This manuscript evaluates the effect of bank slope on the hyporheic flow through meanders. My review concerns the revised version of the manuscript after implementing comments from two other reviewers. I agree with these reviewers that the topic is interesting and relevant to the scientific community. In this review of the revised manuscript, I evaluate whether the comments by the two previous reviewers have been adequately addressed and I provide additional comments.
In general, most of the comments on the original version of the manicurist by reviewers #1 and #2 have been implemented. However, a substantial number of major and minor issues have remained, which I further describe below. The line numbers refer to those in the ‘hess-2023-29-manuscript-version2.pdf’ file.
Major comments
1. Definition of residence or travel time
In this manuscript the term residence time is not defined. There are various definitions of residence time. The common definition of residence time, travel time, or transit time is the time that a parcel or particle spends in a system, subsystem, control volume. Residence time is also often defined as the average time that water spends in a system. The authors do also not explicitly define the system for which they quantify the residence time.
Interpretation of Figure 8 suggests that the authors define the residence time in the hyporheic zone as the travel time passed since the infiltration of river water into the riverbed or bank. However, this is commonly referred to as the age of the water. There is a major risk that the main messages and conclusions of the study are very confusing and hard to understand without a proper definition of these terms. For example, phrases such as ‘the spatiotemporal distribution of the residence time distribution’ (l. 386) are unintelligible if the reader uses the above traditional and common definition of residence time.
The authors should define the term residence time and the system with a proper reference to the literature.
2. Vague and sloppy formulations
The authors tend to use vague and, therefore, meaningless or, at least, incomprehensible phrases throughout the text. To refer to a few examples:
L. 95: ‘By comparing RTD with the timescale of nitrification /denitrification reactions, a meander can be classified as a source or sink of nitrate.’
L. 595: ‘Bank slope could result in longer (near the point bar) or shorter (near the cut bank) pore water travel time s throughout the flood event.’
L. 450-451: ‘… and residence (travel) times of river water in the aquifer would be overestimated or underestimated’.
L. 468: ‘…. show that bank slope can lead to both overestimated and underestimated RT area.’
L. 816: ‘Bank slope could result in longer (near the point bar) or shorter (near the cut bank) pore water travel times at various times of a throughout the flood event.’
These and similar phrases should be rephrased and clarified, so that is clear under what conditions which process or effect occurs.
Furthermore, the authors often use phrases in which they compare situations without referring explicitly to the situation with which they compare. For example, L. 33-35: ‘The impact of bank slope on residence time was more pronounced during a flood event for high transmissivity aquifer conditions’, or L. 450-451 and L. 468 (see phrases mentioned above): Overestimated or underestimated compared with what?
These and similar phrases should also be rephrased and clarified, so that is clear to what the situation is compared.
3. New discussion section 4.2 on the implications for biogeochemical reactions
This section 4.2 is new in the revised version of the manuscript in response to a comment 1.50 by reviewer #1. This new paragraph is weak and it is often hard to understand the logic of the reasoning. This becomes particularly manifest in lines 569-570 in which the authors state that a small residence time to biogeochemical time scale ratio indicate a high reaction potential for that chemical species. This counterintuitive statement should be clarified or should be supported by literature references.
Furthermore, in lines 596-599, the authors state ‘that point bars with bank slopes are more favorable for removing dissolved organic carbon and for nitrification, while cut banks with bank slope may have adverse effects on the groundwater quality near rivers.’ This statement suggests that the authors think that DOC removal and nitrification (nitrate production) has a positive effect on groundwater quality. Given the fact that increasing hyporheic exchange rates result in decreasing travel times in the aquifer, the assumption that long travel times (which occur, amongst others, in sloping point bars) promote the DOC removal and nitrification, would then imply that hyporheic exchange has an adverse effect on groundwater quality. This would be n contradiction with the common paradigm that hyporheic exchange controls water quality in a positive manner.
The authors should rewrite this entire section 4.2 with a sound logical reasoning and consider/discuss the following aspects in their argument:
- The effects of hyporheic exchange rate in addition to the associated effects of residence time on biogeochemical reactions.
- The effects of hyporheic exchange and residence time on denitrification rates, and possibly other important biochemical reactions controlling water quality.
- The effects on river water quality in addition to the effect on groundwater quality.
