Variability in the Negro River Water Level at Manaus, Brazil, Since 1902

Abstract

Long-term observations of water level are scarce in Amazonia. The observations in the Negro River at the port of Manaus provide one of the few time series for more than 100 years in the Amazon Basin. We performed spectrum and trend analyses to understand long-term hydrological changes in the observed river levels at Manaus since September 1902. We obtained a significant long-term increasing linear trend of the order of 0.9 m in 122 years. The series of annual maxima and minima values, respectively, show stronger positive and negative trends. The annual minimum water level has lowered by 3.7 m, and the difference between the annual maximum and minimum levels has widened by 4.2 m since 1973. The spectra of the Fourier decomposition show the dominance of annual and semiannual cycles with amplitudes of the order of 4.1 m and 0.9 m, respectively. Other notable cycles present are the ENSO-related 3 - 6-year cycle, decadal cycle, and multi-decadal cycles of 40 to 60-year periodicity. The wavelet analysis also showed peaks of power at those periodicities. The Pettitt test obtained change points around the 1970s in the daily data series, as well as in the annual maxima series and annual minima series. Changes in the variability characteristics, both in the trends and the amplitudes of the cycles in the water-level series at the Manaus port, suggest changes in the climatic regime of the basin since the 1970s.