| Summary: | Understanding the response of the Himalayan-Karakoram (HK) rivers to climate change is crucial for ~1 billion people who partly depend on these water resources. Hydrological models suggest that glacier and snow melt are important components of HK rivers, with greater hydrological importance for the Indus basin than for the Ganges and Brahmaputra basins. Significant progress has been made, primarily through remote sensing and modelling approaches yet the field-based glacier mass balance and hydro-meteorological studies are very poor in the HK, only available from a few rapid-response type small glacierized catchments. Critical knowledge gaps concern accurate representations of glacierized area/volumes, precipitation distribution, permafrost thaw, sublimation, glacier dynamics; and impacts of debris cover, black carbon, and dust. These gaps severely affect modelled contributions of different runoff components, future runoff volumes and seasonality, and they render wide variability in estimates of stream flow even within the same catchments. Lacking a broad consensus on the region’s glacio-hydrology, planning and management of water demand and supply remain challenging, especially within individual catchments. Comprehensive, long-term field and remote sensing-based methods and models are needed to fill the highlighted knowledge gaps and reduce uncertainties in the HK river runoffs.
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