This manuscript combines data-driven analyses and modeling approaches to understand how catchment and drought characteristics impact annual N export, transit times, and overall catchment N retention across a set of nested sub-catchments in Germany. The manuscript is very well-written and easy to read, and the authors present their ideas and results in a clear and compelling way. The authors combine of data-driven C-Q analyses with catchment modeling which is a unique approach.

Overall, this paper has the potential to be a strong contribution to the literature, but additional information/considerations are needed. Below are some general suggestions, as well as line by line comments.

General comments:

This paper explores how antecedent drought impacts N export and retention. There has been a fair bit of work on this in the US – in particular Davis et al. 2014 (Journal of Environmental Quality) and Loecke et al. 2017 (Biogeochemistry). The idea of “weather whiplash” may be a useful reference and motivation for this work.

The manuscript relies on a previously calibrated model (mHM-SAS, Nguyen et al 2022) for a fair bit of the analysis. While it is beyond the scope of this manuscript to include/replicate all the calibration information in the original modeling paper, the authors need to provide enough detail for readers of this paper to understand the analysis presented in this paper. For example, SAS functions were not introduced until line 202, and there is no description of what they are or how they are defined, and how SAS functions relate to nitrate generally. The a/b ratio (line 292) was never defined or explained, despite it being a central figure in the paper. It seems important to have at least some content explaining the model/SAS functions (and how they relate to nitrate cycling) in the introduction, and then substantially more context in the methods, perhaps even results. For example, the authors mention briefly the relatively low NSE efficiency for the lower basin model in the methods section (line 204) but never expand in results or discussion.

One central component of the analysis is the catchment vs soil N retention capacity. The catchment N retention metric is using a simple mass balance approach. However the soil N retention capacity is a simulated parameter from the mHM-SAS model. After digging back into the original Nguyen 2021 manuscript (which appears to be missing from the reference list), it doesn’t seem like this parameter has been validated in any way. Were there any measurements of root zone leachate made to validate this particular parameter estimate? At the very least, there is likely uncertainty in this parameter estimate, but this is not propagated into the Nret-soil metrics. There are numerous fates of N in the soil, and this seems like it has the potential to be highly uncertain. It would be helpful if the authors could provide more nuance here, as this is potentially an over-simplification.

Finally, the forest change scenarios seemed like side component of the manuscript that wasn’t fully explained, explored, or integrated into the main narrative of the manuscript. The scenarios used were extreme (100% vegetation loss?), and overall it came across as somewhat of a diversion from the main story line which was so focused on drought. I don’t think this is a necessary part of the manuscript.

Line by line comments:

Line 100: is the agriculture in the lower catchment irrigated? If so, how would this impact the findings discussed throughout the rest of the manuscript?

Line 115: I think this description could be clarified/simplified. eg - 1 year = 1 drying/wetting cycle. the year starts in May. Calling it a drying-rewetting cycle instead of year is clunky.

Line 160: This is results, not methods.

Line 195: I think this section needs to be expanded to provide more context about the modeling approach here. For example – what are SAS functions? Why is the NSE so low for the lower catchment?

Line 202: have the authors considered whether/how a model calibrated under moderate climate conditions will perform under extreme climate conditions? Might there be specific hydrologic processes that are more sensitive to drought and therefore do not simulate well during a period with different climate conditions than the calibration period?

Line 220: This sensitivity analysis is good - but do you think that N inputs might systematically have changed during the drought? How might applying the long-term N inputs to the drought period impact your conclusions?

Line 231: instead – word choice?

Line 273: More information on model fits needed (found it in line 209, but it could use a brief description in the results)

Line 292: a/b ratio has note been defined or described; substantially more context is needed here.

Line 295: It is surprising to that the median TTs are dominated by old water in the lower catchment, which is dominated by agriculture. Typically agricultural land use encourages rapid movement of water through the surface/subsurface (via tile drainage, etc), which might lead to younger water fractions. Can you provide more discussion as to why the median TTs are so long here, particularly given the land use? It also would be helpful to have more information in the site description about irrigation practices in the agricultural portion of the watershed, as this might impact TTs and flushing of N.

Line 324: this applies to throughout the manuscript, but the language of upper Selke could be clarified by in parentheses including the site names (SH, MD). It’s hard to remember which sites are upper vs lower based on the initials (which is what is labeled on all the figures).

Line 336-337: This is an incomplete sentence

Line 337: Were the concentrations during the drought exceptionally high? They appear to plot on top of the existing historical data, albeit on the high end – but this makes it sound like the concentrations were much higher than what was previously observed.

Line 455: this is a really interesting point! Very cool!

Line 343: Overall this appears to be true, but it also warrants some more nuanced discussion/context – similarly high concentrations were observed at other times in the historical record. Were those droughts as well?

Line 358: Can you provide more explanation here? Why is the potential for denitrification in groundwater lower in the downstream sub-catchment? Why would the capacity for in-stream retention be greater in this area (if denitrification is reduced)? Why would those two microbial processes respond differently to the same drivers? Reduced flow velocity would like enhance both?

Line 380: this is also a general comment – It seems like there is a disconnect between the upper and lower catchment behavior throughout the manuscript. How much of this might be driven by lower model fit metrics/uncertainty in the model outputs for that spatial scale specifically? Also does the land use change factor in here? How about irrigation in the lower agricultural catchment during the drought?

