chapter - Atmospheric and Oceanic Science

Following are the observed changes:
Hydrological trends
• Mean flow in the modern stage is 37% superior to in the old stage.
• Annual maximums of the modern stage have also increased regarding the old
one. Also, the frequency of extraordinary discharges has increased: Out of the 4
discharges with peak flows higher than 50.000 m3 /s (1905, 1983, 1992, and
1998), 3 took place in the modern stage.
• Annual minimums also show an increment when passing from the old stage to the
modern one.
• The current situation indicates a sequence of annual mean and maximum values
smaller than those of the modern stage. In particular, mean values are similar to
those of the 1970 decade (transition stage), the maximums to those of the old
stage and the minimums to those of the modern stage.
The trend change from the 1970 decade was rigorously established by García
and Vargas (1998), by means of statistical techniques, based on data of monthly
mean flows in stations located upstream from Corrientes, analyzing the period
1901-1992. This was corroborated later by Jaime and Menéndez (2002) using
monthly mean flows in the period 1931-2001 for the section Paraná-Santa Fe.
The increase of mean flows means that there is a larger runoff in the high
basin of the Paraná, where precipitations that feed the river take place. This increase
of the runoff should be bound to the increment of mean precipitations and to the
change in the use of the land.
The increase in frequency of the large discharges indicates changes in the climatic
conditions that favour the generation of extraordinary storms. The increase of
minimum flows is strongly linked to the regulation imposed by the chain of
Brazilian reservoirs developed since the 1960 decade.
The current situation is still uncertain in terms of determining if it is a new
tendency change or if it simply responds to climatic variability matters. Eventually,
the permanence of minimum levels similar to those of the modern stage confirms
that it is due to the regulation imposed by the chain of reservoirs.
Figures 6.2 and 6.3 show annual minimums and maximums of the Paraná
River monthly mean flows, in the section Paraná-Santa Fe, and of the Uruguay
River, in Concordia, for the period 1931-2001, based on the data provided by the
Sistema de Información y Alerta Hidrológico del INA (System of Information and
Alert Hydrological of the INA). Trends observed in the first one are, obviously,
compatible with the ones discussed in relation to figure 6.1. For the Uruguay River
(Fig. 6.3) the increase of mean flow is similar to the one of the Paraná (Jaime and
Menéndez 2002); although due to the variability of its regime this tendency is less
noticeable at first sight.
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