Trends and Variability of Precipitation: Implications for Water Resources in Lake Ziway Watershed, Central Ethiopian Rift

Abstract

Precipitation is the significant climatic variable that governs the availability of water resources in the country, Ethiopia, but it is highly erratic and variable in spatial-temporal scales. The purpose of this paper was to analyse trends and variability of rainfall at seasonal to annual sales in the Ziway Lake Watershed. The non-parametric Mann-Kendall (MK) and Sen’s Slope (SS) estimation were used to detect the trends and compute the magnitudes of slopes respectively. The Coefficient of Variation (CV) was employed to reveal the variations in rainfall. The spatial interpolations mapping was made by using IDW (Inverse Distance Weighting). The results revealed that the western Ziway Lake watershed had experienced more rainfall variability than the eastern Ziway Lake watershed. The results also indicated that the annual and summer rainfall did not exhibit significant evidence of a monotonic trend. On the other hand, almost all rainfall stations in the spring season revealed decreasing trends (significant and non-significant), for instance, there are three stations (Ogalcho, Butajira, and Koshe) showed significantly decreasing trends at 5% of significant level and the two stations (Kulumsa and Meki) indicates significantly decreasing trends at 10% significant level. Moreover, the magnitudes of slope (changes in mm/year) estimated by SS for stations in the spring season that display significantly decreasing trends were as follows: -8.702, - 6.58, -4.018, and -3.681,-3.667 for Butajira, Koshe, Kulumsa, Ogalcho, and Meki respectively. Droughts can be expected to increase if the decline precipitation trend continues. This could lead to over-exploitation of the water resources. The similarities in patterns of the intra-annual variability of precipitation and river discharges imply that any changes in rainfall pattern would have effects on water availability. The results of this investigation can significantly contribute to guiding water managers and decision-makers for more efficient water resources planning and management. Keywords: Central Ethiopian Rift,, Ziway Lake Watershed, Precipitation, Variability, Trends DOI: 10.7176/JEES/10-10-03 Publication date:October 31st 2020