Modeling snowdrift habitat for polar bear dens

Throughout the Arctic most pregnant polar bears (Ursus maritimus) construct maternity dens in seasonal snowdrifts that form in wind-shadowed areas. We developed and verified a spatial snowdrift polar bear den habitat model (SnowDens-3D) that predicts snowdrift locations and depths along Alaska's Beaufort Sea coast. SnowDens-3D integrated snow physics, weather data, and a high-resolution digital elevation model (DEM) to produce predictions of the timing, distribution, and growth of snowdrifts suitable for polar bear dens. SnowDens-3D assimilated 18 winters (1995 through 2012) of observed daily meteorological data and a 2.5m grid-increment DEM covering 337.5km2 of the Beaufort Sea coast, and described the snowdrift depth distributions on 30 November of each winter to approximate the timing of polar bear den entrance. In this region of Alaska, winds that transport snow come from two dominant directions: approximately NE to E (40–110°T) and SW to W (210–280°T). These wind directions control the formation and location of snowdrifts. In this area, the terrestrial, coastal mainland and barrier island banks where polar bear dens are found average approximately 3m high. These banks create snowdrifts that are roughly 2m deep, which historical den analyses suggest is approximately the minimum snow depth required for a polar bear den. We compared observed den locations (n=55) with model-simulated snow-depth distributions for these 18 winters. For the 31 den locations where position accuracy estimates were available in the original field notes, 29 locations (97%) had a simulated snowdrift suitable for denning within that distance. In addition, the model replicated the observed inter-annual variability in snowdrift size and location at historical den sites, suggesting it simulates interactions between the terrain and annual weather factors that produce the snowdrifts polar bears use for dens. The area of viable den habitat ranged from 0.0ha to 7.6ha (0.00–0.02% of the 337.5km2 simulation domain), depending on the winter. SnowDens-3D is available to help management agencies and industry improve their prediction of current polar bear den sites in order to reduce disturbance of denning bears by winter recreational and industrial activities.