ASSESSING THE SPATIAL AND TEMPORAL RAINFALL VARIABILITY AND CONTROLLING GLOBAL OCEANIC INDICES, IN THE RIFT VALLEY BASIN OF ETHIOPIA

Abstract

In this paper, an attempt has been made to study the temporal and spatial rainfall variability, controlling oceanic indices at the rift valley basin in Ethiopia. The data used for the study was 30 years (1986 – 2015) rainfall, SOI, SSTA and OID. Mann-Kendal trend test, Sen Slope estimator and Coefficient of Variation (CV) were used for rainfall trend and variability analysis and Pearson’s correlation test and simple regression analysis were used to assess the relationship between rainfall and oceanic indices. ArcGIS software was used to generate rainfall surface maps. Mann-Kendall trend test shows there is no trend in the annual rainfall total, MAM and JJAS seasons for the study area. However, October and November months show positive trend for cluster III stations which corresponds to the lower part of the basin or at the floor of the rift valley basin. For cluster II and I central and upper part of the basin part February month showed negative trend for the period 1986-2015. Relative to cluster II and I rainfall of MAM, JJAS and annual seasons show low rainfall variability and for ONDJF season show high variation. For cluster I rainfall variability of all seasons and annual total show high rainfall variability. It is found that no correlation found between rainfall variation of the basin and SOI for MAM season. However, weak correlation with r = 0.34 and r = -0.38 for SOI and SST respectively observed for JJAS season over the study area. JJAS season rainfall is negatively correlated with SST over the equatorial Pacific and this negative correlation indicated that warm El Nino event is the cause for reduced JJAS rainfall over the study area. The overall findings of the study underlined the importance of close monitoring and further study on rainfall variability over the Rift Valley Basin.