Eventually, when glaciers vanish completely, the discharge dividend is zero and annual river flows are equal to the precipitation. In a review of data from mountain regions in North and South America, central Asia and the European Alps, Casassa et al. (2009) showed that higher latitude/higher altitude glacier ba- sins are still in the increasing glacier runoff stage, while some lower latitude/lower altitude basins are already experiencing decreased glacier runoff due to climate warming.

River discharge increases up to a point during the shrinking of glaciers, and the contribution to river flow is flow is great- est where glaciers occupy a large fraction of the catchment (i.e. in the high mountains), and progressively decreases downstream. This is because the glacial contribution to run- off decreases with increasing distance from the glacier fronts, as the river flow is supplemented by precipitation, ground- water flow, and tributary rivers from non-glacierized catch- ments. In some publications, estimates have been made of the glacial contribution to river flow for major river systems (e.g. Xu et al., 2009). Such statistics are misleading, because the percentage varies along the course of the river. Detailed analyses of individual river basins are unavailable at the time of writing, and are urgently needed.

Downstream variation in the impact of glacier recession is illustrated in Figure 5, which shows modelled river flows for idealized catchments under climates representative of the western and eastern Himalaya and a 0.06º C per year warm- ing scenario. In the upper parts of the river basins, where glaciers occupy 95% of the catchment area, the impact of gla- cier shrinkage is large. River discharges increase until mid- century, after which they decline to a base level equal to the annual precipitation totals. When larger catchments are con- sidered, the percentage glacierized area is smaller, and the impact of glacier shrinkage is much reduced. For modelled catchments with an area of 5,000 square km and 1% glacier cover, the impact of deglaciation is barely detectable in the ‘eastern’ area, where there is high monsoon precipitation in

 Figure 5: Modelled impact of glacier shrinkage in hypotheti- cal river basins in climates representative of the western (a) and eastern (b) Himalaya. Note that the river discharge (Q) has a logarithmic scale. (Rees and Collins, 2006).

Glacier shrinkage in hypothetical river basins

River discharge, cubic metres per second 100

10

0 100

1990 2010

Western Himalaya 2030

2050

2070

2090

2110

2130

10

0

1990 2010

Eastern Himalaya 2030

2050

Area (square kilometres)

5 000 500 100 52.6

2070

2090

2110

Modelled river basin catchment area Ice extent

(percentage on total area)

1%

10% 50% 95%

Notes: 1. The represented scenario includes idealized catchment under climates representative of western and eastern Himalaya and a 0.06º C annual temperature increase.

2. Logaritmic scale for river discharge.

Source: Rees, H.G., and Collins, D., N., Regional differences in response of flow in glacier-fed Himalayan rivers to climatic warming, Hydrological Processes, 20, 2157–2169, 2006.

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2130