Isotope hydrograph separations and rapid delivery of pre-event water from drainage basins

Abstract

Environmental isotopes, such as oxygen-18 and deuterium, have been used increasingly to separate stormflow into its event and pre-event components in order to elucidate the sources, pathways and residence times of water in drainage basins. The majority of isotopic hydrograph separations indicate that pre-event water supplies at least 50% of streamflow at peak discharge in small- and medium-sized basins; however, there is no consensus as to the means by which pre-event water is rapidly exported from drainage basins. The hydrological processes that have been invoked to explain the observed isotopic response of streamflow to rainfall and snowmelt inputs in various environments are reviewed. These processes include groundwater ridging, translatory flow, macropore flow, saturation overland flow, kinematic waves and release of water from surface storage. Tests of the ability of the hypothesized mechanisms 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. Along with various methodological issues associated with the isotopic hydrograph separation technique, the overall relevance of these hydrograph separations to the understanding and prediction of stream hydrochemistry must be evaluated critically.