Multi-year Seasonal Mass Balance Using Airborne Laser Altimetry, Columbia Basin, Canada.

Strong sensitivity of glaciers to decadal-scale climate change is well documented. This sensitivity originates at the seasonal time scale since seasonal mass balance is strongly related to meteorological conditions that affect alpine glaciers. Many studies detail meteorological drivers of summer melt, but processes controlling glacier nourishment during the winter season are less well understood. High financial and logistical cost of studying seasonal balance, especially during winter, also limits data available to assess alpine snowpacks and glacier mass balance gradients. Here we present seasonal balance data from a five-year study in the Canadian Columbia Mountains. We collected field data and conducted airborne laser altimetry surveys (LiDAR) to derive surface and geodetic balance records from six glaciers in the Columbia River Basin from 2014 through 2018. The LiDAR surveys captured an additional 84 glaciers covering 105 km². Geodetic balance from our study glaciers compared with these proximal glaciers indicate strong overall agreement. Our geodetic and direct seasonal balance estimates agree within 10% over the study period. Differences stem from time gaps between aerial surveys and field visits, fresh snow on glacier surfaces during fall surveying, and spatial coverage inadequacies in field measurements. Field-derived balance gradients between 1800 and 2600 m above sea level (asl) range from 1.3 to 2.1 mm w.e. m^-1. Maximum snow depth typically occurs at 2600 m asl. During the winter of 2017, however, winter balance was 135% of normal and snow depth continued to increase above 2600 m asl. Our results indicate that geodetic methods can produce accurate estimates of seasonal balance without field data spanning multiple years over an entire basin containing four mountain ranges. The relation between the seasonal balance of our study glaciers and surrounding glaciers suggests that a limited sample of spatially distributed glaciers can be used to represent a regional signal of glacier mass change.