Temperatures in the upper Indus, upper Ganges, upper Brahmaputra, upper Salween and upper Mekong basins are projected to increase with considerable certainty between 1–2.2°C up until 2050, compared to the baseline period (1998– 2007). There is more uncertainty about precipitation patterns than temperature, but they are also projected to change between –3.5 to +9.5% for the same period, depending on which upper basin is considered. On average, an increase in precipitation is expected for all of the upper basins, with greater uncertainty for the upper Indus basin, where a decrease in precipitation is also possible. In response to changing precipitation and temperature patterns, the relative contribution of different sources of water – glacial melt, snow melt, rainfall and baseflow – to river flow will change. Under both RCP scenarios, the amount of glacier and snow meltwater will decrease, while the amount of rainfall-runoff will increase, for the upper basins of the Brahmaputra, Ganges, Salween and Mekong. For the upper Indus basin, the contribution of glacial melt is projected to increase in both scenarios, and the contribution of snow melt and rainfall to runoff are projected to decrease for the extreme cases in the RCP 8.5 scenario. Overall, no significant decrease in runoff is projected until at least 2050 for all of the basins. An increase in runoff is projected for both RCP 4.5 and RCP 8.5 scenarios for the upper Ganges (1–27%), Brahmaputra (0–13%) and Mekong (2–20%) basins. Increasing precipitation is the main driver of this change, which will compensate for decreasing contributions of glacial and snow melt. For the upper Indus and Salween basins, the picture is uncertain and varies depending on the scenario. Under the RCP 4.5 ensemble mean, the total upper Indus river flow increases (12%), while under the RCP 8.5 ensemble mean, it decreases (–5%) compared to the reference period. In the upper Salween basin, the projected change in total river flow ranges from –3 to +19%. The difference is mainly due to a reduction in snow melt and rainfall runoff under RCP 8.5, caused by a decrease in precipitation, although glacial melt increases in both scenarios.
From collection: Adaptation in the Himalayas: Knowledge, Action and Results