the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 May 2024
Nitrate and Water Isotopes as Tools to Resolve Nitrate Transit Times in a Mixed Land Use Catchment
Abstract. To understand the transport and fate of nitrate in catchments and its potential hazardous impact on ecosystems, knowledge about transit times (TT) and age of nitrate is needed. To add to that knowledge, we analyzed a 5-year low-frequency dataset followed by a 3-year high-frequency data set of water and nitrate isotopic signatures from a 11.5 km2 headwater catchment with mixed land use within the Northern lowlands of the Harz mountains in Germany. For the first time, a combination of water and nitrate isotope data was used to investigate nitrate age and transport and their relation to water transit times. To do so, the numerical model tran-SAS based on Storage Age Selection (SAS) functions was extended using biogeochemical equations describing nitrate turnover processes to model nitrification and denitrification dynamics along with the age composition of discharge fluxes. The analysis revealed a temporally varying offset between nitrate and water median transit times, with a larger offset at the beginning of wet periods due to higher proportions of young nitrate that is released more quickly with increasing discharge compared to water with larger transit times. Our findings of the varying offset between water and nitrate transit times underline the importance of analyses of solute transport and transformation in the light of projected more frequent hydrological extremes (droughts and floods) under future climate conditions.
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Status: open (until 04 Aug 2024)
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RC1: 'Comment on hess-2024-109', Anonymous Referee #1, 28 Jun 2024
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Review of “Nitrate and Water Isotopes as Tools to Resolve Nitrate Transit Times in a Mixed Land Use Catchment” by Radtke et al.
This paper presents a joint analysis of travel times of water (derived from SAS-based transit time modelling on water isotopes) and nitrate (based on the same SAS model, but with included biogeochemical reactions). This is an important topic and a “next step” in research on combining (conservative) transport of water with non-conservative solute transport. Of course, this is a tricky task, and the study is only able to propose some solutions, but it is nonetheless an important step in the right direction.
I think the paper is a very useful contribution to the community. It is mostly well written, but clarity could be improved in a few sections. With some better explanations, this might become a key paper at some point, and I am providing some suggestions that may help with this.
Specific comments:
Abstract: the mention is of 5 years of low frequency data followed by 3 years of high frequency data. All figures only show five years of data. This is somewhat inconsistent and should be clarified.
Lines 52-63: these are very relevant processes, and it might be helpful for readers if this could be illustrated in a schematic plot.
Line 71: the authors mention here that mobilisation as nitrate “can take up to decades”. This is an important point, and I am glad it is being made. But it is not entirely clear to me how they constrain these inputs and their enormous temporal variability and uncertainty in their model (this is not really explained in the methods – at least not prominently, so I missed it if it was). I would encourage the authors to explain this more clearly in their methods.
Line 80: hypothesis (i): can the authors add an explanation of why they expect this temporal offset?
Line 107: please add units to the kf values.
Lines 107/108: Can you provide information on the depth of the storage/aquitard? This would be helpful information for some of the conclusions drawn later, regarding the contributions from shallow and deeper storage.
Line 118: I find it a bit confusing and distracting that delta15_N values are reported and presented in great detail here, after it is explained in the introduction that it’s the oxygen isotopes of nitrate that are of relevance. Consider adding a note here that only oxygen isotopes are analysed.
Section 2.2: The separate description of the water isotope and nitrate sampling is quite confusing here. It appears that these span the same periods, but it is not explained if these are actually the same samples or samples collected at totally different times during these intervals. If the latter is the case, then it would be useful to present a flow duration curve, to illustrate that similar conditions are covered by the two different sample sets. Else this would question comparability between the water and nitrate travel time analyses.
Figure 2: This figure tries to convey a lot of information, and I’m unsure it helps. I am finding it slightly confusing at least and would recommend simplifying it. I’m not sure what the incorporation of solutes into the “legend” on the left means (e.g. for “leaching” or “discharge”). Does it imply that these compounds are measured or modelled in these fluxes? It would also be helpful if it was explained which of these fluxes and concentrations are measured, and which ones are modelled. Including the relevant equations from the routing storage is likely more confusing than helpful.
