chapter - Atmospheric and Oceanic Science

Introduction
as they pour water over tens of thousands of km 2 with great intensity in relatively
short periods of time, often causing severe floods. SESA has been identified as one
of the world's regions with more frequency of MCSs (Nesbitt and Zipser 200l).
Thus, heavy precipitation is a distinctive feature of the SESA climate, and its positive
trends are resulting in more frequent floods.
Increased precipitation has led to increased river discharge (García and Vargas
1998; Genta et al. 1998); since evaporation -controlled by temperature- appears not
to have changed too much (Berbery and Barros 2002, see also chapter 7). The percent
rate of change of the river discharges was amplified when compared to the corresponding
rate of change of La Plata basin average precipitation (Berbery and
Barros 2002; Clarke 2003; Collishonn et al. 2001). This feature can be attributed
partially to deforestation and land use change that resulted in increased runoff
(Tucci and Clark 1998; Collinshon et al. 2001). However, interannual changes in
rainfall and river discharge occurs also between consecutive years. Since in this
case, the impact of the land use change is irrelevant, this result leads to infer that
the amplification in the streamflows is a natural and intrinsic feature of this system
(Berbery and Barros 2002). This characteristic increases the vulnerability of the
activities that depend on water resources to precipitation changes, which in the current
context of Climate Change is a relevant issue.
The most adverse effect of this change is the greater frequency and severity of
flooding, both at the river valleys and in extensive flat areas of the Pampas. Despite
the better hydro-meteorological forecasts, damages due to intense rainfalls and consequent
flooding has been increasing as a result of the regional climate trend and of
the increased occupation by settlements and agriculture of areas that until recently
had relatively low risk of flooding.
In the alluvial valleys of the Paraná, Uruguay and Paraguay, floods have
become more frequent since the middle seventies. During the 20th century, 12 out
of the 16 highest monthly discharges of the Paraná River in Corrientes occurred in
the last 25 years and four of the five highest monthly discharges have also occurred
during this last period (Camilloni and Barros 2003). In the Paraguay River, eleven
of the 15 largest floods in Asunción during the last century have also occurred after
1975 (Barros et al. 2003) whilst for the Uruguay River, none of the 16 largest peaks
since 1950 occurred before 1970 (Camilloni and Caffera 2003). These examples
indicate the severity of impact that regional climate trends have caused in the intensity
and frequency of floods.
The cost of coping with these changing conditions is extremely high. As but an
example, if losses are measured as a percentage of the Gross Domestic Product
(GDP), Argentina is one of the 14 countries most affected by floods, with losses estimated
as high as 1.1% of its GDP (World Bank 2001). These and others impacts of
the recent trends in the hydrology of the La Plata basin are discussed in chapter 8.
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