Thank you for giving me the opportunity to read your manuscript. It is well written and interesting. Your results provide novel and important information to the scientific community.

You did a great job describing the rather complex methods in a way that is easily understood by the readers. Your discussion on the implication of the findings to catchment water movement is well described, and provides direct, actionable, and targeted advice to hydrologic planners.

Overall, I was very impressed and I only have minor revision requests:

Line 79 Introduction: I recommend breaking the last paragraph of the introduction into two paragraphs. The first should provide more literature on chemical tracers (namely Br and D2O/H2O, used in your study) and any other literature that has studied on water transport as it relates to chemical tracer behavior. Additionally, this should include the literature of Fickian and non-Fickian (anomalous) transport models and how they apply to water behavior, as you have mentioned in your abstract.

The second paragraph should describe your objectives and include the more specific explanation of your methods shown in lines 79 – 85.

Line 258: Explain Fickian and non-Fickian transport in 1-2 sentences.

Conclusion: I recommend providing one more paragraph summarizing the findings that your results may indicate smaller aquifer thickness requirements. I also recommend providing a sentence or paragraph on the applicability of this research to global aquifers/catchments to provide a global perspective/conclusion to this research.

The title of this paper "Transport behavior displayed by water isotopes and potential implications for assessment of catchment properties" is misleading, as it suggests the paper has something to say about water isotopes and their use in assessing catchment properties. I see no evidence of this whatsoever.

The core claims of the paper rest on two points:

1. stable water isotopes behave similarly to inert tracers like Br-

2. The v_T parameter of a CTRW model fit to breakthrough curves showing non-fickian behavior is not the same as the value of v_W calculated from v_W=Q/(nA). 

I have no major issue with either of these points, per se. However the authors then argue that there is some issue with using isotopes to understand water movement through catchments -- as though the movement of water molecules were somehow different from the movement of "water" itself.

To me the issue seems to rest entirely on the assumption that the v_T parameter that arises in CTRW theory is the 'true' mean velocity of the tracer, and that this ought to correspond with the value of v_W calculated as mentioned. Tf there is a mismatch between theory and observation then the issue is not with reality, it is with the theory.

If the discrepancy between the two 'velocities' arises regardless of which tracer is used (which they themselves say their results support) then the use of isotopes as a tracer is not at issue. Rather it seems to me it highlights a theoretical confusion about what v_T and v_W actually mean in relation to one another. Does CTRW theory assert that they ought to be the same? Are the observations in agreement with the theory, or at odds with it? Where exactly does this difference come from? It certainly seems like a paper that disentangles that issue would be useful to the CTRW community.

Meanwhile, the community that uses isotopes to study catchment properties is moving on from the notion of 'mean travel time'. The leading-edge approaches do not require it, and it is reported less often in favor of other more reliable metrics, like those based on storage selection.

I would note in passing that the authors do not seem familiar with the storage selection approach. They seem to be under the impression that it is based on the collapse of the system to one spatial dimension (Line 331). This is not the case -- in storage selection theory the system is collapsed to zero spatial dimensions.