Hydrological Extremes and Their Association with ENSO Phases in Ethiopia

Abstract

In this study, an attempt has been made to determine the occurrence of droughts and floods on monthly basis, by calculating the monthly Standardized Precipitation Index using the available rainfall data during (1975-2005) in Ethiopia.Based on the monthly SPI values computed, the droughts and floods of different intensities; extreme, severe and dry have been determined for all stations. The frequencies of the droughts and floods during the two rainy seasons, Belg (Feb-May) and Kiremt (Jun-Sep) seasons have been determined. For instance, during Belg season, there were 11 extreme (SPI < -2.0) at Nazreth, 10 severe (SPI between -1.99 and -1.50) at Diredawa and 14 moderate (SPI between -1.49 and -1.00) droughts at Kulumsa. The total number of droughts of all intensities over the study period is also highest (22) at Kulumsa and lowest (8) at Mekelle. Moreover, at Kulumsa the numbers of droughts and floods during Belg are more, which shows that the rainfall variability is highest at this station.The association of the hydrological extremes during the two rainy seasons, belg and kiremt with the ENSO phases has also been examined, which forms a basis for the prediction of the occurrence of droughts and floods at individual station using ENSO phases.