Assessing the state of the glaciers in the Parvati basin of the Himalaya under a changing climate

Abstract

The Himalayan cryosphere is one of the most climate-sensitive components of the Earth's system, acting as a critical water tower for millions of people across South Asia. Snow and glacier melt from this region contribute significantly to the headwaters of major rivers. However, rising temperatures and changing precipitation regimes over recent decades have led to changes in the cryosphere, including rapid glacier retreat, altered snowfall patterns, and the formation of glacial lakes, raising concerns over long-term water security, hazard risks, and ecological stability. Despite the importance of these dynamics, many existing assessments lack regional-scale resolution and overlook spatial heterogeneity in cryospheric processes. Hydrological modelling in the Himalaya has traditionally focused on larger river basins, offering limited insight into sub-basin scale responses and local community perspectives. This thesis addresses these gaps through a comprehensive, basin-specific inquiry into the cryospheric and hydrological evolution of the Parvati basin, Western Himalaya. The research is structured into three interlinked components: (i) quantification of glacier volume and mass changes, (ii) assessment of glacial lake formation, and (iii) simulation of future runoff under climate change scenarios using the Spatial Processes in Hydrology (SPHY) model, contextualized with field-based societal observations. The overarching aim is to enhance understanding of the coupled cryospheric-hydrological processes under present and projected climate scenarios, while incorporating perspectives from downstream communities. Chapter 2 evaluates glacier volume and mass changes in the Parvati basin to understand how regional glaciers have responded to recent climate shifts. Glacier volume for the year 2000 was estimated using a laminar flow-based approach and scaling relationships, providing a basin-wide assessment of ice reserves. To quantify mass loss over time, two complementary methods were applied: the Improved Accumulation Area Ratio (IAAR), based on climatic inputs, and the Geodetic method, using elevation data derived from satellite imagery. Both approaches indicate substantial glacier thinning and mass loss between 2000 and 2015, with an estimated reduction in glacier mass of around 14 percent. The results reveal marked spatial variability in glacier response, reflecting differences in elevation, slope, and debris cover. Together, the volume and mass balance estimates offer a regional-scale understanding of cryospheric change in the basin and establish a critical baseline for assessing future water availability and glacial hazards. Chapter 3 shifts focus to the potential formation of glacial lakes in response to continued glacier retreat and thinning due to climate change. As ice mass is lost and glacier surfaces lower, topographic depressions may become exposed, allowing meltwater to accumulate and form lakes, which can pose a significant risk of glacial lake outburst flood (GLOF). A semi-automated geospatial tool has been developed that identifies future lake formation sites by integrating modelled ice thickness with surrounding terrain features. Ice thickness is estimated using a laminar flow-based method that incorporates surface slope and ice velocity, while probable lake locations are inferred from bed topography and over-deepened glacier zones. Application of the tool to 132 glaciers in the Western Himalaya resulted in the identification of numerous potential glacial lake sites with implications for future water storage and downstream risk. These findings contribute to a better understanding of where and how deglaciation may reshape the Himalayan landscape and provide essential inputs for GLOF hazard assessments and regional hydrological modelling. Chapter 4 integrates the cryospheric insights into a distributed hydrological modelling framework using the SPHY model. The model incorporates dynamic glacier change and simulates runoff under four Coupled Model Intercomparison Project (CMIP) 6 scenarios (Shared Socio-economic Pathways (SSPs) 126, 245, 370, and 585) until 2050. Current simulations indicate a snow-dominated regime, with snowmelt and glacier-melt contributions to be around 42-47% and 21% of total runoff, respectively. Spatial decomposition shows increasing cryospheric contribution with elevation, with upstream sites like Tosh receiving higher contribution of their runoff from snowmelt. Future projections suggest show a progressive decline in snowmelt contributions and a rise in runoff from rainfall and baseflow, with notable increases in glacier runoff under higher emission scenarios. These trends imply reduced seasonal buffering, particularly during the late summer, with potential implications for water supply reliability. To contextualize these hydrological changes at the community level, a social survey was conducted in Pulga village, situated within the Parvati basin. The survey assessed water sources, agricultural practices, and perceptions of climate change. Findings reveal heavy dependence on snow and glacial-fed streams, with current changes in snowmelt timing raising uncertainty about the reliability of water sources in the future. Villagers reported diminished snowfall and earlier snowmelt, consistent with model outputs. Lack of irrigation infrastructure makes agriculture more vulnerable during dry months, despite sufficient drinking water. These insights highlight the need for basin-specific assessments that capture village-level variations in water availability and socio-aspects, which are often missed in broader hydrological studies. Collectively, this thesis makes a multi-spatio-temporal resolution assessment of climate change impacts on a Himalayan catchment. It (i) provides a detailed, spatially distributed record of glacier mass change, (ii) identifies zones of glacial lake development, and (iii) incorporates glacier information into a calibrated runoff model that accounts for cryospheric retreat. The inclusion of local knowledge through social surveys offers a novel linkage between scientific modelling and community-level water realities. By structuring the thesis as a staged assessment, from glacier retreat to lake formation potential, future hydrological changes, and social aspects, the research delivers an integrated narrative of Himalayan cryosphere-hydrology interactions under climate change. The work lays a foundation for further research and decision-support systems in Himalayan basins. It emphasizes the need for long-term in situ measurements, improved representation of permafrost and debris-covered ice in models, and enhanced coordination between physical science and regional water governance. Methodologically, it demonstrates the value of combining high-resolution remote sensing, empirical terrain analysis, distributed modelling, and community engagement to inform sustainable water management. The thesis contributes to broader climate resilience efforts by offering insights into how local hydrological regimes are being reshaped by global climate dynamics.