Climate change in mountainous river basins: Understanding climate change impacts and challenges across different spatial scales

Climate change is expected to impact the cryosphere and hydrology of mountainous river basins. Meltwater, rainfall runoff, and groundwater recharge will likely be affected and the frequency and intensity of hydrological extremes are expected to change. It can therefore be expected that climate change will have a large impact on the society and the environment; impacts that will be further amplified with the anticipated socio-economic developments and associated changes in water demand. Understanding climate change impacts in mountainous river basins is challenging due to the complexity of the mountain environment and the wide range of scales on which hydrological processes can occur. This makes it necessary to examine these effects at different spatial scales. The research described in this thesis aims at understanding the impacts and challenges of climate change across different spatial scales. To this end, novel modelling approaches have been developed and applied to assess these impacts from the catchment- to the regional/basin-scale. At the catchment scale, it is shown that the loss of ice of two glaciers in a different climatological setting (the debris-covered Langtang Glacier (Himalaya, Nepal) and the clean-ice Hintereisferner (Ötztal Alps, Austria)) can mainly be attributed to anthropogenic climate change in both cases. It is also shown that the debris-covered glacier shows a limited retreat and tends to lose less mass due to insulation of the glacier surface by a layer of supra-glacial debris, where a clean-ice glacier tends to respond faster to climate change and shows a larger retreat. Another zooming in on the hydrological response of the Ötztal glacierized catchments (Austria) to future climate change shows that glaciers will continue to lose mass in the future. Combined with diminishing snow reserves it is expected to result in changing discharge volumes and regimes, accompanied by shifts in the seasonality of flood peaks and changes in the frequency of low flows. As a result, the local seasonal water availability will be affected and risks for local floods and droughts will increase. At the regional/basin-scale, outcomes from an assessment to the impacts of climate change on future hydrological extremes in the upper Indus, Ganges, and Brahmaputra (IGB) river basins show that changes in the (large-scale monsoonal) precipitation climatology are very likely. Precipitation amounts and extremes will increase, which, in combination with increased snow- and ice melt, is expected to contribute to higher discharge rates, more frequently occurring peak flows and less frequently occurring low flows. These changes will consequently result in increased surface water availability. As is shown in another assessment to the combined impacts of climate change and socio-economic developments on the future blue water gap in the IGB, the increase in surface water availability is, however, not sufficient to counterbalance the anticipated strong socio-economic developments and associated rise in water demand. Socio-economic developments are therefore considered to be the key driver in the evolution of the future South Asian water gap. The findings of the presented research can help us to better understand the future impacts of climate change in mountainous river basins