Monitoring Agricultural Drought in Mainland Portugal - Monitoring and Predicting Agricultural Drought: A Global Study - page 184

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46

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 (June-
August). During the transition months (March-May, October-November),
precipitation is highly variable.
A statistical analysis of long-term records (1931-2002) of annual precip-
itation data over mainland Portugal shows that, since 1982, only 6 years
had precipitation values above the 1961-90 mean. Precipitation signifi-
cantly declined during the spring season, slightly declined during the win-
ter 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.

[184

Line
——
0.0
——
Norm
PgEn

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 (trop-
ical 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 re-
vealed 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 tem-
perature above normal (i.e., above the 1961-90 mean). There has been a
reduction in the frequency of extreme low temperatures without an equiv-
alent increase in the frequency of extreme high temperatures.

[184

D roughts in Mainland Portugal
Droughts are common occurrences in mainland Portugal. During the
1976-2002 periods, severe droughts occurred during 1980-81, 1982-83,
1991-92, 1994-95, and 1997.
Between September 1991 and May 1992, the absence of rainfall caused
a long drought that directly affected winter cereals and pastures. This