Monitoring Agricultural Drought in Mainland Portugal - Monitoring and Predicting Agricultural Drought: A Global Study - page 183

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

There is a lag between the rainy season and the growing season. The
critical period for water requirement by crops is in the spring. The lack of
the coincidence between radiation and heat on the one hand, and soil water
availability on the other, limits the agricultural productivity and crop yields
in drier regions that do not have irrigation facilities (Pinto and Brand ao,
2002). If in some years the cold season is dry, it leads to water stress and
damages dry land crops, mainly wheat, rye, oats, and barley.

Vu lnerability to Drought
An aridity index (the ratio of annual precipitation to potential evapotran-
spiration) was computed for the 1961-90 period (figure 14.1). Evapo-
transpiration was computed using the Penman method (Penman, 1948),
as proposed by the United Nations Environment Programme (UNEP). In
accordance with this criterion, dry subhumid regions represent 23%, and
semiarid regions represent 11% of the territory. 1 This means that measures
should be taken in about 34% of the territory to prevent further degrada-
tion of the soil in the most drought prone areas subject to desertification.
These problem areas are located in the southeastern zone (Alentejo) and
the northeastern zone (figure 14.1).
Alentejo represents almost one-half of the total cropland of the main-
land. However, 60% of irrigated land is in the northern and central regions
(GPPAA, 1999). In Alentejo, few areas are irrigated, although there has
been an increase in the last few years. This region of Portugal is particu-
larly vulnerable to droughts, which have frequently occurred here during
the last few years. Due to the seasonal distribution and high variability in
precipitation, agricultural practices are highly dependent on irrigation dur-
ing spring and summer seasons (IHERA, 1999). A strong Mediterranean
influence and declining spring precipitation have contributed to the de-
sertification of some rural areas, mainly in the southern and northeastern
regions. In addition, conventional farming practices have contributed, to
some extent, to land degradation and subsequently to desertification.

[183

Line
——
0.5
——
Long
PgEn

[183

M ain Causes of Drought

G eneral Meteorological Conditions

The geography of the mainland Portugal favors the occurrence of droughts.
The subtropical anticyclone in the North Atlantic is situated in a block-
ing position that prevents disturbances on the polar front from reaching
the Iberian Peninsula. Although the overall climate for mainland Portu-
gal 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