Improving Hydrological Modelling Practice through Componentization: A BMI-Based HBV Implementation within the NextGen Framework

Hydrological modelling practice increasingly demands transparent, reproducible, and flexible workflows that enable systematic evaluation of model structure, process representation, and coupling strategies. This study presents a refactoring and componentization of a conceptual hydrologic model and its internal routines using the Basic Model Interface (BMI) as a practical mechanism for improving modelling practice through modularity, interoperability, and reproducibility. As a case study, an existing R implementation of the Hydrologiska Byråns Vattenbalansavdelning (HBV) model was reimplemented in Python using object-oriented design and exposed through BMI, a standardized interface widely adopted in Earth system modelling.

BMI components were developed at two complementary levels of granularity: (1) a component representing the complete HBV model, and (2) individual components representing the Snow, Soil, Response, and Routing routines. This dual-level design enables transparent reconstruction of the full model from its constituent processes while supporting controlled experimentation with alternative structural configurations, such as the inclusion or exclusion of internal routing. The BMI-enabled components were integrated within the Next Generation National Water Model (NextGen) framework, facilitating consistent execution, standardized variable exchange, and reproducible multi-model simulations. Applications include both standalone HBV simulations and multi-model mosaic formulations in which HBV components are coupled with other hydrologic and land-surface models.

The results demonstrate how interface-driven model design can improve hydrological modelling practice by enabling systematic model comparison, structural sensitivity analysis, and reusable workflows across modelling environments. More broadly, this work provides a transferable roadmap for converting Python-based hydrologic models into BMI-compliant components, supporting community efforts toward more transparent, interoperable, and reproducible hydrological modelling.