APPLICABILITY OF ISOTOPIC HYDROGRAPH SEPARATION IN A SUBURBAN BASIN DURING SNOWMELT

Methodological issues associated with isotopic hydrograph separations (IHSs) in built-up environments are explored using results from the 19 90 spring melt in a suburban basin in Peterborough, Ontario, Canada. T he heterogeneous nature of suburban environments complicates the selec tion of appropriate isotopic signatures for event and pre-event waters . Near-stream groundwater delta(18)O sampled from wells was poorly mix ed, such that the pre-event water signature was best characterized by delta(18)O in pre-melt baseflow or discharge from a headwater spring. The event water signature during snowmelt can be characterized using d elta(18)O in the pre-melt snowpack, surface runoff samples or meltwate r from lysimeters. However, the use of snowpack delta(18)O may be inap propriate in suburban basins where meltwater from thin snowcover may e xhibit pronounced responses to delta(18)O in rainfall contributions. I ntensive sampling of the spatial variability of runoff or meltwater de lta(18)O may be required to characterize the average event water signa ture adequately. Rainfall delta(18)O provided an appropriate event wat er signal during a large rain on snow event, and differences between t his IHS and one generated using an event water signature that included meltwater contributions from snow-covered surfaces were within the un certainty attributable to the analytical error in delta(18)O values. E vent water supplied 55-63% of the peak discharge and 48-58% of total r unoff from the basin during the melt, which is consistent with the fra ction of the basin that has been developed. These results contrast wit h IHSs conducted in forested basins that suggest that stormflow is dom inated by pre-event water contributions.