Analysis of storm run‐off sources using oxygen‐18 in a headwater swamp

Abstract A combination of hydrometric data and observations of natural isotope (oxygen‐18) variations in saturation overland flow and stream discharge were used to investigate the sources of storm run‐off in a headwater swamp located in a permanent groundwater discharge zone near Toronto, Canada. The results of a two‐component hydrograph separation indicated that pre‐event water formed 80–90% of the stream hydrograph volume for six of the seven storms analyzed in June–November 1990. However, the instantaneous event water contribution showed considerable variability, ranging from maximum values of 20–25% for four moderate intensity storms to 63% for a high intensity thunderstorm with a return period of two years. The relative contribution of event and pre‐event water to storm run‐off from saturated areas and small streamlets within the swamp was similar to the main outlet stream. The dominance of pre‐event water during storms could be accounted for by the mixing of a small volume of event water with a large pool of pre‐event water on the surface of permanently saturated areas within the swamp. Occasional storms of high intensity or long duration produced a greater shift towards an event water signature in the saturated areas and a larger event water contribution to the outlet stream hydrograph.