Bonacina et al. (2026) Marked thermal heterogeneity within a small mountain stream network and its response to climate warming

Identification

• Journal: Journal of Hydrology
• Year: 2026
• Date: 2026-10-31
• Authors: Luca Bonacina, V. Mezzanotte, Riccardo Fornaroli
• DOI: 10.1016/j.jhydrol.2026.136713

Research Groups

• Department of Earth and Environmental Sciences (DISAT), University of Milano-Bicocca, Milan, Italy

Short Summary

This study characterized the thermal heterogeneity of five mid-altitude mountain streams in the Serio River catchment, Orobic Alps, finding pronounced differences driven by contrasting water sources and hydrological modifications (reservoirs), and projected these patterns to change under future climate warming scenarios.

Objective

• To characterize the water thermal regime of five watercourses and untangle the influence of key meteorological, hydrogeological, and anthropogenic drivers.
• To assess thermal heterogeneity at the catchment scale and determine how it diverges from that observed in other mountain basins.
• To evaluate the potential effects of climate warming on future thermal patterns at both the watercourse and catchment scales under different scenarios, testing the persistence of different thermal refugia.

Study Configuration

• Spatial Scale: Upper Serio River catchment, Orobic Alps, Northern Italy (383 km²), covering five mid-altitude streams (Serio River, Sanguigno, Goglio, Ogna, Nossana). Water temperature was monitored at 16 sites ranging from 456 meters to 1,395 meters above sea level.
• Temporal Scale: Water temperature was continuously monitored every 10–60 minutes from July 2017 to September 2022. Analyses for current thermal regimes focused on the quinquennium 2017–2021. Future thermal patterns were projected for the period 2011–2100.

Methodology and Data

• Models used:
  • Generalized Additive Models (GAMs) with a Gaussian Location-Scale framework (gaulss) for estimating missing daily water temperatures and projecting future water temperatures.
  • Principal Component Analysis (PCA) for dimensionality reduction and identifying major axes of thermal variation.
  • Permutational multivariate analysis of variance (PERMANOVA) for testing differences in thermal regimes among watercourses.
  • Permutational analysis of multivariate dispersions (PERMDISP) for quantifying interannual variability within sites.
  • Six CMIP6 Global Climate Models (EC-Earth3, CESM2, ECHAM6.3, HadGEM3, MIROC6, CMCC-CM2) for climate projections, downscaled using a monthly Delta Method.
• Data sources:
  • Water temperature: Continuous measurements using iButton devices (DS1925L and DS121Z) at 16 sites.
  • Air temperature: Daily means from five meteorological stations in the upper Serio catchment (www.arpalombardia.it) and statistically downscaled projections from CMIP6 Global Climate Models.
  • Electrical conductivity: Measured at each monitoring site using a HACH HQ40d probe.
  • Channel width: Measured at bankfull level using a tape.
  • Topographic variables (altitude, catchment area, distance from source): Derived from a Digital Terrain Model (DTM, 2015 edition) with 5 meter spatial resolution, provided by the Regional Environmental Geoportal, using QGIS (version 3.4).
  • Climate scenarios: Shared Socioeconomic Pathway (SSP) scenarios from the IPCC AR6 framework (SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5).

Main Results

• Watercourses exhibited distinct thermal responses: siliceous, unregulated streams (e.g., Sanguigno) were thermally reactive (slope of 0.66 between air and water temperature), closely tracking air temperature. Groundwater-fed (e.g., Nossana, slope near 0) and reservoir-regulated streams (e.g., upper Goglio, Serio) were thermally buffered and less sensitive to warming (slopes of 0.29–0.34).
• Catchment-scale thermal heterogeneity was pronounced, with annual mean temperatures varying between 5.5 °C and 9.1 °C across sites. Maximum differences reached 3.6 °C in annual means, up to 8.7 °C across maxima, and 7.6 °C across minima, comparable to or exceeding that of alpine systems influenced by glaciers.
• Spatial thermal patterns were primarily driven by differences in annual and daily thermal variability, and secondarily by mean annual temperatures, closely associated with lithology (siliceous vs. limestone) and topography (altitude).
• Future projections indicate a general warming across all scenarios, with mean annual temperature increases ranging from 0.2 °C to 0.6 °C under SSP1-2.6 and 1 °C to 2.6 °C under SSP5-8.5 over 90 years. The Nossana stream showed minimal warming (<0.2 °C).
• Annual thermal variability (V6) is projected to increase from 0–1.6 °C under SSP1-2.6 to 0.9–3.7 °C under SSP5-8.5, with the largest increases in thermally reactive streams like Sanguigno.
• The frequency and duration of warm conditions (e.g., days with mean daily temperature > 15 °C) are projected to increase significantly, particularly in reactive streams.
• Catchment-scale thermal heterogeneity is projected to increase under strong warming scenarios (SSP5-8.5), enhancing the diversity of thermal habitats within small spatial scales despite regional climate warming.

Contributions

• Provides a comprehensive framework for assessing catchment-level thermal heterogeneity by integrating sub-daily monitoring, multi-year and multi-metric analyses, and high-resolution climate projections.
• Demonstrates that pronounced thermal heterogeneity, comparable to glaciated alpine systems, can exist in mid-altitude mountain streams due to the interplay of contrasting hydrogeological features and anthropogenic regulation (reservoirs).
• Highlights that climate change will lead to an asymmetrical thermal response across the catchment, increasing overall thermal heterogeneity and enhancing the diversity of thermal habitats, which can serve as climate refugia for aquatic species.
• Underscores the critical importance of understanding fine-scale thermal dynamics for interpreting ecological patterns and guiding effective conservation and management strategies for alpine aquatic biodiversity.

Funding

• Department of Earth and Environmental Sciences of the University of Milano-Bicocca (grant Cod. 23CE169 within project ID 2021-ATEQC-0017/PER).

Citation

@article{Bonacina2026Marked,
  author = {Bonacina, Luca and Mezzanotte, V. and Fornaroli, Riccardo},
  title = {Marked thermal heterogeneity within a small mountain stream network and its response to climate warming},
  journal = {Journal of Hydrology},
  year = {2026},
  doi = {10.1016/j.jhydrol.2026.136713},
  url = {https://doi.org/10.1016/j.jhydrol.2026.136713}
}