It is known that changes in catchment runoff variability are a function of changes in climate as well as catchment behavior. For proper management of a certain watershed it is important to have a good understanding of the main causes of variability. Specifically, changes in extreme conditions of water resources are imperative as their consequences are far reaching. This paper attempts to identify the cause of hydrological extremes variability in the upper Blue Nile basin of Ethiopia. A method is proposed to utilize conceptual hydrological models to simulate long term (41 years) hydro-meteorological data and analyse the
outputs using the Quantile Perturbation Method (QPM) specially designed for investigation of the temporal variability of extreme values in time series over multi-annual to (multi-)decadal time scales. Two conceptual hydrological models were calibrated and evaluated for their performance to simulate extreme high flows and changes in these flows for corresponding changes in rainfall conditions. The temporal variability results show similar patterns for simulated and observed extreme flows. This indicates the major influence of climate variability in extreme flows as demonstrated by the rainfall input in the
models. There is no discernible change in the catchment response, e.g. quick runoff coefficient as a function of soil saturation state, between periods of the 1960–1970s, the 1980s and the 1990–2000s, which are attributed to land policy changes. This shows the influence of changes in catchment characteristics is minimal. (Multi-)decadal climate variability is identified as the main cause of temporal variation in hydrological extremes of the Blue Nile basin.