Figure 1: This figure is very data dense and could potentially be split apart into panel a/b and a separate figure for panel c. Further, the flipped double axis is difficult to read – typically discharge is not shown as a hanging plot (usually this is used for precipitation). One suggestion is to pull the hydrographs for all 3 onto a single panel of this figure. And finally, panel C seems like it should be in the results section – it is showing the calibration results of the model.

Figure 2: The color of the slope lines for panel a-c is confusing, because the color scheme flips (grey line corresponds to orange points, red line corresponds to grey points). Also, the discussion starting around line 335 is drawing attention to differences in these C-Q plots by wet vs dry season. The colors here are differentiated by year, and obscure the point about wet vs dry season (within a year). In contrast, there isn’t a discussion of how the C-Q slope changes between 2018 and 2019. So you could consider shifting the colors to show the wet/dry season comparison and not accentuate year. This might fit the discussion better.

Figure 3: suggest labeling y axis in panels d-f in same units used in written text/results section (years, not days).

Referee report on: Droughts can reduce the nitrogen retention capacity of catchments

By Carolin Winter, Tam V. Nguyen, Andreas Musolff, Stefanie R. Lutz, Michael Rode, Rohini Kumar, Jan H. Fleckenstein

I agree with the first referee that this is a nicely written manuscript with clear hypotheses and a good concept to analyse altered N dynamics under drought conditions within a mesoscale catchment. In particular, I like the strategy to combine a data-driven and model-based analysis. Although I am no native speaker, I think that language issues are largely settled, which is why a reviewer can concentrate on the most important issues: science and novelty behind the presented analysis. So I congratulate all authors for this work.

This said, I still have some major points that I think should be addressed prior to publication in egushere.

1. Information about N-Inputs

You state that exported N loads have generally decreased (e.g. L 268). What about the input? I propose to present the entire N-balance for the different subcatchments instead of only four components in Table 2. Moreover, a diagram would be more intuitive than a table.

2. Stability of catchment properties

This is a general drawback: If you compare N-retention–discharge relations during the recent drought to longterm values, you assume that catchment properties stayed the same. Can you really do this? There might be landuse changes (forest diebacks after storm events, or due to preceding droughts, e.g. 2003) and there is for sure a rising trend in temperatures. This trend per se alters the N-cycle, prolongs vegetation periods and presumably intensifies processes. I would like to see this point in the discussion section.

3. Scenario of forest dieback

This point was already raised by referee #1: Omit the simplistic scenario of forest dieback. You only simulate reduce N uptakes but there are surely more effects on the N cycle here: e.g. mineralisation of dead organic material, altered soil characteristics, etc.

4. Increased N-mineralization during droughts

You speculate that mineralization during droughts might increase, based on a single study that found increased rates of depolymerisation during droughts in a montane grassland in Austria. I would be more careful when transferring these findings to the forested Selke catchment.  E.g. depolymerisation in montane grasslands might principally be temperature dependent. In forest soils, I would rather argue that mineralization is hampered by soil moisture deficits during droughts and subsequently reduced microbial activity. Your data only shows increased mineralization during the entire dry-wet cycle which could be due to onset of strong mineralization during re-wetting. Then mineralization could be more intense, because you have an organic N-pool accumulated during the drought.  

5. Groundwater data to illustrate longer term N-effects

You hypothesize that in the upper Selke short TTs lead to visible effects of N-retention rather than in the low Selke, where longer TTs might cause longer term effects. This is a logic outcome of your transit time model. I think this could nicely be proved by groundwater data. Do you have well or spring data that could be used to support your hypothesis here? Recent papers have looked on groundwater nitrate responses after droughts and e.g. found instantaneous or delayed reactions depending on aquifer types.

6. Shape of the Retention-Discharge-Relationship

I disagree with the shape of the function between retention capacity and log-scaled discharge, as presented in Figure 4 and in the conceptual framework in Figure 5. A linear relationship is not meaningful here, maximum retention is 1 (at zero discharge), and your figure implies higher values. The data of HD shows this shape shape quite nicely. So put in the upper limit and an asymptotical approach of the function to this value when it comes to very low Q.

Minor issues (line by line):

L99: Check throughout the manuscript if you introduce abbreviations, I did not find this for TTs

L103 (Fig. 1): Colourscale for discharge anomaly could be clearer

L128: you speak about multi-year drought, although you only analysed the first two years of this event. I agree that the drought lasted longer, in some areas this event is somehow present up to now. But in your analysis this is a two-year event..

L204: these numbers refer to the subcatchments? This is not clear..

L292: I understand that more details of the mHM-SAS-model are given in the supporting material. But still I would like to have a couple of sentences also in the main document that explain what a/b ratios and SAS functions are and why they are indicative of water age.

L297: a median of a median, really?

L303: you present a conceptual drawback of the model: it cannot handle TTs that are larger than the simulation period. Nevertheless you model fits are quite nice on the entire time series from the very begin of grab sampling. Does this mean that your model is right for the wrong reasons and that you overcome structural deficiencies by calibration? I think this should be a point for discussion.

L339: I think the post-drought nitrate pulses in soils do not only propagate through catchments and find their way to rivers, but that runoff generation processes are altered, too. This has been documented before, i.e. a change of HOF/SOF to more SSF in a forest catchment after drought (https://doi.org/10.1016/j.jhydrol.2012.07.010).

L351, L398: You claim that upper Selke is “very low in nitrate” during droughts. What is “very low”? I think in forest catchments a value of 1 mg/l Nitrate-N is not exceptional. Also here you should compare with other studies in forest streams.

L467: N uptake by denitrification?