The authors responded in detail to the comments written down in the review file and I appreciate the effort they put into implementing many of the suggested changes. However, none of the 57 comments I made in the annotated manuscript were (directly) addressed which makes it hard for me to fully evaluate the revised version. I would highly recommend to take these comments also into consideration.
Therefore, I included the annotated manuscript from the original submission again.
In addition I prepared a new annotated manuscript with comments on the revised version.
Also, I copied the comments from both annotated manuscripts to the end of this file.
The manuscript has already improved since the initial submission, still language and sentence structure would benefit from another round of careful checks.
The discussion needs more clarity and precision in explaining the processes, concepts, methods and results. The structure is also not that clear in certain places.
I was not able to follow some of the conclusions because there is not enough explanation on how they were drawn. For example, I understand that there will be an immediate decrease in solutes in the stream if we assume that the catchment TTD is exponential (this is trivial). Now why exactly does this immediate reduction not occur if the MTT is very large – this does not make sense to me – there should still be an immediate decrease, just a slower one. So there must be another process involved that turns the exponential TTD into a different kind of TTD (like, e.g., an exponential piston-flow TTD). It could be explained by making the difference between catchment and groundwater TTD more prominent (an exponential GW-TTD preceded by a constant time delay in the unsaturated zone is basically creating an overall exponential-piston flow TTD).
In the end I am still wondering about the exact results and conclusions that this study provides.
And although, as I said in the beginning, the manuscript has improved since the last round of reviews, in my opinion it requires another iteration.
Comments in the annotated manuscript of the original submission
• I do not understand the title. There is no specific focus or relation to 'aged streams' in the paper. The term is not even mentioned or explained anywhere. So why is this part of your title?
• time series?
• This last sentence makes me wonder what the actual novelty of this study is. You should definitely explain better what you mean by 'groundwater distributes water in time and space' and 'groundwater makes it possible for different waters to mix'.
• Mediums are actually persons who talk to ghosts. Media are substances that water passes through.
• Maybe add some more details on the scenarios here?
• Does 'the upstream' exist as a word?
• Sampling surface and groundwater quality
• What does that mean exactly?
• Are you discussing the drawbacks of this method anywhere later in the manuscript?
• What are strong and weak sinks? Why were the particles stopped there? Where they removed? What happened to the other 50%?
• Are these subcatchment outlets?
• Can you estimate the TTs through this layer?
• How was this included?
• Curve sounds awkward.
• Any ideas on why that could be?
• Why don't you show any of these dynamic TTDs (individually not like in Figure 3)?
• What are possible implications?
• You applied this factor where/when? During the model run? After the model run?
• You should achieve this by changing the model setup (different aquifer thickness, porosity , etc.) not by applying a factor.
• What delay? Some more years, months, days?
• If the peak was kept in 1985, how did you achieve the delayed input function? Did you change the skewness, the variance?
• Does this not directly correspond to the fractions of agricultural land in the subcatchments? If not, why? Vegetation?
• Why 'although'? The shorter the time series, the smaller the chance to not capture certain events...
• How was that done exactly? Was it added at the end? Was it done in the model? If so, how?
• delete 'a'
• Made it longer or thicker or both (added weight to the tail)?
• no apostrophe (TTDs not TTD's)
• The agricultural input to the GW? To the streams?
• Does that mean that nitrate was not removed at all for all other particles flowing only through shallower layers?
• Is that because you assume that nitrate is not removed under unsaturated conditions?
• ...and THUS removing this field only...
• Why? Maybe discuss it in more detail.
• the majority of the yearly variation
• Could you derive it by combining tritium and NO3 measurements (transport through the unsaturated zone would cause more removal - lower peak concentrations - than transport through the saturated zone)?
• Rephrase. The nitrate concentrations cannot be lower than the measurements, because you measured the measurements.
• How does this promote denitrification?
• What about changes to your denitrification calculations to improve model performance? I think this analysis falls a little short...
• Why is it not possible? You should try it to provide a more complete analysis.
• Can you quantify the uncertainty caused by the grid cell size?
• This does not correspond to any other shape in this figure.
