Abstract. In response to climate change, most glaciers are losing mass and hence contribute to sea-level rise. Repeated and accurate mapping of their surface topography is required to estimate their mass balance and to extrapolate/calibrate sparse field glaciological measurements. In this study we evaluate the potential of sub-meter stereo imagery from the recently launched Pléiades satellites to derive digital elevation models (DEMs) of glaciers and their elevation changes. Our five evaluation sites, where nearly simultaneous field measurements were collected, are located in Iceland, the European Alps, the central Andes, Nepal and Antarctica. For Iceland, the Pléiades DEM is also compared to a lidar DEM. The vertical biases of the Pléiades DEMs are less than 1 m if ground control points (GCPs) are used, but reach up to 7 m without GCPs. Even without GCPs, vertical biases can be reduced to a few decimetres by horizontal and vertical co-registration of the DEMs to reference altimetric data on ice-free terrain. Around these biases, the vertical precision of the Pléiades DEMs is ±1 m and even ±0.5 m on the flat glacier tongues (1σ confidence level). Similar precision levels are obtained in the accumulation areas of glaciers and in Antarctica. We also demonstrate the high potential of Pléiades DEMs for measuring seasonal, annual and multi-annual elevation changes with an accuracy of 1 m or better if cloud-free images are available. The negative region-wide mass balances of glaciers in the Mont-Blanc area (−1.04 ± 0.23 m a⁻¹ water equivalent, w.e.) are revealed by differencing Satellite pour l'Observation de la Terre 5 (SPOT 5) and Pléiades DEMs acquired in August 2003 and 2012, confirming the accelerated glacial wastage in the European Alps.

Abstract. The flow of glaciers and ice streams is strongly influenced by the presence of water at the interface between ice and bed. In this paper, a hydrological model evaluating the subglacial water pressure is developed with the final aim of estimating the sliding velocities of glaciers. The global model fully couples the subglacial hydrology and the ice dynamics through a water-dependent friction law. The hydrological part of the model follows a double continuum approach which relies on the use of porous layers to compute water heads in inefficient and efficient drainage systems. This method has the advantage of a relatively low computational cost that would allow its application to large ice bodies such as Greenland or Antarctica ice streams. The hydrological model has been implemented in the finite element code Elmer/Ice, which simultaneously computes the ice flow. Herein, we present an application to the Haut Glacier d'Arolla for which we have a large number of observations, making it well suited to the purpose of validating both the hydrology and ice flow model components. The selection of hydrological, under-determined parameters from a wide range of values is guided by comparison of the model results with available glacier observations. Once this selection has been performed, the coupling between subglacial hydrology and ice dynamics is undertaken throughout a melt season. Results indicate that this new modelling approach for subglacial hydrology is able to reproduce the broad temporal and spatial patterns of the observed subglacial hydrological system. Furthermore, the coupling with the ice dynamics shows good agreement with the observed spring speed-up.

Abstract. The grounding line is a key element of coastal outlet glaciers, acting on their dynamics. Accurately knowing its position is fundamental for both modelling the glacier dynamics and establishing a benchmark for later change detection. Here we map the grounding line of the Astrolabe Glacier in East Antarctica (66°41' S, 140°05' E), using both hydrostatic and tidal methods. The first method is based on new surface and ice thickness data from which the line of buoyant floatation is found. The second method uses kinematic GPS measurements of the tidal response of the ice surface. By detecting the transitions where the ice starts to move vertically in response to the tidal forcing we determine control points for the grounding line position along GPS profiles. Employing a two-dimensional elastic plate model, we compute the rigid short-term behaviour of the ice plate and estimate the correction required to compare the kinematic GPS control points with the previously determined line of floatation. These two approaches show consistency and lead us to propose a grounding line for the Astrolabe Glacier that significantly deviates from the lines obtained so far from satellite imagery.