Main Causes of Drought

General Meteorological Conditions

The geography of the mainland Portugal favors the occurrence of droughts. The subtropical anticyclone in the North Atlantic is situated in a blocking position that prevents disturbances on the polar front from reaching the Iberian Peninsula. Although the overall climate for mainland Portugal varies little, significant variations, temporal as well as spatial, exist in temperature and precipitation, causing droughts.

Precipitation Mean annual precipitation in mainland Portugal is around 900 mm. The northwest region of Portugal is one of the wettest spots in Europe, with mean annual precipitation exceeding 3000 mm, but the average rainfall in the interior of Alentejo and in the northeastern part of the territory is about 500 mm and is characterized by high variability.

On average, about 42% of the annual precipitation falls during the 3-month winter season (December-February). The lowest precipitation (only 6% of the annual precipitation) occurs during the summer season (JuneAugust). During the transition months (March-May, October-November), precipitation is highly variable.

A statistical analysis of long-term records (1931-2002) of annual precipitation data over mainland Portugal shows that, since 1982, only 6 years had precipitation values above the 1961-90 mean. Precipitation significantly declined during the spring season, slightly declined during the winter season, and slightly increased during other seasons. The decrease in annual precipitation in Portugal during 1976-2002 accompanied the same percentage decrease in the number of wet days. The number of consecutive dry days for several stations in Portugal showed an increasing trend from the 1970s onward. This trend was more evident in the southern region. In general, the number of wet days and dry days show a weak tendency for more extreme events during 1976-2002, especially in the south of Portugal.

Temperature The mean annual air temperature varies between 7°C in the inner highlands of central Portugal and 18°C in the southern coastal area. The annual number of days with minimum temperature below 0°C (frost days) reaches a peak in the highlands of northern and central inland, with more than 100 days/year, and is nil in the western coastal and southern zones. The number of days with minimum temperature above 20°C (tropical nights) and maximum temperature above 25°C (summer days) and above 35°C (hot days) is higher in the inner center of the country, the eastern part of Alentejo and the seaside Algarve. This indicates cold and warm spells in the Portuguese climate, which have significant impacts on agriculture.

An analysis of temperature data (1931-2002) in mainland Portugal revealed a general trend toward an increase in the mean annual temperature since 1972. The year 1997 was the hottest of the last 70 years, and it was a year with a severe drought. The 6 warmest years occurred in the last 12 years, and 2002 was the 16th consecutive year with a minimum temperature above normal (i.e., above the 1961-90 mean). There has been a reduction in the frequency of extreme low temperatures without an equivalent increase in the frequency of extreme high temperatures.

0 0

Post a comment