chapter - Atmospheric and Oceanic Science

Hydrological trends
6.4. Relationship between precipitations and EHE
Camilloni and Barros (2003) studied the relationship between the precipitations
and extreme discharges of the Paraná River. They determined that the extreme
peaks at Corrientes usually originate in the central and south areas of the High
Paraná River basin, especially in the central area. Also, they established that the
contribution of the north area of the high basin of the Paraná is not only generally
small, but rather, sometimes negative.
A similar study for the Uruguay River was carried out by Camilloni and
Caffera (2003). Daily extreme flows during the warm season are related to intense
rains in the upper basin, particularly in the period from 9 to 12 days before the maximum
of flow takes place in Salto Grande. On the other hand, the daily extreme
flows during the cold season are due mostly to intensive rains over and close
upstream of Santo Grande in two separate periods: from 9 to 12 and from 1 to 4
days before the date of the flow peak at the Salto Grande station. From these results,
it comes out that floods at the lower Uruguay River can be predicted by hydrological
forecasts during warm season, while meteorological forecasts are needed for
the cold season.
An interesting additional result of the study of Camilloni and Caffera (2003)
it is that around 50% of largest discharges of the Uruguay River can be the result
of the precipitation increment due to the convergence of fluxes of humidity in the
region of the South America Low Level Jet (SALLJ). Also, they observe that the
frequency of occurrence of the SALLJ causing large discharges is slightly higher in
the cold season than in the warm one.
For the Paraguay River, Barros et al. (2004) found that the origin of largest
discharge peaks are in the high and middle Paraguay River basin, and that its occurrence
doesn't depend on the volume of water stored in the Pantanal. Additionally,
they verified that the contribution of the Pantanal does not correlate considerably
with the contribution of the high and middle basins. They explain that the situation
is different for annual ordinary discharges, since the annual peak of June takes place
because the slow contribution of the Pantanal, loaded with the precipitations of the
Summer, on top of the contribution of the high and middle basins of the Paraguay
River caused by the Autumn precipitations. Besides, the decrease of flow from June
to February is due, in Winter, to the small precipitation and, in Spring and Summer,
to the great evaporation.
6.5. Relation between El Niño and EHE
In the work by Camilloni and Barros (2003) about the Paraná River is shown
that two thirds of the major monthly peaks (and of the largest contributions from
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