Glacial Budgets

The glacial budget
It is a system
A glacier can be viewed as a system with inputs, stores, transfers and outputs:
Inputs: are precipitation in the form of snow and ice, avalanches which add snow, ice and debris from the valley side
Storage: represented by the glacier itself
Transfer (throughout): is the way that the ice moves
Outputs: are water vapour (from evaporation of water on the ice surface and sublimation – the direct change of state from ice to water vapour), calving (the formation of icebergs), and water in liquid form from ablation (melting). The debris deposited at the snout (moriane) can also be considered a output.
The upper part of the glacier, where inputs exceed outputs and therefore where more mass is gained than lost over a year, is known as the zone of accumulation
The lower part, where outputs exceed inputs, and where mass is lost rather than gained is known as the zone of ablation.
Between the two zones is the line of equilibrium which separates net loss from net gain and represents the snow line on the glacier. A glacier that is characterised by large volumes of gains and losses will discharge a large volume of ice through its equilibrium line to replace mass lost at the snout and will therefore have a high erosive capacity.
The net balance is the difference between the total accumulation and the total ablation during 1 year.
Net balance over one year
The net balance is the difference between the total accumulation and the total ablation during 1 year.
In temperate glaciers, there is a negative balance in summer when ablaition exceeds accumulation, and the reverse in winter. If the summer and winter budgets cancel each other out, the glacier appears to be stationary. In other words, the snout of the glacier remains in the same position, although ice is still advancing down the valley from the zone of accumulation into the zone of ablation.
If the ‘supply’ begins to exceed the losses, then the snout moves down the valley. This is known as glacial advance. when the reverse is true, the glacier begins to shrink in size and the snout moves its position up the valley. this is glacial; retreat. Even though the position of the snout is moving mving backwards (retreating), ice continues to move down from the upper parts of the system.