On the change of the annual streamflow cycle of the Paraná River

The Paraná River basin expands over a large area of Southern Brazil and Northeastern Argentina. This river is the most important of the Río de la Plata basin, both in terms of the magnitude of its flow and of the human settlements along the riversides. Floods along the Argentine riversides are associated to huge economic and social damages as well as important environmental impacts. The considerable number of big dams on the Upper and Medium Paraná River have been reported as responsible for the modification of some features of the annual cycle of the river discharge. The greatest impact should be around 1983 when Itaipú, the largest dam of the world and other dams started to operate. It has been shown that after that year the annual cycle of the Paraná River is characterized by a reduction of the summer maximums and an increase in the maximums of the early winter.

Monthly and annual mean streamflows of gauging stations on the Paraná River were analyzed for the periods 1904-93 and 1931-92 respectively. Positive trends were found in the annual mean river discharge. In order to detect the year of change in the annual mean streamflow trends, a two-phase regression model was applied. It was found that the changepoint year was around 1980 and that the annual mean discharge increased significantly after that date. In order to determine the causes of the increase of the Paraná River discharge, rainfall anomalies trends were analyzed for the Upper and Medium Paraná River basins and for two of its tributaries: the Iguazú and Paraguay basins. It was found that the increment of the winter streamflow was due to enhanced autumn and early winter precipitation over the Upper and Medium Paraná, Iguazú and Lower Paraguay basins after 1980. These results indicate that dams are not the only factor responsible of the change of the Paraná River discharge annual cycle.