Diurnal variations in vertical strain observed in a temperate valley glacier

During a period of diurnal fluctuations in glacial flow speed, vertical strain was measured with sub‐daily temporal resolution on Unteraargletscher, Switzerland. Mean vertical strain in boreholes up to 300‐m deep in 400‐m‐thick ice was found to fluctuate diurnally. Vertical strain rates were tensile in the daytime and compressive at night, with a magnitude of up to 10−3 day−1. Horizontal surface strain was observed to fluctuate in a manner consistent with the vertical deformation. Diurnal surface flow speed variations correlated well with basal water pressure suggesting a basal control on temporal flow variations. Nevertheless, the strain rate measurements indicated that changes in surface flow speed are affected by internal ice deformation and not a direct measure of local basal motion. Basal conditions in the surrounding neighborhood and their temporal variations take an important role in short‐term glacial flow fluctuations.