Agriculture Reference
In-Depth Information
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the time series, especially in the last 20 years. In Beja, 50% of the extreme
droughts occurred after 1980. In fact, during the 1901-2000 period, this
station recorded the greatest frequency of extreme droughts (4.0%), with
Lisbon and Évora showing similar frequencies (2.0%; Pires, 2003).
The PDSI time series for the three meteorological stations is shown in fig-
ure 14.3. They reveal a high-frequency oscillation of the PDSI between neg-
ative and positive values, superimposed by periods of consecutive months
with negative or positive values, which are almost coincident for the three
stations presented. With respect to the change in variability of the PDSI,
the negative values seem to dominate the last 20 years of the 20th century,
especially in the south inland stations of Évora and Beja. The 1980s begin
with a sudden and large decrease in the PDSI, maintaining a trend for nega-
tive values through several years. According to figure 14.3, the values of the
PDSI in the cooling period 1946-75 are less negative than in the warming
period 1976-2000, suggesting an increased frequency of droughts in the
south of Portugal (Pires, 2003).
A Geographical Information System (GIS) is used to map the PDSI and
monitor the historical evolution of the index in the southern regions of
Portugal that are the most affected by droughts (Pires, 2003). The percent-
age of time with mild, moderate, and severe drought was mapped for the
southern region from 1961 to 2000 (Pires, 2003). During this period of
40 years, the percentage of time with mild drought (PDSI values below -
1.0) in a great part of the region occurs between 30 and 40% of time. The
percentage of time in moderate drought is lower; however, a large area with
15-20% of time in moderate drought is still observed. The percentage of
time with severe drought is low, although in a small region it reaches nearly
10%.
A statistical analysis of long climatological series of the PDSI was made
for the southern region of mainland Portugal. The PDSI average was cal-
culated for the last four decades since 1961. An increase in severity is ob-
served in most of the months in the first three decades, while in the last
decade the drought intensity, although it has increased in some regions,
does not increase significantly as compared with the previous one. In the
period from February to April the increase is more significant, changing
from normal conditions (PDSI with small positive values) to conditions
of mild and moderate drought, especially in the months of February and
March (figure 14.4). As these are decade-average values, this change is very
significant. No relevant change is noticed during the summer period (June
to August) over the four decades because this period is normally very dry
(Pires, 2003).
As a means of validation of the PDSI as a tool to monitor agricultural
droughts in Portugal, three situations, which occurred in the 1990s (1991-
92, 1994-95, and 1997), were analyzed. The example of the meteorolog-
ical station of Beja is presented in table 14.1 (Pires, 2003). In these three
periods of drought, the PDSI values show the occurrence of severe and
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