In this study, we assess the accuracy and precision of digital elevation models (DEM) retrieved from aerial photographs taken in 2011 and from Very High Resolution satellite images (WorldView-2 and Pléiades) from the period 2012–2017. Additionally, the accuracy of the freely available Strip product of ArcticDEM was verified. We use the DEMs to characterize geometry changes over Hansbreen and Hornbreen, two tidewater glaciers in southern Spitsbergen, Svalbard. The satellite-based DEMs from WorldView-2 and Pléiades stereo pairs were processed using the Rational Function Model (RFM) without and with one ground control point. The elevation quality of the DEMs over glacierized areas was validated with in situ data: static differential GPS survey of mass balance stakes and GPS kinematic data acquired during ground penetrating radar survey. Results demonstrate the usefulness of the analyzed sources of DEMs for estimation of the total geodetic mass balance of the Svalbard glaciers. DEM accuracy is sufficient to investigate glacier surface elevation changes above 1 m. Strips from the ArcticDEM are generally precise, but some of them showed gross errors and need to be handled with caution. The surface of Hansbreen and Hornbreen has been lowering in recent years. The average annual elevation changes for Hansbreen were more negative in the period 2015–2017 (−2.4 m a−1) than in the period 2011–2015 (−1.7 m a−1). The average annual elevation changes over the studied area of Hornbreen for the period 2012–2017 amounted to −1.6 m a−1. The geodetic mass balance for Hansbreen was more negative than the climatic mass balance estimated using the mass budget method, probably due to underestimation of the ice discharge. From 2011 to 2017, Hansbreen lost on average over 1% of its volume each year. Such a high rate of relative loss illustrates how fast these glaciers are responding to climate change.

Albedo is one of the variables controlling the mass balance of temperate glaciers. Multispectral imagers, such as MODerate Imaging Spectroradiometer (MODIS) on board the TERRA and AQUA satellites, provide a means to monitor glacier surface albedo. In this study, different methods to retrieve broadband glacier surface albedo from MODIS data are compared. The effect of multiple reflections due to the rugged topography and of the anisotropic reflection of snow and ice are particularly investigated. The methods are tested on the Saint Sorlin Glacier (Grandes Rousses area, French Alps). The accuracy of the retrieved albedo is estimated using both field measurements, at two automatic weather stations located on the glacier, and albedo values derived from terrestrial photographs. For summers 2008 and 2009, the Root Mean Square Deviation (RMSD) between field measurements and the broadband albedo retrieved from MODIS data at 250 m spatial resolution was found to be 0.052 or about 10% relative error. The RMSD estimated for the MOD10 daily albedo product is about three times higher. One decade (2000–2009) of MODIS data were then processed to create a time series of albedo maps of Saint Sorlin Glacier during the ablation season. The annual mass balance of Saint Sorlin Glacier was compared with the minimum albedo value (average over the whole glacier surface) observed with MODIS during the ablation season. A strong linear correlation exists between the two variables. Furthermore, the date when the average albedo of the whole glacier reaches a minimum closely corresponds to the period when the snowline is located at its highest elevation, thus when the snowline is a good indicator of the glacier equilibrium line. This indicates that this strong correlation results from the fact that the minimal average albedo values of the glacier contains a considerable information regarding the relative share of areal surfaces between the ablation zone (i.e. ice with generally low albedo values) and the accumulation zone (i.e. snow with a relatively high albedo). As a consequence, the monitoring of the glacier surface albedo using MODIS data can provide a useful means to evaluate the inter-annual variability of the glacier mass balance. Finally, the albedo in the ablation area of Saint Sorlin Glacier does not exhibit any decreasing trend over the study period, contrasting with the results obtained on Morteratsch Glacier in the Swiss Alps

