Short-term surface velocity variations of the Argentière glacier monitored with a high-resolution continuous GNSS network

The motion of glaciers with a temperate base is highly variable in time and space as a result of glacier basal sliding being strongly modulated by subglacial hydrology. Here we investigate short term (diurnal to multi-diurnal) changes in horizontal velocity and vertical displacement caused by melt and rain water input events on the Argentière Glacier (French Alps). We use up to 13 permanent GNSS stations that have been operating continuously over three full years (since April 2019). We report observations of strong surface acceleration events occurring in response to late summer storms, during which a velocity pulse propagates from up to down glacier at a migrating speed of about 0.1 m/s, which is typical of flow drainage speeds in the distributed system. We thus suggest that transient changes in the surface velocity field during intense and short-term water input events reflect transient changes in the distributed system flow properties. Although the efficient drainage system is expected to be well developed at this time of the year, this latter does not appear to play a primary role in our observations. Using concomitant observations of subglacial flow discharge and seismic tremor amplitudes we are able to estimate the average height of cavities and the associated cavity-drainage conductivity. Examination of the vertical displacement suggests that a vertical motion of the glacier (uplift) is largely controlled by the volume increase in subglacial water cavities rather than by the vertical strain rate change. These observational constraints may be crucial to test subglacial drainage and transient friction theories.