Rainfall Variability and Trends over Central Ethiopia

Abstract

The rainfall variability and trends over North Shewa investigated using gauge as well as gridded rainfall data from 1985 to 2018. The variability of rainfall in both annual and seasonal scales were evaluated using coefficient of variation (CV), standardized rainfall anomaly, precipitation concentration index (PCI), and standardized precipitation index. Mann-Kendall test and Sen’s slope estimator were used to assess the rainfall trends. The rainfall in North Shewa was found to be highly variable both in space and time; i.e., irregular rainfall distribution was observed (PCI = 20%). The coefficient of variation showed moderate variation in both annual and Kiremt (June-September) rainfall as compared to the rainfall in Belg (February-May) and Bega (October-January) seasons. Mann-Kendall test resulted a decreasing trend in Belg and Bega seasons and an increasing trend for annual and Kiremt rainfall. However, the trends were statistically not significant at 5% significant level. The onset and cessation dates showed a non-significant decreasing and increasing trends, respectively. Whereas, the length of growing period showed a significant increasing trend. Overall, in North Shewa, the wet season (Kirmet) has been wetter while the dry and small rainy seasons (Bega and Belg) have been drier; North Shewa has been vulnerable to drought during Belg season (CV > 30%). Due to the high contribution of Kiremt season for annual rainfall amount of about 75%, the annual rainfall has also showed an increasing trend.