Lecture Revision Questions (Glaciers)

What is glacier mass balance, and what are the three ways of collecting data to calculate it? (1)

Glacier mass balance is the relationship between accumulation and ablation, from this an equilibrium line can be calculated that determines the topography of a glacier over time. Data can be collected using the glaciological method (in-situ, surface measurements using stakes), geodetic (satellites, GPS) or geodetic methods (GRACE and GRACE-FO).

What are the key considerations you need before beginning modelling glacier mass balance? (1)

Spatial domain (what dimension you will model in), mathematical descriptions (which model and calculation will be used), inputs and observations that are used to calibrate models.

What does the degree day model calculate, what are the pros and cons of this model? (1)

The degree day model calculates the empirical relationship between air temperature and melting per positive degree day and also tracks refreezing due to air temperatures. This is useful as it is easy, only requires air temperature measures and can predict future melt but is empirical, offers limited insights into processes and gives limited information on temporal patterns.

What does the energy balance model calculate, what are the pros and cons of this model? (1)

The energy balance model calculates how much energy is available at the glacier surface to melt ice or snow taking into account many variables and calculating conduction. This is useful as it provides good insight into processes, useful over spatial and temporal scales and can be applied to other glaciers but is expensive and requires many inputs.

What have hybrid models been used to calculate, how have these been useful? (1)

Hybrid models are variants on the degree day model and have been used to account for potential clear sky radiation (patterns of shading) and cloud cover. This is useful for future predictions and accounting for alternative spatial methods.

Where do inputs to models come from, and how are the models then used to reveal information about glaciers? (1)

Inputs can come from automatic weather stations, climate models and observations but these must be extrapolated over whole glaciers and calibrated. Once this has occurred models can be used to understand glacial sensitivity to other factors, inter-comparison of other models and global assessments of contributions to processes such as sea level rise.

How does supraglacial snow dampen variations in surface water inputs, what factors influence this? (2)

Snow stores and releases water with a lesser effect in the summer as it becomes thinner and more porous; there is a greater delay in colder months, with greater depth and due to changes in albedo changing the amount of energy absorbed by the snowpack.

How does water flow within a glacier, why does this occur? (2)

Water flows through conduits and channels as veins between ice crystals are impermeable.

How do boreholes, radars and glacio-speleology provide insight into englacial hydrology? (2)

Boreholes can have cameras or instruments to measure electrical conductivity sent down them in order to map stratigraphy of the inner glacier and find water concentrations. Radars use reflections from radio waves to determine where changes mean there is a conduit or cavity, different angles of reflection can also show what kind of sediment or water is being transported at different times of year. Glacio-speleology can reveal characteristics of englacial passages and how they form.

What does the hydraulic potential tell us about subglacial hydrology, what assumptions do we have to make in order to use this equation? (2)

The hydraulic potential enables us to calculate where water would end up on the bed theoretically as a function of gravitational potential and pressure potential; this does require an assumption about the k value which measures the pressure of basal water compared to the pressure exerted by the ice pack above. This is often not at 1 but it can be found by comparing empirical data to results from various assumptions of k.

What different configurations can water flow in subglacially, what key differences come as a result of these different morphologies? (2)

Water can flow horizontally, through pore flow, pipe flow, in a thin film, in linked cavities, braided canals or channels; this changes the pressure and distribution of water with implications for the movement and dynamics of glaciers.

What are the processes by which channels and cavities enlarge and close? (3)

Channels are enlarged by melt rates where cavities are enlarged by sliding. Both channels and cavities close due to creep deformation, but critically there are different controls on enlargement.

Why is discharge key to these processes and how does this affect pressure? (3)

The rate of opening and closure rates is a function of discharge; therefore, the volume of discharge will allow the system to tend towards channels or cavities. Different states of discharge have different effective pressure relationships seen as discharge increases, water pressure will increase in a stable linked cavity system which will initially increase the cross sectional area but at a lower rate than discharge rises. The opposite is true for channels, if discharge is increased the channel will enlarge at a faster rate than the discharge. Therefore, water pressure will be lower in a channel system than a linked cavity system for a given volume of discharge.

What is the idealised process of drainage system evolution modelled by Schoof? (3)

There is an idealised process of seasonal variation mapped where the drainage system will begin as cavities, then become unstable, then become channels and then close up again and return to cavities. Throughout the spring to summer the level of diurnal fluctuations will increase as the system becomes more effective at transporting meltwater. It becomes more responsive to changes occurring on smaller time scales such as higher melt rates during the day due to higher temperatures and solar radiation and lower in the evening when melt rates fall.

How have borehole pressure changes and dye tracing at Haunt Glacier d’Arolla in Switzerland helped to back up the theory of subglacial drainage system evolution? (3)

Water pressure in boreholes show a decline in water pressures from the spring to summer as well as heightened diurnal cycles which is indicative of a switch from a distributed to channelised subglacial drainage system. Dye tracing also provides this by measuring the velocity of water movement with much faster velocities in the late summer as drainage becomes more effective in a channelised system.

What other empirical data has been used to prove these theories of channelisation? (3)

Further empirical data is found through the use of velocity measures, GPS data and catchment modelling which are all used to support theories for evolution of subglacial drainage systems.

What kind of glaciers do we know the least about, where has there been an exception to this trend realised? (3)

The least is known about debris covered glaciers such as those in Nepal and High Mountain Asia, dye tests in Nepal were inconclusive. However, measures of temperatures and drainage in Italy show that there is an evolution of drainage systems through the summer seeing rises in diurnal fluctuations indicative of changes to channels.