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This paper examines the long-term historical changes in frequency and amplitude of hydroclimatic extremes in the Blue Nile basin using data from the second half of 20th century. The temporal variability of basin-wide rainfall extremes and river flow extremes from four gauging stations was investigated under the hypothesis of no trend and no persistence in time. On the basis of a quantile anomaly analysis method, decadal variations in extreme daily, monthly, and annual quantiles were studied, and the periods of statistical significance were identified. The analysis showed that high and low river flows and rainfall depths do not vary in time in a fully random way but show a 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 of 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 the Pacific and Atlantic Oceans and the Blue Nile rainfall and 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 Blue Nile water availability for Ethiopia, Sudan, and Egypt. [ABSTRACT FROM AUTHOR]

Summary: Investigation was made on the hydrological extremes of the upper Blue Nile basin in Ethiopia through the use of high and low flow probability distributions and flow-duration-frequency (QDF) relations. The dependence of QDF predictions on the period used for the analysis was investigated considering the possibility that short historical records might cover periods with a cluster of wet/dry years or where there was a high/low climate oscillation. In such cases, the derived relationships might not be representative of the long term natural variability and might lead to significantly different (biased) predictions. The results show that the high and low flow extremes of the basin could be best described by exponential and Fréchet distributions, respectively. The 1960s–1970s and the 1990s–2000s were identified as wet periods while the 1980s had more dry years. The constructed QDF curves using wet and dry periods reveal significant variations on their estimation specific to the sub-basins. For the entire upper Blue Nile basin, the wet/dry periods could be evaluated relative to the long term 1964–2009 data available at the downstream location, El Diem station. High flow QDF estimations using wet periods are in similar order of value with the long term QDF predictions. The exception is when data concentrates on the dry period (1980s); then the QDF statistics are not representative. It was found that bias correction of around 15% in the flow quantiles is required to shift the QDF statistics to the long term values. The required shift could be fully explained by the climate oscillations. On the contrary, low flow QDFs show shifts that do not necessarily correspond to the changes observed in the climate; rather they are influenced by combined effect of changes in climatic variables and changes in land/water management practices. Since high flow extremes appear to have link with large scale atmospheric variables, it would be interesting to further investigate how the temporal changes in high flow statistics can be linked to the atmospheric variables. [ABSTRACT FROM AUTHOR]

This study analyses the relation of forest cover and stream flow on the 266 km2 Koga watershed in a headwater of Blue Nile Basin using both observed hydrological data and community perception. The watershed declined from 16% forest cover in 1957 to 1% by 1986. The hydrological record did not reveal changes in the flow regime between 1960 and 2002 despite the reduction in forest area. This agrees with the perception of the downstream community living near the gauging station. The upstream community, however, reported both decreases in low flows and increases in high flows shortly after the forest cover was reduced. The upstream deforestation effect appeared to have been buffered by a wetland lower in the watershed. This study concludes that community perception can be a complement to observational data for better understanding how forest cover influences the flow regime. [ABSTRACT FROM AUTHOR]