Trends in the Flow Regime of the Southern African Rivers as Visualized from Rescaled Adjusted Partial Sums (RAPS)

Abstract

In an attempt to investigate possible trend in the annual river runoff of 502 rivers in the region of southern Africa, possible trends were first visualized and examined by the use of rescaled adjusted partial sums (RAPS) rather than the actual time series plots of runoff. A simulation experiment of the technique has been conducted to show how the RAPS reveal and be used for detecting the mode of underlying trend in the hydrological time series. The plot of the RAPS offers a reasonable visualisation of the readily apparent features, which may be hidden from the standard time series plots. The dominantly visualized trends have been linear and declining in general. A subsequent linear trend test conducted by fitting a linear trend model to the annual time series of river discharge revealed a predominantly negative slopes which vary between -6.8% to -0.2% thus suggesting existence of declining trends in some of the rivers of the Southern African Region. Of the 502 time series of river runoff record ranging from early 1950s to late 1990s considered in this study, 137 time series are deemed to have statistically significant decreasing trend and the remaining 96 series are deemed to have significant increasing trend; 269 series seem to have no trend at all. Those regions with evidence of declining runoff appear to be in parts of Zambia, Angola, Mozambique and the High Veld in South Africa.

(Af. J. of Science and Technology: 2002 3(1): 70-79)