TREND ANALYSIS OF MONTHLY RAINFALL DATA IN CENTRAL ZONE

Abstract

Proper trend analysis of rainfall data is necessary for social and economic planning especially in the tropics where rainfall is a major limiting factor for crop production. In spite of the growing number of studies on climatic variables including rainfall in recent years, in the context of Tanzania little is known as to whether or not the observed time trend in rainfall in recent years is statistically significant. This study examines the evolution of rainfall in central zone focusing on Dodoma region and ascertains whether the observed time trend in rainfall is significant in statistical terms. Also, the study establishes whether there was a shift in the distribution of rainfall for both short and long rainy seasons in the study area. The data for the analysis consists of monthly rainfall (mm) records covering the period from January, 1981 to December, 2010. Linear regression analysis through OLS estimation and the Kruskal-Wallis and Mann-Kendall’s tau test statistics were employed. The data demonstrate that rainfall in the study area is unpredictable both in terms of onset and cessation. The mean annual rainfall during the years 1981-2010 displayed a decreasing trend with the years 1981-1989 recording the highest mean annual rainfall as opposed to the succeeding two periods: 1990-1999 and 2000-2010. Overall, the mean annual rainfall has declined by about 4% between the periods 1981-1989 and 2000-2010. Parametric and nonparametric analysis results did not find any statistically significant evidence of a trend in the amount of rainfall in Dodoma region over the last 30 years. Nevertheless, there was a shift in the distribution of rainfall for both short and long rainy seasons. The study recommends for further research that will include rainfall data from the various zones of Mainland Tanzania in order to permit a comparative analysis of the issues examined in this study.