| 英文摘要: | Rivers originating in the high mountains of Asia are among the most meltwater-dependent river systems on Earth, yet large human populations depend on their resources downstream1. Across High Asia’s river basins, there is large variation in the contribution of glacier and snow melt to total runoff2, 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 change3. Here we use a large-scale, high-resolution cryospheric–hydrological model to quantify the upstream hydrological regimes of the Indus, Ganges, Brahmaputra, Salween and Mekong rivers. Subsequently, we analyse the impacts of climate change on future water availability in these basins using the latest climate model ensemble. Despite large differences in runoff composition and regimes between basins and between tributaries within basins, we project an increase in runoff at least until 2050 caused primarily by an increase in precipitation in the upper Ganges, Brahmaputra, Salween and Mekong basins and from accelerated melt in the upper Indus Basin. These findings have immediate consequences for climate change policies where a transition towards coping with intra-annual shifts in water availability is desirable. In general, the climate in the eastern part of the Himalayas is characterized by the East-Asian and Indian monsoon systems, causing the bulk of precipitation to occur during June–September (Supplementary Fig. 4). The precipitation intensity shows a strong north–south gradient caused by orographic effects4. Precipitation patterns in the Hindu Kush and Karakoram ranges in the west are characterized by westerly and southwesterly flows, causing the precipitation to fall more equally distributed over the year5 (Supplementary Fig. 4). In the Karakoram, up to two-thirds of the annual high-altitude precipitation occurs during the winter months6, 7. In addition, basin hypsometry determines the ratio of solid and liquid precipitation within a basin. Solid precipitation can be stored long-term as perennial snow, and ice or short-term as seasonal snow before turning into runoff by melting, whereas liquid precipitation runs off directly. Each of these runoff components can be further delayed by infiltration into the soil and recharge to groundwater. The magnitude of the contribution of each of these runoff components to the total runoff determines a basin’s runoff composition and to a large extent also its response to climate variability and change. Climate change impact assessments are characterized by large uncertainties stemming from large variation in climate change projections between different general circulation models8 (GCMs), large regional variation in climate projections and uncertainties in the associated response of the cryosphere9, 10. In addition, the present-day hydrological regime is not well understood, constituting a major source of uncertainty in the assessment of climate change impact for hydrology in High Asia. Thus, detailed and comprehensive assessments of the future water availability in the region are only possible once the present hydrological regime is better quantified3. Although methods to quantify meltwater contribution exist, high-resolution modelling studies focus on small-scale watersheds11. High-resolution approaches that explicitly simulate ice dynamics, necessary to simulate the transient response to climate change, are even scarcer12. On the other hand, large-scale assessments in the region are often qualitative2, 13 or include crude assumptions and simplifications to simulate the response of the cryosphere to climate change, which cannot be resolved at low resolution1, 14, 15, 16. In this study we close this scale gap by implementing a large-scale modelling approach at such a resolution that allows accurate simulation of key hydro-cryospheric processes. Only by using a distributed hydrological modelling approach incorporating transient changes in climate, snow cover, glacier dynamics and runoff, appropriate adaptation and mitigation strategies can be developed17. Here we use a fully distributed, high-resolution cryospheric–hydrological model (Supplementary Methods and Data) to assess upstream runoff composition in five major Asian river basins (Fig. 1) and we demonstrate how runoff composition and total runoff volume are expected to change until 2050 by forcing this model with an ensemble of the latest GCM outputs.
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