AbstractWhen sizing reservoirs with high regulation levels,by using syntherically generated inflows, the latters’Hurst coefficient h,expressing their percistence, must be preserved by properly selecting the flow-generating model. The influence on sizing of using the FFGN (Fast Fractional Gaussian Noise) model, which allows preservation of h, instead of the widely used Thomas-Fiering model, which fails to preserve h, is shown to be significant. The former results to a more conservative and reliable design than the latter (or any other Markovian) model. The analyzed data refer to the Kremasta reservoir on the Acneloos river in Western Greece with 27-year long Inflow series, which is supposed to be fully regulated for an operational horizon of 50-years.