<
0.05
7.5
Arid
0.05-0.20
12.5
Semi-arid
0.21-0.50
17.5
Dry subhumid
0.51-0.65
9.9
Humid
>
0.65
39.1
Cold
>
0.65
13.5
In these instances, the
effective precipitation
is high enough to sustain plant growth,
regardless of the absolute amount.
Tabl e 1 . 1
shows that deserts and their semi-arid margins thus occupy 37.5 per cent
of the land area of the globe, and if we include the dry subhumid regions, where
mean annual rainfall may range from 750 to 1,500 mm, 47.4 per cent of the terrestrial
surface. Given that roughly one in five persons now on this earth live in these drylands,
it is important to understand how these lands have evolved through time and how they
may change in the future.
1.3 Polygenic nature of desert landscapes
Desert landscapes are akin to ancient palimpsest maps in that they consist both of
very young depositional landforms and of very old erosional landforms (Mabbutt,
1977
; Frostick and Reid,
1987a
;Cookeetal.,
1993
; Abrahams and Parsons,
1994
;
Thomas,
1997
; Williams,
2002a
; Laity,
2008
; Parsons and Abrahams,
2009
; Thomas,
2011
; Goudie,
2013
). The young landforms include dunes, alluvial fans, salt lakes and
alluvial channels. The old landforms include mountains, hills and plateaux (
Figures
1.2
to
1.6
). It is misleading to assume that the landform assemblages that we find in
present-day deserts has developed under entirely arid conditions. In fact, few have
done so, because most of the major erosional landforms were shaped under previously
wetter climates and have been preserved from further erosion by the onset of aridity.
Many desert landforms are exceedingly old. The vast desert plains of the central
Sahara and western Australia have been exposed to subaerial denudation for far more
than 500 million years, under very different climates from those prevalent today (Wil-
liams,
2009a
). Desert monoliths such as Ayers Rock (Uluru) (
Figure 1.7
) in central
Australia or the granite inselbergs of the Sahara (
Figure 1.8
), far from being dia-
gnostic of aridity, owe their present morphology to prolonged and repeated phases
of weathering and erosion under a succession of former climates, few of which were
particularly arid. The abrupt juxtaposition of very ancient erosional landforms and
