Hydrologic flowpaths during snowmelt in forested headwater catchments under differing winter climatic and soil frost regimes

Changes in hydrologic flowpaths have important impacts on the timing, magnitude and hydrochemistry of run-off during snowmelt in forested catchments, but how flowpaths are affected by variation in winter climate and the irregular presence of soil frost remains poorly understood. The depth and extent of soil frost may be expected to increase as snowpack decreases or develops later because of climate change. In this study, we used end-member mixing analysis to determine daily contributions of snow, forest floor soil water and groundwater to stream run-off during snowmelt under different soil frost regimes resulting from interannual and elevational variation at the Hubbard Brook Experimental Forest in New Hampshire, USA. We observed greater routing of run-off through forest floor flowpaths during early snowmelt in 2011, when the snowpack was deep and soil frost was minimal, compared with the early snowmelt in 2012 under conditions of deep and extensive soil frost. The results indicate that widespread soil frost that penetrated the depth of the forest floor decreased the flow signal through the shallowest subsurface flowpaths, but did not reduce overall infiltration of melt waters, as the contribution from the snow-precipitation end-member was similar under both conditions. These results are consistent with development of granular soil frost which permits vertical infiltration of melt waters, but either reduces lateral flow in the forest floor or prevents the solute exchange that would produce the typical chemical signature of shallow subsurface flowpaths in streamwater.