Water balance modeling with explicit ice-flow dynamics

Abstract

Mountains, the water towers of the world, are capable of storing water and releasing it to lower elevations, hence ensuring water supply to the the lowlands. In mountainous regions, higher precipitation amounts and low temperatures favor the presence of glaciers, which act as vast reservoirs of fresh water being stored in the form of snow, firn and ice. Glaciers primarily release water due to melt, resulting in a decrease in their volume and area. In the matter of changing climate, anthropogenic causes lead to rising temperatures, which are projected to continuously do so until the end of the century. This inevitable results in glacier volume loss and increased glacier runoff, which at some point will reach a maximum (peak water). Beyond this point, glacier volume continues to decrease and the water balance surplus cannot be sustained due to the lack of glacier runoff. This matter contributes to sea-level rise on a global scale, while changes in the runoff regime and water shortage can be awaited at the local and regional levels. Accurate prediction of glacier evolution and runoff becomes crucial for the assessment of changes in catchment hydrology. In this sense, glacio-hydrological models are extremely valuable as they can predict both glacier and hydrological processes. Yet, the limited available glacier measurements (e.g. long-term mass balances) poses a challenge to model glacier evolution. To address this issue, simplified and empirical methods, such as the Volume-Area (VA) scaling approach, are commonly utilized. Even though good estimates can be expected for hundreds of glaciers, its application at the catchment scale is somewhat controversial due to the difficulty in selecting representative scaling parameters (e.g. representative thickness) for a single glacier. On the other hand, the emergence of global glacier datasets enhances the development of (global) glacier models that implement more complex approaches to account for glacier evolution. Essentially, the flux of ice along the glacier’s flowline is contemplated, thus relying on the actual physical forces that underlie the processes. Despite efforts to couple standalone glacier models to hydrological models, major limitations still persist: Either a single variable is transferred between models (e.g. glacier area), or inconsistencies in the driven climate dataset can be recognized. For these reasons, the aim of this study is to go beyond existing offline coupled glacio-hydrological models (offline means that the models run separately but variables are transferred between them) by ensuring a consistent exchange of state variables between two independent models. More precisely, and being both models driven by the same climate dataset, glacier states (i.e. areas, ice thickness distribution, volumes and mass balances) are produced annually with the help of the Open Global Glacier Model (OGGM), based on explicit ice-flow dynamics, and later integrated into the fully-distributed and physically-based Water Flow and Balance Simulation Model (WaSiM). The developed WaSiM-OGGM coupling scheme, applied to the Gepatschalm catchment (Austria), demonstrates greater reliability in predicting glacier evolution and runoff when compared to the original WaSiM model with integrated VA scaling. Despite rather pessimistic results (with nearly 19% more glacier area loss by the end of the century under severe warming conditions), they share more affinity with other studies carried out in the European Alps. Furthermore, the proposed coupling scheme could be implemented in catchments lacking glacier observations while providing a physically-based and fully-distributed representation of hydrological processes, including glacier evolution from any year in the past. Finally, the WaSiM-OGGM coupling scheme might serve as a powerful tool for the effective water resources management in vulnerable regions, especially in face of a changing climate.