Variability and trends in rainfall amount and extreme event indices in the Omo-Ghibe River Basin, Ethiopia

This study evaluates variability and trends in rainfall amount and extreme event indices in the Omo-Ghibe River Basin of Ethiopia based on observed records from six stations. Nine daily rainfall indices were defined and analyzed based on the World Meteorological Organization guideline. Sen's slope estimator and Mann-Kendall's trend test were used to determine the magnitude and statistical significance of the rainfall changes, respectively. The results show complex patterns of rainfall variability and local-scale trends. Statistically significant increasing changes were observed at Key Afer for most of the indices. On the other hand, the increasing trends in the number of very heavy rainfall events (R20 mm) were statistically significant at three (Jinka, Key Afer and Wolaita Sodo) out of the six stations. On the other hand, significant decreasing trends were found at Sawla for the maximum one day (Rx1 day) and the number of very heavy rainfall event (R20 mm). No systematic trends were found for the number of wet and dry day-related rainfall indices. However, the annual total rainfall averaged for the entire basin and at two stations (Key Afer and Wolaita Sodo) showed statistically significant increasing trends, both at the 0.01 level. Generally, the results of this study contradict with previous studies that reported significant declining trends in annual and Kiremt (main rainy season) rainfall amounts over this part of the country, suggesting the reviving of rainfall during the last two decades. Also, the results of this study agree with previous researches that reported the absence of systematic patterns of trends in daily rainfall indices in different parts of the country. The study has important implications for an effective management of water resources and climate risk management particularly in view of the fact that five hydropower plants and large-scale sugarcane plantations are planned, and some are under construction in the basin.