The motion of temperate and polythermal glaciers is influenced by the seasonal input of meltwater to the basal hydrological system. Spatial differences in the bed sliding velocities lead to stress and strain in the glacier ice, and the related changes can be measured on the glacier surface. This study analyzes the motion of three Global Navigation Satellite System (GNSS) stations installed on the glacier surface of Holtedahlfonna over the period September 1st 2014 – August 31st 2015. In order to detect the small-scale changes on the glacier surface, the error sources affecting the GNSS positioning need to be reduced or eliminated. By comparing precise point positioning (PPP) and different setups of relative positioning, this study finds that finds that a network setup with kinematic relative positioning has the best combination of precision and ability to capture the short-term changes of the glacier. The approximated uncertainties for each estimated position on the glacier was ± 18 mm and ± 69 mm (95% confidence level) in the horizontal and vertical directions, respectively.
The observations during winter reveals surface velocities in the range of 0.12 - 0.28 m/day. By estimating the runoff with a surface mass balance model, the influence of meltwater on the glacier motion is evaluated during the summer season. Two major events of increasing horizontal and vertical motion is evident at all three stations, and coincides with significant increases in the estimated runoff. A prominent supraglacial lake is identified on optical satellite images and its volume is estimated from a digital elevation model (DEM). The drainage of the lake occurs in the same period as rapid uplift, increasing surface velocities and horizontal translation can be observed at all three stations. The middle and lower GNSS station sustains elevated vertical positions over a two-week period after the first major event, and this indicates local storage of water at the glacier bed. During the two major events, significant variations in the longitudinal strain was observed, with both compression and extension between the three stations. Changes in elevation due to vertical strain and rates of bed separation are estimated, but the absolute magnitude of these values are uncertain. Although the complexity of glacier dynamics gives a range of uncertainties, this study has shown the potential of high resolution GNSS for these applications.