Spatial Rainfall Variability of Oromia Region, Ethiopia

Rainfall over Ethiopia shows high spatial and temporal variability and has significant effects on Ethiopia’s economy in general and on the agriculture sector in particular. This study describes the rainfall climatology of Oromia Region of Ethiopia using over the past 30 years (1983-2016) of monthly rainfall data from 20,475 points of 4km×4km grids. The data are created by blending station records with meteorological satellite estimates and were obtained from the National Meteorological Agency of Ethiopia. Methods of analysis used include calculation of longterm averages and indices of variability, and detection of trends over the period of observation. We found that mean annual areal rainfall over Oromia Region is 1009 mm, varying from 816 mm in the lowlands (locally known as Kolla, <1500 m asl) agroecological zone to 1240 mm in the midlands (locally known as Weyna-Dega, 1500–2300 m asl) agroecological zone. Inter-annual rainfall variability is generally low, and its intra-annual variability is characterized by moderate to high concentration. Annual rainfall shows statistically non-significant increasing trend. March-May (Belg) rainfall shows statistically non-significant decreasing trend except for Weyna-Dega agroecological zone where there is no clear trend. June-September (Kiremt) rainfall shows statistically significant increasing trend in Weyna-Dega and Dega (>2300 m asl) agroecological zones at p=0.05 level; and at p=0.1 level in the entire Oromia Region. The October-February (Bega) rainfall shows increasing trend in the Kolla agroecological zone at p=0.1 level. February and September show statistically significant increasing trends at p=0.05 level. The correlation between altitude and rainfall in Oromia Region is positive and statistically significant at p = 0.01 except in the Dega agroecological zone which showed statistically significant negative correlation at p=0.01 level. We present results by agroecological zones, and we believe this gives useful insights for development of context-specific agricultural planning, climate risk management and water resource management strategies in this topographically and climatologically diverse Region.