Estimation of Glacier Mass Balance at Basin Scale in the Himalaya for Recent Decades and Future

Abstract

The Himalayan glaciers are a major source of perennial river systems which support the livelihood of millions of people in south Asia. Therefore, it is important to understand changes in water resources due to changes in glacier-stored water. In order to understand long-term changes in glacier-stored water, mass balance studies at larger spatial scale are needed. Therefore, in this dissertation annual mass balance of glaciers at basin scale in the current and projected future climate is assessed. In this dissertation, for the first time, the long-term investigations of glacier mass balance for a period of 1980s to 2090s in the Chandra basin, western Himalaya are analysed. The mass balance of 146 glaciers in the basin, occupying an area of ~654.29 km2 and the total water volume of ~62.1 ± 16 Gt are analysed. Results of this study suggest that mass loss in the Chandra basin has accelerated from mid-1990‘s to the current decade and could persist till the end of century. Sensitivity analysis suggests that a 20% increase in precipitation can offset changes in mass balance from a 1°C temperature rise. The basin has lost ~19% of its volume during 1984-2013 and the volume loss is ~67% if only the twenty-nine low elevation glaciers are considered. The ensemble mean of climate models considered in this dissertation project a temperature rise of 2.2- 2.9°C (RCP 4.5, the moderate emission scenario) and 4.3- 6°C (RCP 8.5; the high emission scenario) by 2090‘s near the basin with a steady or decreasing trend in snowfall. In response to the projected climate change, the basin will retain only 40-43% (RCP 4.5) and 29-34% (RCP 8.5) of the current glacier stored water volume stored by the end of century. However, the volume loss is very large (~97% of the present volume) for low altitude glaciers indicating a need for effective water management strategies in mountain communities in the future. To calculate the annual mass balance of glaciers at larger spatial and temporal scales, the Accumulation Area Ratio (AAR) method is used in this analysis. However, a new approach is proposed to the conventional methods of equilibrium line altitude (ELA) estimation in AAR method. Satellite-derived transient snowlines, in-situ meteorological observations and a snow-melt model are combined to model the position of ELA. When the field estimates are used for validation, the improved AAR method reduces the biases in mass balance estimates by 46% compared to the traditional technique. Results from the improved method are also in the good agreement with the geodetic estimates for recent decades. The possible climate change impacts on glaciers during 21st century are quantified using the improved AAR method and the glacier geometry model driven by climate projections from fine resolution multiple climate models. The projected future values of mass balance, area and volume (along with uncertainties) in the present dissertation are within the range of results from previous studies at different spatial scales and resolutions. Overall, this study highlights the likely severe impacts to water resources in the Himalaya if CO2 emissions follow the high-emission scenario of RCP8.5.