Geoscience Reference
In-Depth Information
to remain very low ('soil moisture utilization' and 'soil
moisture deficit') until the rains of October and
November. Available soil moisture, together with temper-
ature, is the dominant control of the productivity of
the region's vegetation. An additional limitation on
ecosystems is that precipitation varies greatly from year
to year; interannual variations, measured by the
'interannual coefficient of variation', reach 25-35 per
cent), and available records of rainfall show significant
periods of wetter and drier rainfall.
In addition to the limitations of the summer drought
and rainfall variability from year to year, a further
important characteristic is the intensity of rainfall from
Mediterranean depressions, falling on bare, dry soils in
autumn. A raindrop can reach 6 mm diameter in size,
giving a terminal velocity (maximum sustained speed) of
10 m s
-1
. The amount of work done and the erosion
caused by such storms, infrequent though they are, is out
of all proportion to the relatively small amounts of
precipitation involved, as discussed later in this chapter.
Temperature, precipitation and
evaporation
Figure 25.4
shows the average temperature and moisture
climatic elements for Heraklion, Crete. The conventions
used are those proposed by Walter and Lieth (
Chapter
20).
Precipitation shows a unimodal distribution, with
one peak in January. North Africa and the eastern
Mediterranean show this simple regime with a single
winter maximum. A bimodal annual regime with two
peaks in November and March is characteristic of the
western and central Mediterranean in Spain, southern
France, Italy and the Balkans. High summer temperatures
lead to the high levels of potential evapotranspiration (PE)
shown in
Figure 25.4
.
The monthly potential evapotran-
spiration totals are calculated by a method developed by
C. W. Thornthwaite on the basis of air temperatures (see
Chapter 5).
It gives a general guide only, but illustrates how
the higher soil moisture levels of winter ('soil moisture
recharge' and 'soil moisture surplus') fall rapidly in April
50
100
Absolute maximum recorded - 45.7°
Mean of warmest month - 26.4°
Altitude - 40m
Mean annual temperature - 19°
Mean annual precipitation - 492mm
40
80
30
60
mm
Rainy
season
20
40
Dry
season
°C
20
10
Air frost
possible
Mean of coldest month - 12.2°
Absolute minimum recorded - -0.5°
0
0
SONDJFMAMJJAS
Source: After Rackham and Moody (1996)
