| 07 Aug 2026
Deriving Subsurface Stormflow Mechanisms with Isotopes and ERT
Abstract. Subsurface stormflow (SSF) is a dominant process in the generation of runoff in many catchments. However, the mechanisms by which new event water triggers the activation and release of stored pre-event water are not well understood. To identify the flow paths and mechanisms governing SSF generation, we conducted a controlled sprinkling experiment using deuterium- labeled water alongside time-lapse electrical resistivity tomography (ERT), groundwater monitoring and soil core sampling at two trenched hillslopes in the Black Forest, Germany. Water was applied for three hours, with the supply switching to deuterated water halfway through the experiment. Both hillslopes responded rapidly, with event runoff coefficients of approximately 0.6, and tracer breakthrough occurred at the trenches within 7–18 minutes of application, even at depths greater than 70 cm. However, post-irrigation soil cores detected the tracer only in the uppermost 20 cm of the soil matrix, where approximately 45 % of the labeled water remained stored. Time-lapse ERT revealed that the wetting front was largely confined to the upper 0.2–1.5 m; meanwhile, the deeper, near-saturated zone exhibited negligible resistivity change despite sustained lateral discharge as measured at the trenches.
Together, these observations suggest the presence of a dual-domain flow system, whereby a small proportion of event water bypasses the matrix via vertically and laterally connected preferential pathways. Meanwhile, the majority of SSF consists of pre-event water, which is mobilized by pressure-driven (piston) displacement within the saturated zone. Subsequent natural rainfall remobilized the stored tracer, raising the cumulative recovery rate to approximately 30 % within one month, thereby confirming the soil matrix to be both a sink and a delayed source of event water.
Competing interests: One of the Co-Authors (Markus Weiler) is on the editorial Board of HESS
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.
This is an interesting paper with a very nice experimental setup. Combining a sprinkling experiment, deuterium tracer, trench flow, groundwater wells, soil cores, and ERT gives the authors a strong data set. The main idea is valuable, and I think the paper can make a useful contribution to understanding subsurface stormflow generation.
However, the manuscript needs more work in organization and interpretation. In several places, the authors present interpretations as if they were directly observed. One important example is the distinction between “unlabeled water” and “pre-event water.” Because the first half of the sprinkling experiment used unlabeled stream water, the authors need to show clearly how different this water was from pre-event trench water and whether the two could be separated isotopically. If their isotope compositions were similar, then the isotope data would mainly distinguish labeled water from unlabeled water, rather than event water from pre-event water.
This does not weaken the experiment, but it does mean that the wording around “pre-event water” should be more careful.The paper would also benefit from a clearer separation between results and interpretation. Some mechanisms, such as piston displacement, preferential flow, and matrix storage, are plausible but they are still interpretations. The authors should make this clearer throughout the abstract, discussion, and conclusions.
Line by line Comments
Introduction
The introduction needs some organization. It is hard to follow some of the ideas because many concepts are introduced one after another without enough transition. Reworking the introduction could greatly improve the manuscript and help the reader understand why this experiment is important.
L36-42: The discussion of isotopes and the old-water paradox is important, but the transition from preferential flow to old water feels abrupt. The authors should add one or two sentences explaining why preferential flow and old-water dominance can happen at the same time.
L43-50: This paragraph about difficulties with natural rainfall experiments needs a better connection with the paragraph above. It currently reads as a new topic. The authors could first say that one reason the mechanism remains difficult to identify is that natural rainfall inputs are hard to constrain.
L56: The transition into ERT feels disconnected. The paper moves from controlled irrigation to direct observation of flow pathways very quickly. I suggest adding a sentence explaining why isotope data alone are insufficient.
L68-74: The limitations of ERT may fit better in the Methods or Discussion. The introduction is already quite long, and this part slows down the motivation of the paper.
L80: The main knowledge gap should appear earlier. The mechanism by which event water triggers release of stored water is the central motivation, and it should be introduced near the beginning of the introduction.
L82: The third research question seems partly answered already in the introduction, where the authors explain that isotopes and ERT are complementary. I suggest reformulating Q3. Maybe something like: “To what extent can the combination of isotopic tracers and ERT improve the interpretation of SSF generation mechanisms?”
Methods
L100-110: The trenches are described here, and then section 2.2.1 describes the trenches again. Consider merging these parts or reducing repetition.
The trench setup is very important for the paper, but it is still difficult to understand exactly what each trench captures. A more detailed schematic would help. Figure 1 is currently somewhat rough (it literally says “rough sketch).” The authors should replace this with a cleaner figure showing soil horizons, trench layers, bedrock/refusal depth, spring location at Trench 1, and the collection depths.
The sprinkling experiment needs more information about spatial uniformity. Was rainfall distribution measured with collectors? How variable was sprinkling intensity across the plot? This matters because the authors later interpret spatial differences in ERT and soil tracer storage.
The isotope design should be explained more carefully. Because the first 1.5 hours of sprinkling used unlabeled stream water, the isotope data clearly distinguish labeled water from unlabeled water, but may not fully distinguish pre-event water from the initial unlabeled irrigation water if their isotope compositions were similar. The authors should state the isotope composition of the initial sprinkling water and pre-event trench water, and clarify how this affects the interpretation of “pre-event” fractions.
