ISOTOPE HYDROGRAPH SEPARATIONS AND RAPID DELIVERY OF PRE-EVENT WATER FROM DRAINAGE BASINS

Environmental isotopes, such as oxygen-18 and deuterium, have been use d increasingly to separate stormflow into its event and pre-event comp onents in order to elucidate the sources, pathways and residence times of water in drainage basins. The majority of isotopic hydrograph sepa rations indicate that pre-event water supplies at least 50% of streamf low at peak discharge in small- and medium-sized basins; however, ther e is no consensus as to the means by which pre-event water is rapidly exported from drainage basins. The hydrological processes that have be en invoked to explain the observed isotopic response of streamflow to rainfall and snowmelt inputs in various environments are reviewed. The se processes include groundwater ridging, translatory flow, macropore flow, saturation overland flow, kinematic waves and release of water f rom surface storage. Tests of the ability of the hypothesized mechanis ms to explain the isotopic signature of stormflow from drainage basins will require a more complete integration of hydrometric methods with the use of environmental isotopes than has been achieved previously. A long with various methodological issues associated with the isotopic h ydrograph separation technique, the overall relevance of these hydrogr aph separations to the understanding and prediction of stream hydroche mistry must be evaluated critically.