River flow trends in La Plata Basin: relationship with ENSO phases.

MOIRA DOYLE; VICENTE BARROS; RICARDO VIDAL

Perugia, Italia

Congreso; IUGG, XXIV. Earth: Our changing planet; 2007

IAMAS - International Association of Meteorology and Atmospheric Sciences

During the second half of the twentieth century trends were observed in the river flow series of the principal rivers that compose the La Plata Basin, i.e. Paran, Uruguay, Paraguay and Iguaz rivers. The relationship of these trends with the different phases of El Nio-Southern Oscillation (ENSO) is explored.As river flow changes are characterized by linear trends, under certain conditions, the total linear trend is the result of adding the partial linear trends corresponding to each of the ENSO phases, i.e. El Nio, La Nia and Neutral events. When splitting a series in various sub series, the linear trends of these sub series add up to the same value as the linear trend of the entire series if each of the sub series has its elements ordered in time around the same mean value and variance as the complete series. A linear trend is a special case of a linear regression slope when the independent variable is time. These conditions were approximately fulfilled during the period 1960-1999. This property permits to isolate the linear trends of any variable corresponding to each phase, and therefore, helps to highlight or discard any possible relationship between the linear trends of the variables and the ENSO phases.Annual river flow linear trends were calculated applying a linear regression model to 8 gauging stations: Ladario and Asuncin on the Paraguay River, Salto Caixas on the Iguaz River, Jupi, Itaip and Corrientes on the Paran river and Paso de los Libres and Salto Grande on the Uruguay river; regression coefficients were tested for significance through a t-test. There are positive linear trends in all rivers. Corrientes, located in the lower Paran Basin receives the contribution from three of the rivers: Paran, Paraguay and Iguaz rivers, and therefore having the highest discharge values. Senn´s slope values indicate an increase of 172 m3/s per year during the last 40 years at Corrientes which is about 40% of the initial value. These annual trends took place predominantly during the neutral phase of the ENSO, except in the Brazilian stations of Jupi and Itaip where the slope values are higher during El Nio events. At Saltos Caixas slope values are quite similar during both the neutral and El Nio phases. Almost everywhere in the region, no trends or very small ones took place during La Nia periods. Hence, in most of the La Plata Basin river flow trends during the extreme phases of the ENSO constitute only a small part of the trends of the last 40 years of the twentieth century. These results are consistent with those found when analyzing rainfall trends during the same period.