Glacier Hydrology - An Overview
The surface of a glacier consists of seasonal snow, multi-year firn, and glacial ice.
Firn is present in the upper accumulation area, where it can be tens of metres thick. In theory, firn can be found everywhere above the glacier’s equilibrium line altitude (ELA), increasing in thickness with altitude. During negative mass‐balance years, however, firn ablates above the ELA. A sequence of negative mass‐balance years can remove much of the firn, and 20th century warming has caused many small alpine glaciers to lose their firn layers entirely.
On a glacier surface, meltwater channels carve into the ice and wind sinuously downslope, sometimes pouring off the front of the glacier and sometimes plunging into deep moulins and crevasses. Water that disappears into the glacier may be stored internally or transported through a drainage network within the glacier. In the latter case, it typically descends to the glacier bed to join the subglacial drainage system. At the glacier front, the water usually emerges as a few major streams flowing out of tunnels.
Most of the water in mountain glaciers is produced from melting at the glacier surface. Englacial and basal melting also contribute to the water budget, while rainfall and groundwater provide additional, external sources of water to a glacier. Exchanges between the groundwater and the subglacial drainage system can be two‐way, but these are seldom measured and are not well‐understood.
Early in the melt season, glacier hydrology is not significantly different from that of seasonal snow in non-glacial environments. Surface meltwater percolates and refreezes until the snowpack becomes saturated and isothermal (reaches 0 degrees C throughout), at which point, runoff begins.
There are two major differences between non‐glacial and glacial snowpacks, both involving the underlying surface.
- First, the underlying firn or ice never has a temperature exceeding the melting point, and this limits how much heat can flow up into the snowpack; there is no geothermal heat flux into the seasonal snowpack.
- The second difference is the presence of an impermeable ice surface where glacier ice underlies the seasonal snowpack, a situation analogous to snowpacks on landscapes of bare bedrock or permafrost. Meltwater that drains to the "snow‐ice interface" ponds or flows along this interface, depending on the surface gradient. This also occurs at the "snow‐firn interface" due to the permeability contrast between the seasonal snowpack and the underlying firn.
As the melt season progresses, ablation of the seasonal snowcover exposes glacier ice in the ablation area. The area of ice exposure expands upslope through the summer. Water drains efficiently in the bare‐ice zone, typically through sinuous, ice‐walled surface channels. These channels can discharge off the front or lateral margins of a glacier, but they more commonly drain into crevasses or moulins.