Environmental Engineering Reference
In-Depth Information
in the western basin - yields the important result that, although winter is a rainy period,
there are relatively few rain days. Figure 26.3 shows the rain days for Malaga, Spain
(precipitation 447 mm) and, for comparison, Los Angeles, California (precipitation 386
mm). Mean annual rainfall varies between 300 mm and 750 mm in Mediterranean
regions, falling on forty to eighty rain days. Variations in precipitation totals result from
altitude, with orographic rainfall being added to frontal, from rain shadow effects, and
from the exposure of coastal areas to onshore winds from areas of cyclogenesis in the
Mediterranean Sea.
Figure 26.4 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 (pattern) with a single winter
maximum. A bimodal annual regime with two peaks in November and March is
characteristic of Spain, southern France, Italy and the Balkans. The double maxima are
interpreted as transitional between the Continental Interior type, with a summer
maximum, and the Mediterranean type, with the winter maximum. The high summer
temperatures lead to the high levels of potential evapotranspiration (PE) shown in Figure
26.4. The monthly potential evapotranspiration 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 to remain very
low ('soil moisture utilization' and 'soil moisture deficit') until the rains of October and
November. Available soil
Figure 26.3
Rain days (precipitation of 0ยท25 mm or more) in
Los Angeles, California, and Malaga, Spain, during the year.
