Process-based water balance modelling with explicit ice-flow dynamics and multi-data set calibration: the WaSiM-OGGM coupling scheme

The water balance of high-alpine glacierized catchments is largely dominated by snow and ice processes. When modelling the hydrological response of such catchments, a reliable representation of snow/ice accumulation and melt should be ensured, especially when studying the effects of climate change. Even though numerous state-of-the-art hydrological models are able to adequately represent the contribution of snow melt into the total runoff with the use of complex approaches (e.g. energy balance models), glacier dynamics are still based on conceptual or empirical methods, which exhibit some limitations compared to more sophisticated models (e.g. explicit ice-flow dynamics).
The Water Flow and Balance Simulation Model (WaSiM) is a process-based hydrological model that includes an empirical volume-area scaling approach for describing the glacier’s evolution. Although acceptable estimates can be obtained with this approach, an integration to a more complex glacier representation is still missing. For this reason, a coupling scheme between WaSiM and the Open Global Glacier Model (OGGM) is developed, hence accounting for explicit ice-flow dynamics.
The workflow consists mainly on three steps: i) a first WaSiM run to obtain monthly values of temperature and precipitation that serve as input for the ii) second step, which is running OGGM. Finally, iii) a dynamic model run of WaSiM with the updated output from OGGM (annual glacier outlines and ice thickness) is performed. Within this last step, the glacier’s volume internally calculated by WaSiM (i.e. with the VA-scaling approach) is replaced by OGGM’s output, while performing a simultaneous multi-data set automatic calibration. In this calibration, only WaSiM parameters are adjusted and simulation results are compared against glacier mass balances (OGGM) and observed runoff. The performance of the calibration is then evaluated in terms of a weighted multi-objective function. Although the best fit between observed and simulated runoff is achieved when considering only runoff observations (single-data calibration), glacier components are better represented when calibrating the coupled model with the multi-data set (i.e. also including glacier mass balances). Therefore, a trade-off is made between general model performance and accurate runoff prediction. 
This coupling scheme is aimed for hydrological modellers with no additional expertise on glacier modelling, since OGGM is set up according to its default parameters. Finally, it could serve as a tool not only to predict the hydrological response of any glacierized catchment (even without any available glacier data), but also to make predictions under future climate projections with a more reliable representation of glaciers.