• Are these urban areas?
• What is this? The hydrological base?
• I would label the individual panels.
• Better connect the upper left corners (so you don't draw a line directly through the data).
• I do not see a difference in the scale of the x-axes.
• fraction of area
Comments in the annotated manuscript of the revised version
• I still do not understand the title. There is still no specific focus or relation to 'aged streams' in the paper. The term is not even mentioned or explained anywhere. So why is this part of your title? I would remove it.
• limited to what?
• does it depend on the travel time in the unsaturated zone or on the slow release of organic N or on the travel time in the unsaturated zone only if it includes slow release of organic N? This is not clear to me...
• not clear either
• Which processes?
• This list could use more explanation. How does the time lag change in response to the listed properties?
• You mean the location of delivery or the timing of delivery or the amount of delivery?
• Backward TTDs, to be precise.
• Input curve sounds wrong. Input time series would be better.
• What do you mean by 'adding curves to the flow paths'?
• 'principally' is not equal to 'in principle'
• Do you mean time invariant? Also, do you mean backwards TTDs?
• What does that mean? A short explanation of the nitrate transformation factor method would be nice.
• This is not a result of your modeling study. Just a description of your input.
• Again, this is not a result of your modeling study but rather a description of the measurements you use. It would be a results section if you described how your model was or was not able to reproduce certain parts of the measured time series.
• This should be named figure 3.
• You mean the input of solutes to the stream?
• 'also' used twice
• This entire section needs some careful language/sentence structure editing. It is sometimes hard to understand what you want to express. I recommend more consistency in your terminology - always refer to the same processes, solutes, methods, concepts with the exact same name.
• Be more specific, please.
• This is one of the sentences that are quite imprecise in their terminology. What change in what input? Which stream concentrations? What reduction rates in which input?
• Is that addition necessary?
• If the TTD is exponential there should never be a time lag in the arrival of the peak (because the maximum of each exponential TTD is always at the beginning). How exactly do you explain the observed time lag? It must come from something else in the model structure.
• Why do you spell out Travel Time distribution and Mean Travel Time and reintroduce abbreviations (MTTs) here?
• Why? This does not make sense to me? Please explain this behavior.
• But you are not looking at catchments with an exponential TTD. You are looking at a catchment with an exponential groundwater TTD. Adding a lag for the transport through the unsaturated zones makes the total catchment TTD become a piston flow-exponential TTD.
• This is where it becomes interesting and therefore it needs more explanation/analysis.
• Isn't this a different effect from the one you describe above? You say that if denitrification occurs predominantly close to the stream the time lag would increase. But is this not more a function of where the nitrate input takes place? Closer to the stream or farther away? I can imagine that the amount of damping is also influenced by these scenarios.
• This should be expressed more precisely and with more details:
• Longer MTTs in the unsaturated zone cause a larger time lag...
• Which combinations of MTT and solute input reduction cause which dynamics in the time lag?
• ...and so on...
• I do not understand this new definition. '...where water entering the groundwater-flow system is discharged' Is this water entering or being discharged? Maybe a figure would help.
• I do not understand your point here. How are the two contributing areas different?
• Why not call it 'recharge area' if you're using this term to explain it?
• How about calling it subsurface flow contributing area and overland (or surface) flow contributing area? In the end your groundwater contributing area is also contributing to the runoff in the stream.
• Which consequences does this have on your model results? What about particles that reinfiltrate after seeping out?
• Hey, but above you said that the lag to the peak is extended for some scenarios. How does that fit together?
• Again, you should be more precise. It is a trivial result (and not surprising at all) that there is an immediate reduction if the catchment TTD is exponential. But were you even looking at that or were you also exploring the lag caused by the unsaturated zone?
• To me this is not a convincing argument, because you give too little information on the specific processes that cause your conclusions.
• I would like to see a summary on how exactly each of these parameters influences the time lag. (Not just a statement that they can influence the time lag).
• Again, if you expect an exponential TTD then you always expect an immediate reduction.
• You didn't show them in enough detail or with enough explanation.
• What about the blue line on panel c? Should also be mentioned in the captions.