Response of streamflow to climate variability and changes in human activities in the semiarid highlands of northern Ethiopia

Climate variability and human activities are two major drivers influencing changes in streamflow response of a watershed, and thus assessing their relative effect is essential for developing sustainable water resources planning and management strategies at watershed-scale. In this study, a runoff model driven by rainfall and potential evapotranspiration was established to estimate the effect of climate variability on the changes in annual streamflow of Agula watershed in northern Ethiopia. Significant decreasing trends were observed for annual and wet season streamflow between 1992 and 2012, while dry season streamflow showed an increasing trend. Analyses of seasonal and annual rainfall records showed no significant trends. The change-point test revealed that an abrupt change in annual streamflow occurred in 2000. In the period 2000–2012, the mean annual and wet season streamflow decreased by 36 and 49%, respectively compared with 1992–1999, while dry season streamflow increased by 57%. Climate variability was estimated to account for 22% of the total reduction in mean annual streamflow, whereas human activities (e.g., proper watershed management practices and associated changes in land use/land cover among other factors) were responsible for 78%; indicating that human activities were the major drivers of changes in the streamflow response. The results of this study point to the potential that reduced wet season flow and improved dry season water availability can be achieved by proper planning and implementation of appropriate watershed management practices.