Trends and Periodicities in East African Rainfall Data

Abstract

Annual rainfall values from 35 stations in parts of East Africa (Kenya, northern Tanzania, and south-eastern Uganda) are analyzed for trend, periodicities, and variability. The data cover time periods ranging from 44 to 83 years. After normalizing the series by dividing by the long-term mean, areal averaged series are constructed. Spectral analysis by the Blackman-Tukey method shows in most cases peaks at frequencies corresponding to the following time periods: 2–2.5, about 3.5, and 5–5.5 years. The area distribution and the statistical significance of these peaks are briefly discussed. Smoothing by binomial coefficients is applied to the data series to filter out short period fluctuations. The resulting series show no definite long term trends, except possibly at some stations in northern Kenya where a trend towards increased precipitation in the recent years is indicated.