L180-185: The isotope mixing model section needs more detail. What isotope values were used for each end member? Were source uncertainties included? What priors and MCMC settings were used in simmr? How was convergence checked?
L187-192: The storage estimate in the upper 20 cm is based on strong assumptions, e.g., full saturation, homogeneous porosity. However, the authors collected very few soil cores. This should be presented as an approximate estimate, not as a precise value.
L193-204: The ERT inversion description is useful, but the choice of regularization parameters needs more justification. Saying that values were selected after “several tests” is not enough for reproducibility.
L210-211: The 3 %, 5 %, and 10 % thresholds need a clearer explanation. Why is 3 % considered meaningful given measurement and inversion uncertainty? A noise or repeatability test would help.
Results
The results are generally interesting, but some parts already include interpretation. I suggest keeping the Results more descriptive and saving mechanism language for the Discussion.
L214-228: The description of the SSF response is good. However, “event runoff coefficient” should be used carefully. The trenches may not capture all water, and later the authors discuss possible deep or lateral bypass. Maybe call this “trench-recovered runoff coefficient” or define the limitation clearly.
L249-263: The tracer breakthrough is one of the strongest results. However, the authors report tracer arrival within 7-18 minutes, while samples were collected every 20 minutes. Please clarify how these arrival times were calculated.
L265-272: The sample size for the soil cores is small. The conclusion that tracer was only stored in the upper 20 cm should be limited to the sampled locations. The authors should avoid implying that the entire plot was characterized by only three cores.
L271: The estimate that 45 % of labeled water was stored in the topsoil seems too precise. It depends heavily on porosity, saturation, and spatial representativeness. Please add uncertainty or present this as a rough mass-balance estimate.
L274-323: The ERT results are detailed, but the section is hard to follow. There are many percentages, locations, and times. A summary table or simplified interpretation figure would help a lot.
Discussion
The wording on the dual-domain interpretation is too strong in places. The data support rapid movement of labeled water through preferential pathways and storage of some labeled water in the shallow matrix. However, the statement that most SSF was pre-event water is less certain because unlabeled irrigation water cannot be separated from pre-event water.
L349-356: This is a strong paragraph and one of the best parts of the paper. The contrast between tracer in deep trench outflow and no tracer in deeper soil cores is convincing evidence for bypass flow. However, the authors should still acknowledge that the soil cores are spatially limited.
L357-364: The manuscript says the tracer budget shows preferential flow carried only a small fraction of applied water. This is probably true, but recovery depends on trench capture efficiency and storage estimates. Please make the uncertainty more visible.
L365-370: The piston-flow interpretation is plausible, but ERT non-response in the deeper saturated zone does not by itself prove piston displacement. It could also reflect low ERT sensitivity under saturated conditions or low contrast between waters. The authors discuss this later, but the interpretation here should be softer.
L399-420: The “unaccounted labeled water” section is useful and should maybe be emphasized more. It shows that the mass balance is still uncertain. This uncertainty should be connected back to the main conclusions.
L421-463: The methodological uncertainty section is good. I think some of this should appear earlier because it affects how the results are interpreted. The issue of unlabeled irrigation water versus pre-event water is not a small limitation, it is central to the main claim.
Conclusions
The conclusions should be rewritten with more careful wording. The study shows that labeled event water reached the trenches quickly and that much of the labeled water was retained or delayed. It also supports a dual-domain interpretation. But the authors should avoid saying that the majority of SSF was definitively pre-event water unless they can better account for the unlabeled irrigation water.
The final suggestion about using multiple tracers, including a conductive tracer detectable by ERT, is very good. This is one of the clearest take-home messages from the study and could be highlighted more.
Minor comments
L9-10: The statement that 45 % of labeled water remained stored in the upper 20 cm should include uncertainty or be softened.
L17: “confirming” is too strong. Maybe use “suggesting” or “supporting.”
L84-85: “gives insights of where” should be “provides insight into where.”
Figure 5: Add more explanation of how the reference profile is used and whether the upper 5 cm were discarded.
Figures 6-7: Consider adding depth-of-investigation or sensitivity masks for ERT results.
L399: “According to the retrieved tracer data” could be clearer. Maybe “Based on the tracer recovery and storage estimates…”
L412-419: The discussion of bedrock uptake and lateral flow beneath the trench is important, but it also means the trench water balance is incomplete. This should be stated more explicitly.
Data availability: The authors mention data and supplementary ERT data, but code for isotope mixing and ERT processing could also be made available if possible.
Questions for the authors
How different were the isotope values of the initial stream water, pre-event trench water, and labeled water?
How were breakthrough times shorter than the sampling interval estimated?
Was sprinkler uniformity measured across the 200 m2 plots?
How sensitive is the 45 % storage estimate to porosity and saturation assumptions?
Could the first unlabeled sprinkling phase have changed the flow pathways before the labeled water was applied?
How much confidence do the authors have that the lower trench discharge represents water from within the sprinkled area rather than lateral inflow from outside it?
Overall, I think this is a strong and interesting experiment, but the interpretation needs to be tightened. The paper will be much more convincing if the authors are clearer about what is directly observed, what is inferred, and where uncertainty remains.