Evaluation of the safe operating space during low flow for water allocation and climate change scenarios in the Danube

A basin-to-local evaluation framework, quantifying safe operating space (SOS) for low-flow water allocation and climate change along the Danube, is presented, linking basin hydrology to local operations for navigation, ecology, run-of-river hydropower, drinking water, and nuclear power plant cooling. The water allocation model (RIBASIM), forced by the hydrological model (wflow_sbm), embeds explicit demands on the mainstem under ‘what-if’ scenarios, while the numerical model (RSim-3D) simulates corresponding water levels at an Upper Danube case study. SOS was determined with sector-specific indicators and thresholds, such as the number of days per year with interruptions or disconnections. The results show competing demands and upstream-downstream effects, even for local abstractions. Navigation interruptions increase (on top of morphodynamic challenges), and side channel connectivity decreases under ‘what-if’ scenarios. A reconnection measure improves, whereas bed degradation reduces side channel connectivity, worsening SOS. Hydropower production declines, with seasonal shifts mitigating winter deficits. Nuclear power plant cooling was sensitive to low flow, extending under a more extreme climate scenario, while SOS for drinking water was not exceeded at the investigated cities. Across sectors, the interaction of water allocation and sediment management with fairway depths, lateral connectivity, sediment continuity, hydropower production and cooling-water reliability under changing climate and demand needs to be discussed.