Content

This manuscript describes a parsimonious model application for instream DOC concentrations along a stream reach, basically consisting of a mixing model and an instream uptake model. While the DOC concentrations were directly taken from groundwater sampling at each of the 25 stream segments, the amount of GW inflow was calculated from the discharge discrepancy between inflow and outflow of the reach. The distribution of water inflow per segment was either equally distributed (diffuse inflow scenario) or according to the area of specific sub-catchments contributing to groundwater inflow at a segment (UCA, upslope contributing area). The model is used to test the hypothesis, that previously defined ‘discrete riparian inflow points’ (DRIPs, Ploum et al. 2020) have a major influence on DOC stream concentrations and that this is connected with spatially variable groundwater inputs.

General comments

The topic of DOC mobilization and fate is well within the scope of HESS, but the manuscript needs to be improved substantially before publication is possible.

Some explanations of the model structure and setup are unclear. The paragraph explaining the strategy to account for uncertainty in the simulations (L.232-244) is on the one hand hard to follow and some decisions seem arbitrary. On the other hand, simulation uncertainties are not discussed at all in the manuscript, even though they are shown in the main results figure 3.

The model performance is rated by using four different likelihood measures and pre-defined thresholds for behavioural models. When the model performance is discussed, it seems arbitrary which likelihood measure is highlighted. The discussion would be stronger if the model performance evaluation would be more systematic. Further, the influence of DRIPs on DOC concentrations is only visible (in the sampling data) if the gain in streamflow is > 40% (Figure S1). Otherwise, the lake outflow dominates, which was also stated in the discussion. Thus, events A, B, F, G and H are the most important for the testing of the hypothesis. These events should be discussed in more detail. For example, ‘diff_nobio’, which does not consider UCAs, often performs better or similar good compared to the UCA-version. Is the UCA-concept really helpful or are the stream concentrations rather driven by the groundwater concentrations and not the inflow volumes? If the UCA-concept is helpful, in which hydrological situations? After a structured analysis and discussion, more specific conclusions should be drawn.

Specific comments

L.1. Title: In my opinion, the title does not correctly represent the content. In the manuscript, the model itself is rather used as a tool to analyse processes. The title, however, suggests that the main focus is on model structure and testing.

L.16. Change ‘concentration’ to ‘concentrations’

L.29. The word ‘Importantly’ seems inappropriate here.

L.55. Change ‘By contrast’ to ‘In contrast’

L.58. Remove ‘a’

L.79. Remove ‘what’ and ‘is’

L.89. Remove ‘than’

L.89. The term ‘passive pipe’ might be misleading. Pipe flow is a specific type of hydraulic flow, which is not addressed here at all. Probably better refer to ‘non-reactive’ or ‘recalcitrant’.

L.136. Change ‘talweg’ to ‘thalweg’.

L.141. What kind of filters were used for filtering (filter material)? How long was the time between sampling and lab-analysis? Is there a reference explaining DOC sampling and analysis in more detail?

L.165. How were the reaches classified into DRIP and non-DRIP zones?

L.166. What do you mean by ‘…was weighted based on the mean…’? Did you want to say ‘…was derived by using the average of all…’?

L.168. Change ‘upslope contributing are’ to ‘UCA’. It was already expanded in the introduction.

L.217. Remove ‘yielded’

L.232ff. In this paragraph, the method for uncertainty estimation is explained and the uncertainty bounds are shown in figure 3. However, the uncertainty of the model results is not discussed at all in the results and the discussion sections. Please add it.

L.233-234. ‘…we compared the range of uncertainty in our simulations to the range of uncertainty in the observations.’ Where did you do that? I can’t find it in the manuscript.

L.234 The number of model runs (100 for each scenario) appears really low, especially for such a simple and fast model (in terms of computational time). Usually, several thousands of runs are performed for uncertainty estimation. How did you check that 100 runs are enough for stable uncertainty bounds?

L.236. Why did you choose to use 66% uncertainty bounds? More common are 80% (10^th -90^th quantile) or 90% (5^th-95^th quantile).

L.246ff. I understood that 100 model runs were performed for each scenario. Which one was evaluated with likelihood measures? All? Only the best? Which values are shown in table 2?

L.252-254. The description of R² is - at least – unusual. I would recommend using a more straight-forward description. In this context, the most important thing might be, that R² shows the ability of the model to simulate the dynamics of the measured time series, but not the absolute value.

