FIGURE 2.1 The global hydrological cycle, or water cycle, is the process by which water moves through a series of reservoirs, including the ocean, lakes, groundwater, atmosphere, snowpack, and glaciers. Water can be in any phase (solid, liquid, gas) in these reservoirs. Water moves from the terrestrial and oceanic reservoirs to the atmosphere through transpiration, evaporation, or sublimation. Water moves from the atmosphere to the terrestrial and oceanic reservoirs through precipitation. Precipitation can occur in liquid form (rain) or solid form (snow, sleet, other types). SOURCE: U.S. Geological Survey.

there is an annual net gain in mass, and the ablation area is the lower elevation zone where there is an annual net loss in mass. The equilibrium-line altitude (ELA) is the elevation where the accumulation and ablation zones meet and where the annual net mass balance is zero (Figure 2.2). The annual mass balance is the net difference between accumulation and ablation (cf. Cuffey and Paterson, 2010).

Accumulation includes all processes by which glaciers increase in snow and ice mass, such as snowfall, condensation, refreezing of rainfall, avalanche transport onto the glacier, and blowing snow transport onto the glacier. Ablation includes all of those processes by which glaciers lose snow and ice mass, such as snow-melt, icemelt, sublimation, blowing snow transport off the glacier, calving and avalanche removal (cf. Cuffey and Paterson, 2010).

When viewed as water supply systems, glaciers are analogous to lakes. Water storage in glaciers is analogous to the total quantity of water stored in a lake. Glacial accumulation is analogous to water input to a lake, which includes processes such as precipitation and water carried into the lake by streams, rivers, and groundwater channels. Glacial ablation is analogous to water removal from a lake, which includes processes such as evaporation, water carried out of the lake by streams, rivers, and groundwater channels, and extraction by humans. When water input sources equal water output sources, the lake is in steady state and the lake level does not change. With glaciers, when accumulation equals ablation, the volume of water stored in the glacier does not change and the ELA does not move. Glacial volumes decrease when ablation persistently exceeds accumulation, the ELA moves up, and the glacier in question ultimately disappears. This is analogous to a lake where persistent overdraft, in which extractions exceed water input, is always self-terminating.

Several important principles follow from this discussion. First, it is the change in the volume of the glacier, not the change in its downhill extent or areal extent that determines whether the net change is positive or negative. However, it is difficult to directly measure the volume of a glacier; thus measurements of glacial volumes are scarce throughout the world. Second, where the entirety of the glacier is below the equilibrium line, there will be no accumulation and with time the glacier will disappear. Third, glacial mass balance information will provide an important

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