• ground penetrating radar;
  • subglacial hydrology;
  • monitoring

[1] We demonstrate that automated GPR techniques can monitor, at repeat timescales of minutes, hydrological processes beneath glaciers experiencing perennial surface melting. At Grubengletscher, Swiss Alps, melt penetrates into porous near-surface ice during the day, modifying the transmitted radar energy and thus the amplitudes of the targeted subglacial reflections. Normalising these reflections by early-time radar arrivals, integrated over a suitable time window, minimises such artefacts. In mid afternoon peak surface ablation, a diagnostic pulse in englacial reflectivity, sharp increases in subglacial reflectivity and glacier surface uplift precede the onset of transient glacier acceleration. Sliding terminates as the glacier surface lowers and the magnitude of subglacial reflectivity decreases. We infer a prominent episode of basal sliding as subglacial water pressure rises rapidly in response to englacially-routed melt delivery, jacking the glacier off its bed and modifying the observed reflectivity. Quantification of such processes is pertinent for any measurement and interpretation of basal reflection strength or bed reflection power from a GPR dataset.