Development of a Harmonized Water Balance Modelling System for the Upper Sava Pilot Area – Danube Water Balance Interreg Project

Effective and harmonized water balance modelling is crucial for sustainable water management, particularly in transboundary river basins like the Danube. Within the framework of the Interreg Danube Transnational Programme 2021–2027, the "Danube Water Balance" project aims to standardize methodologies and improve water balance assessment capacities across the region. A key pilot study is focused on the Upper Sava sub-basin, covering 12,319 km², with 86% of the area in Slovenia and 14% in Croatia.

This poster presents the joint efforts of the Slovenian Environment Agency (ARSO) and Croatian Waters to develop a harmonized modelling system for the Upper Sava. The area is hydrologically significant, with the average long-term (30-year) discharge at the Jesenice na Dolenjskem gauging station amounting to 267 m³/s. To ensure methodological consistency and comparability, both institutions have agreed to use the Community Water Model (CWatM), which simulates the full hydrological cycle at daily resolution and supports integrated water demand and supply assessments.

In Slovenia, we are currently using the mGROWA model for national-scale water balance assessments, while Croatia has employed custom modelling approaches and long-term surface water balance analyses, along with groundwater classification systems established since 1992. The transition to a unified model is an attempt to align national practices, improve data exchange, and support basin-wide decision-making.

The poster will present the main steps of the project, key challenges related to input data comparability and model calibration, and the expected outcomes. This approach offers a practical example of transboundary cooperation in operational hydrological modelling and contributes to enhancing the overall resilience of the Danube region to water-related risks under both current and future climate conditions.