Spatial and temporal runoff oscillation analysis of the main rivers of the world during the 19th–20th centuries

The annual discharge time series of selected large rivers in the world were tested for wet and dry periods. The 28–29-years cycle, as well as 20–22-years cycle of extremes occurrence were identified. From the trend analysis it follows that the hydrological characteristics of the rivers must be stated at least for one 28-year period. If we want to identify any trend uninfluenced by the 28-year periodicity of the discharge time series, we must determine the trend during a single or multiple curve cycle, starting and terminating by either minima (e.g. 1861–1946 in West/Central Europe) or maxima (e.g. 1847–1930 or 1931–1984 in West/Central Europe). Trends determined for other periods are influenced by the periodicity of the series and depend on the position of the starting point on the increasing or recession curve. Long-term trends during the period 1860–1990 have not been detected for the West/Central European runoff. Further, the temporal shift in the discharge extremes occurrence (both, maxima and minima) was shown to depend on the longitude and latitude. The time shift between Neva and Amur discharge time series is about four years, between Amur and St Lawrence is about 16 years, and between St. Lawrence and Neva is about nine years. The time shift between Congo and Amazon is about seven years.