Glacier Hydrology Flashcards
Sources of Meltwater
- Snow and ice melt at glacier surface. (up to 10cm a day).
- Melting of glacier bed (basal, frictional, geothermal, pressure melting) (estimated 1cm/yr Paterson, 1994).
- Groundwater from sediments/bedrock.
- Wet precipitation.
What can cause variability in melt production?
- Seasonality changes in solar insolation.
- Air temperature
- Albedo, snow is more reflective than ice, debris can change albedo too or insulate.
Where does water go after it has been produces?
The supraglacial conditions are critical to answer this:
- Do you have snow or firn layers?
- Is there bare glacier ice?
- Are Crevasses or moulins present?
- What is the thermal regime.
We know water gets to the bed via moulins and crevasses.
Why is melt/runoff evolution important?
Public: global warming & SLR, a hazard and resource.
Glaciologys: ice dynamics, sediment transport, influence on glaier-fed river flow.
Melt influence on river flow.
As glaciers store precipitation in winter and release it in summer, summer glacier rivers get heavily supplemented by ice melt. This leads to ‘compensation effect’ due to more river flow when there is less likely to be wet precipitation.
What did Gordon et al. 1998 notice?
Melt increased 3 fold when snow cover was removed. Hence the snow pack suppresses melt.
What do diurnal flow cycles look like?
There is a base flow that represents slowly routed melt, and daily peaks that represent rapidly produced and routed ice melt. This represents a combination of quick and delayed flow components.
Moulin Formation
The easiest way for water to get into the ice is through fractures, e.g. crevasses from the ice flow. The stream develops the crevasse ice into a semi-circular hole with friction-generated heat, and the flow maintains or enlarges the hole.
Temperate ice internal plumbing system
Liboutry (1971): in temperate ice there is always some free water at the boundaries between ice crystals. These are a very small route ways for water that are enough to allow water to flow under gravity and work its way through the ice. yet bubbles and deformation strongly restricts this permeability.
How does the free water at ice crystal boundaries create a drainage system?
Moving water dissipates heat, so even microscopic flow paths should begin to enlarge. Rothlisberger (1971) and Shreve (1972) made the accepted theory of englacial conduit evolution, predicting properties of the conduit system and the size of flow paths, to help understand the direction water will flow.
Englacial Conduit Growth?
The larger the conduit, more water flows with more efficient heat transfer to the ice, counteracting the inward ice flow. Size of the conduit is a reflection of the inward deformation of the surrounding ice and the melting of the walls by frictional heat.
- Rate of melting is proportional to discharge, bigger conduits enlarge faster.
- Balance of melting vs closure dictates that bigger conduits have lower water pressure.
- Water flow is perpendicular to planes of equal water potential (equipotential surfaces) effectively being dictated by the ice surface gradient.
Equipotential Surfaces
These are planes of equal water pressure. These dip upglacier at an angle roughly 11x the ice surface slope. Water flows perpendicular to the equipotential lines.
Implications of Equipotential surfaces
- Flow id directed by ice surface slope.
- Flow can climb uphill is the slipe is 11x shallower than the ice slope.
- Larger conduits have lower water pressure and therefore capture smaller ones, forming a branching (aborescent) network or increasingly large conduits.
What are the two main types of subglacial drainage system?
- Channelized systems
- Distributed systems
We know channels exist as we see water exit a glacial system in a single large channel.
What is a channelized drainage system?
Consists of ice-roofed channels. These are effectively conduits at the bed so they entrain and drop sediments, and their roof is the glacier base. Rise in runoff results in a rise in basal water pressure.
