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Sources of river water: Glacier melt, snow melt, rainfall, groundwater

Rivers originating in the HKH are among the most meltwater dependent in the world. 21 It has been known for quite some time that there is a large variation between rivers basins in terms of the relative contributions of glacial melt, snow melt and rainfall to river flow, although this has often been poorly quantified. As the science and availability of measurements has improved, the importance of glacial melt to total river flow has become better understood. 22,23 Estimates of the number of people who depend (either directly or indirectly) on meltwater from glaciers have also gradually been revised downwards from hundreds of millions (or even billions) to millions of people. 24 Generally speaking, the contribution of glacial melt to river flow is less important in river basins experiencing monsoon-dominated precipitation regimes and more important in the drier, westerly- dominated river basins such as the Indus. 25,26 Recent findings from HICAP research within upper river basins indicate that stream flow in the Indus river basin is dominated by glacial melt (up to 41%), while rainfall is of greater importance to the other four river basins. 27 Average runoff composition within each upper basin are: • Upper Indus: runoff is dominated by meltwater: glacial melt (41%) and snow melt (22%), and rainfall is of minor importance to total runoff (27%) • Upper Ganges: runoff is dominated by rainfall (66%) and meltwater contributes about 20% to total runoff • Upper Brahmaputra: runoff is dominated by rainfall (59%) and meltwater contributes about 25% to total runoff • Upper Salween: runoff is dominated by rainfall (42%), but snow melt is also important contributing 28%; glacier melt contributes about 8% to total runoff

• Upper Mekong: runoff is dominated by rainfall (44%), but snow melt is also important contributing about 33%; glacier melt contributes approximately 1% to total runoff In general, the relative contribution of different runoff sources can be explained by the weather systems and altitudinal effects. The climate in the eastern part of the Himalayas is driven by the East- Asian and Indian monsoon systems, where most precipitation falls between June and September. In the upper Ganges basin, despite the quite large glaciated area, rainfall runoff dominates due to the

monsoon rains. The situation is quite similar for the upper Brahmaputra basin. Because a greater proportion of the upper Brahmaputra’s basin’s area is at high altitude than in the upper Ganges basin, there is a larger contribution of snow and glacier melt. Although rainfall is dominant in the upper Salween and upper Mekong basins, there is a large contribution of snow melt in these areas, because large parts of these basins are located on the Tibetan plateau. The magnitude of the contribution of each component to total runoff partly determines the basin’s response to climate variability and change. 28

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