the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 May 2025
Bedrock geology controls on new water fractions and catchment functioning in contrasted nested catchments
Abstract. We still lack substantial understanding on how landscape characteristics shape the storage and release of water at the catchment scale. Here we use 13 years of fortnightly precipitation and streamflow δ18O measurements together with hydrometeorological data from 12 nested catchments (0.5 to 247.5 km2) in the Alzette River basin (Luxembourg) to study bedrock geology and landcover controls on streamflow generation.
Streamflow responses to precipitation were highly variable. Runoff coefficients were typically higher in catchments dominated by less permeable bedrock (i.e. marls and claystones, Rc = 0.43 to 0.52) than in catchments with a high fraction of permeable bedrock (i.e. sandstones and conglomerates, Rc = 0.19 to 0.40). The fraction of new water (Fnew, water younger than ~16 days in this study) determined via ensemble hydrograph separation was strongly related to differences in bedrock geology. Fnew was highest in impermeable bedrock catchments (i.e. with a dominance of marls and claystone, Fnew = 4.5 to 11.9 %), increasing with higher specific daily streamflow (Fnew up to 45 % in one catchment). In catchments with an important fraction of permeable sandstone and conglomerates, high Fnew variability with specific streamflow (Fnew rising to 25 % in one catchment) was also found, despite a damped and delayed hydrograph response to precipitation and low Fnew (means of 1.3 to 2.7 %). In the weathered bedrock catchments (i.e. dominated by schists and quartzites), rapid infiltration led to large fractions of water that was older than 12 weeks (~80 %) and very small fractions of water younger than two weeks (~3.5 %). Fnew variability with streamflow was near zero, contrasting with the rapid response of the hydrograph to precipitation events. At high specific streamflow, Fnew was also correlated with bedrock geology and certain land use types. The extensive data set of streamflow δ18O enabled us to link water storage and release to bedrock geology. Such information is key for a better anticipation of water storage and release functions under changing climate conditions, i.e. long dry spells and high-intensity precipitation events.
Competing interests: Some authors are members of the editorial board of journal Hydrology and Earth System Sciences.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2025-1530', Anonymous Referee #1, 11 Jun 2025
This manuscript presents a robust and well-structured investigation of how bedrock geology influences hydrological responses—particularly new water fractions (Fnew)—across 12 nested catchments in the Alzette River basin (Luxembourg), using 13 years of δ¹⁸O data, high-resolution environmental datasets, and ensemble hydrograph separation (EHS). The findings provide strong empirical support for the role of lithological heterogeneity in shaping flow pathways and water storage, contributing meaningfully to our understanding of subsurface–hydrology linkages. I recommend the manuscript for publication once the following issues are satisfactorily addressed.
General Comments:
- The authors define Fnew as water younger than approximately 16 days, based on their fortnightly sampling interval. While the authors note that fortnightly sampling limits Fnew resolution, a brief discussion on how the chosen sampling frequency may bias or underrepresent fast-response components would strengthen the interpretation of Fnew.
- While the manuscript rightly identifies bedrock geology as a key control on flow partitioning and Fnew, the analysis is limited to areal lithological classification. Crucial structural factors—such as bedrock depth, regolith thickness, and the presence of fractures or faults—are not considered, despite their known influence on subsurface storage and connectivity. If such data are available, they should be integrated into the analysis; otherwise, their absence should be acknowledged as a limitation.
- The manuscript emphasizes the dominant role of bedrock geology in shaping hydrological response, however, the potential confounding influence of co-varying catchment attributes (e.g., slope, land cover, elevation) is not adequately resolved.
- The manuscript would benefit from a clearer explanation of the methodological constraints of the Ensemble Hydrograph Separation (EHS) approach. Specifically, does EHS impose minimum requirements on sampling interval, time series length, or end-member stability?
Specific Comments:
Line 163: The phrase “i.e.i.e.” is repeated. Please delete the redundant “i.e.” to correct the typo.
Line 221: Please clarify how α limits evaporation.
Line 224: The manuscript defines Smax as the highest 0.5% of daily catchment storage values, please justify why this specific quantile was chosen
Citation: https://doi.org/10.5194/egusphere-2025-1530-RC1 -
AC1: 'Reply on RC1', Turk Guilhem, 05 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1530/egusphere-2025-1530-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2025-1530', Anonymous Referee #2, 22 Jul 2025
This paper focuses on the stable isotope-based estimation of the proportion of new water (Fₙₑw) and the watershed response mechanism, and constructs a multiscale analytical framework covering different lithological, climatic, and geomorphological features, with clear ideas and standardized structure, which is of strong scientific significance and regional adaptability. However, there is still room for improvement in the transparency of methodological details and parameter interpretation.
1、Section 2.5 clearly outlines the EHS method, but its reliance on equations may pose difficulties for readers unfamiliar with isotope-based hydrograph separation. I recommend adding a simplified conceptual diagram or schematic to illustrate the EHS workflow.
2、I recommend that the authors provide a brief rationale or literature-based justification for choosing the 16-day threshold as the representative time scale for defining “new water.” It would also be helpful to clarify whether and to what extent this threshold might influence the interpretation of the results and the robustness of the study’s conclusions.
3、The assumption of a 200 mm threshold for field capacity warrants further clarification. I recommend that the authors briefly justify whether this value reflects region-specific soil and climatic conditions, and whether it is based on measured soil data or literature from comparable settings. Clarifying this point would enhance the robustness of the catchment storage estimates.
4、Please note that the abbreviation “i.e.” appears redundantly in both line 163 and line 253. Please delete the redundant "i.e.".
Citation: https://doi.org/10.5194/egusphere-2025-1530-RC2 -
AC2: 'Reply on RC2', Turk Guilhem, 05 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1530/egusphere-2025-1530-AC2-supplement.pdf
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AC2: 'Reply on RC2', Turk Guilhem, 05 Aug 2025
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