L.265. The reference to ‘Fig. 3F, K-M’ should rather be ‘Fig. 3E, K-M’

L.277. change ‘NSE<0’ to ‘NSE was <0’

L.295-296. Probably it is worth mentioning that R² generally has problems with evaluating uniform values such as D and I.

L.315. Please re-format the table. Floating point numbers and scenario identifiers are in two lines, which makes them hard to read.

L.319-328. This paragraph reads like a conclusion/summary. At this point, however, the results were not yet discussed. The paragraph should be removed.

L.337. change ‘influence stream’ to ‘influence on stream’

L.341. Remove ‘of’

L.347 The mentioned UCA_BIO is not the best model under the conditions of 3E (Neff=0.5; R²=0.84). DIFF_BIO performs much better with Neff=0.81 and R²=0.86.

L.364. Change ‘that’ to ‘than’

L.364-365. Change ‘… spatial variability in groundwater …’ to ‘‘… spatial variability in groundwater concentrations…’

L.402. ‘…demonstrated that UCA can be useful to identify “reactive” reaches…’ Actually, the model results don’t show a clear signal towards UCA explaining the variability better. Often, the Diff-version is similar good or even better.

L.419ff. In the conclusions, UCAs are not mentioned at all. Since they play an important role in the whole manuscript, please add your conclusions about this concept.

L.425. ‘occasionally’ is imprecise language. Please, be more specific.

L.429-431. ‘This study …. (Mineau et al., 2016)’. This is not a conclusion of this study. Delete or shift the sentence.

Figure S1. I would recommend adding figure S1 to the main text. I think the different hydrological conditions, even though they are provided in the text, are much easier to understand with Figure S1.

This is a potentially interesting paper but one that needs some attention before publication. There are several aspects of the paper that rely on results from previous papers. Some, particularly the locations of DRIPs, need more explanation here. Other aspects (such as the discussion of uncertainties) also need more detail.

I also do not get a clear sense of how this paper helps address important problems or knowledge gaps. A clearer explanation in the Introduction of where those gaps are and the Conclusion of the broader aspects would help here. It is an important topic, but the importance needs to be better explored.

The paper is generally clear but has minor grammatical errors throughout and occasional odd phrasing. I have sympathy with having to write in a second language, but checking the English would help communicate the research better.

I hope that these comments are useful.

Title.

Not sure that this reflects the paper’s contents well. There is no explanation of parsimony in the paper (ie what are the alternative ‘non-parsimonious’ approaches?) If parsimony is important it needs to be reflected in the Introduction and Discussion.

Abstract

The Abstract is a reasonable summary of the paper, but there is a lot of focus on what was done rather than what was concluded. Also as with the rest of the paper try to add some indication of importance in the statements at the start and the concluding statements at the end of the abstract.

Introduction

The introduction is reasonably comprehensive but suffers from some odd terminology. More importantly, it does not convey a sense that the paper is addressing an important general question. What is it that we really don’t know that this paper will help understand? DOC (and C in general) in rivers is important but there have been many studies over recent years. If you explain what the key gaps in our understanding are and how this study helps address those, then the paper will have more impact with the scientific community. Your aims should also refer back to those broader themes.

L35. “Running waters” is odd terminology if you specifically mean rivers.

L39-43. Point not clearly made. There are obviously differences in sources of water, retention times, and flowpaths in headland streams but HOW do those result in differences in DOC? Terms such as ‘dynamics’ are a bit vague, better to explain exactly what you mean (sources, loads, reactions etc).

L52. What is a ‘spiralling framework’?

L61-66. Is this universally true? Reactions in groundwater likely reduce DOC loads and, in many catchments, the deeper groundwater seems to have lower DOC concentrations than surface runoff. In which case groundwater inflow may dilute surface water DOC concentrations. Perhaps be more circumspect here?

L81-90. Your aims are fine as far as they go, but I don’t get a good idea of how these fit in to a broader understanding of the science.

L89. Not sure the ‘passive pipe’ argument is valid. It is generally accepted that streams are dynamic environments, particularly around reations involving compounds such as DOC. This makes it seem that you are addressing a problem that may not exist.

