Applicability of isotopic hydrograph separation in a suburban basin during snowmelt

C. H. TAYLOR Methodological issues associated with isotopic hydrograph separations (IHSs) in built‐up environments are explored using results from the 1990 spring melt in a suburban basin in Peterborough, Ontario, Canada. The hetrogeneous nature of suburban environments complicates the selection of appropriate isotopic signatures for event and pre‐event waters. Near‐stream groundwater δ 18 O sampled from wells was poorly mixed, such that the pre‐event water signature was best characterized by δ 18 O in pre‐melt baseflow or discharge from a headwater spring. The event water signature during snowmelt can be characterized using δ 18 O in the pre‐melt snowpack, surface runoff samples or meltwater from lysimeters. However, the use of snowpack δ 18 O may be inappropriate in suburban basins where meltwater from thin snowcover may exhibit pronounced responses to δ 18 O in rainfall contributions. Intensive sampling of the spatial variability of runoff or meltwater δ 18 O may be required to characterize the average event water signature adequately. Rainfall δ 18 O provided an appropriate event water signal during a large rain on snow event, and differences between this IHS and one generated using an event water signature that included meltwater contributions from snow‐covered surfaces were within the uncertainty attributable to the analytical error in δ 18 O values. Event water supplied 55‐63% of the peak discharge and 48‐58% of total runoff from the basin during the melt, which is consistent with the fraction of the basin that has been developed. These results contrast with IHSs conducted in forested basins that suggest that stormflow is dominated by pre‐event water contributions.