Abstract:
Trend analysis of rainfall and stream flow is essential for effective water resources planning and
management. The objective of this research is to determine the trend of rainfall and stream flow
time series in the upper of Wabe Shebelle River basin. The time series of seven rainfall stations and
seven runoff stations were taken and analysed. The trend of rainfall and stream flow data was
analysed individually for each station using Modified Mann-Kendall package called mmkh, which
was installed in to the R software. Magnitude of trend was observed from Sen’s slope value. From
annual rainfall trend analysis, rainfall time series at Adaba, Assassa, Dodola and Meraro stations
showed significant decreasing trend. From Spring season rainfall trend analysis rainfall time series
at Adaba and Meraro stations showed significant decreasing trend. From summer rainfall trend
analysis rainfall at Adaba, Assassa and Kofele stations showed significant decreasing trend. From
Autumn season rainfall at Dodola and Meraro stations showed significant decreasing trend. From
Winter season, rainfall at Adaba, Assassa, Dodola and Meraro stations showed significant
decreasing trend. From monthly rainfall trend analysis most of the stations showed significant
decreasing trend. The trend results of the average rainfall of the seven stations showed significant
decreasing trend only during summer and winter seasons. The trend magnitude of the selected
rainfall stations ranges from -1.29mm/annual up to -14.88mm/annual and the average of trend
magnitude of the seven rainfall stations was -8.43mm/year. From runoff trend analysis; Leliso and
Ukuma stations showed significant increasing trend during annual period and Winter season.
Leliso station showed significant increasing trend during spring season. Ukuma and Wabi stations
showed significant increasing trend during summer season. Leliso, Ukuma and Wayib stations
showed significant increasing trend during Autumn season. From high flow trend analysis Assassa
and stations showed significant decreasing trend and Ukuma station showed significant increasing
trend. From low flow trend analysis results only Leliso station showed significant increasing trend.
In this study the correlation between rainfall, runoff, high flow and low flow was evaluated using
Linear regression model. The result demonstrated no correlation, weak correlation, moderate
correlation and negative correlation at different stations. The result of this study can provide
essential information for current and future plan in Wabe Shebelle River Basin.
