Modelling climate change impacts on river temperatures in two Norwegian catchments

Study region: Gaula and Nausta are two Norwegian river catchments that differ in size, hydrological setting, and degree of hydropower regulation. Gaula includes regulated tributary sections affected by diversions and reservoir operation, whereas Nausta represents a smaller, largely unregulated western Norwegian catchment. Together, the catchments provide contrasting test cases for assessing climate-change impacts on river discharge and water temperature at management-relevant spatial scales. Study focus: This study applies the HYPE model to simulate daily discharge and water temperature at river-segment scale. The model was calibrated and validated against observed discharge and river water temperature, with satisfactory performance for both variables. Future hydroclimatic conditions were assessed using an ensemble of regional climate projections under RCP4.5 and SSP3-7.0. New hydrological insights for the region: The simulations indicate substantial changes in seasonal discharge regimes, including reduced spring flood magnitude and increased winter discharge. Simulated river water temperatures increased across both catchments and all future periods, with long-term warming at river mouths reaching approximately 0.20 °C/decade under RCP4.5 and 0.30 °C/decade under SSP3-7.0. Warming generally intensified downstream and towards the late 21st century. In regulated parts of the Gaula system, bypass reaches downstream of hydropower diversions showed thermal differences to unregulated conditions, suggesting reduced thermal buffering under low-flow conditions. These results indicate that environmental flow design in regulated rivers should consider thermal conditions alongside discharge.