Discharge trends and flow dynamics of South American rivers draining the southern Atlantic seaboard: An overview

Abstract

Summary River discharge time series in southern South America (between ∼15° and ∼50°S) show dissimilar historical trends. In the Rio de la Plata drainage basin (RPDB) the record shows a runoff trend change from decreasing to increasing, around the mid 20th century. Independently of the seasonal variation, currently-observed discharge change in the region is mainly accounted for by increased discharge during low-flow periods. In contrast, most Patagonian rivers show a negative discharge trend, which mainly occurs in some austral summer months and is particularly significant in the Negro River. When considering the historical discharge variability in the southern part of South America, the spectral analysis produced evidence of interannual, quasi-decadal, and quasi-bidecadal periodicities. Most rivers in the region show El Nino-like interannual periodicities with the exceptions of the Bermejo and Pilcomayo rivers. In the RPDB, during the 20th century, interannual periodicity changed from the ∼2 to ∼5-year band to the ∼5 to ∼7-year band between ∼1935 and ∼1970. In Patagonia, the interannual oscillation showed maximum power ∼1930 and ∼1980 in the Colorado, whereas further south (Negro and Chubut rivers), maximum power appears ∼1950. The quasi-decadal signal is strong in the RPDB, in general, and particularly in Patagonia’s Colorado, which runs close to the so-called Arid Diagonal and bears this signal presumably due to the influence of the low-level jets (LLJs). This periodicity is conspicuously absent in other Patagonian rivers. Finally, a quasi-bidecadal signature appears to be restricted to the Bermejo and Iguacu rivers, in the RPDB, and in Patagonia’s Colorado and Santa Cruz rivers.