Comment on hess-2021-536

The Abstract provides a good summary of the paper. However, as with the paper as a whole, provide one or two sentences at the end which explain why this is important.


Introduction
As noted above, the first paragraph of the introduction introduces some important general issues that need to be addressed throughout the paper, especially at the end.
The referencing in parts of the introduction can be improved. For example, several of the references in L33-45 are reports and Bieroza et al., 2018;D'Amario et al., 2021 deal with techniques. Try to add a few key papers that discuss these general issues. L46-47. Not clear what you mean here.
L50-54. Some of the concepts here could also use better referencing. Evans & Davies (1998: Water Resources Research, 34, 129-137) and Walling and Foster (1975: Journal of Hydrology, 26, 237-244) present some of the framework for these studies. Lloyd et al. (2016: Hydrology and Earth Systems Sciences, 20, 625-632) develop a framework for characterising hysteresis and also reference other literature. The recent paper by Knapp et al. (2020: Hydrology and Earth System Sciences, 24, 2561-2576 (2010: Soil Sci Plant Nutr, 56, 72-85). Other papers (eg Knapp et al., 2020) apply similar methodology to other solutes (including other nutrients such as P). The way that this is written implies that there is little work being done in this space, when there is a large body of work that needs acknowledging. L70-85. The objectives are very specific and parochial. While they describe what you have done and why it is locally important, can you reframe them so that they have a more general focus (ie understanding differences in the behaviour of TKN and NO3 that we do not understand in general) L73-74. The studies cannot become increasingly rare -do you mean that there have not been many thus far? L74-76. If understanding TKN is important, you could mention it in the main part of the introduction with a few more details rather than in your objectives section. It is a bit lost here.

Materials and Methods
The description of the Study Site (Section 2.1) is comprehensive.
L105-108. Is this the meteorology of the site or of the region (not sure if there are 10045 stations, which seems a lot, or if that is the station identifier and the sentence is slightly misworded).
L108. What do you mean by "pluvial" in this context? L113. How was the hydrological year defined -specify the date / month when it starts. L125-128. The preservation and storage of samples presumably is only after they are retrieved from the autosampler. How much of a delay between collection and storage was there and does this have an impact (I presume not).
L125-132. Minor point, but here and elsewhere be consistent with specifying valences or not.
L125-132. You should report the precision and lower detection limits of the analyses.
L135-139. For clarity, can you include a figure of a typical event showing rainfall, streamflow, and concentration data. That would help visualise the data and interpretation of events etc.
L140-151. These is some repetition here as you say that there are three groups of variables, describe broadly what they are, and then explain that in more detail. Some of these variables do not seem to have been explained -how did you calculate KE and I presume that WY is Q / catchment area? Some of the parameters associated with the Q-C loops would be clearer if they were on a figure (previous comment).
L145-146. Qmax-Qb will not give you a % L151. Metal? L154-159. There are probably better references. Yu & Schwartz (1999, Hydrol. Process., 13, 191-209) has an early general explanation. For these general statements, try to quote some of the early papers that develop the techniques or review-style papers.
L154-168. The use of EC in this way makes a range of assumptions (e.g., that pre-rainfall EC represents water from within the catchment, not a mix of that water and recent prior rainfall; that we know the EC of surface runoff; that the contrast in rainfall and catchment water EC is high). These are discussed in many papers that have used that technique (e.g.  , 2017, Hydrol. Process., 31, 4705-4718). Some comments are needed here if you are going to use that technique. Actually, I am not sure whether you really need this parameter -there is a little discussion in section 3.3 but not much else (?). Given the considerable uncertainties in using an EC mass balance, you probably could safely omit it.
Did you consider looking at the EC hysteresis? Notwithstanding the comments above, the EC hysteresis may be useful in understanding hydrology even if Qb is not calculated. In particular, if subsurface flow is important then that may show up.
L172-179. This could be more tightly worded. How close is "close to the peak" and does it vary with Q? Rotational directions can probably be defined by 5 samples (although a figure of eight might be hard to see), but what about the concentration and discharge differences?
L203 Linear not lineal L206-210. This just says that these methods were used. In particular is there any reason that the Redundancy Analysis offers more than the correlations? Any details that we should know? Figure 2. Without some more details, it is difficult to interpret this diagram. I presume a & b have similar rotation but different slopes? How close to the start of the events are the first points and what time periods do these events depict (do the points within each graph and between graphs represent the same timesteps). More could be done to make it clear what is going on here.

Results
L223-233. There are a lot of generalities here ("highs", "lows" etc) and Table 1 only shows summary statistics. Add a few more details to the text to explain and consider adding a Supplement Table with more details in it.
L235. It would be worth showing the TKN data on Fig. 2 also to illustrate this point.
L240-242. In the methods you introduced the +/-10% cut off for neutral loops, so you just use that here from the outset. So just note that 13% have DR values within the 10% limit and are classified as neutral. As written, it is not very clear how many of the loops you consider to be what type and whether you are being consistent in definitions. . Similar comments about the +/-10% apply here and again the text is not very clear.

L257
. Not sure what "response controls" means.
L258-261. You should discuss that you are using a Pearson correlation matrix and any relevant derails in the methods.

Discussion
This is a long section and a brief introduction at the start guiding the reader through what you intend to discuss would be good. There is also a tendency to interpret your data by reference to other studies (so on L304-305 you explain the results of others and then discuss your interpretations in the next few sentences). It would be preferable to discuss your interpretations and justify them using your data and then note whether they are similar or different to those elsewhere. Otherwise it looks like you are trying to fit your data into an existing framework, which I don't think is the case.
L293. Section heading is confusing L294-303. This seems to be off topic with the title of this section. Does it belong here? Perhaps it go be fore the sectiion heading and act as a general lead-in? L304. What is an "accretion pattern"? L310-321. There is a lot of speculation here (deep drainage, high NO3 in soils, rapid transport). Can you provide justification -there seems to be a number of prior studies on this area that may help.
L324-L330. In section 2, you say that overland flow is unusual, so how can that be? L334-350. Similar to the comments above, there is a lot of speculation here. While studies elsewhere may help with that, is there evidence from this catchment that you could use to firm these ideas up.

Discussion (Conclusions?)
As noted at the outset, this is where you should provide more of a global context. There are parts of Section 4 (e.g. L351-365 and L366-371) that are more general and which could form the basis of the Conclusions. Once a reader has gotten the basic idea of a paper, they may skip to the end to see what broader insights there are. This makes this last section important for communicating your ideas.