Abstract. As the behavior of subglacial water plays a determining role in glacier dynamics, it requires particular attention, especially in the context of climate warming, which is increasing ablation and generating greater amounts of meltwater. On many glaciers, water flowing from the glacier's surface is the main source of supply to the subglacial drainage system. This system is largely influenced by the supraglacial drainage system, which collects meltwater and precipitation and rapidly delivers it to discrete points in the glacier bed via moulins and crevassed areas, called water input areas (WIAs). Models of patterns of subglacial conduits mainly based on the hydrological potential gradient are still regularly performed without taking into account the supraglacial drainage system. We modeled the pattern of subglacial channels in two glaciers located in Svalbard, the land-terminating Werenskioldbreen and the tidewater Hansbreen during the 2015 melt season. We modeled a spatial and a discrete water recharge in order to compare them. First, supraglacial catchments were determined for each WIA on a high-resolution digital elevation model using the standard watershed modeling tool in ArcGIS. Then, interpolated water runoff was calculated for all the main WIAs. Our model also accounts for several water pressure conditions. For our two studied glaciers, during the ablation season 2015, 72.5 % of total runoff was provided by meltwater and 27.5 % by precipitation. Changes in supraglacial drainage on a decadal timescale are observed in contrast to its nearly stable state on an annual timescale. Nevertheless, due to the specific nature of those changes, it seems to have a low impact on the subglacial system. Therefore, our models of subglacial channel are assumed to be valid for a minimum period of two decades and depend on changes in the supraglacial drainage system. Results showed that, for Svalbard tidewater glaciers with large crevassed areas, models of subglacial channels that assume spatial water recharge may be somewhat imprecise but are far from being completely incorrect, especially for the ablation zone. On the other hand, it is important to take discrete water recharge into account in the case of land-terminating Svalbard glaciers with limited crevassed areas. In all cases, considering a discrete water recharge when modeling patterns of theoretical subglacial channels seems to produce more realistic results according to current knowledge.

Abstract. Being a determinant factor of the glacier's dynamic, subglacial water behavior needs a special attention. Water flowing from the glacier's surface is the principal source supplying the subglacial drainage system. Therefore, insight into the state and evolution of the supraglacial drainage system is crucial for recognition of recharge pattern of the englacial and subglacial drainage pathways. Climate warming causes increased ablation generating higher amount of meltwater and thinning of glacier.Decadal timescale evolution of the supraglacial drainage leads to some modifications of the system in opposition to its nearly 5 stable state on an annual timescale. For two studied glaciers Hansbreen and Werenskioldbreen in southern Svalbard surface meltwater is the main runoff component. During the ablation season 2015, 72.5 % of the total amount was provided by meltwater and 27.5 % by precipitations. Supraglacial catchments were determined on the high resolution digital elevation model using standard watershed modelling tool in ArcGIS, for each water-input area (WIA). Spatialized water runoff calculations for all the main WIAs have been done. Having data on the water sources from catchments delimited on glacier's surface, mod-10 elling of a theoretical pattern of subglacial conduits was done considering discrete water recharge via moulins, shear fractures or crevasses. Classical modelling with an assumption of homogeneous water supply was done for comparison. Several water pressure conditions have been taken into account as well. Results show that models of subglacial drainage system with homogeneous water recharge are more realistic for tidewater glaciers with rather broad permeable firn areas and creased frontal zones, while discrete water recharge models are better for land-terminating glaciers with almost continuous impermeable su-15 perficial cold ice layer. Subglacial channel models are assumed to be valid for a minimum period of two decades taking into account evolution of supraglacial drainage system and ice thickness changes of Svalbard polythermal glaciers.

A single week-long warm event in midwinter in Svalbard flooded an inefficient en-and subglacial drainage system and led to a 2.5x velocity increase that remained in effect for the remainder of the winter -more than 3 months. Because of the long winter season, changes in winter velocity have a large impact on the annual average velocity. As the climate warms and surface melt and rain events increase during winter months, sustained high winter glacier velocities are likely to occur more often. Increasing glacier velocity near the terminus leads to additional ice entering the fjord, and an increase of ice dynamics contribution to sea level rise during winter.