| Term: | Mass balance |
| Definition: |
The difference between accumulation and ablation on a glacier; usually calculated on an annual basis. NSIDCCryosphere
Mass balance describes the net gain or loss of snow and ice through a given year. It is usually expressed in terms of water gain or loss. A measure of the change in mass of a glacier at a certain point for a specific period of time. The balance between accumulation and ablation. Also called Mass Budget. The change in the mass of a glacier, or part of the glacier, over a stated span of time; the term mass budget is a synonym. See mass-balance units for recommended units. The span of time is often a year or a season. A seasonal mass balance is nearly always either a winter balance or a summer balance, although other kinds of season are appropriate in some climates, such as those of the tropics. The definition of year depends on the method adopted for measurement of the balance. See time system. The reference in the definition to a glacier means that a particular volume of space is being studied. A properly delineated glacier has no mass transfer of ice across its boundary other than as ice discharge. However, the mass balance is often quoted for volumes other than that of the whole glacier, for example a column through the glacier, the part of the glacier upglacier or downglacier from the grounding line, or a band defined by two contours of surface elevation. It is necessary in such cases to make clear that the study volume is something other than the whole glacier, and also to make clear which components of the mass balance are being reported. The quantity reported may be the climatic mass balance or the climatic-basal mass balance, but will often be the surface mass balance. In all cases the need for a defined study volume is fundamental because without it the principle of conservation of mass cannot be invoked. The study volume may change over the study period. The surface and bed elevations may change, and the areal extent is unlikely to be the same at the end of the period as it was at the beginning. Whether these changes are significant will depend not just on their magnitude and the accuracy with which they can be determined but on the purpose of the investigation. See conventional balance, reference-surface balance. IHPGlacierMassBalance (1) In a hydrosystem, equivalence between the total mass of water entering the system and that leaving the system. (2) The necessary condition in a chemical reaction for mass conservation of each element regardless of the component in which the element participates. WMOHydrology (or mass budget) A year-by-year measure of the state of health of a glacier, reflecting the balance between accumulation and ablation. A glacier with a positive mass balance in a particular year gained more mass through accumulation than was lost through ablation; the reverse is true for negative mass balance. Swisseduc Usually, a model that employs the limitation that the system observed maintains a constant mass, that is, total mass divergence for the entire system is zero, but may be nonzero within the system. AMSglossary The relative balance between the input and output of material within a system. PhysicalGeography The balance between the mass input to the ice body (accumulation) and the mass loss (ablation and iceberg calving) over a stated period of time, which is often a year or a season. Point mass balance refers to the mass balance at a particular location on the glacier or ice sheet. Surface mass balance is the difference between surface accumulation and surface ablation. The input and output terms for mass balance are: Accumulation All processes that add to the mass of a glacier. The main contribution to accumulation is snowfall. Accumulation also includes deposition of hoar, freezing rain, other types of solid precipitation, gain of wind-blown snow, and avalanching. Ablation Surface processes that reduce the mass of a glacier. The main contributor to ablation is melting with runoff but on some glaciers sublimation, loss of wind-blown snow and avalanching are also significant processes of ablation. Discharge/outflow Mass loss by iceberg calving or ice discharge across the grounding line of a floating ice shelf. Although often treated as an ablation term, in this report iceberg calving and discharge is considered separately from surface ablation. (Massbudget (of glaciers or ice sheets)) IPCC2013 GCW |