Temporal Variation of Trend in Seasonal and Annual Rainfall in Southwest Ethiopia in Case of Drought Prone Wolaita Zone

Abstract

Monthly total rainfall data of ten stations were obtained from the south region meteorology office for cash. Stations data vary in time of record and the number of missing values. Stations which have more than ten percent  missing data over their recording period are excluded from the study. This reduces the number of stations to eight. Missing values were estimated using linear regression of neighbor stations with none missing value.  After infilling, seasonal Spring ('Belg', February-May) and summer ('Kiremt', June-September) and annual total data were prepared by summing monthly values  in respective time scale. Following the test for  the presence of autocorrelation, Mann–Kendall (Modified Mann–Kendall) test was applied to non-auto correlated (auto correlated) series to detect the trends in rainfall data. Theil and Sen's slope estimator test was used for quantifying  the magnitude of change over a time period. In addition, the percentage change of the mean over a period of time was determined  too. In annual rainfall data three stations show decreasing trend while the rest five stations show an increasing trend, however, all trends observed in annual data were non-significant. Based on seasonal trend analysis, non-significant trends were observed in seven stations(four increasing and three decreasing) and one station show a significant decreasing  trend in spring. During summer  seven (six increasing and one decreasing) non-significant and one significant decreasing  trends were observed.

Keywords: Trend, Mann–Kendall test, Sen's slope estimator, Rainfall, Wolaita zone