The Paranparaguay River Basin Interannual Variability And Extreme Events

The Paraná-Paraguay River Basin in South America encompasses about 84% of the La Plata Basin with a population of about 100 million (1992 figures). It is the most developed agricultural and industrial zone in South America, accounting for 80% of the economic production in Argentina, Brazil, and Paraguay. The period of relatively small numbers of high floods between 1905 and 1960 coincided with the expansion of urban areas onto the valley bottoms. Three major changes have taken place in the basin since the 1960s (Penning-Rowsell, 1996). First, agricultural and industrial production has increased. The Upper Paraná Basin in Brazil has been converted from coffee plantations, developed in the 1940s, to fields of soybeans and sugar cane (for alcohol fuel production). Second, deforestation to establish cropland and pasture has been extensive in both Brazil and Paraguay. In Paraguay forested regions declined from about 45% in the eastern region in 1945 to about 15% in 1992. In northern Argentina and southern Brazil forest losses range from 60 to 90% in this century. Third, about 20 hydroelectric power plants have been built on the river, significantly changing system hydrology. For example, the width of the Paraná River varies dramatically from 4 km north of Guaira (Brazil) to about 60 m below the Itaipu Dam. As in other parts of the world, the increasing damages caused by extreme rainfall events result from urban and agricultural encroachment onto the floodplains, as much as from the events themselves. Most of these floodwaters (up to 85% on the Paraná) come from the Upper Basin in Brazil. Accompanying these changes are heavy sediment flows from agricultural lands bordering the Paraná.

The Upper Paraguay Basin has low river banks and is prone to flooding, creating a zone known as the 'Pantanal'. Flooding in the Paraná-Paraguay Basin has become both more frequent and more severe in recent years. In particular, the 1983 flooding cost an estimated US$1.8 billion, while 1992 flooding caused serious damage to infrastructure and capital stock resulting in estimated damages of up to US$ 1 billion and affecting 3.1 million people. The cost estimates of more recent and widespread flooding during the 1998 to 2000 period are as yet incomplete. Ten-year flood discharge rates are now over 15% greater than those in the early twentieth century (Anderson, 1993). In addition, the low-river flows in the watershed have been less frequent and less extreme (i.e., not as dry) in the latter part of the twentieth century. Five of the 10 largest floods recorded during this century (as measured by daily peak discharge) have occurred since 1982. At present El Niño (warm) events may provide the best explanation as a driver of increased precipitation during the rainy season

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(March-May). The relative contribution of El Niño as opposed to La Niña-related teleconnections results in a difference of about 20% of annual streamfiow. However, it is clear from the record that the general trend in streamfiow is upward, and the hydrological regime of the rivers appears to have been changing primarily since 1940. It should be noted that there are also references to high flood events during the nineteenth century.

The human occupancy of these floodplains reflects their economic value for agriculture, communications, and transportation. Flood losses will increase because people are concentrated in flood-prone urban areas, there is increasing migration from rural areas, and land use is poorly regulated. Wet-season rainfall in the region has also been relatively high since the early 1980s. Good relations among Brazil, Paraguay, and Argentina are important for shared use of flood-forecasting data and mitigation strategies, including land-use changes and reforestation. Interagency rivalries within each of these countries restrict the capacity to act internationally, especially under crisis conditions. The prognosis for implementation of sustainable, preventive, nonstructural alternative approaches to reducing exposure (property at risk) and social vulnerability over the long-term does not appear promising.

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