Glaciers an overlooked water balance component in global hydrological modeling

Although the share of glacier coverage is generally limited in large river basins, glaciers act as large water storages and contribute to runoff in the summer months. Due to climate change, glacier runoff is undergoing considerable change and is expected to decrease significantly by the end of the century. Thus, glaciers are a water balance component with a strong seasonal pattern which is rapidly changing in the future. However, glaciers have been mostly omitted in large-scale hydrological models so far, which limits climate impact studies on global water resources. We aimed to improve the glacier representation in regional and global hydrological modelling and assess the contribution of glaciers to runoff. Therefore, we sequentially coupled the global glacier model OGGM (Maussion et al., 2019) with the large-scale hydrological model CWatM (Burek et al., 2020). Coupling a glacier model with a hydrological model for global application comes with multiple challenges, such as precipitation data adjustment, different spatial and temporal resolutions, different snow process representations and model calibration. Here we elaborate on our experience of combining glacier and hydrological modelling, its challenges and uncertainties. Moreover, we show results of glacier contributions to runoff in the past and under future scenarios. Glacier contributions to runoff are largest close to the glaciers and decrease downstream. Nevertheless, the runoff contribution from glaciers at the outlet of large river basins often remains important, especially in dry periods. We analyzed projected changes in glacier contribution to discharge at the outlet of 56 glacierized river basins globally. Our analysis suggests that the relative glacier contributions to discharge will decrease drastically towards the end of the century, also under the low-emission scenario SSP1-2.6. Thus, including glaciers in regional and global assessments of water availability is especially relevant when assessing future changes, particularly on seasonal or shorter timescales. Otherwise, future changes in discharge are likely underestimated in glacierized basins. The hydrological and glacier modelling communities should foster continued collaborations to include glaciers in the modelling of the water cycle and address the associated challenges. Burek, P., Satoh, Y., Kahil, T., Tang, T., Greve, P., Smilovic, M., Guillaumot, L., Zhao, F., and Wada, Y.: Development of the Community Water Model (CWatM v1.04) a high-resolution hydrological model for global and regional assessment of integrated water resources management, Geosci. Model Dev., 13, 32673298, https://doi.org/10.5194/gmd-13-3267-2020, 2020. Maussion, F., Butenko, A., Champollion, N., Dusch, M., Eis, J., Fourteau, K. et al..: The Open Global Glacier Model (OGGM) v1.1, Geosci. Model Dev., 12, 909931, https://doi.org/10.5194/gmd-12-909-2019, 2019.