Water Resources Research vol:48 pages:W03513-13 p.
This paper examines the long term historical changes in frequency and amplitude of hydro-climatic extremes in the Blue Nile basin using the second half of 20th century data. The temporal variability of basin-wide rainfall extremes and river flow extremes from four gauging stations were investigated under the hypothesis of no trend and no persistence in time. Based on quantile anomaly analysis method, decadal variations in extreme daily, monthly and annual quantiles were studied and the periods of statistical significance identified. The analysis showed that high and low river flows and rainfall depths do not vary in time in a fully random way because they have particular variation pattern. Their extremes show significant decadal variations. The 1980s had statistically significant negative anomalies in extremes in comparison with the long term reference period 1964-2009, while the 1960s-1970s and the 1990s-2000s had positive anomalies although less significant. There is neither consistent increasing nor decreasing trend in rainfall and flow extremes of recent years. Therefore, anticipated trends due to global warming could not be identified. Conversely, low flow extremes show an increasing trend during the last decade, which could be related to the effect of water regulation works at the outlet of Lake Tana. Moreover, similar patterns and statistically significant correlations were found between climatic indices representing Pacific and Atlantic oceans and the Blue Nile rainfalls/flow extremes. Changes that occur on the Pacific Ocean appear to be a main driver for the decadal oscillations in climate and related high and low water availability.