Trade-offs between regionalised and non-regionalized calibration of a continental model for estimating European coastal river inflow

One of the current goals of large-scale hydrological modelling within the FOCCUS project (https://foccus-project.eu) is improved estimation of water and matter runoff to the coastal regions of Europe. The main challenges and decisions that are typically made for physically based modelling are as follows: the complexity of the model, the level of process representation in the model, the number of models involved in the modelling chain, and a level of regionalization of model parameters. Creating a parsimonious model and using enough data to achieve the desired results are also challenging. In order to account for geographical variability of the model parameters, calibration at a large number of subbasins is necessary when implementing increasingly complex hydrological models at large scale. Our research focuses on the level of regionalization of parameters and assessment of model performance for coastal vs full domain calibration.

The Hydrological Predictions for Environment (HYPE) model (1), used here for large-scale hydrological analyses, is a dynamic process-based rainfall-runoff and water quality model. Regional precipitation corrections tested in the previous European-domain HYPE (E-HYPE) version improved the model performance by helping to compensate for systematic biases in the water balance (2). However, the addition of these precipitation corrections also introduces uncertainty to the model calibration, as the corrections could just compensate for model biases instead of addressing underlying problems in the model structure or process representation. The calibration strategy for the current E-HYPE version 4 has one “global” parameter set that is used for the European domain to limit the amount of parameter regionalization. E-HYPE calibration was further improved using performance filters for parameter sets and assessment of model behaviour including snow water equivalent, reservoir sedimentation, and stream resuspension. We explore the effects of regional calibration against the coastal gages on the overall model performance and process description as well as on the freshwater inflows to coastal areas. We discuss the trade-offs between regionalised coastal-oriented calibration and full-domain model tuning, point out the main factors limiting model performance, and investigate the effect of diversified calibration workflow based on soil/landuse dependent parameters.

References:

• SMHI, 2023. HYPE Model Documentation. 〈http://www.smhi.net/hype/wiki/〉
• Brendel, C., Capell, R., & Bartosova, A. (2023). To tame a land: Limiting factors in model performance for the multi-objective calibration of a pan-European, semi-distributed hydrological model for discharge and sediments. Journal of Hydrology: Regional Studies, 50, 101544.