Jorge Lopez Laborde· Gustavo J. Nagy
7.2.2
Tributary River Discharge
Given the vast area of the drainage basin, its relief and precipitation characteristics
are highly variable, with elevations ranging from 100-2000 m and annual average
precipitation ranging from 200-3000 mm. Although snow-rainfall hydrological behaviour can be observed at the upper basin the hydrological characteristics can be
considered as purely rainfall related. Table 7.1 shows the principal variables of the Rio
de la Plata drainage basin and their values.
The analysis of the hydrologic contributions of the main tributaries to the Rio de
la Plata (Mazio and Martinez 1989) showed significant differences for the period studied (1960-1980). The Uruguay River provides an average flow rate of 4700 m 3 S-1
(Hervidero Gauging Station), with maximum flow rate (6500 m3 S-I) during the winter months and minimum flow rate (3500 m3 S-I) during summer. The Parana River
provides an average flow rate between 15400 m3 S-1 (Rosario Gauging Station) and
17000 m3 S-1 (Corrientes Gauging Station); however, maximum flow rates occur during the summer months and minimum during the fall. This creates a damping effect
on peak flow rates for the combined discharge of the two rivers, producing a combined annual average discharge between 20 000 and 23000 m3 S-1 with minimum and
maximum values of 10 800 and 30600 m3 S-I.
Recently, many authors have reported an increase in the river discharge; Garcia and
Vargas 1994; Genta 1996; Nagy et al. 1996); during the second half of the century (1950S)
began a continuous lineal trend expansion of normal and maximum values, well distinguished from the previous behaviour. In fact, Parana River average flow rate (Rosario
Gauging Station) increased from 14600 m3 S-1 (1884-1975) to 18400 m 3 S-1 (1975-1994);
meanwhile Uruguay River average flow rate (Hervidero Gauging Station) increased
from 4400 m3 S-1 (1916-1975) to 5600 m3 S-1 (1975-1991). Such increments have been
associated to many causes:
a deforestation of the upper river basins and, consequently, reducing of the water retention capacity,
b the development of an excessive moisture hydrological period producing greater
drain off, and
c "El Nino" phenomenon (Mechoso and Perez Irribaren 1992; Nagy et al. 1996).
7.2.3
Meteorology
The general atmospheric circulation is controlled by the influence of the quasi-permanent South Atlantic high pressure system and the continuous passage of low pressure systems that come from the south. This general circulation is modified by a low
pressure system located in northern Uruguay, which has a NW to SE movement generating NE to SE winds.
Rio de la Plata normal weather evolution is controlled by several factors. One of
them is the passage of polar air masses. They penetrate into the continent from the
Patagonian region with a NE direction. When air masses cross over the Rio de la Plata,
wind direction changes from North to South, generating strong wind gusts and rainy
7.2.2
Tributary River Discharge
Given the vast area of the drainage basin, its relief and precipitation characteristics
are highly variable, with elevations ranging from 100-2000 m and annual average
precipitation ranging from 200-3000 mm. Although snow-rainfall hydrological behaviour can be observed at the upper basin the hydrological characteristics can be
considered as purely rainfall related. Table 7.1 shows the principal variables of the Rio
de la Plata drainage basin and their values.
The analysis of the hydrologic contributions of the main tributaries to the Rio de
la Plata (Mazio and Martinez 1989) showed significant differences for the period studied (1960-1980). The Uruguay River provides an average flow rate of 4700 m 3 S-1
(Hervidero Gauging Station), with maximum flow rate (6500 m3 S-I) during the winter months and minimum flow rate (3500 m3 S-I) during summer. The Parana River
provides an average flow rate between 15400 m3 S-1 (Rosario Gauging Station) and
17000 m3 S-1 (Corrientes Gauging Station); however, maximum flow rates occur during the summer months and minimum during the fall. This creates a damping effect
on peak flow rates for the combined discharge of the two rivers, producing a combined annual average discharge between 20 000 and 23000 m3 S-1 with minimum and
maximum values of 10 800 and 30600 m3 S-I.
Recently, many authors have reported an increase in the river discharge; Garcia and
Vargas 1994; Genta 1996; Nagy et al. 1996); during the second half of the century (1950S)
began a continuous lineal trend expansion of normal and maximum values, well distinguished from the previous behaviour. In fact, Parana River average flow rate (Rosario
Gauging Station) increased from 14600 m3 S-1 (1884-1975) to 18400 m 3 S-1 (1975-1994);
meanwhile Uruguay River average flow rate (Hervidero Gauging Station) increased
from 4400 m3 S-1 (1916-1975) to 5600 m3 S-1 (1975-1991). Such increments have been
associated to many causes:
a deforestation of the upper river basins and, consequently, reducing of the water retention capacity,
b the development of an excessive moisture hydrological period producing greater
drain off, and
c "El Nino" phenomenon (Mechoso and Perez Irribaren 1992; Nagy et al. 1996).
7.2.3
Meteorology
The general atmospheric circulation is controlled by the influence of the quasi-permanent South Atlantic high pressure system and the continuous passage of low pressure systems that come from the south. This general circulation is modified by a low
pressure system located in northern Uruguay, which has a NW to SE movement generating NE to SE winds.
Rio de la Plata normal weather evolution is controlled by several factors. One of
them is the passage of polar air masses. They penetrate into the continent from the
Patagonian region with a NE direction. When air masses cross over the Rio de la Plata,
wind direction changes from North to South, generating strong wind gusts and rainy