Dorthe et al. (2025) The thermal future of a regulated river: spatiotemporal dynamics of stream temperature under climate change in a peri-Alpine catchment

Identification

• Journal: Hydrology and earth system sciences
• Year: 2025
• Date: 2025-11-17
• Authors: David Dorthe, Michael Pfister, Stuart N. Lane
• DOI: 10.5194/hess-29-6309-2025

Research Groups

• School of Engineering and Architecture of Fribourg, HES-SO University of Applied Sciences and Arts Western Switzerland, Fribourg, Switzerland
• Faculty of Geosciences and Environment, Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland

Short Summary

This study investigates the future thermal regime of a peri-Alpine regulated river under climate change using a high-resolution process-based model. Projections indicate mean annual water temperatures may rise by up to 4 °C by 2080–2090 under RCP 8.5, with river regulation introducing distinct spatial and seasonal warming patterns, particularly in autumn and winter due to reservoir thermal inertia.

Objective

• To assess climate change-induced temperature variations along a regulated river.
• To evaluate the evolution of thermal alterations caused by hydropeaking.

Study Configuration

• Spatial Scale: A 22 km long river reach, divided into 436 computational segments, each spanning 50 m.
• Temporal Scale: Simulations for current climate (average from 2019 to 2021) and future periods (2055–2065 and 2080–2090) at 10 min intervals.

Methodology and Data

• Models used:
  • One-dimensional process-based stream temperature model based on the HEC-RAS framework.
  • Statistical model for lake water temperature estimation based on past air temperature.
  • Delta-change downscaling method for meteorological and hydrological climate scenario data.
• Data sources:
  • Meteorological: MeteoSwiss station at Fribourg/Grangeneuve (GRA) for current climate (air temperature, incoming solar radiation, relative humidity, atmospheric pressure at 10 min resolution). CH2018 dataset for future climate projections (daily air temperature, relative humidity for GRA; solar radiation from Payerne station).
  • Hydrological: Residual flow from Rossens Dam (2.5 m³/s to 3.5 m³/s). Hydropower plant (HPP) discharges (15 min resolution). Tributary discharges (Gérine and Glâne) from fribourg.swissrivers.ch (1 h resolution). Hydro-CH2018 dataset for future tributary runoff projections.
  • Water Temperature: Measured lake temperatures from the operator (three depths, 6 h resolution). Measured tributary temperatures (10 min resolution over 7 years).
  • Environmental: Topographic and vegetation data for shading factors (Digital Surface Model). Sediment layer properties.

Main Results

• Mean annual water temperatures are projected to increase by up to 4 °C by 2080–2090 under RCP 8.5.
• Daily mean temperatures are projected to exceed 15 °C for nearly half the year under RCP 8.5 by 2080–2090, raising ecological concerns (e.g., for proliferative kidney disease).
• The ratio of water-to-air temperature increase is 1.1 ± 0.2 for RCP 4.5 and RCP 8.5, suggesting regulated rivers may be more vulnerable to climate change than unregulated ones (ratio ~0.8).
• River regulation introduces distinct spatial and seasonal patterns:
  • The residual flow reach (low discharge) is particularly susceptible to warming.
  • Deep reservoir releases help moderate climate change impacts downstream of the dam and hydropower plant.
  • The strongest warming is projected for autumn and winter, unlike unregulated rivers where it typically occurs in summer, due to the thermal inertia of the reservoir.
• Indicators used to assess thermopeaking impacts (TT90 and AT90) remain largely unaffected by climate change, assuming hydropower operations remain unchanged.

Contributions

• Provides a high-resolution, process-based assessment of climate change impacts on the thermal regime of a regulated peri-Alpine river, including thermopeaking.
• Highlights that river regulation creates distinct spatial and seasonal thermal responses to climate change, differing from those in unregulated rivers (e.g., dominant autumn/winter warming).
• Demonstrates the dual role of reservoirs in shaping river thermal dynamics, exacerbating vulnerability in some areas (residual flow) while mitigating impacts in others (deep water releases).
• Offers crucial insights for guiding decision-making and optimizing hydropower operations under future climate conditions.

Funding

• Groupe E
• Ribi SA ingénieurs hydrauliciens
• School of Engineering and Architecture of Fribourg (HEIA-FR, Funding Ra&D)
• Haute école spécialisée de Suisse occidentale (HES-SO)

Citation

@article{Dorthe2025thermal,
  author = {Dorthe, David and Pfister, Michael and Lane, Stuart N.},
  title = {The thermal future of a regulated river: spatiotemporal dynamics of stream temperature under climate change in a peri-Alpine catchment},
  journal = {Hydrology and earth system sciences},
  year = {2025},
  doi = {10.5194/hess-29-6309-2025},
  url = {https://doi.org/10.5194/hess-29-6309-2025}
}