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Downstream impacts of changing glaciers, snow and ice

The importance of glaciers for freshwater supply to downstream populations

Glacier melt contributes to river flow in many parts of the world, so changes in glacier mass can affect the availability of freshwater supply at diurnal, seasonal and multi-annual timescales. The importance of the glacial contribution to runoff depends on the magnitude of other components of the hydrological cycle, and is thus regionally highly variable.

Water is stored in glaciers in the form of snow (short-term reservoir) and ice (long-term reservoir), and is released by melting after some delay. At first, the most recent snow cover (e.g. from the previous accumulation season) melts and later snow from previous years that is located in the accumulation area melts. Both contributions do change the storage of short- term reservoirs. The ice that melts at the same time in the ablation region is much older (tens to thousands of years) and reduces the long-term reservoir that is in principal replaced by glacier flow. For a glacier extent that is in balance with cli- mate, the amount of ice that is melting at the glacier tongue is replaced by the supplies from ice flow. The latter changes only slowly and in response to the long-term mass balance history. In this regard the mass balance in a specific year can be seen as the direct and un-delayed response to the annual atmospheric conditions, whereas the advance or retreat of a glacier tongue is a delayed, filtered and enhanced response to a (more long-term) change in climate.

On an annual time scale, the water that comes from a glacier- ized catchment is the sum of the precipitation and the melted snow and ice (minus some evaporation). Both contributions can have a pronounced seasonality (e.g. spring-time snow

melt, glacier melt in summer, highest precipitation in autumn) and will thus strongly vary by region and degree of glacieriza- tion. On a decadal time scale, also a change in the long-term reservoirs will have an impact on runoff characteristics, as a di- minishing glacier cover will produce less meltwater. However, in the coming decades there can also be first an increase of the glacier contribution to runoff (e.g. Huss et al. 2008), in particu- lar when large glaciers are located in the catchment, that adjust their size only with some delay to the climatic forcing.

The role of glaciers on river flow can therefore be summarized as: (i) influencing daily and seasonal patterns of river flow, and (ii) increasing or decreasing total annual flows through long-term changes in the reservoirs, i.e. the area covered by glaciers and their respective volume. The impact of both ef- fects on river flows further downstream depends crucially on the magnitude of other components of the hydrologi- cal cycle (e.g. rainfall, evaporation, groundwater flow), and thus varies greatly between regions (Immerzeel et al., 2010; Pellicciotti et al., 2010).

The influence of glaciers on seasonal distribution of river flow is strongly dependent on annual temperature and pre-

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