Impact of climate change on the hydrology of High Mountain Asia

Abstract

In Asia, water resources largely depend on water generated in the mountainous upstream parts of several large river basins and hundreds of millions of people depend on their waters downstream. The large-scale impacts of climate change for the water resources in High Mountain Asia are poorly understood, because the area has a complex climate, which is poorly monitored. Climate change may have large consequences for water availability, seasonal changes in runoff generation and the frequency and magnitude of hydrological extremes. In the research described in this thesis robust climate change impact assessments for water resources in High Mountain Asia have been made. To achieve this, novel approaches have been developed to improve the understanding of the impacts future climate change may have in this region. A new selection procedure is presented aiming at defining an ensemble of climate models that covers the full range of possible futures in terms of changes in mean temperature and precipitation, and changes in temperature and precipitation extremes, while at the same time only including climate models that have sufficient skill in simulating historical climate. A novel regionalized glacier mass balance model, which is suitable to estimate future changes in glacier extent under data scarce condition at river basin scale is presented. Forcing this model with climate change projections shows that the glacier extent in High Mountain Asia will be reduced strongly during the 21st century. Downscaled climate change scenarios and associated projections of future glacier extent are used to force a new, high-resolution, fully distributed cryospheric-hydrological model to assess changes in future hydrology. The modeling results show that water availability is expected to increase in five large Asian river basins, despite the large differences in hydrological regimes between the basins. For the upper Ganges, Brahmaputra, Salween and Mekong river basins this is due to increased precipitation, whereas for the upper Indus basin the main driver is accelerated melt. A second impact assessment zooming Asia’s climate change hotspot, the upper Indus basin, reveals that the upper Indus basin faces a very uncertain future in terms of water availability in the long run. Projections of changes in water availability at the end of the 21st century range from -15% to +60% with respect to the reference. This uncertainty mainly stems from the large spread in the precipitation projections throughout the 21st century. Despite the large uncertainties in future climate and water availability, basin-wide patterns and trends of intra-annual shifts in water availability are consistent across climate change scenarios. For the near future (2021-2050), these trends mainly consist of minor increases in summer flows combined with increased flows during other seasons. For the far future (2071-2100) the trends show decreases in summer flows combined with stronger increasing flows during the other seasons. Furthermore, increases in intensity and frequency of extreme discharges are found for most of the upper Indus basin and for most scenarios and models considered, implying increases in flooding events during the 21st century.