Geoscience Reference
In-Depth Information
Table 5.1
Quantitative classifications of arid zones. (Based on Thomas 1997a.)
Classification
and use
Data used
Comments
Classification
UNESCO
1979
Uses Meigs' (1953) classification
together with Thornthwaite's (1948)
indices of moisture availability (
Im
)
Im
Semi-arid
Arid
Hyper-arid
Does not include areas
too cold for crops
(e.g. polar deserts)
60
D
)/
PET
where
S
is the moisture surplus,
M
the
moisture deficit and
PET
is potential
evapotranspiration.
S
and
M
are
aggregated on an annual basis from
monthly data, taking stored soil
moisture into account
Meigs 1953 classification, with rainfall
figures
=
(100
S
−
Grove 1997
Semi-arid
=
200-500 mm yr
−1
Approximate rainfall
figures. Figures available
over widely varied time-
scales spatially
PET
calculated by
Penmans formula using
data not globally available
Includes dry subhumid
areas
Arid
25-200 mm yr
−1
Hyper-arid
=
=<
25 mm yr
−1
UN 1977
P
/
PET
index: where
P
is annual
precipitation and
PET
is potential
evapotranspiration
Aridity index (
AI
)
AI
Semi-arid 0.20
≥
AI
<
0.50
Arid 0.03
≥
AI
<
0.20
Hyper-arid
AI
0.03
Dry-subhumid 0.50
<
UNEP 1992
≥
AI
<
0.65
P
/
PET
where
P
is annual precipitation and
PET
is potential evapotranspiration.
PET
is calculated using the
Thornthwaite method. Data are taken
from time-bounded study
=
Semi-arid 0.20
≥
AI
<
0.50
Arid 0.05
≥
AI
<
0.20
Hyper-arid
AI
<
0.05
occur over several days but generate only limited
runoff, to high-intensity rainstorm events, which
generate flash flood events but may occur only
two or three times in a 100-year period. Regions
of such restricted rainfall are most commonly
related to four main individual climate causes,
which may be related in part to tectonics (see
section 5.1.2.2). In addition the individual causes
listed below may interact with each other, rein-
forcing conditions of aridity.
1
Global atmospheric circulation
. Areas dom-
inated by high-pressure cells and hot (such as
the tropics of Cancer and Capricorn) or cold dry
subsiding air (the North and South Poles) will
have limited rainfall capability.
2
Continentality
. In large continents (e.g. the
interior of Asia) available moisture may have
been precipitated out in the more coastal areas
of the landmasses.
3
Rain-shadow created by high mountain areas
.
In coastal ranges such as the Rockies, USA, moist
air derived from evaporation over the ocean
rises and is cooled adiabatically. Thus vapour
saturation is reached quickly and precipitated as
rain or snow. The desiccated air will then descend
across the mountain range, becoming warmer
and drier into the area of rain shadow.
4
Cold upwelling ocean currents.
The most
prominent examples are the Humbolt current of
the Atacama Desert and the Benguela current
of the Namib Desert. Both these ocean currents
are derived from the colder, southern polar
latitudes and flow northwards. Their associated
cool moist ocean air reaches relatively warmer
land, where its relative humidity decreases and
its capacity to absorb moisture is increased. Thus
the oceanic air mass tends to desiccate these
coastlines. The main source of moisture in such
areas is coastal fogs such as the 'camanchaca' of
the Atacama.
Moisture in arid areas will not be solely
sourced from rainfall events. Coastal fog may
be important in arid regions that border oceans
(e.g. the Atacama Desert of South America and
