Türk et al. (2026) Bedrock geology controls on new water fractions and catchment functioning in contrasted nested catchments

Identification

• Journal: Hydrology and earth system sciences
• Year: 2026
• Date: 2026-01-23
• Authors: Guilhem Türk, Christoph J. Gey, Bernd R. Schöne, Marius G. Floriancic, James W. Kirchner, Loïc Léonard, Laurent Gourdol, Richard F. Keim, Laurent Pfister
• DOI: 10.5194/hess-30-343-2026

Research Groups

• CATchment and ecohydrology research group, Environmental sensing and modelling unit, Luxembourg Institute of Science and Technology, Belvaux, Luxembourg
• Faculty of Science, Technology and Medicine, University of Luxembourg, Belval, Luxembourg
• Institute of Geosciences, University of Mainz, Mainz, Germany
• Department of Civil, Environmental and Geomatic Engineering, ETH Zürich, Zürich, Switzerland
• Department of Environmental Systems Science, ETH Zürich, Zürich, Switzerland
• Swiss Federal Research Institute WSL, Birmensdorf, Switzerland
• Department of Earth and Planetary Science, University of California, Berkeley, CA, USA

Short Summary

This study investigates how bedrock geology controls water storage and release in 12 nested catchments in the Alzette River basin, Luxembourg, using 13 years of stable isotope data. It finds that bedrock permeability is the primary control on the fraction of new water (water younger than approximately 16 days) and catchment hydrological functioning, with impermeable bedrock leading to higher new water fractions and more rapid streamflow responses.

Objective

• To establish relationships between metrics of catchment functioning (i.e., the release of water to streamflow) and landscape attributes (e.g., catchment geological properties).
• To test the hypothesis that bedrock geology controls hydrological functioning and the time scales over which precipitation is transformed into streamflow.
• To assess how precipitation intensity affects streamflow and transit times across multiple nested catchments.
• To explore whether fast transit times are associated with specific types of bedrock geology.

Study Configuration

• Spatial Scale: 12 nested catchments ranging from 0.5 km² to 247.5 km² within the Alzette River basin, Luxembourg.
• Temporal Scale: 13 years of fortnightly measurements (June 2011 to March 2023).

Methodology and Data

• Models used:
  • Ensemble Hydrograph Separation (EHS) method (Kirchner, 2019) to calculate the fraction of new water (Fnew).
  • Water balance equation to calculate catchment storage S(t).
  • Thornthwaite (1948) approach for monthly potential evapotranspiration (PET).
  • Ordinary Kriging for spatial interpolation of hourly precipitation.
• Data sources:
  • Fortnightly precipitation and streamflow δ¹⁸O measurements from 7 rain gauges and 12 stream gauges.
  • Hourly hydrometeorological data (precipitation, specific streamflow) from the LIST monitoring platform.
  • Stable isotope analysis performed using an off-axis integrated cavity output laser spectrometer (Los Gatos TIWA-45-EP, OA-ICOS).
  • Geologic maps of Luxembourg (APC, 2022) and other sources (Hellebrand et al., 2007; BRGM, 2020).
  • Physiographic characteristics (catchment area, elevation, land use, bedrock geology, alluvial deposit cover) from the CAMELS-LUX database (Nijzink et al., 2025).

Main Results

• Runoff coefficients (Rc) were higher in catchments dominated by less permeable bedrock (marls and claystones; Rc = 0.43 to 0.52) compared to those with permeable bedrock (sandstones and conglomerates; Rc = 0.19 to 0.40).
• The fraction of new water (Fnew, water younger than approximately 16 days) was strongly related to bedrock geology.
• Fnew was highest in impermeable bedrock catchments (mean 4.5 % to 11.9 %), increasing significantly with higher specific daily streamflow (up to 45 % in one catchment).
• In permeable layer interface catchments (sandstone/conglomerates with underlying marl/claystone), Fnew showed high variability with specific streamflow (up to 25 % in one catchment) despite generally damped hydrograph responses and low mean Fnew (1.3 % to 2.7 %).
• Weathered layer catchments (schists and quartzites) exhibited rapid infiltration, with approximately 80 % of streamflow being older than 12 weeks and only about 3.5 % being younger than two weeks; Fnew variability with streamflow was near zero.
• At high specific streamflow rates (top 20 %), Fnew was significantly correlated with land use (fraction of forests: r = -0.59, p = 0.044; grassland: r = 0.61, p = 0.036), mean catchment elevation (r = -0.81, p = 0.001), permeable to impermeable bedrock fraction difference (r = -0.68, p = 0.016), and mean slope (r = -0.67, p = 0.025).
• A "variance collapse" in Fnew was observed for catchments larger than approximately 150 km², suggesting an averaging effect with increasing catchment size.

Contributions

• Provides the first quantification of the proportion of water traveling to streams via flow paths with different transit times in the Alzette River basin using extensive stable isotope data.
• Demonstrates a clear and systematic link between bedrock geology and catchment hydrological responses, isotope signatures, and new water fractions (Fnew).
• Validates the mechanistic conceptualization of young water fractions in relation to streamflow, corroborating findings from other inter-catchment studies in different settings.
• Identifies specific stream types (e.g., impermeable layer catchments) that would benefit most from high-frequency sampling campaigns to capture rapid hydrological responses.
• Proposes a conceptual framework illustrating how bedrock permeability predominantly controls the variability of Fnew and fast flow path contributions across different hydro-lithological categories.
• Highlights that even relatively small proportions of distinct bedrock types can significantly shape the overall catchment response, suggesting the importance of focusing on distinguishable sections rather than just spatially averaged properties.
• Suggests that catchment sensitivity to a changing climate can be inferred from landscape features, particularly bedrock geology, for improved predictions in ungauged basins and under non-stationary conditions.

Funding

• Fonds National de la Recherche Luxembourg (grant no. C20/SR/14757154)
• Deutsche Forschungsgemeinschaft (grant no. SCHO793/25)
• MUSES project (Freshwater pearl mussels as stream water stable isotope recorders)

Citation

@article{Türk2026Bedrock,
  author = {Türk, Guilhem and Gey, Christoph J. and Schöne, Bernd R. and Floriancic, Marius G. and Kirchner, James W. and Léonard, Loïc and Gourdol, Laurent and Keim, Richard F. and Pfister, Laurent},
  title = {Bedrock geology controls on new water fractions and catchment functioning in contrasted nested catchments},
  journal = {Hydrology and earth system sciences},
  year = {2026},
  doi = {10.5194/hess-30-343-2026},
  url = {https://doi.org/10.5194/hess-30-343-2026}
}