Other comments
L 34: ‘This decreases’: To what does “this” refer? Please rephrase and clarify.
L. 36: ‘More complex’ : more complex than what? (see also my comment about comparisons under my major comment 2)
L. 104: ‘will likely result in’ = show? (use present tense) (with regards to the entire sentence, see also my comment about comparisons under my major comment 2)
L. 115-116: What are ‘globally spreading processes’? It is hard to imagine that riverbank erosion has global consequences. Please clarify.
L. 130: analytical and numerical model solutions of what? Please clarify.
L. 130: ‘therefore’: The claim that Neglecting bank slope may have a significant influence on the prediction accuracy of HEF and RTD does not logically follow from the previous sentence. Please rephrase.
L. 141: ‘evolution’: I would avoid the term ‘evolution’ as a synonym for ‘temporal change’, since its formal definition is strictly biological. Here, but also elsewhere in the manuscript, the term is used incorrectly, as it does not even refer to a change in time. In this particular sentence, it refers to a change in in RTD in response to a change in aquifer transmissivity. Such changes only occur in model simulations and not in reality. Please use a different word for evolution in the entire manuscript.
L. 156: parameterization metrics = parameterization? (i.e. omit ‘metrics’).
L. 159: I would recommend writing out the abbreviation DGM (deformed geometry method) when used for the first time in the methods section.
L. 174 (Equation (1)): please explain M(t) directly underneath the equation (following the explanation of Y0(x)).
L. 183: Provide a reference to COMSOL.
Figure 1-3, 6-14: define t* in the main text as dimensionless time relative to the duration of the flood event. Use t* consistently without the formula in the figures.
L 264: evaluate = evaluated (use past tense)
Figure 3a: please use small letters when referring to the dimensionless times in figure 4
Figure 4: In the figure caption, refer explicitly (i.e. in numbers) to the dimension times for figure a-e.
L. 385: Please rephrase the section title, since ‘Spatiotemporal evolution of mean residence time distribution’ is nonsense. It has multiple issues. First, it is totally unclear clear what is meant by both ‘mean residence time’ and ‘distribution’ (see also my major comment about the term residence time). Second, the term ‘evoluation’ should be avoided (see my above comment on L. 141).
Figure 8: In the figure caption, it should be mentioned that this is a plan view of the model area. With regards to ‘mean residence time distribution’, see my above comment about the section title.
L. 526-528: 526 ‘Furthermore, the stronger impact of smaller bank slope angles can both extend the time over which and increase the magnitude with which younger water was discharging along the downstream meander.’ This sentence is incomprehensible. Please rephrase.
L. 570-572: The ranges of values of the first-order reaction rate constants reported by Hunter et al. (1998) refer to the different fractions of organic carbon with varying degrees of persistence and not on the spatial variation or site-specificness.
L. 572: ‘nitrite’: I assume that the authors mean nitrate here. If so, please correct.
L. 573: Investigated range of biogeochemical time scales (BTS) is not the full range of previously reported BTS’s but the overlapping part of the BTS’s. Please rephrase.
Figure 15: In the figure caption, it should be mentioned that this is a plan view of the model area. Furthermore, unlike the previous figures with a plan view, in this figure the spatial scale matters as the temporal scale not dimensionless but expressed in days.
L. 585: ’will impact’: use present tense.
L 587-588: ‘For sloping bank conditions, the reaction hotspots (areas) expanded into the aquifer’: What are ‘reaction hotspots? Moreover, this formulation seems to refer to a temporal change in the position of ‘hotspots’ under the condition of sloping bans. However, I think the authors mean that the hotspots are at a position further away from the riverbank compared to a steep bank. ‘identical’ = similar? Please rephrase.
Figure 15 scale bar of spatial scale. In contrast to the other figure, does scale matter because of the dimension of the biogeochemical time scale (in days)
L. 592: sloppy formulation. I cannot think of a mechanism that increases aquifer transmissivity, at least not at the time scales considered in this study. An increase in aquifer transmissivity can only occur in model simulations. Please rephrase.
L. 599-604: These inferences are platitudes; see also my major comment 3 about this section.
L. 648: bank slope angle
L. 649: What is reasonable river water infiltration? Please clarify or rephrase. |
