Geoscience Reference
In-Depth Information
100%
80%
60%
40%
20%
0%
SW USA
Sahara
Arabia
Libyan Desert
Mountain
Desert flat
Regions bordering through flowing rivers
Fan and bajada
Badland
Playa
Bedrock field
Dry wash
Aeolian dune
Volcanic features
Fig. 5.3
The percentage of
landscape units found within four
typical deserts. (Data from Clements
et al. 1957.)
associate deserts with areas dominated by exten-
sive sand dune systems (sand seas). In reality,
however, these landscape elements are relatively
minor compared with other elements such as
mountainous areas (Fig. 5.3).
Drylands
are classified quantitatively into
hyper-arid, arid, semi-arid and dry-subhumid
using meteorological data. These classifications
are defined using the balance of moisture inputs
(precipitation) and losses (evapotranspiration)
expressed as an aridity or moisture index. This
index is typically expressed as the ratio of
P
(precipitation) to
PET
(potential evapotranspi-
ration). Classifications vary, however, depend-
ing on the data used (Table 5.1). For example
Meigs (1953) used aggregated monthly to
annual moisture surplus and deficit data to rep-
resent
P
and the Thornthwaite approach to
calculate
PET
. These data are not widely avail-
able from arid regions. More recent definitions
therefore have tended to use annual precipita-
tion for
P
and the Penman method of calculating
PET
, which utilizes an understanding of the
diffusion of water vapour and aerodynamic
functions, data which are universally avail-
able. To further facilitate a comparable global
coverage the data sets used in the most recent
definitions also tend to be time-bounded to
avoid data bias from regional data sets where
the records are less continuous. The time period
spans some 30 years to take account of inter-
annual and interdecadal variations in climate.
Variation in the data sets used means that
figures on the modern extent of drylands range
from 26.3 to 47.2% of global land area,
depending on the classification used (Thomas
1997a).
5.1.2 Causes of aridity
5.1.2.1 Climate
Arid environments are typified by variable rain-
fall (characteristically more than 30% of the
mean, Fig. 5.2b). This rainfall will range from
a number of low-intensity events, which may
