chapter - Atmospheric and Oceanic Science

Hydrological trends
The most important floods in the Paraguayan territory happened during June
1997 (with an area flooded of 31400 ha). An event of similar happened during
November 1998, flooding 29400 has. This covered a third of the total area of grassland
and palm tree savannah in the vicinity of the Paraguay River (approximately
90000 has area). The extension of these habitats suggests that, in exceptional cases,
all the area could be flooded. The flooded minimum area was in November 1999,
with 6870 has. It is interesting to notice that only one month later (December 1999)
the flooded area had almost increased 4 times, to 22600 has, which illustrates the
speed of the changes in the local regime.
To characterize the intensity of the EHE of the Paraná and Uruguay rivers,
Jaime and Menéndez (2002) established as criterion to take the excess (for floods)
or defect (for ebbs) flow (monthly mean) with respect to of each of floods waters
and ebbs thresholds, respectively. To define these thresholds an analysis of frequency
of occurrence of extreme events was made and were chosen those corresponding
to a 4 year recurrence, representative of El Niño's periodicity. But the three hydrological
periods identified by García and Vargas (1998) were distinguished: mean up
to 1943, dry up to 1970 and humid up to 2001, so thresholds values are different for
each one of them. These values are shown in table 6.2 (Jaime and Menéndez 2002).
Table 6.2 Thresholds of floods and ebbs events for Paraná and Uruguay rivers
in m 3 /s (Jaime and Menéndez 2002).
Table 6.2. Thresholds of floods and ebbs events for Paraná and Uruguay rivers in m 3 /s
Río Umbral 1931-43 1944-70 1970-2001
Paraná Floods 27800 28500 32000
Paraná Ebbs 8200 8250 11000
Uruguay Floods 13700 12000 17300
Uruguay Ebbs 600 600 850
[Jaime and Menéndez 2002]
Figures 6.11 and 6.12 show the results for the Paraná and Uruguay rivers,
respectively, identifying the month of occurrence. For the Paraná River (Fig. 6.11)
it is observed that from 1970 on, not only larger floods have occurred, but also relatively
more intense ebbs. This indicates an increase of the variability mentioned in
the previous section. For this period, the intensity of the larger floods of the Paraná
River were between 15 and 20 thousand m 3 /s while that of the ebbs were between
500 and 1000 m 3 /s, with a singular negative peak of 2000 m 3 /s in December 1971.
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