Climate change impacts on glacier hydrology and river discharge in the Hindu Kush-Himalayas: a synthesis of the scientific basis

Rising temperatures and changing precipitation patterns across the Hindu Kush–Himalaya (HKH) region resulting from climate change have an influence on water resource availability and food security for the downstream population. This review seeks to objectively assess the available evidence of the impacts of climate change on glacier hydrology and the wider implications upon water resources within the Indus, Ganges, and Brahmaputra basins. Glacier meltwater contribution to river flows is scale dependent and varies considerably across the east–west climatic zones of the HKH. For the Ganges and Brahmaputra this contribution is estimated to be significantly less than for the Indus to the west, with summer monsoon rains dominating flows from central and easterly areas, whereas meltwater remains a significant contributor to downstream flow of westerly basins, which receive most precipitation during winter. No corroborated trends exist in observed discharge for any basin, and such analyses are hindered by a lack of good-quality long-term data. Predicted increases in temperature will drive increased shrinkage of glaciers, leading to initial increases in meltwater produced, followed by subsequent declines with reduced glacier mass. The impacts of such changes are predicted to be minimal for the overall discharge of the Ganges and Brahmaputra, where increases in rainfall may in fact lead to increased flows but with greater variability. Within the Indus basin, reduced meltwater will have significant impacts upon available runoff; however, increased uncertainties surrounding precipitation and socioeconomic changes limit any conclusive assessment of how water availability will be affected; moreover, seasonality of runoff may be a more important factor. Scientific challenges and research recommendations are identified for the region. This review proposes the need for the scientific evidence pertaining to the region's glacier systems to be approached objectively in the future, such that a robust assessment of change can be attained

Rates of rockwall slope erosion in the upper Bhagirathi catchment, Garhwal Himalaya

Rockwall slope erosion is defined for the upper Bhagirathi catchment using cosmogenic Beryllium-10 (10Be) concentrations in sediment from medial moraines on Gangotri glacier. Beryllium-10 concentrations range from 1.1 ± 0.2 to 2.7 ± 0.3 × 104 at/g SiO2, yielding rockwall slope erosion rates from 2.4 ± 0.4 to 6.9 ± 1.9 mm/a. Slope erosion rates are likely to have varied over space and time and responded to shifts in climate, geomorphic and/or tectonic regime throughout the late Quaternary. Geomorphic and sedimentological analyses confirm that the moraines are predominately composed of rockfall and avalanche debris mobilized from steep relief rockwall slopes via periglacial weathering processes. The glacial rockwall slope erosion affects sediment flux and storage of snow and ice at the catchment head on diurnal to millennial timescales, and more broadly influences catchment configuration and relief, glacier dynamics and microclimates. The slope erosion rates exceed the averaged catchment-wide and exhumation rates of Bhagirathi and the Garhwal region on geomorphic timescales (103−105 years), supporting the view that erosion at the headwaters can outpace the wider catchment. The 10Be concentrations of medial moraine sediment for the upper Bhagirathi catchment and the catchments of Chhota Shigri in Lahul, northern India and Baltoro glacier in Central Karakoram, Pakistan show a tentative relationship between 10Be concentration and precipitation. As such there is more rapid glacial rockwall slope erosion in the monsoon-influenced Lesser and Greater Himalaya compared to the semi-arid interior of the orogen. Rockwall slope erosion in the three study areas, and more broadly across the northwest Himalaya is likely governed by individual catchment dynamics that vary across space and time. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons, Ltd