Spatial and Temporal Rainfall Trend Analysis: A Case Study of South Western, Ethiopia

Abstract

Based on the 31 year period rainfall record, analysis was carried out to extract the trends of seasonal and annual rainfall in south western Ethiopia. Eventhough, Nekemte and Jima are found in the same rainfall regime called single maxima; the analysis demonstrates that Nekemte stations are shown a slight decreasing trend in rainfall magnitude. Cross-correlation of annual rainfall between the two stations in the period of 1971-2002 is around 11.07, which is low. Though they are in the same rainfall regime, the correlation shows that they may not be subjected to the same climatic change influence. The coefficient of variation and standard deviation for 1951 to 1990 ranged from 0.32 to 0.2, and 60 mm to 90.4 mm, respectively, confirming the moderate variability of the mean monthly rainfall over the two stations. This shows that outside of the 4-months rainy period, there is little rainfall during the rest of the year and are highly variable. Even though the data was inadequate for the analysis of rainfall series, the temporal rainfall distribution shows that the dry spells are more frequent than the wet spells. In this regard, hence, the mean annual rainfall coefficient of variability (CV) clearly indicated moderate to high rainfall variability.

Keywords: Trends, Climate change, Rainfall Variability, South Western Ethiopia