Methods

Section 2.1

Given the importance of understanding the groundwater inflows to the stream and the origins of the groundwater, more details are needed here. It seems that some of these are in other papers but better explanation would make this paper more convincing. Specifically:

• L112-114. It is not clear what you mean by “route 60% of the upslope contributing areas to 5% of the stream length”. I presume that your DRIPs occur along 5% of the stream but where does the 60% area come from. This is probably in the Leach et al. paper but more details are needed here
• What exactly is the groundwater? Are you sampling only shallow riparian zone waters or a mix of those and deeper groundwater? Explain how deep the bores are and what units they are screened in.
• How are the DRIPs identified and do you have diffuse discharge between the DRIPs?
• Some descriptions (eg shallow and near-surface) could be more specific (ie are the groundwater levels above the land surface?)

L98. What underlies the regolith and soils? Are there deeper aquifers that can contribute groundwater and if so do we know anything about that groundwater?

L100-110. This description is difficult to envisage. You should illustrate the variations in river flow and groundwater responses on a Figure. Most of those details are in Fig. S1. Given that you have only 3 figures in the paper (and only 1 is multipart) you could easily fit Fig. S1 into the main text (and also Fig. S2 if you wanted) – figures in supplements rarely get looked at.

Figure 1. What are the contours (I presume elevation)? Given the topic of the paper, can you show groundwater flow paths – you refer to these in the caption (L120) so presumably you know something about them.

Section 2.2

L123-127. That is not really a drought. Droughts imply that the catchment as a whole dries up, which impacts more than just river flows (groundwater levels, soil moisture are also impacted). While, this analysis is useful, ‘artificial low flows’ or something similar would be better.

L132-135. Not clear how the DRIPs were identified from the wells (which is what you imply). Or do you mean that you located the wells following identification of DRIPs? Again, I think that these details are in other papers but would be better summarised in section 2.1.

L136. “thalweg”

L137. “evacuated” not “vacuumed”.

L146. Report the typical or range of analytical uncertainties here.

Section 2.3

L165-169. As noted above, the classification of reaches and location of DRIPs needs more justification. It is important to the study. It seems that much of this comes from the geometry of the catchment, specifically the UCA., with the details only in other papers. It may be correct, but as presented here it is not very convincing.

L176-177. The interpretation that all increase in streamflow is due to groundwater inflows is also important and needs more detail. From Figure 1 there looks not to be any significant tributaries, but what about overland flow, small rivulets, draining pools etc (especially at high flows). You should be more specific in ruling those out if they do not exist.

L198. see comment earlier about ‘passive pipe’ and perhaps come up with a better term.

L199-201. Not very clear what you mean by ‘upstream’, do you mean above the lake?

L232-255 Do you present the uncertainties in the paper (it is not clear that you do)? There should be more discussion in the results or discussion sections.

L234-239. Is there any reason for only using 100 model runs? I would have thought that the model is not computationally limited and more runs would be possible. If increasing the number of runs resulted in the same rand and distribution of outcomes than you should state that. I agree that studies should not do redundant analyses, but knowing where redundancy occurs is useful.

Results

L259-265. This is really a summary of the results and would make more sense after you have described the data from the different studies. Suggest moving to the end of this section or to the beginning of the Discussion.

L283. See comments earlier about calling this a ‘drought’

Table 2. Difficult to read (final version needs line breaks fixing)

Sections 3.1 to 3.3. These are detailed but a bit dense to read. The level of detail also tails off a bit. In 3.1, you report the DOC concentrations but not in the other sections. Given the level of detail here, it is difficult to have to keep looking at Figure 3 to see the changes. Adding the values in here would help.

Discussion

Here I think that a better discussion of the uncertainties (which are the most important, does that differ between the different conditions is needed). All I understand to this point is that the uncertainties were used to generate the predictions in Figure 3, but not what the main uncertainties are.

L325-329. Using the model names introduced above would help to key this discussion into the previous sections and Figure 3.

L347-349. Do you mean that DIFF BIO is the better model (not UCA BIO)? The NSE and R2 in the table would indicate that was the case.

L355. ‘drought’ and ‘pipe’ comments again.

L361-368. This is written as if the catchment is really considered to have been in drought (rather than just the flows being artificially low). Are these findings what you would expect if the catchment was relatively wet and just the flows were impacted?

Conclusions

As with the Introduction, there is no broad overview here. As it is the Conclusions are understated. A final paragraph or two explaining the broader relevance of this study or outlining what you think you have done which is novel is needed. Be more specific how you contribute to the “greater goal” of understanding these processes. That would make the study more interesting and impactful to a wider audience. Perhaps speculation on what this means for DOC if flow regimes change would be interesting and useful here.