ISOTOPIC HYDROGRAPH SEPARATION DURING SNOWMELT EVENTS: FACT OR FICTION

Streamflow can originate from new water associated with a rain or snowmelt event, or old water, which is any water that was in the basin prior to that event. Event hydrographs are commonly separated into source components using chemical tracers to evaluate flow paths and timing of new water delivery to a stream. Hydrograph separations for snowmelt events require more effort than rain events because several classic assumptions do not apply, including: 1) the new water chemical signature is significantly different than old water, and 2) the new water chemical signature is constant throughout the melt event. This study documents high spatial and temporal variability in d¹⁸O in a 0.6 km² snow-dominated headwater catchment in the Boise Front. Such variability in the chemical signature makes the above stated assumptions invalid. Hydrograph separations must be performed at a time step equal to new water collection. At this time step, average new water concentrations must be different than old water concentration so that a mixture of the two will produce the stream water concentration. Estimation of new water concentration is difficult considering significant temporal and spatial variability. Sampling schemes implemented dictate whether daily averages or area weighted daily averages should be used for new water concentrations.