Trend and variability in observed hydrometeorological extremes in the Lake Victoria basin

Abstract

Summary In the Lake Victoria basin hydrology, trend analysis has mainly been limited to the mean of the hydrological variable without explicit consideration of extremes, which are very crucial in understanding the behaviour of disastrous hydrometeorological events. Since the effects of climate change are unleashed, more through the occurrence of extremes, analysis of both monotonic and cyclic trends in hydrological extremes is very crucial. The presence of a significant linear trend, in a long-term hydrometeorological record of extremes, may provide evidence of a shift from the natural trend to that which is enhanced by, for example, anthropogenic forcing. In addition, cyclic trends analysis of hydrological extremes provides information on the cyclic behaviour of the extreme anomalies that have occurred over and above the natural climate variability and may link them to past consequences and their drivers. Analysis of long term records of extremes for rainfall, temperature and streamflows for selected stations in the Lake Victoria basin, were carried out based on a linear trend test, to detect significant monotonic trends, and quantile perturbation analysis, to detect significant temporal extreme anomalies. In addition, correlations between change in rainfall extremes and that for the other extremes, as well as sunspot maxima, were investigated. The findings indicated that extremes in the Lake Victoria basin are, generally, experiencing positive linear trends. Albeit positive trend was generally demonstrated, the presence of significant linear trend was manifested in the extremes of the data obtained from the stations located in the northern and eastern parts of the Lake Victoria basin. This may suggest that the monotony in the positive trend is a result of an ever increasing and consistent external enhancement of the natural climate agitation. The latter has implications for flood risks if the trend persists in the near future. The cyclic analysis of the behaviour of extremes indicated that the 1940s and the 1970s experienced significantly low extremes. Furthermore, the higher significant anomalies for the 1990s, compared to that for the 1960s, may suggest a more intense enhancement of the change in the natural variability in the recent climate. Correlation between change in the extremes for rainfall and that of the minimum daily temperature was demonstrated to be stronger (c.f. maximum temperature and sunspot maxima) implying that if such correlation persists in the future then change in the extremes of daily minimum temperature can be used as an indicator for the change in rainfall extremes. The investigation of the correlations between climate indices/solar activity and hydrometeorological extremes suggests that oceanic and solar influences are part of the explanation of the variability observed in rainfall and temperatures extremes in the Lake Victoria basin.