ENSO-triggered exceptional flooding in the Paraná River: Where is the excess water coming from?

SummaryThe Paraná River has been increasing its annual flow during the last ∼30 years. The relative contribution of its major tributaries (i.e., the upper Paraná and Paraguay rivers) is uneven in as much as the Paraguay is increasing its annual discharge at a higher pace than the upper Paraná does. Contrastingly, the upper Paraná has been increasing significantly its flow during the second half of the year (i.e., historical low water period) whereas the Paraguay River has been amplifying its flow throughout the hydrological year. The variability of δ 18O measured in the Paraná River middle reach tends to follow Paraguay’s relative contribution to Paraná’s total discharge. A simple model built on the basis of the mean δ 18O signature of rainfall (Global Network of Isotopes in Precipitation) shows significant coherency with the relative contribution time series during non-El Niño periods but it is necessary to invert the mean isotopic values in precipitation during the occurrence of a major ENSO event (i.e., assume that δ 18O in upper Paraná River water becomes more negative than usual) to improve the resemblance to the observed variability.