Variations in rainfall and extreme event indices in The wettest part of Ethiopia

Abstract

Changes in daily rainfall indices in the wettest (south-western) part of Ethiopia, commonly referred to as 'year-round' rainfall region were examined, by using records from nine stations over the period 1978-2007. Sixteen rainfall and dry spell related indices were defined and analyzed for trends; least squares regression was used to quantify trend and Kendall's tau test was used to test statistical significance of trend. A complex picture of rainfall variability emerges from the analysis. Only one station (Gore) shows a statistically significant decline. The same station (Gore) shows significant decreasing trends in very wet day (R95p) and extremely wet day (R99p) amounts, whereas another station (Jimma) shows significant increasing trends in these parameters (total rainfall, very wet day and extremely wet day amounts); the other stations show no significant trends in these indices. Two of the stations (Asendabo and Sokoru) show statistically significant increasing trends in the maximum number of consecutive dry days. The results generally support previous studies in Ethiopia that there are no consistent patterns or trends in daily rainfall characteristics or seasonal rainfalls with the additional finding that choice of study stations strongly influences results of trend analysis. This suggests that regional scale conclusions may not be provided by analysis of few selected stations, given the high level of spatial variability at sub-regional scales in Ethiopia.