Glacial Systems - the Glacial Budget

The glacial budget refers to the balance between the amount of inputs versus outputs affecting the glacial system.

In the upper part of the glacier, often found in mountainous areas, inputs exceed outputs as there will be snowfall and avalanches adding to the glacier. Evaporation and meltwater rates are low, so in the colder, upper zone most of the glacier remains as ice that is periodically added to. This part of a glacier is known as the zone of accumulation and results in an increase in mass within the glacier.

In the lower part of the glacier, near the snout, the reverse will occur and outputs exceed inputs. Outputs occur lower down the glacier system where the temperatures are warmer. The lower part of the glacier is often at a much lower altitude resulting in lower levels of precipitation as snow, and limited input from avalanches as the glacier is now lower down in the valley. The glacier loses mass as evaporation will increase due to warmer temperatures and there will be melting at the snout. This part of a glacier is known as the zone of ablation.

Between the two zones is the line of equilibrium which is the point at which inputs and outputs are balanced. During winter or colder climatic periods, the equilibrium line falls further down the glacier, and occurs higher up in summer or warmer periods.

The net balance is the difference between the total accumulation and the total ablation during 1 year. It represents the annual change in the ice store. A glacier which is moving downhill, but where the output matches the input is in a state of dynamic equilibrium: the total store remain the same, but the actual ice mass is in a state of continual transformation.