Page 83 - Arabian Studies (V)

Water Resources and Agriculture in Qatar 73
speed whilst temperatures remain very high. This period is the most
uncomfortable and trying time of the year.
Climatic phenomena of particular concern to agriculture are the
very high summer maximum temperatures that exceed 42°C for a
period of forty-five days in July-August and thus inhibiting photo­
synthesis, pollination and growth; the hot dessicating winds of
May-July which impose a high water stress on plants; and the high
temperatures and humidity of August-September which provide an
ideal environment for insect pests and disease outbreaks which
reach a maximum at this time of the year. During the winter
growing season the large diurnal range in temperature (7°-25°C)
and the high light intensity has an adverse effect on vegetable
quality. Evapo-transpiration, as calculated by a modified Penman
equation, shows values that range from a minimum of less than 2
mm./day in December to a maximum of 10 mm./day in June.
During the main growing season of November-May, evapo-
transpiration values fall within the range 2 mm./day—7 mm./day.
While attempts have been made to extend and increase production
during the summer months the basic climatic parameters impose a
limit to summer cultivation. Very few plants are capable of taking
up water to meet an energy demand of more than 9 mm./day and
the application of additional water at this time is merely wasteful.
Rainfall over Qatar is derived chiefly from Mediterranean-type
depressions and from a localised weather phenomenon known as a
boundary front (Fisher, 1950). These depressions occasionally
retain sufficient energy for their associated cold fronts to have an
attenuated effect on the weather over Qatar but, in general, the
peninsula lies, in winter, in a region of warm subsiding air associa­
ted with the winter position of the sub-tropical jet stream. (Walker,
1967). The probability of widespread rain over Qatar is therefore
low because this upper air subsidence inhibits convection and
causes a blocking effect to Mediterranean type cold fronts moving
across northern Saudi Arabia and Iraq. Occasionally however these
fronts do penetrate as far as Qatar, on an average of perhaps twice
during a winter season, causing widespread rain of light to medium
intensity and sometimes accompanied by high intensity cells. Rain­
fall from boundary fronts may however be of high intensity parti­
cularly during the early and late winter season, such as in December
1964 when 190 mm. fell over a five-day period and again in April
1972 when 68 mm. fell in just over one hour over a small storm area
of 3000 km2, of northern Qatar. From a probability analysis of
existing rainfall data (Pike et al., 1975) it is apparent that there is a
90 per cent probability that at least one significant storm over 10
mm. will occur each winter season, a 50 per cent probability that