Peter D. Morse

Evaluation of RADARSAT-2 DInSAR Seasonal Surface Displacement in Discontinuous Permafrost Terrain, Yellowknife, Northwest Territories, Canada

Abstract. Differential Interferometric Synthetic Aperture Radar (DInSAR) is an increasingly viable method for assessing permafrost terrain stability, but the accuracy and performance within discontinuous permafrost terrain has not been well studied. We used a RADARSAT-2 DInSAR data stack for a 120-day period in the summer of 2010 to map seasonal surface displacement in the discontinuous permafrost terrain of Yellowknife, Northwest Territories. Calculated displacement was compared to surficial geology and municipal land use zones. Displacement results reveal that glaciofluvial, glaciolacustrine, humanly modified, and organic terrain are increasingly unstable, in contrast to predominantly stable bedrock. Within municipal zones, increased proportional displacement is related to higher proportions of glaciolacustrine sediments and organic terrain. Organic terrain, associated with the highest proportion of the moderate downward displacement (−3.0 cm to −6.0 cm), occupies less than 6% of the total area. Widespread glaciolacustrine sediments (30% total area) are associated with most of the downward displacement in municipal zones. Semi-quantitative field and geotechnical validations indicate that most areas of moderate seasonal downward displacement in developed areas also represent areas of long-term subsidence. This work shows that even a short InSAR data stack and a simple stack processing method can yield information that is useful for municipal knowledge and planning. Résumé. L’interférométrie différentielle par radar à synthèse d’ouverture (DInSAR) est une méthode de plus en plus viable pour évaluer la stabilité des terrains en zone de pergélisol, mais la précision et la performance dans les zones de pergélisol discontinu ne sont pas bien étudiées. Nous avons utilisé une pile de données DInSAR de RADARSAT-2 d’une période de 120 jours au cours de l’été 2010 pour cartographier le déplacement saisonnier de la surface du sol dans la zone de pergélisol discontinu de Yellowknife, Territoires du Nord-Ouest. Le déplacement calculé a été comparé à la géologie de surface et les zones municipales d’utilisation des terres. Les résultats de déplacements révèlent qu’en ordre de stabilité, du plus stable au moins stable, on trouve les zones fluvio-glaciaires, glacio-lacustres, humainement modifiées et organiques, tandis que le substrat rocheux est essentiellement stable. Dans les zones municipales, le déplacement proportionnel accru est lié à des proportions plus élevées de sédiments glacio-lacustres et du terrain organique. Le terrain organique, associé à la plus forte proportion du déplacement modéré vers le bas (−3.0 à −6.0 cm), occupe moins de 6% de la superficie totale. Les sédiments glacio-lacustres répandus (30% de la superficie totale) sont associés à la plupart des déplacements vers le bas dans les zones municipales. Les validations semi-quantitatives de terrain et géotechniques indiquent que la plupart des zones de déplacement saisonnier modéré vers le bas dans les régions développées représentent également des zones d’affaissement à long terme. Ce travail montre que même une pile de données InSAR de courte durée et une méthode de traitement de pile simple peuvent donner des informations utiles pour la connaissance et la planification municipale.

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 (21,887 km2) with a semiautomated approach utilizing late spring Landsat archival images (1985 to 2014). Individual icing size, ranging 3 orders of magnitude (1.8 × 10−3 km2 to 4.1 km2), is related to return frequency. Infrequent ice (25% return frequency) accounts for 94% of the total icing area (86 km2). Winter warming intervals (≥5°C; typically over 1–3 days) 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.

Spatiotemporal impacts of wildfire and climate warming on permafrost across a subarctic region, Canada†

Field observations show significant impacts of wildfires on active layer thickness and ground temperatures. However, the importance of fires to permafrost conditions at regional scales remains unclear, especially with climate warming. This study evaluated the regional impacts of fire on permafrost with climate change from 1942 to 2100 using a process-based model in a large subarctic region in the Northwest Territories, Canada. Climate warming is shown to be the dominant factor for permafrost reduction. The warming trend of climate reduces permafrost extent in this region from 67% at present to 2% by 2100. For burned areas, fire increases the reduction of permafrost extent by up to 9% on average, with up to 16% for forest, 10% for tundra and bogs, and 4% for fens. Fire accelerates permafrost disappearance by 5 years on average. The effects of fire on active layer thickness and permafrost extent are much larger in forest areas than in tundra, bogs, and fens. Since active layer is thicker after a fire and cannot recover in most of the areas, the fire effects on active layer are widespread. On average, fires thickens active layer by about 0.5 m. The fire effects on active layer increased significantly after 1990 due to climate warming.

Great Slave Lowland: The Legacy of Glacial Lake McConnell

The Great Slave Lowland of the Taiga Shield is an 11,000 km2 low-elevation granitic bedrock plain along the north shore of Great Slave Lake, Northwest Territories. It is characterized by a mosaic of coniferous and deciduous forest cover, wetlands, sparsely vegetated bedrock outcrops, and peatlands. The region was glaciated until about 13,000 years ago and then inundated by Glacial Lake McConnell and by ancestral Great Slave Lake, which gradually declined towards the present lake elevation. Consequently, fine-grained glacilacustrine and nearshore lacustrine sediments are broadly distributed across the region. Permafrost is widespread within forest-covered sediments and peatlands, but is not sustained beneath bedrock outcrops, leading to an extensive, but discontinuous, permafrost distribution. Lithalsas, which are permafrost mounds up to 8 m in height and several hundred metres in length, are also abundant. These form by ice segregation within mineral soil, as permafrost aggrades into the fine-grained sediments following lake level recession. Lithalsas are most common within the first few tens of metres above the present level of Great Slave Lake, indicating that many are late Holocene in age and some <1000 years. These elevated surfaces favour the establishment of deciduous forests with thin organic ground cover and with mean annual ground temperatures typically between −0.5 and −1.5 °C. With annual mean air temperatures consistently warming since the 1940s, this terrain is vulnerable to thawing and subsidence, with impacts on the ecology, hydrology, and population of the region.