Residence Times of Water Contributing to Headwater Streams Determined Using 3H

Headwater streams in the Ovens catchment, southeast Australia, have 3H activities between 1.63 and 2.45 TU that are lower than those of local rainfall (∼3 TU). The variation of 3H activities and major ion concentrations with streamflow implies that simple dilution of older groundwater with rainfall during high flow does not occur. Rather different stores of water from within the catchment (e.g. from the soil or regolith) are mobilized during rainfall events. Mean residence times estimated using an exponential-piston flow model12 to 36 yearsduringsummer baseflow and 6 to 14 yearsduring recession from higher flows. A broad correlation between 3H activities and the percentage of rainfall exported from each catchment may reflect local differences in recharge and evapotranspiration; this correlation allows a broad prediction of likely residence times. These streams are buffered against rainfall variations on timescales of a few years;however, the impacts of changes to landuse or longer timescale changes to rainfallmay take years to decades to manifest itself in changes to streamflow or water quality.