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cipitation cycles, and the proportion of the catchment occu- pied by glacier ice. Figure 4 compares precipitation and river flow data for heavily and lightly glacierized catchments in the European Alps and Peru. In the European Alps, runoff is greater than precipitation in summer in both heavily and lightly glacierized catchments. This is because the Alps expe- rience a large seasonal temperature cycle, and precipitation falls as snow in winter, providing a source of meltwater into the summer even where no glaciers are present. In Peru, on the other hand, temperature varies less throughout the year and there is less annual variation in snow cover. Thus, runoff is much greater than precipitation in the dry season (June– August) in heavily glacierized catchments, but not in lightly glacierized catchments. The implication is that the removal of glaciers from catchments will have a much larger impact on water resources in Peru than in the European Alps. Without a glacial source of meltwater, river discharges closely follow the pre- cipitation cycle, with very little runoff in the dry season.

In general, the impact of glaciers on the seasonal distribution of river flow is greatest where: i) ice melt occurs during a dry season; ii) glacier meltwater flows into semi-arid areas; and/or iii) small annual temperature cycles mean that there is little sea- sonal variation in snow cover. Conversely, the seasonal effect is smaller where there is significant precipitation during the melt season, such as the monsoonal central and eastern Himalaya.

Reduction of the mass of glacier ice in a catchment will increase the annual river flow, so it will be greater than the precipita- tion total. This extra component of river flow has been called the ‘deglaciation discharge dividend’ (Collins, 2008), and will increase if melt rates rise in a warming climate. As glaciers shrink, however, the discharge dividend decreases because a smaller area of ice contributes meltwater. This will be the long term result. For the coming decades the areas will not change much, but the melt will nevertheless strongly increase due to downwasting of the flat and thick tongues from large glaciers.

Seasonal patterns of precipitation and runoff Coefficient

Vent catchment

0 1 2 3

Tumpen catchment

0 1 2 3

October December February April June August October December February

Source: Kaser, G., et al., The impact of glaciers on the runoff and the reconstruction of mass balance history from hydrological data in the tropical Cordillera Blanca, Peru, Journal of Hydrology, 2003

Figure 4: Seasonal patterns of precipitation (Cp) and runoff (Cq) coefficients in heavily glacierized (top panels of the Vent (41% glacierized) and Llanganuco (33% glacierized) and lightly glacierized catchments (bottom panels Tumpen (2% glacierized) and Querococha (3% glacierized)), in the European Alps (left) and Peru (right), respectively (Kaser et al., 2003).

18 April June August Querococha catchment Lightly

glacierized catchments

Llanganuco catchment Heavily

glacierized catchments

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