Trend Analysis Of Rainfall And Stream Flow; A Case Of Upper Of Wabe Shebelle River Basin, Ethiopia

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.