25 © Institution of Engineers Australia, 2013 * Paper W12-028 submitted 18/11/12; accepted for publication after review and revision 11/06/13. † Corresponding author Dr Tony Ladson can be contacted at tony.ladson@moroka.com.au. A standard approach to baselow separation using the Lyne and Hollick ilter * AR Ladson † Department of Civil Engineering, Monash University, Melbourne, Victoria R Brown, B Neal and R Nathan Sinclair Knight Merz, Melbourne, Victoria ABSTRACT: The digital iltering approach to baselow separation suggested by Lyne & Hollick (1979) has been widely used and is available in a number of computer packages. However, details of the approach used by different authors vary and so do the results. This means baselow volumes and indices reported by different authors, and at different times, are dificult to compare. We propose a standard method for baselow separation using the Lyne and Hollick digital ilter. This includes relecting the low series at the start and end of the record to reduce “warm up” effects and the adoption of speciic starting values for each ilter pass. KEYWORDS: Baselow separation; digital ilter; Lyne and Hollick; baselow index. REFERENCE: Ladson, A. R., Brown, R., Neal, B. & Nathan, R. 2013, “A standard approach to baselow separation using the Lyne and Hollick ilter”, Australian Journal of Water Resources, Vol. 17, No. 1, pp. 25-34, http://dx.doi.org/10.7158/W12-028.2013.17.1. 1 INTRODUCTION Separating the baselow and quicklow components of hydrographs is a common undertaking in hydrology and is used as part of rainfall runoff modelling (Jakeman & Hornberger, 1993), contaminant generation modelling (Merritt et al, 2003), assessments of groundwater recharge and discharge (Arnold et al, 2000), and flood hydrograph estimation (Graszkiewicz et al, 2011). There is a wide range of approaches that have been developed to separate baselow from quicklow using measured streamlow data (Lyne & Hollick, 1979; Boughton, 1988; Nathan & McMahon, 1990a; Chapman, 1999; Brodie & Hostetler, 2005). It is generally acknowledged that these numerical techniques are not closely related to the underlying physical processes that lead to rapid and delayed responses in streamlow but they offer a way forward for hydrologic practice if they can provide objective and repeatable results. 1.1 The Lyne and Hollick ilter Lyne & Hollick (1979) proposed a recursive digital ilter for baselow separation. Although recognised as lacking a physical basis (Chapman, 1991; 1999) the technique is easy to automate, objective and repeatable (Nathan & McMahon, 1991). This makes it particularly useful for comparative hydrology and for regionalisation as it can be used to characterise differences between catchments in a consistent manner. The limitation of the digital iltering approach is that the derived series do not relect any underlying physical processes in shape, timing or quantum so it is not possible to make quantitative inferences. The basic ilter equations are:          1 1 1 for 0 2 0 otherwise f f f q i qi qi q i q i                   (1) q b (i) = q(i) – q f (i) (2) where q f ( i ) is the quickflow response at the i th sampling instant; q(i) is the original streamlow at Australian Journal of Water Resources, Vol 17 No 1