The manuscript "Tracing near-surface runoff in a pre-Alpine headwater catchment" investigates overland flow and interflow dynamics at two Alpine sites using sprinkler experiments and different tracers. Overall, this is a fascinating and highly valuable study. The authors have clearly dedicated substantial effort to producing a unique and rich set of qualitative and quantitative observations, which significantly advance our understanding of runoff generation in Alpine catchments.

The authors' work is a significant contribution to the field, and the manuscript is comprehensive and well written. While reading the draft, however, I noted a few points that could benefit from further clarification or refinement. These are outlined below and, once addressed, will help strengthen the manuscript for publication in HESS.

Specific Comments

Page 3, Line 101-104: The claim seems to be that fast flow pathways are a key factor because streamflow reacted earlier than groundwater in 'about half of the events'. This raises the question if that is special compared to other catchments, or is this typical? And how does this relate to velocity (fast flow paths) vs celerity (earlier rise)?

Page 6, Line 185-187: Do these statistics still hold when taking the duration of rainfall into account? If the return period of 24 mm/h over one hour is 1 to 2 years, the return period of a rainfall of 24 mm/h over 24 hours will be much higher. It would be helpful if you could include this information, and discuss the possible implications.

Page 10, Line 259: Please be more specific: how were the samples selected? Were not all samples analysed for all tracers?

Page 12, Line 341: where did the remaining third of the water go?

Page 15, Line 398: NaBr was applied to shallow piezometers. Did you observe any overflowing of these boreholes?

Page 17, Table 5: Why are the velocity estimates from the Deuterium results not included here?

Page 18, Line 445-454: Could you elaborate more on this topic? What does the incomplete recovery mean for your conclusions? Are the differences in tracer transport and recovery only because of the nature of the different flow paths, or are properties of the compounds also an explanation that needs to be considered. e. g, different adsorption characteristics?

Page 19, Table 6: Unfortunately, it is not fully clear what is shown here exactly. Is the recovery expressed cumulatively? Is it the percentage of the total applied mass, or only for the first tracer lines in the first columns? Please add explanation to the table caption. Maybe consider moving the last two sentences from the caption to the discussion.

Page 22, Line 564: Would this not require a similar velocity of transport at both sites to be a fair comparison?

Page 23, Line 594 - 601: how about ambient temperature? Water viscosity changes a lot with temperature, and that influences the flow velocity.

Page 25, Line 604-635: This part of the discussion could be more elaborate. The OF velocity will be determined by slope and surface characteristics (roughness, infiltrability), which in turn will be determined by the types and states of vegetation and soil. Also, the temperature (viscosity of the water) and other experimental conditions like length of the flow paths also play a role. It is thus not only 'vegetated' vs 'bare' soil. Comparing mean (see below) velocities without normalizing for these factors is not really conclusive.

How were the velocities averaged? Arithmetic or harmonic mean? This also applies to the other average velocities reported here. Example: When the time that overland flow needs to travel a distance of 2 m would be 1 minute and 2 minutes, the average velocity would be 1.3333 m per minute (harmonic mean).

How would the measured flow velocities compare with theoretical estimates, e.g., Gauckler-Manning-Strickler formula? Would you get realistic roughness values when inverting the formula?

Page 26, Line 685: The data should be uploaded before publication. In fact, it would be helpful if they could be included in the review. Otherwise, chances are too high that this will never happen.

Minor Comments / Clarifications

Page 3, Line 103: What does 'close' mean, in m?

Page 5, Line 125-127: Please be a bit clearer: 10 cm organic rich AND another horizon rich in organic with 30 cm thickness, or up to 30 cm depth? Would that be A and B horizon, or litter layer and A horizon, or something else?

Page 5, Line 128: Figure S1 is not about roots

Page 8, Table 3: Tracer volumes are given for Uranine and Deuterium - what were the masses? Please specify to align with the table header

Page 18, Figure 7: Is this the from the Deuterium experiment? Please add more info to the caption.

Page 20, Fig 8: These are great images. Perhaps make them a bit larger (page width)?

Page 22, Line 543: Does this refer to Deuterium labeled water? Please clarify.

Page 22, Line 562: That could possibly be exfiltration from biomat flow, right?

Page 22, Line 564 – Page 23, Line 566: This requires a little more explanation. Would that mean that concentrations were much higher with the lower flow rates?

Page 24, Line 609: What does this flow rate should tell the reader? Isn't the surface area/wetted perimeter equally important?

Page 25, Line 642: Please clarify why the saturated and steady-state conditions would make comparisons difficult?

Page 25, Line 673: info that these are 'trenched' maybe more important than the width

Page 26, Line 680: maybe include a comment on the difference in OF and TIF velocities - both are fast, but also OF still is significantly faster

Technical Corrections

Page 6, Line 160: “(see Gauthier et al. (2025))“ - Consider avoiding the double parentheses - check style guide

Page 10, Line 283: “h” -  variables are set in italic, please check style guide – also variables elsewhere

Page 10, Line 270: “containing the 3 mg L-1 brilliant blue dye” – check wording/sentence structure

Page 10, Line 272: “tree” - typo

Page 10, Line 279: “was able to see” - check wording/sentence structure

Page 18, Line 450: “large” - Please check

Page 25, Line 651: Check sentence structure

This manuscript uses rainfall simulation and tracer experiments, including NaCl, Uranine, Bromide and deuterium, to understand the magnitude and spatial distribution of overland flow (OF) and topsoil return flow TIF) at two plots in a pre-Alpine catchment, one with clear-cutting, one with grassland. They also explore the celerity and velocity for OF and TIF. Overall, they put in very significant effort to conduct all the experiments. I find it is overall hard to follow all the experiments.

Specific Comments:

It seems that the mean intensity for experiments in clearing and grassland is quite different (Table 2). I would guess that might also impact the partitioning between OF and TIF, where high intensity for grassland will result in a higher OF. So I am not sure which plays a bigger role, intensity or soil macropores.

Does that matter if two sprinklers contribute more total deuterium mass at the overlapped area (in the middle of Figure 3a)? 

How do you determine the first increase in the flow rate in Figure 5? I think there are flow rate up and down before and after the points you labelled. Why are the locations you labelled the response to the water pulse? Thank you.

It took me a long time to really understand what Figure 6 represents: I guess you can unify with NaCl/Uranine/Bromide using red lines, and only with deuterium with grey shading in Figure 6. 

For Table 3, you can list the duration of each tracer experiment and their start time if possible.

Also if possible, add a table for sample collections and collection intervals.