Baia de Guanabara, Rio de Janeiro, Brazil
111
(June-August) season. During the austral summer, heavy rains can produce intense runoff and flooding of low-lying fringe areas (Alcantara and
Washington 1989).
During the austral winter, polar fronts propagate northward at
500 km d- 1 from the South Atlantic and produce very intense, short-duration southerly winds. A satellite study indicated that an average of 13 cold
fronts of polar origin, one per week, impacted the Baia de Guanabara
coast during the austral winter (Stech and Lorenzzetti 1992). The frontal
system normally passes the bay in less than 24 h, and winds from the
south and southwest may occasionally exceed 25m s- 1 along the adjacent
ocean beaches, at the same time that the air temperature may drop by
5-10°C. Frontal passages are however not limited to the winter, but occur
on a year-around basis with an average of 46 fronts annually. Ocean
swells, 2-4m high with periods of 8-12 s, at times precede the arrival of
fronts. Breaking waves on the adjacent ocean beaches are translated into
occasional, damaging low-frequency oscillations in the lower bay. The
median significant wave height in deep water varies between 1.3 and
1.8 m and has a period of 7 s during normal periods of anticyclonic dominance (Souza 1988). The most frequent winds blow across Baia de Guanabara from the southeast (21% of the time), from the south (17%), and
from the north (14% ). Twenty-four percent of the time winds are less
than 1m s- 1 • The mean wind speed measures is 3m s- 1 , and the maximum
wind in 1990 was recorded in April, blowing from the south at a speed of
14.4m s- 1 •
8.5 Runoff
The Baia de Guanabara drainage basin measures only 4,080 km 2 • It is
drained by 45 small rivers, although six of these are responsible for 85% of
the total runoff. The monthly freshwater discharge rate was modeled by
Kjerfve et al. (1997) based on monthly measurements of temperature and
rainfall form multiple stations in the Baia de Guanabara catchment. The
calculations yielded varying runoff ratios, from a low 0.01 at sea level in the
austral winter to 0.57 at high elevations in the austral summer. The areaweighted mean runoff ratio measured 0.20, which corresponds to a mean
fresh water discharge into Baia de Guanabara of 100±59 m 3 s- 1 , with a
range from 33m 3 s- 1 in July to 186m 3 s- 1 in January, based on the 30-year
normal temperature and rainfall data. To provide sufficient quantities of
potable water for metropolitan Rio de Janeiro, Companhia Estadual de
Agua e Esgoto (CEDAE) pumps 40m 3 s- 1 from Rio Paraiba do Sul, the ad-
111
(June-August) season. During the austral summer, heavy rains can produce intense runoff and flooding of low-lying fringe areas (Alcantara and
Washington 1989).
During the austral winter, polar fronts propagate northward at
500 km d- 1 from the South Atlantic and produce very intense, short-duration southerly winds. A satellite study indicated that an average of 13 cold
fronts of polar origin, one per week, impacted the Baia de Guanabara
coast during the austral winter (Stech and Lorenzzetti 1992). The frontal
system normally passes the bay in less than 24 h, and winds from the
south and southwest may occasionally exceed 25m s- 1 along the adjacent
ocean beaches, at the same time that the air temperature may drop by
5-10°C. Frontal passages are however not limited to the winter, but occur
on a year-around basis with an average of 46 fronts annually. Ocean
swells, 2-4m high with periods of 8-12 s, at times precede the arrival of
fronts. Breaking waves on the adjacent ocean beaches are translated into
occasional, damaging low-frequency oscillations in the lower bay. The
median significant wave height in deep water varies between 1.3 and
1.8 m and has a period of 7 s during normal periods of anticyclonic dominance (Souza 1988). The most frequent winds blow across Baia de Guanabara from the southeast (21% of the time), from the south (17%), and
from the north (14% ). Twenty-four percent of the time winds are less
than 1m s- 1 • The mean wind speed measures is 3m s- 1 , and the maximum
wind in 1990 was recorded in April, blowing from the south at a speed of
14.4m s- 1 •
8.5 Runoff
The Baia de Guanabara drainage basin measures only 4,080 km 2 • It is
drained by 45 small rivers, although six of these are responsible for 85% of
the total runoff. The monthly freshwater discharge rate was modeled by
Kjerfve et al. (1997) based on monthly measurements of temperature and
rainfall form multiple stations in the Baia de Guanabara catchment. The
calculations yielded varying runoff ratios, from a low 0.01 at sea level in the
austral winter to 0.57 at high elevations in the austral summer. The areaweighted mean runoff ratio measured 0.20, which corresponds to a mean
fresh water discharge into Baia de Guanabara of 100±59 m 3 s- 1 , with a
range from 33m 3 s- 1 in July to 186m 3 s- 1 in January, based on the 30-year
normal temperature and rainfall data. To provide sufficient quantities of
potable water for metropolitan Rio de Janeiro, Companhia Estadual de
Agua e Esgoto (CEDAE) pumps 40m 3 s- 1 from Rio Paraiba do Sul, the ad-