Long-term thermal projections for the regulated Tisza River under limited future hydrological information

Study region River warming threatens aquatic ecosystems in temperate Europe. The Tisza is a large regulated lowland river influenced by barrages and a reservoir. The study covered its 571 km Hungarian reach at 11 monitoring stations. Study focus This study assessed changes in water temperature, their seasonal and spatial differences, and projection uncertainty where future discharge and operating conditions are unknown. An equilibrium-temperature model was calibrated using daily observations from 2001 to 2023 and independently validated for 2024–2025. Fourteen climate-model chains were run separately under RCP4.5 and RCP8.5 for 2030–2050, 2051–2080 and 2081–2100. Historical relationships with air temperature and discharge were examined. New hydrological insights for the region The equilibrium temperature concept showed robustness for climate change assessment providing R² = 0.951 and RMSE = 1.821 °C for the validation. By 2081–2100, annual mean water temperature was projected to increase by 1.0–1.3 °C under RCP4.5 and 2.0–3.0 °C under RCP8.5 relative to 2030–2050. Under RCP8.5, station-specific median frequencies of days above 25 °C increased from 28.9 to 56.8–71.0–86.9 days year⁻¹ between these periods. Warming varied across seasons and river sections, while differences among climate-model chains were substantial. The study provides an assessment of future warming and prolonged warm-water conditions to support river management and ecosystem protection. These projections describe the response to atmospheric warming under the calibrated conditions; future changes in discharge and barrage operation remain unquantified.