Glacier recession, debris cover development and the implications for water resources in Afghanistan

Résumé 0

Rapid climate change is impacting water resources in Afghanistan, a country in the western Himalayas. It is a semi-arid to arid country of Central Asia where livelihoods and economies have developed to be strongly dependent upon mountain water resources, and where snow and glacier melt deliver 80% of Afghanistan’s water supply. It is also poorly-developed in terms of scientific research and environmental monitoring. Average global temperatures are rising and glaciers are shrinking significantly, potentially impacting water supply to a critical degree. This effect is likely hidden for the time being as glaciers are still larger in volume than that associated with a warmer and warming climate. However, once glaciers shrink to a certain size, “peak water” will be reached. Water supply will decline. If winter snowfall declines, or becomes more variable, glaciers are less likely to compensate for the associated water shortage that results. Recent research has emphasized the complexity of identifying when peak water develops, not least because as glaciers retreat they may develop extensive debris cover, which can significantly modify the relationship between climate change and glacier melt. Given the lack of research in Afghanistan on this topic, the primary aim of this thesis is to quantify the impacts of ongoing and future climate change on Afghan glaciers and the associated consequences for water resources in Afghanistan. To this end, a multidisciplinary approach is used, based on the combination of remote sensing, a restricted field campaign, hydrological modeling, archival data, and locally adjusted climate change scenarios. The PhD thesis addressed the following four research questions: R1: Is it possible to improve remote sensing techniques for glacier monitoring in Afghanistan to allow mapping of not only ice cover but also debris-covered ice at the scale of the entire country? R2: How have the total, bare ice and debris-covered ice extents of Afghanistan glaciers changed in recent decades with the onset of rapid warming? R3: Is it possible to develop a glacier-snow melt runoff model suitable for application over large scales in the data poor context of Afghanistan? R4: How has and how will climate change influence future glacier runoff in Afghanistan? This study is contextualized through a systematic review of more than 130 scientific articles, reports and data sources to assess the potential impacts of climate change on the cryosphere, streamflow, groundwater and hydrological extremes in Afghanistan and to highlight knowledge gaps. This review shows the importance of the cryosphere for Afghan water resources and identifies the need to develop a better inventory of glacier cover in Afghanistan, notably taking into account debris-cover given the geology and geomorphology of the region. To do this, the thesis develops two new debris-cover mapping indices based on thermal and near Infrared Landsat 8 bands. The indices are calibrated with field data, and validation both within and beyond Afghanistan suggests that they have a high level of accuracy. Principal components analysis was applied to 9 glacier parameters to identify the most influential drivers of debris-covered ice extent. This analysis showed that lower proportions of debris cover were associated with glaciers with a higher elevation range that were larger, longer and wider. However, these patterns were statistically clearer when the dataset was broken down into climate zones and geological regions. This methodology is then applied to two timespans (2000-2008 and 2008-2020) to assess recent glacier changes in Afghanistan. Three glacier inventories were developed for the years 2000, 2008, and 2020. In 2020, a glacier area of 2684±100.7 km2 was mapped, split into 75±0.7 % clean ice area and 25±3.0 % debris-covered ice. Total glacier area retreated by -4.5±0.5 km2 yr-1 (-0.15±0.01 % yr-1) between 2000 and 2008 and -12.3±1.5 km2 yr-1 (-0.43±0.05 % yr-1) between 2008 and 2020. However, the analysis revealed substantial spatial variation in these retreat rates based upon geographical region, glacier size and climate region. The results point to the environmental complexity of Afghanistan and suggest that certain regions may pass through peak water much sooner than others. Debris cover changes were also found to be complex, with debris-covered ice extent increasing much more for smaller glaciers ⁓0.8). Glacier runoff dominated the Sust catchment (76%); rain and snow runoff the Taqchakhana catchment (50%) and baseflow the Bamyan catchment (61%). Projected streamflow under RCP 2.6 suggests that mean annual glacier runoff for the Sust and Taqchakhana catchments will increase until 2050, following an increase in temperature (0.9 ˚C). Then runoff declines to the end of the 21st century. The Bamyan is predicted to have declining runoff throughout the 21st century. However, under RCP 8.5 glacier runoff increases more markedly in the Sust and Taqchakhana catchments as temperature rises are larger, climate change mitigation measures do not reverse them. These catchments are likely to pass through a phase of peak water not due to temperature limitation on runoff as under RCP 2.6, but due to progressively diminishing glacier size.