A similarity index for storm runoff due to saturation excess overland flow

•We propose and demonstrate an index for similarity in the storm event response of catchments to rainfall.•The similarity index is based on the steady state assumption.•The index is applicable to catchments where runoff is generated by saturation excess overland flow.•The index was applied to 236 independent storm events on eight catchments in southeast Australia.•The index’s effectiveness was assessed by comparing its groupings with those obtained from a flood frequency analysis. An index for the determination of hydrologic similarity is proposed and demonstrated. The index is based on the steady state assumption and is applicable for small- to medium-sized catchments where storm runoff is generated principally by the saturation excess mechanism – a key runoff generation mechanism in humid regions. The index uses variables that can be derived from rainfall and streamflow measurements and topographic and soil hydraulic attributes. The index is applied to eight gauged catchments located in southeast Australia. Comparisons of similarity index values with groupings obtained from a peaks-over-threshold (POT) analysis of daily maxima of hourly runoff series show good agreement. A sensitivity analysis of the index and the POT series indicated that these results are reasonably robust provided catchments are allowed to have partial memberships in all of the groupings identified.