Filtered smoothed minima baseflow separation method

The smoothed minima baseflow separation method of the United Kingdom Institute of Hydrology (UKIH) is coupled with the recursive digital filter (RDF) to develop the filtered smoothed minima baseflow separation (FUKIH) method in which the daily streamflow time series is first separated by the UKIH method and then filtered by the RDF. In the FUKIH method, the sharp peaks and troughs generated by linear interpolation in the UKIH method are removed and a smooth hydrograph representing the baseflow generating mechanisms is obtained. The application to daily streamflow time series from the Filyos river in the Western Black Sea region, Turkey, shows that the FUKIH baseflow sequence traces the UKIH method during the recession period of the hydrograph while it traces the RDF during the ascension. The FUKIH baseflow sequence has delayed peaks rather than peaks taking place right under the total streamflow peaks. This is plausible as it is known baseflow consists of delayed flow with longer time of concentration than the total streamflow hydrograph. A low-flow characteristic used in the comparison of baseflow separation methods, the ratio of the quantiles with exceedance probabilities of 90% and 50%, Q₉₀/Q₅₀, is found to be the same for FUKIH and RDF. This suggests the FUKIH method is able to reproduce the results of the RDF method. The advantage of the FUKIH over the RDF is that it requires only one pass filtering in the forward direction instead of three (forward-backward-forward filtering) in the RDF.