Hydrological controls on diurnal ice flow variability in valley glaciers

This paper uses a combination of field data and three‐dimensional modeling to investigate the spatial variability in basal conditions required to induce observed fluctuations in diurnal ice velocity at Haut Glacier d'Arolla, Switzerland. A network of surface velocity markers was observed at intervals of as little as four hours over diurnal cycles in both winter and late summer. Winter motion showed limited diurnal variability, presumably due to the absence of supraglacial meltwater inputs. By contrast, diurnal fluctuations in ice motion were recorded in summer across the lower and upper glacier. In the lower glacier, surface velocities were intimately linked to hydrological forcing in the vicinity of a subglacial channel. Previously observed diurnal excursions of meltwater away from the channel should reduce areas of basal drag adjacent to the channel thereby impacting on ice dynamics. Using a first‐order ice flow approximation, we investigated the distribution of basal shear traction adjacent to the channel necessary to replicate the observed surface velocity field during periods of rapid ice motion. The modeling suggests that the observed variations in diurnal velocity will only occur with extensive reductions in basal drag across a transverse zone of up to 560 m across, well beyond the immediate vicinity and previously observed extent of diurnal excursions of meltwater away from the subglacial channel.