Snowmelt Partitioning in Discontinuous Permafrost:
A Component of GEWEX-MAGS (Global Energy and Water Cycle Experiment- Mackenzie GEWEX Study)

Collaborators: Drs. A. Pietroniro & T.D. Prowse (NHRC, Saskatoon)

Excerpt from October 1998 Progress Report

Temporal changes in the volume of snowmelt contributions to streamflow are being compared in five streams near Fort Simpson to examine the effect of basin characteristics such as gradient, size, and vegetation coverage and distribution on the hydrograph response. A two-point mixing approach has been employed, which relies on the distinct isotopic signatures of snow versus summer streamflow. Extensive snow surveys, including isotopic sampling, were conducted prior to snowmelt for each terrain classification unit used in the WATFLOOD model. These results, in combination with land classification data for each basin, have been used to estimate snow water equivalent and snowpack isotopic compos ition for each basin. Importantly, the isotopic composition of the snowpack in open-water and wetland areas is found to be substantially more variable than in forested areas. This is attributed primarily to incorporation of lake or fen water into the snowpack by overflow/subsidence of ice over the course of the winter. The isotopic composition of summer streamflow, collected during late July through early October, is found to be distinct for each basin, primarily reflecting differences in the fraction of contributions from evaporated versus non-evaporated source waters. Isotopic data for snowpack and summer streamflow are used to partition snowmelt contributions to streamflow between the onset of melt and late July. An example of results obtained for the Blackstone River is shown below. Most notably, the abrupt decline in snowmelt contributions around day 150 corresponds to the observed transition from runoff via macropore/ interflow to groundwater flow. Such information will be useful to compare with the model results.

Time-series of discharge from the Blackstone River during the spring freshet of 1997 (Water Survey of Canada), and snowmelt discharge based on d¹⁸O of streamflow (grey circles) and surveys of snowpack and summer streamflow.

Similar results were obtained in all five basins, however, the timing of the snowmelt hydrograph is also found to be distinct for each basin, due not only differences in the mean transit times of water, but also potentially due to variations in the relative importance of flow pathways. Importantly, variability in basin isotopic response appears to be systematically related to basin gradient and land classification units which supports the use of a distributed model approach in the present setting. Very different responses were observed for individual basins for 1997 and 1998 (not shown), primarily due to a lighter-than-normal snowpack during the latter year.

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