The contribution of old and new water to a storm hydrograph determined by tracer addition to a whole catchment

Two tracer experiments have been carried out at an enclosed catchment in so uthern Norway. The catchment was brought to steady state with respect to ra infall and runoff prior to the tracer addition. A known concentration of li thium bromide was then added to the rainfall for the duration of each event . The tight control on tracer concentration and rainfall amount enabled ass essment of the contribution of old and new water to runoff, the dominant fl ow pathways and soil water residence times during a storm event. A signific ant volume of 'old' water contributes to runoff despite the hydrologically responsive nature of the catchment and several hours of tracer injected rai nfall are required before 'new' water becomes the dominant runoff source. A fter 34 h of tracer injection,'new' water apparently contributes c. 83% to instantaneous flow and c. 55% of the total tracer input to the catchment ha s been lost in runoff. Recovery of the tracer from soil water indicates tha t the organic soil surface layer is the dominant flow pathway for rainwater through the catchment and that a significant pathway also exists at the so il-bedrock interface. New water is retained in deep pockets of soil for sev eral days. Assessment of the conservative behaviour of the tracer suggests that 10-14% of the input Br- is retained in the soil and the tracer is not conservative. Laboratory experiments indicate that sorption of Br- to organ ic soil is the likely mechanism of retention. This process is probably conc entration dependent and will have occurred predominantly during the initial period of tracer application. Copyright (C) 2000 John Wiley & Sons, Ltd.