Increases in River Runoff Projected for High Mountain Asia’s River Basins during the 21st Century

Abstract:

Rivers originating in the high mountains of Asia are among the most melt water dependent river systems on Earth, yet large human populations depend on their resources downstream. Across High Asia’s river basins, there is large variation in the contribution of glacier and snow melt to total runoff, which is poorly quantified. The lack of understanding of the hydrological regimes of High Asia’s rivers is one of the main sources of uncertainty in assessing the regional hydrological impacts of climate change. In this study we typify contrasting hydrological regimes at two different scales in contrasting climates in Asia: small scale high altitude glacierized catchments and large scale upstream basins. Subsequently we analyze the hydrological impact of climate change using the latest climate model output at both spatial scales. Conceptually different glacio-hydrological models are used at catchment and basin scale. The largest difference is the spatial resolution (90 m versus 1 km) and the fact that at the catchment scale we explicitly include glacier dynamics whereas at the large scale we parameterize future retreat. Despite the conceptual differences the conclusions are remarkably similar. The average annual runoff will increase or remain stable in the coming decades as a result of projected increases in precipitation in combination with sustained higher glacier melt, e.g. the glacier retreat is counterbalanced by an increase in melt rate per unit area for years to come. We conclude that the challenge for the future is coping with intra-annual shifts in the water balance and a possible increase of extreme events.