Geological and meteorological controls on icing (aufeis) dynamics (1985 to 2014) in subarctic Canada

Icings are widespread yet poorly understood winter hydrological phenomena that develop over the winter by freezing successive overflows of groundwater to the surface. Groundwater hydrology in arctic regions is constrained by geological setting and permafrost extent, and overflows are possibly driven by cold winters, winter warming intervals, high antecedent autumn rainfall, and low early winter snowfall. Consequently, icings are spatially recurrent but not necessarily annually nor to the same extent. We test the significance of identified meteorological forcing variables against a long-term data set of icing dynamics and distribution we developed for the Great Slave region around Yellowknife, Northwest Territories. Climate is regionally consistent, but variable geology and permafrost create hydrological conditions representative of much of the subarctic. We mapped 5500 icings in the study area (21,887km(2)) with a semiautomated approach utilizing late spring Landsat archival images (1985 to 2014). Individual icing size, ranging 3 orders of magnitude (1.8x10(-3)km(2) to 4.1km(2)), is related to return frequency. Infrequent ice (25% return frequency) accounts for 94% of the total icing area (86km(2)). Winter warming intervals (5 degrees C; typically over 1-3days) and autumn rainfall (September and October) explain 28% of icing density interannual variation overall. Interannual icing variation and significant meteorological forcing variables differ among ecoregions where varied geological settings and permafrost conditions influence the hydrological regime. Future icings may develop less frequently due to decreasing winter warming intervals, but increasing autumn rainfall may increase icing density where Canadian Shield leads to strong threshold-mediated runoff generation processes.