Multi-objective calibration and extended validation of the National Hydrological Model of Denmark

The National Hydrological Model of Denmark (DK-model) is a distributed, integrated hydrological model coupling 3D groundwater flow with root zone processes, overland flow, and river routing, including major human impacts such as groundwater abstraction. It is a central tool for water management in Denmark, supporting water resources assessments, climate change impact studies, or nutrient transport. Hence, the DK-model calibration and development seek to comply with a variety of metrics and concepts to meet requirements of the diverse end-users. Recently, calibration has been performed using the Pareto Archived Dynamically Dimensioned Search (PADDS) algorithm, enabling multiobjective optimization with a global parameter search under computational constraints. Calibration targets are conventional groundwater head and streamflow observations, complemented by spatial information on evapotranspiration and drain flow. While such an approach represents a state-of-the-art calibration, the integrated and multi-purpose nature of the DK-model requires that key hydrological processes and phenomena are represented adequately, which is not guaranteed by calibration metrics alone due to equifinality and model structural deficiencies. Therefore, we perform an extended validation of the DK-model, evaluating whether it reproduces hydrological system behavior beyond conventional performance metrics. This includes assessment of functional relationships such as precipitation-runoff partitioning or multi-annual system memory. Such extended validation provides additional, process-oriented information on model realism, helping to identify structural deficiencies that remain hidden in conventional calibration and validation. This is particularly critical for multi-purpose integrated models like the DK-model, where credible projections and scenario analyses depend on a physically consistent representation of interacting hydrological processes.