Relationship between rainfall and streamflow in the La Plata Basin: annual cycles, interdecadal and multidecadal variability

The aim of this study is to understand the interaction between rainfall and streamflow variability in the La Plata basin (LPB) along a wide range of timescales. LPB is divided in six sub-basins associated to the main rivers (Paraguay, Paraná, Uruguay and Iguazú). The amplification of the streamflow response is addressed in order to evaluate to what extent river discharges variability can be explained by precipitation fluctuations. Mean annual cycles corresponding to 1931-2010 period and to each of the decades comprising it are analyzed. Streamflow interdecadal changes are observed in most of the gauging stations. In addition, an 11-year moving-average filter is applied to the normalized annual time series. Results exhibit a considerable higher percentage of explained variance in the streamflow filtered series, highlighting the predominance of low frequency variability present in these compared to those of precipitation. Consistently, river discharges show higher spectral density over decadal/interdecadal frequencies compared to precipitation analysis. A simple statistical approach to advance in the understanding of the complex rainfall-streamflow physical relationship is addressed with promising results: streamflow spectrums are derived directly from the precipitation spectrum, transformed by a 'basin' operator, characteristic of the basin itself. It is assumed that watersheds acts on precipitation as spatio-temporal integrators operating as low-pass filters, like a moving average. Streamflow power spectrums are simulated assuming that the underlying process is an autoregressive moving average (ARMA). Considering as the only input the sub-basin areal-averaged precipitation timeseries, results show that simulated streamflow spectrums fits effectively the observations at the sub-basin scale.