INFERRING HYDROLOGICAL PROCESSES IN A TEMPERATE BASIN USING ISOTOPIC AND GEOCHEMICAL HYDROGRAPH SEPARATION - A REEVALUATION

Simultaneous monitoring of conservative and non-conservative tracers i n streamflow offers a valuable means of obtaining information on the a ge and flow paths of water reaching the basin outlet. Previous studies of stormflow generation in a small forested basin on the Canadian Shi eld used isotopic (IHS) and geochemical hydrograph separations (GHS) t o infer that some event water during snowmelt reaches the stream via s ubsurface pathways, and that surface water runoff is generated by dire ct precipitation on to saturated areas (DPSA) in the stream valley. Th ese hypotheses were tested for rainfall inputs using simultaneous IHS (O-18) and GHS (dissolved silica) of basin stormflow, supplemented by hydrochemical and hydrometric data from throughflow troughs installed on basin slopes. Comparison of pre-event and subsurface water hydrogra phs did not provide conclusive evidence for subsurface movement of eve nt water to the stream, owing to the appreciable uncertainty associate d with the hydrograph separations. However, IHSs of runoff at the soil -bedrock interface on basin slopes indicated that event water comprise d 25-50% of total runoff from areas with deep soil cover, and that the se contributions supplied event water flux from the basin in excess of that attributable to DPSA. The surface water component of stormflow e stimated from the GHS was also largely the result of DPSA. GHS assumes that dissolved silica is rapidly and uniformly taken up by water infi ltrating the soil and that water moving via surface pathways retains t he low dissolved silica level of rainfall; however, neither assumption was supported by the hillslope results. Instead, results suggest that the observed depression of silica levels in basin stormflow previousl y attributed to dilution by DPSA was partly a function of transport of dilute event water to the channel via preferential pathways. Implicat ions of these results for the general use of simultaneous IHS and GHS to infer hydrological processes are discussed. (C) 1997 by John Wiley & Sons, Ltd.