Modeling glacio-hydrological processes in the Himalayas: A review and future perspectives

The Himalayas and their surrounding areas boast vast glaciers rivaling those in polar regions, supplying vital meltwater to the Indus, Ganges, and Brahmaputra rivers, supporting over a billion downstream inhabitants for drinking, power, and agriculture. With changing runoff patterns due to accelerated glacial melt, understanding and projecting glacio-hydrological processes in these basins is imperative. This review assesses the evolution, applications, and key challenges in diverse glacio-hydrology models across the Himalayas, varying in complexities like ablation algorithms, glacier dynamics, ice avalanches, and permafrost. Previous findings indicate higher glacial melt contributions to annual runoff in the Indus compared to the Ganges and Brahmaputra, with anticipated peak melting in the latter basins — having less glacier cover — before the mid-21st century, contrasting with the delayed peak expected in the Indus Basin due to its larger glacier area. Different modeling studies still have large uncertainties in the simulated runoff components in the Himalayan basins; and the projections of future glacier melt peak time vary at different Himalaya sub-basins under different Coupled Model Intercomparison Project (CMIP) scenarios. We also find that the lack of reliable meteorological forcing data (particularly the precipitation errors) is a major source of uncertainty for glacio-hydrological modeling in the Himalayan basins. Furthermore, permafrost degradation compounds these challenges, complicating assessments of future freshwater availability. Urgent measures include establishing comprehensive in situ observations, innovating remote-sensing technologies (especially for permafrost ice monitoring), and advancing glacio-hydrology models to integrate glacier, snow, and permafrost processes. These endeavors are crucial for informed policymaking and sustainable resource management in this pivotal, glacier-dependent ecosystem.