RIBASIM Danube: Modeling water allocation in the Danube Basin with a focus on low-flow conditions

Climate change is leading to alterations in low-flow conditions and droughts, while societal factors like urbanization are intensifying these challenges. This underscores the urgent need for a deeper understanding and more effective management of water shortages in the Danube main stream. Innovative modelling tools addressing water demand are able to assess the allocation between relevant demands including agriculture, industry, hydropower, ecology and navigation during low flow periods. The STARS4WATER project deals with the complex challenges faced by the transboundary Danube river basin. The study area encompasses 19 riparian countries, covering a total area of 801,463 km² with diverse topographic and climatic characteristics. Stakeholders actively participated in identifying drought and low flows as key issues for water management in the basin. The upper Danube is particularly influenced by glaciers and snow, which are significant for low flows during summer, and these dynamics are expected to change under future climate scenarios. Lower Danube is expected to face increased drought risk in combination with rising agricultural water demand. As a result, the River Basin Simulation (RIBASIM) model was initiated for the entire basin. The RIBASIM model, a node-link model for simulating and balancing water availability, allocation and use, was employed including the present state. Inflow nodes for sub-catchments representing the boundary conditions regarding water availability were determined using the wflow_sbm model, a grid-based rainfall-runoff model. Input data comprised discharges from defined sub-basins for the period 2010 to 2020 at a resolution of 10 days. On the demand side, the focus was on the Danube main stream, including water supply for cities, major industrial demands in Germany and Austria as well as nuclear power plants, and specific irrigation areas. Those are represented by water demand and water abstraction nodes. Critical low flow nodes, essential for minimum flow for navigation, were also identified. Explicit demands were collected from statistical authorities, non-governmental organizations, academic papers and established consensus. Simulated discharges, demands, supplied demands (i.e. water use), and shortages for the period 2010 to 2020, were verified by a plausibility check and sensitivity analysis. It serves as a starting point for future scenario-based analyses including e.g. the effect of glacier retreat and water allocation and use priority setting during low flows. The study emphasizes the need for comprehensive local water demand data collection river basin-wide and enhanced transboundary cooperation to tackle water management challenges in the Danube River basin, including adherence to national and EU-wide statistical standards considering water use and demand. In particular, data on water demand from industries and agriculture are essential to identify hotspots for shortages in the Danube during droughts and low flows more effectively. The finding is relevant for implementing future scenarios related to climate, hydrology, socio-economic factors, and water resource management. By better unlocking data availability and improving data resolution and incorporating future projections, more accurate and practical insights for managing water resources in the face of evolving environmental and societal pressures are achievable.