Hydrologic pathways beneath ice sheets and glaciers play an important role in regulating ice flow. Antarctica has experienced, and will continue to experience, changes in ice dynamics and geometry, but the associated changes in subglacial hydrology have received less attention. Here, we use the GlaDS subglacial hydrology model to examine drainage evolution beneath an idealised Antarctic glacier in response to steepening ice surface slopes, accelerating ice velocities and subglacial lake drainages. Ice surface slope changes exerted a dominant influence, redirecting basal water to different outlet locations and substantially increasing channelised discharge crossing the grounding line. Faster ice velocities had comparatively negligible effects. Subglacial lake drainage results indicated that lake refilling times play a key role in drainage system evolution, with lake flux more readily accommodated following shorter refilling times. Our findings are significant for vulnerable Antarctic regions currently experiencing dynamic thinning since subglacial water re-routing could destabilise ice shelves through enhanced sub-shelf melting, potentially hastening irreversible retreat. These changes could also affect subglacial lake activity. We, therefore, emphasise that including a nuanced and complex representation of subglacial hydrology in ice-sheet models could provide critical information on the timing and magnitude of sea-level change contributions from Antarctica.

This study presents the first light detection and ranging (LiDAR)-based morphometric description of a water blister from a past ice-sheet bed caused by rapid supraglacial drainage. The blister formed during the rapid early Holocene deglaciation of the Fennoscandian Ice Sheet (FIS). It is located in southwest Finland within a subglacial meltwater route interpreted to represent the transition from a distributed to a channelized drainage system. A LiDAR digital elevation model was supplemented with sedimentological and ground-penetrating radar data on blister outflow channels and sedimentology of downflow polymorphous mounds and ridges (PMRs). Unlike the water blisters recorded from the rapid drainage of supraglacial lakes on the Greenland Ice Sheet, the smaller blister size here was either due to crevasse or moulin drainage, or was a supraglacial lake drainage that tapped into a pre-existing, relatively efficient drainage system and related semi-sorted sediments, promoting rapid drainage and reworking of PMRs along the meltwater route. The preservation potential or exposure probability of blister marks is presumably low but they can provide important information about evolution of subglacial drainage systems that is of value to modern interpretations of glacial hydrology.

Subglacial hydrology is an important component of the ice dynamic system in Antarctica but is challenging to investigate due to the large spatial scales of the catchment systems, the ice thickness, and remote location. Here I discuss key discoveries about Antarctic subglacial hydrology from the Glacier Drainage System (GlaDS) model, including the presence of long, often high-pressure, subglacial channels. These channels pump tens of cubic metres per second of freshwater into ice-shelf cavities and directly affect melt rates at the critical grounding zone regions. Future ice dynamics and ice-shelf cavity models should take subglacial hydrology into account if they are to accurately predict future behaviour of the Antarctic Ice Sheet.

Supraglacial drainage networks play an integral role in both glacier dynamics and run-off timing, and mapping them provides insight into their role in glacial systems. Here we present a reproducible approach for semi-automated mapping of supraglacial hydrologic features, which complements existing work in automated and manual mapping by providing clear definitions for identification of features. This framework uses a digital terrain model (DTM) to identify potential flow routes on the glacier surface, which are then classified using a set of standardized rules based on the DTM and an orthomosaic. We found that the normalized difference water index calculated from digital imagery was influenced by image brightness and introduce a new approach using average RGB values to correct for this. Using this framework we mapped supraglacial drainage networks at Nàłùdäy and Thores Glacier, Canada. The framework was easier to implement with high-resolution (0.5 m) imagery and DTMs, compared to data with lower resolution (10 m), due to the increased detail in topography and feature boundaries at high-resolution. Lower-resolution data captured larger streams (>2 pixels wide), however, indicating that the framework can still be used at this resolution. Mapping supraglacial hydrology using standardized methods opens possibilities for investigating many questions relating to changes in supraglacial hydrology over time.

Liquid water can exist at temperatures well below freezing beneath glaciers and ice sheets, where subglacial water systems, fresh and saline, have been shown to host unique microbial ecosystems. Geophysical techniques sensitive to fluid-content contrasts, e.g. electromagnetics, can characterize subglacial water and its salinity. Here, we assess the ground-based transient electromagnetic (TEM) method for deriving the resistivity and salinity of subglacial water. We adapt an existing open-source Bayesian inversion algorithm, which uses independent depth constraints, to output posterior distributions of resistivity and pore fluid salinity with depth. A variety of synthetic models, including a thin (5 m), conductive (0.16 Ωm), hypersaline (147 psu) subglacial lake, are used to evaluate the TEM method for imaging under 800 m-thick ice. The study demonstrates that TEM methods can resolve conductive, saline bodies accurately using external depth constraints, for example, from radar or seismic data. The depth resolution of TEM can be limited beneath deep (>800 m), thick (>50 m) conductive, water bodies and additional constraints from passive electromagnetic (EM) methods could be used to reduce ambiguities in the TEM results. Subsequently, non-invasive active and passive EM methods could provide profound insights into remote aqueous systems under glaciers and ice sheets.

