Evaluating snowmelt runoff generation in a discontinuous permafrost catchment using stable isotope, hydrochemical and hydrometric data

Research on snowmelt runoff generation in discontinuous permafrost subarctic catchments has highlighted the role of: (i) permafrost in restricting deep percolation and sustaining near-surface water tables and (ii) the surface organic layer in rapidly conveying water to the stream. Conceptual models of runoff generation have largely been derived from hydrometric data, with isotope and hydrochemical data having only limited application in delineating sources and pathways of water. In a small subarctic alpine catchment within the Wolf Creek Research Basin, Yukon, Canada, snowmelt runoff generation processes were studied during 2002 using a mixed methods approach. Snowmelt timing varied between basin slopes, with south-facing exposures melting prior to permafrost-underlain north-facing slopes. The streamflow freshet period begain after 90% of snow had melted on the south-facing slope and coincided with the main melt period on the north-facing slope, indicating that contributing areas were largely defined by permafrost distribution. Stable isotope (δ18O) and hydrochemical parameters (dissolved organic carbon, specific conductivity, pH) suggest that, at the beginning of the melt period, meltwater infiltrates soil pores and resides in temporary storage. As melt progresses and bare ground appears, thawing of soils and continued meltwater delivery to the slopes allows rapid drainage of this meltwater through surface organic layers. As melt continues, soil thawing progresses and pre-event water mixes with melt water to impart streamflow with a gradually decreasing meltwater contribution. By the end of the melt period, the majority of water reaching the stream is displaced water that has resided in the catchment prior to melt. For the entire study period, approximately 21% of freshet was supplied by the snowpack, and the remaining majority was pre-melt water stored in the catchment slopes over-winter and displaced during melt. Hydrochemical data support hydrometric observations indicating the dominant flow pathway linking the slopes and the stream is through the organic horizon on permafrost-underlain slopes.