Evaluating the Effect of Historical Climate Change on Extreme Stream Flow Cases of Gumara Watershed, Upper Blue Nile Basin, Ethiopia

Abstract

Climate and land cover change are very important issues in terms of global context and their responses to environmental and socio-economic drivers. The dynamic of these two factors is currently affecting the environment in unbalanced way including watershed hydrology. The change of observed steam flow is the effect of the combined change of climate and land use land cover of the catchment. In this paper the impact of climate change on stream flow specifically on extreme flow events were evaluated through application of Soil and Water Assessment Tool (SWAT) model in Gumara watershed, Upper Blue Nile basin Ethiopia. The trend of regional climate, like temperature, rainfall and evapotranspiration of the past 40 years in the study area were tested and then the extent of changes has also been evaluated in terms of monthly bases by using two decadal time periods. The period between1973-1982 was taken as baseline and 2004-2013 was used as change study. The efficiency of the model was determined by Nash-Sutcliffe (NS) and Relative Volume error (RVe) and their values were 0.66 and 0.72% for calibration and 0.64 and 1.23% for validation respectively. Both the high and low flows of the catchment have been taken from the simulated stream flow. The high flow has been identified using Annual Maximum (AM) method and the low flow was also identified by using Seven Day Sustained (SDS) minimum annual flow of the river. The impact of climate change was more significant on high stream flow than low flow of the catchment. Due to climate change, when the high flow was increasing by 17.08%, the low flow was decreasing by 6%. The overall results of the study indicated that Climate change is more responsible for stream flow during wet season than dry season.

Keywords: Climate, High flow, Low Flow, SWAT, Gumara, Blue Nile.