Traditional hydrograph separation techniques split an observed storm hydrograph into two main components representing ‘storm runoff’ and ‘baseflow’. In this paper a new separation technique is described which makes an initial split into two main components, quickflow and slowflow, which are each then subsequently split into two further subcomponents. The resulting procedure is termed the ‘four component hydrograph separation technique’. Various ways of recombining these four subcomponents to build up a curve that represents the observed storm hydrograph are possible, of which two ways are examined in further detail. If it is assumed that the four component separation technique provides a promising representation of an observed storm hydrograph, these two ways allow theoretical and practical insights to be gained into four existing hydrograph separation techniques. A conclusion, common to all four, is that much more care is required in naming the flow lines separating out each of the suggested subcomponents making up the observed storm hydrograph. This paper also emphasises the key role played by the slowflow storm runoff subcomponent, which has not been given sufficient prominence in existing event-based models in the past. A procedure for estimating each of the four subcomponents is illustrated for an observed event.

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