Glacier surficial melt rates are commonly modelled using surface energy balance (SEB) models, with outputs applied to extend point-based mass-balance measurements to regional scales, assess water resource availability, examine supraglacial hydrology and to investigate the relationship between surface melt and ice dynamics. We present an improved SEB model that addresses the primary limitations of existing models by: (1) deriving high-resolution (30 m) surface albedo from Landsat 8 imagery, (2) calculating shadows cast onto the glacier surface by high-relief topography to model incident shortwave radiation, (3) developing an algorithm to map debris sufficiently thick to insulate the glacier surface and (4) presenting a formulation of the SEB model coupled to a subsurface heat conduction model. We drive the model with 6 years of in situ meteorological data from Kaskawulsh Glacier and Nàłùdäy (Lowell) Glacier in the St. Elias Mountains, Yukon, Canada, and validate outputs against in situ measurements. Modelled seasonal melt agrees with observations within 9% across a range of elevations on both glaciers in years with high-quality in situ observations. We recommend applying the model to investigate the impacts of surface melt for individual glaciers when sufficient input data are available.

The Drygalski Ice Tongue in East Antarctica stretches 90 km into the Ross Sea and influences the local ocean circulation, and persistence of the Terra Nova Bay Polynya. We examine the controls on the size of this floating ice body by comparing the propagation of six large fractures on the ice tongue's northern side using 21 years of Landsat imagery with hydrostatic ice thickness maps and strain rate calculations. We also apply a subglacial hydrology model to estimate the location and discharge from subglacial channels over the grounding line and compare these with basal channels identified along the ice tongue using remote sensing and airborne radar data. Our results suggest that large fractures are inhibited from full-width propagation by thicker ice between basal channels. We hypothesize that only once the ice tongue thins towards the terminus, can fractures propagate and cause large calving events. This suggests an important relationship between the melting of floating ice from subglacial and ocean sources and the expansion of fractures that lead to ice tongue calving.

We describe elongate, wet, subglacial bedforms in the shear margins of the NE Greenland Ice Stream and place some constraints on their formation. Lateral shear margin moraines have been observed across the previously glaciated landscape, but little is known about the ice-flow conditions necessary to form these bedforms. Here we describe in situ sediment bedforms under the NE Greenland Ice Stream shear margins that are observed in active-source seismic and ground-penetrating radar surveys. We find bedforms in the shear margins that are ~500 m wide, ~50 m tall, and elongated nearly parallel to ice-flow, including what we believe to be the first subglacial observation of a shear margin moraine. Acoustic impedance analysis of the bedforms shows that they are composed of unconsolidated, deformable, water-saturated till. We use these geophysical observations to place constraints on the possible formation mechanism of these subglacial features.

Subglacial hydrology plays a key role in many glaciological processes, including ice dynamics via the modulation of basal sliding. Owing to the lack of an overarching theory, however, a variety of model approximations exist to represent the subglacial drainage system. The Subglacial Hydrology Model Intercomparison Project (SHMIP) provides a set of synthetic experiments to compare existing and future models. We present the results from 13 participating models with a focus on effective pressure and discharge. For many applications (e.g. steady states and annual variations, low input scenarios) a simple model, such as an inefficient-system-only model, a flowline or lumped model, or a porous-layer model provides results comparable to those of more complex models. However, when studying short term (e.g. diurnal) variations of the water pressure, the use of a two-dimensional model incorporating physical representations of both efficient and inefficient drainage systems yields results that are significantly different from those of simpler models and should be preferentially applied. The results also emphasise the role of water storage in the response of water pressure to transient recharge. Finally, we find that the localisation of moulins has a limited impact except in regions of sparse moulin density.

To investigate the subsurface hydrological characteristics of an overdeepened cirque glacier, nine boreholes were drilled to the bed of West Washmawapta Glacier, British Columbia, Canada, in summer 2007. All holes were surveyed with a video camera, and four were subsequently instrumented with a combination of pressure transducers, thermistors and conductivity sensors. Diurnal pressure and temperature records indicate the presence of a hydraulically connected subglacial drainage system towards the northern glacier margin. Hydraulic jacking in the overdeepening, controlled by changing water volume in the marginal zone, potentially impacts basal ice flow and erosion. The presence of a sediment layer underlying the glacier also likely impacts hydrology and ice dynamics. Influx of warm groundwater into the basal system raises subglacial water temperatures above the pressure-melting point (pmp) and induces diurnal water temperature fluctuations of as much as 0.8°C; water temperatures above the pmp could affect basal melt rates and the development of subglacial drainage systems. These observations suggest that the characteristics of the subglacial drainage system substantially affect patterns of flow and erosion by this small cirque glacier.

Borehole instrument records from a cirque glacier with an overdeepened bed are examined to assess the validity of widely held glacial hydrological assumptions. At this glacier, hydraulic-potential calculations suggest water below overburden pressure will flow into the overdeepening, where the steepness of the riegel causes water to pool in the basin and increase in pressure. Our subglacial water pressure data also show high consistent pressures in the overdeepening and the presence of an active, variable-pressure drainage system towards the margin of the cirque. Therefore, we find that although uniform hydraulic-potential calculations are not directly applicable, they can still be useful for interpretation of the subglacial hydrological system. We also examine supercooling assumptions under different pressure and temperature regimes for water flowing over a riegel, driven using our borehole records of subglacial water temperatures that are consistently above the pressure-melting point during the late melt season. Our results show that even a slight increase in basal temperatures relative to the local pressure-melting point is sufficient to prevent a reduction in basal hydraulic conductivity as a result of supercooling freeze-on.