Agriculture Reference
In-Depth Information
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agricultural year was classified as extremely dry in the whole territory
(Report on Climate Conditions, 1992).
Between September 1994 and October 1995, a severe drought occurred;
this period was classified as extremely dry, especially in the southern region
of Portugal. These conditions caused serious damage to winter cereals
(Report on Climate Conditions, 1995).
The meteorological drought event of 1997 was so intense that, within
three months, it gave rise to a severe agricultural drought. In these three
months (February, March, and April) it did not rain at all and the tem-
perature was well above the average, which caused dryness of the soil that
affected winter cereals. In Alentejo, 70% of the cereal production was lost,
and a state of crop calamity was declared (Report on Climate Conditions,
1997).
[185
Yi
eld Losses of Main Crops
Line
——
0.0
——
Norm
PgEn
Figure 14.2 shows the variation in wheat yields for Portugal for the 1970-
99 period. A higher average yield with higher variability can be observed
for the second half of the period. From 1977 to 1979, too much rainfall
caused a very low yield for wheat, as well as for other crops. Yield losses
from 1981 to 1983 were caused by a prolonged and severe drought. In
1992 and 1995 also, drought caused significant yield losses. The low yields
in 1997 and 1998 were caused by irregular rainfall distribution (i.e., sur-
plus in autumn and in the beginning of winter, but deficit in February and
March) and untimely heat during critical growing phases (GPPAA, 1999).
[185
D
rought Monitoring
Drought is monitored using different indices. To characterize drought
spells in mainland Portugal, some simple indices such as the percentage
of normal have been used. To monitor drought situations in Portugal, the
Palmer drought severity index (PDSI) (Palmer, 1965) is now being used.
The next step will be the implementation of the standardized precipitation
index (McKee et al., 1995) and its comparison with PDSI.
Pr
ecipitation Deviation and Deciles
The percentage of the normal precipitation is one of the simplest methods
of quantifying rainfall for a given location, and it is very effective when
used for a single region or a single season. However this criterion can be
misleading, because it is not standardized for varying environments.
The decile was developed by Gibbs and Maher (1967). The distribution
of occurrences over a long-term precipitation record is divided into tenths
of the distribution. Each of these categories is called a decile. The first decile
is the rainfall amount not exceeded by the lowest 10% of the precipitation,
and so on, until the rainfall amount identified by the tenth decile is the
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