Supraglacial, Englacial and Subglacier Hydrology Revision

What does supraglacial hydrology measure?

How water flow over the surface, and within the snowpack and firn

How is supraglacial hydrology measured?

Themistors, dye tracing, automatic weather station data

What are the differences between snow and ice permeability in supraglacial hydrology?

Where the snowpack is frozen, often in cold seasons or high latitudes, permeability is greatly reduced. This means water is forced to flow over the top of the glacier in supraglacial streams, or form lakes and ponds at the surface. However, when the snowpack is dominated by snow, permeability is much greater, as a result of increased air pockets within the snow. This allows water to begin to permeate into the glacier body, and enter the englacial system

What did Campbell and others (2006) reveal about different permeabilities of snow and ice (and where)?

At Haut Glacier D’Arolla in Switzerland dye tracing was used to track the movement of water within the glacier. It was found that the presence of ice had an important effect as where ice was present water began to pool and form internal pounds at layers of ice. This was due to refreezing within the snowpack which decreased permeability and impacted the ability of water to continue to flow into the snowpack

What is theorised about seasonal variations in supraglacial hydrology, why is this important?

The supraglacial snowpack is theorised to increase in permeability throughout the melt season as a result of warming and increased water content/air pockets within the snow. This is important for the dampening effect of the snowpack as its ability to store water and regulate flow is key for controlling the water pressure within conduits and at the bed which controls sliding and surging

What did Campbell and others (2006) find about seasonal variation in supraglacial hydrology (and where)?

At Haut Glacier D’Arolla in Switzerland dye tracing proved that after was more efficiently channeled through the snowpack during the spring and summer, as warmer temperatures formed a better connected layer than the previous cold and relatively impermeable winter surface

What did Samimi and Marshall (2017) find about diurnal variation in the supraglacial snowpack (and where)?

At Haig Glacier in the Rocky Mountains, Canada dye tracing was used to show that more water was held in the supraglacial snowpack during the daytime where it slowly percolated through to the englacial system. Overnight the water content was much lower due to a lower melt rate at lower temperatures overnight and decreased permeability of the snowpack

What does englacial hydrology measure?

The manner in which water flows within the body of the glacier in conduits, moulins and channels

What measures are used to investigate englacial hydrology?

Borehole video imaging, glacis-speleology and ground penetrating radar (GPR)

What are theories of englacial hydrology influenced by?

Theories of englacial hydrology are largely influenced by Shreve’s 1972 theorisation of these systems, based on idealised perspectives of hydraulic potential and channel form. However, Shreve’s assumptions are now being challenged through evidence collected from various measures of englacial hydrological systems

What did Gulley and others (2009) find about the formation of englacial channels (and where)?

Widespread glacio-speleology at 27 global englacial systems began to dispute Shreve’s classical conceptions of englacial hydrology, attempting to the dispute the idea that efficient englacial systems are unable to form at thick, cold ice settings. This study revealed that this was not correct, and instead identified three key manners that englacial channels were able to form: through cut-and-closure, exploitation of joints and hydrofracturing. This was found to allow water to travel though the body of the glacier and recharge the bed

What did Stuart and others (2003) reveal about the character of conduits (and where)?

GPR profiles at Broggerbreen glacier in Svalbard were used to reveal the great variation in the characteristics of conduits, with large semicircular conduits around 5 metres wide present within the accumulation zone, but closer to the outlet much thinner (around 2.5 metres wide), vertically elongated conduits were revealed

What did Fountain and others (2005) reveal about the mechanisms of englacial hydrology (and where?)

At Storglaciaren, this dispute was extended to show that it was not conduits that made up the key mechanism of flow within the glacier at all; instead borehole video imaging revealed that water was transmitted through a network of hydraulically connected fractures.

What is theorised about seasonal variation in englacial hydrology?

It is expected that the englacial system will increase in efficiency throughout the melt season as larger volumes of water will enlarge channels and allow for faster transmission to the bed

What did Church and others (2020) reveal about the seasonal variation of englacial systems (and where)?

At Rhonegletscher in Switzerland GPR surveys revealed that during the summer englacial channels were wide and active; however, in the winter, channels were much thinner and some closed completely

What did Gordon and others (2001) reveal about the impact of boreholes on englacial hydrology (and where)?

At Haut Glacier D’Arolla results showed that boreholes were not neutral measures of the drainage system, but were forming hydraulically connected systems themselves to alter water pressures in parts of the glaciers; this finding is important for problematising measures of glacial systems, and encouraging a deeper investigation into their accuracy, and potential impact on the systems they seek to explore

What does subglacial hydrology measure?

The manner in which water flows at the base of a glacier, and how it is channeled out as meltwater

What are the key measures of subglacial hydrology?

Borehole water pressures, GPR and dye tracing

What is subglacial hydraulic potential?

This describes the manner in which water flows as a function of the gravitational pressure and ice overburden pressure raised to a power of k. The k value is important in determining where water is located at the bed of the glacier, what kind of drainage system may form and how this impacts glacier dynamics such as sliding and surging

What did Willis and others (2012) show about the k value (and where)?

Digital elevation models and dye tracing at Mitdalsbreen glacier in Norway revealed that the k value was closer to 0.7 in the hydraulic potential equation to critique Shreve’s claim to universality

What did Fischer and others (2005) show about the k value (and where)?

The k value at Unteraargletscher in Switzerland was closer to 0.5, showing a greater influence of the bed topography as well as the variation between glaciers

What is theorised about the seasonal variation in subglacial hydrology?

The subglacial drainage system is theorised to transition from an inefficient linked cavity system to an efficient channelised system by the end of the melt season

What did Hock and Hooke (1993) show about seasonal variation of the subglacial drainage system (and where)?

Dye tracing at Storglaciaren showed that the efficiency of the drainage system increased throughout the melt season, with water velocities increasing from 0.07 m/s to 0.29 m/s from the beginning to the end. This was indicative of the switch of the drainage system, with the channelised, efficient morphology reached by late August

What did Miles and others (2020) show about the impact of debris-cover on the subglacial drainage system (and where)?

Dye tracing revealed that under Khumbu Glacier there was a channelised system present, but there was no evidence of this transitioning from or to a linked cavity system

What did Hubbard and others (1995) show about the co-existence of subglacial drainage systems (and where)?

Borehole water pressures at Haut Glacier D’Arolla shows that there was a central, efficient channel where water pressures were low alongside a linked cavity system surrounding it where water pressures were high due to inefficient drainage

How does the seasonal shift in subglacial hydrology impact basal water pressure and flow behaviour?

The linked cavity system is inefficient with small channels that can be easily blocked leading to a low efficiency and a high subglacial water pressure, this can match the ice overburden pressure leading to a low effective overburden pressure and lifting the glacier off the bed, accelerating via ice flow and enhanced basal sliding or widespread sediment deformation to occur (due to losing shear strength); when the morphology shifts to channelised it is efficient and large volumes of water are evacuated out to reduce subglacial water pressure and allowing the glacier to settle back to the bedrock, increasing friction and slowing basal sliding in the ‘spring deceleration’

Why are measures of subglacial hydrology important?

These measures are particularly important in not only revealing the seasonal variation in the subglacial hydrology system beneath glaciers, but also how this differs between glaciers, based on their differential characteristics. For glaciers such as Khumbu Glacier in High Mountain Asia, this is important in modelling water security for downstream communities, and increasing resilience through shifts in irrigation and water use patterns