Glacial budgets

• The mass balance of a glacier is a concept critical to all theories of glacier flow and behaviour. It is simple enough, really: mass balance is simply the gain and loss of ice from the glacier system. A glacier is the product of how much mass it receives and how much it loses by melting.
• glaciers losing more mass than they receive will be in negative mass balance and so will recede. Glaciers gaining more mass than they lose will be in positive mass balance and will advance. Glaciers gaining and losing approximately the same amount of snow and ice are thought of as ‘in equilibrium’, and will neither advance nor recede.

•Inputs – precipitation in the form of snow and ice, and avalanches willdd snow, ice and debris from the valley side.

•Storage – represented by the glacier itself. •Transfer – the way ice moves – the various types of movement described above. •Outputs – water vapour (from evaporation of water on the ice surface and sublimation – the direct change of state from ice to water vapour), calving, ablation. The debris deposited at the snout can also be considered an output.

•The zone of accumulation is where inputs exceed outputs and therefore where more mass is gained than lost over a year. This is the upper part of the glacier.

The glacier system receives snow and ice through processes of accumulation. Surface accumulation processes include snow and ice from direct precipitation, avalanches and windblown snow. There may be minor inputs from hoar frost. The snow and ice is then transferred downslope as the glacier flows. Precipitation falling as rain is usually considered to be lost to the system. Internal accumulation may include rain and meltwater percolating through the snowpack and then refreezing. Basal accumulation may include freezing on of liquid water at the base of the glacier or ice sheet.

•The zone of ablation is  where outputs exceed inputs and therefore more mass is lost rather than gained. This is the lower part of the glacier.

Glaciers lose mass through processes of ablation. Surface ablation processes include surface melt, surface meltwater runoff, sublimation, avalanching and windblown snow. Glaciers on steep slopes may also dry calve, dropping large chunks of ice onto unwary tourists below. Other processes of ablation include subaqueous melting, and melting within the ice and at the ice bed, which flows towards the terminus.