Section 2.4 (Model setup): I think it is very useful that the authors are trying to provide a good explanation of the used SAS approach. However, to be truly beneficial, a bit more information needs to be added. For example, it is not explained anywhere what the backward transit time distribution is (line 165) or what the implications and definitions of time variant and time invariant SAS are (line 172). The variables T and deltat are also not explained (equations 2+3), and neither is the wetness index (line 177), or the parameters kQ1, kQ2, kL1, and kL2 (lines 180, 181).
Line 219: “as a time stamp” – this would require that every point in time has a unique isotope signal, but this isn’t the case as the isotope signal is variable over time. Instead, a better wording may be “used as a finger print” or similar.
Lines 217-219: This is a really helpful explanation, but I would recommend bringing it much earlier to limit confusion that arises before this point regarding the methods and sampling.
Lines 224-258 (and lines 505-506): it may well be true that an average value of 23.5 permil is adequate for most approaches. However, the isotope approach is mostly based on isotope fluctuations, hence employing an average value has the potential to artificially dampen the fluctuations, resulting in an overestimation of the age. I would encourage the authors to employ a random sampling approach from a normal (?) distribution of values within this range to evaluate the effect of the soil isotopic signature. At the very least, the possible range of variability in the soil gas should be placed in context with the range of variability in the isotopic signal of the leaching water, to evaluate how significant the potential damping effect of using the average value is.
Line 250: Could the authors use a different letter to define the denitrification rate to avoid confusion with the parameter k introduced in line 174?
Table 1: the entries in the parameter column should be formatted consistently, with consistent use of subscripts etc.
Figure 3: why are d15N presented when they aren’t relevant for the analysis (see above)? And what is the cause of the high variability in isotope values of N mid 2019? These seem to be the same four outlier points in Figure 4. Is it likely that these samples aren’t useful/something went wrong during the analysis/handling?
Figure 3: Please add in the caption that these are measured values.
Line 315-317: It would be helpful if an explanation was added here of how a case of denitrification would be identified based on the monthly distributions.
Line 322: please add the statistical significance (p-value) of the Pearson correlation coefficient.
Figure 6: which of the boxes indicates the “measured range of d18o-N in streamwater”? The first one that is labelled NO3? And which ones are calculated? And how? Do the authors mean modelled? Please clarify.
Line 345: change to “range in nitrate in streamflow”.
Lines 341-654: This is a lot of text to explain that denitrification likely didn’t play a major role at this site. While this is an important explanation, I wonder if it could be communicated in a more concise way.
Figure 7b: I would encourage that the authors add the R2 value to panel b (like they did in panel a), to support the statement they make in the text, i.e. that the relationship is essentially non-existent.
Line 373: “highest proportion of young water”. What does this refer to? Do the authors mean young water fractions? I found no place where they were actually calculated, even though he sinusoidal fit in Figure 8 implies that they may have been thinking along these lines. If so, then I would caution against them in this context, because they are not the right measure to establish short-term changes.
Lines 384-385, and 458-460: I’m not certain that the findings here are logical. After a very dry summer like 2018, the older water stores should have been depleted, meaning there would be expected to be MORE young water in streamflow after a precipitation event following a dry spell. There is also higher variability in streamwater isotopes in 2019, supporting shorter travel times (less damping means less mixing). For nitrate, I would expect the opposite: Nitrate gets added to the surface even while it’s dry, hence the nitrate clock starts ticking, resulting in older (compared to water) nitrate ages after droughts. The findings here are opposite to this, and the explanation isn’t completely coherent.
Technical comments:
Line 63: I think the authors mean “along its flow path” rather than “on its flow path”.
Line 65: delta symbols are not displaying correctly here.
Line 193: should be “through” rather than “leaving” the routing.
Line 284: delta symbols are not displaying correctly here.
Citation: https://doi.org/10.5194/hess-2024-109-RC1
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