Detection of trends in hydrological extremes for Ethiopian watersheds, 1975–2010

Abstract

This study investigates trends in streamflow variables for 57 gauging stations distributed across the Ethiopian highlands for the period 1975–2010. We used the Mann-Kendall’s test to detect trends and the Sen’s slope estimator to calculate trend magnitudes. The findings show that more than 70% out of 513 test cases have shown increasing signals, and 32% of the tests were globally field significant at 0.05 level. Increasing change in low-flow magnitudes and decreasing change in low-flow frequency that exceeded 80 percentile (Qmin80p) were more prevalent than the others. Global field significant increasing changes were observed for 40% out of 228 test cases for low-flow amounts, while Qmin80p has shown decreasing trend at 46 out of 57 stations, and 26 of these were statistically significant. The general tendency is towards upward change, but there were some stations that showed field significant decreasing trends for high-flow indicators. General trend signals (upward or downward) and stations with significant changes did not show any spatial pattern. There were even adjacent gauging stations within the same river basin or adjacent river basins that showed statistically significant opposite trends for some test cases. The complex spatial pattern of trend signals is partly attributable to the very complex topographic, climatic, and land cover variations in the country that are well documented in previous studies. Also, the observed trends are difficult to fully explain in terms of climate change or land cover conversion. Generally, the results of this study contradict with previous studies that reported no significant trends in streamflow variables over Ethiopia. The study has important implications for climate change adaptation planning, water-related disaster risk management, and water sector development activities in the country.