A similarity index for storm runoff due to saturation excess overland flow Bryson C. Bates a,⇑ , Santosh K. Aryal b a CSIRO Marine and Atmospheric Research, Wembley, Western Australia, Australia b CSIRO Land and Water, Canberra, Australian Capital Territory, Australia article info Article history: Received 18 October 2013 Received in revised form 1 February 2014 Accepted 7 March 2014 Available online 19 March 2014 This manuscript was handled by Andras Bardossy, Editor-in-Chief, with the assistance of Axel Bronstert, Associate Editor Keywords: Hydrologic similarity Topographic analysis Hillslope saturation Baseflow separation Flood frequency Sensitivity analysis summary 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 measure- ments 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. Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved. 1. Introduction Hydrologic similarity is a key concept for fields as diverse as surface water quality management, aquatic ecology, catchment conservation planning, and engineering hydrology (Huang and Ferng, 1990; Hosking and Wallis, 1997). The concept is based on the premise that if the rainfall–runoff processes in two catchments are alike their hydrologic responses to rainfall will be similar (Blöschl, 2005). It provides a basis for catchment classification, informing the design of experimental and monitoring networks, regionalisation, improved understanding of the dominant controls on catchment function, and greater insight into the potential im- pacts of environmental change on hydrologic response (Wagener et al., 2007, 2008; Sawicz et al., 2011). Although considerable effort has been put into empirical and theoretical studies of catchment classification and hydrologic similarity during the last four decades, no universally agreed measures have been identified (e.g. Chery, 1967; Rodriguez-Iturbe and Valdes, 1979; O’Loughlin, 1981, 1986; Dooge, 1986; Sivapalan et al., 1987, 1990; McDonnell et al., 2007; Wagener et al., 2007, 2008; Lyon and Troch, 2010). A number of approaches have been proposed including (Blöschl, 2005; He et al., 2011): using geo- graphical proximity as an indicator of similarity under the assumption that the rainfall–runoff relationship is likely to vary smoothly in space (e.g. Merz and Blöschl, 2005; Zhao et al., 2012); clustering of catchments based on their landscape, climate and streamflow attributes under the assumption that if these characteristics are similar across catchments their hydrologic responses will also be similar (e.g. Nathan and McMahon, 1990; Hosking and Wallis, 1997; Bates et al., 1998; Rao and Srinivas, 2008); and similarity indices (defined as one or more dimension- less or dimensional numbers reflecting the structure of runoff generation and routing) that are based on the presumption that catchments could be expected to a similar hydrologic response if they are associated with the same value of a similarity index (e.g. Chery, 1967; Rodriguez-Iturbe and Valdes, 1979; O’Loughlin, 1981, 1986; Sivapalan et al., 1987, 1990; Larsen et al., 1994; Berne et al., 2005; Harman and Sivapalan, 2009; Lyon and Troch, 2010). Many of these studies considered the infiltration excess (Horton) mechanism, which is most common in semi-arid and arid lands, or subsurface flow. http://dx.doi.org/10.1016/j.jhydrol.2014.03.021 0022-1694/Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved. ⇑ Corresponding author. Address: CSIRO Marine and Atmospheric Research, Private Bag No. 5, Wembley, Western Australia 6913, Australia. Tel.: +61 8 9333 6554; fax: +61 8 9333 6499. E-mail address: Bryson.Bates@csiro.au (B.C. Bates). Journal of Hydrology 513 (2014) 241–255 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol