Influence of Floodplain Forest Structure on Overbank Sediment and Phosphorus Deposition in an Agriculturally Dominated Watershed in Iowa, USA

Round 1

Reviewer 1 Report

Dear Authors

I have reviewed the manuscript entitled "Influence of Floodplain Forest Structure on Overbank Sediment and Phosphorus Deposition in Agriculturally-Dominated Watersheds, Iowa, USA", submitted for consideration for publication in the Hydrology (MDPI) Journal. The research work is quite original, engaging, and has some established merit. I do have a quite positive opinion on the work carried out.

I have laid out some minor comments to the manuscript that will guide the authors in improving the manuscript. Please consider adressing these comments, point-by-point.

Comments for author File: Comments.pdf

The English is fine, moderate editing is required.

Author Response

Thank you for your feedback. See our responses in the attached document.

Author Response File: Author Response.docx

Reviewer 2 Report

This is an interesting and potentially useful paper, which reports on a study investigating the role of vegetation in enhancing floodplain sediment and nutrient deposition. The context provided was clear and the methods were mostly well described. The overall quality of the presentation was high.

Most of our understanding of the role vegetation plays in sedimentation processes comes from laboratory or small scale studies, so to see this investigated at the landscape scale was welcome. Of course, the number of confounding variables increases at this scale, adding ambiguity to the results: this was reflected in the findings, and is the basis for the one significant issue I have with this paper.

Where this paper needs strengthening is in increasing the degree to which it considers the geomorphology of the floodplain. As Figure 6 shows, the associations between vegetation and sedimentation are relatively weak - this is largely because there are lots of factors controlling floodplain sedimentation, some working together, some working against each other. Existing research, which is hardly acknowledged in this paper, has shown that irrespective of vegetation differences, sedimentation increases where the grain size settling out of suspension is coarser and the floodplain water column is deeper. Grain size variation is not described in this paper, and the only proxy for water depth is the fairly crude use of three different recurrence intervals. Not taking into account grain szie and only using the three proxy depths as opposed to estimating the actual water depth associated with each location means these factors cannot be factored into the analysis. It is impossible to say whether these geomorphological/hydrological factors explain the variations seen, or indeed, whether they're bigger controls on sedimentation than the vegetation differences that exist between the sites. Alongside this lack of consideration of these factors is an absence of literature considering them. I was surprised to read a paper on floodplain sedimentation and pollutants which failed to cite any of the vast number of papers written by Des Walling and co-workers - incorporating this wide base of existing geomorphological knowledge would have enabled to authors to give the research more context and include (or eliminate) a range of relevant factors beyond the ones described in the study as it stands. (The authors might point out that much of this research is on un-forested floodplains: that's the point - by not considering vegetation, this existing research has elucicated what the other, non-vegetation related factors are. These factors need including as a baseline before vegetation is added back into the analysis as well).

I'm unsure how the authors go about broadening the perspective of this paper (although I'm sure it needs doing). Bringing in the factors mention above means either rethinking the analysis (and its data requirements) or rethinking the way they interpret the data. The extent to which this is possible is for the editors and authors to decide.

Other issues I identifed as I read the paper are detailed below:

In the methods, how were the 2, 5, and 10 year flood identified, and how were their extents mapped across the floodplain? This is not made clear. Very late in the paper (line 435) it is revealed that LiDAR data was used, but this doesn't explain how the depths were identified or how the extents were extrapolated across the landscape.

Line 156: the units are given in inches here, in a paper which otherwise uses metric/S.I. units. It would be better to be consistent if possible.

Line 221: The way this sentence is phrased is odd. "Though the distribution was similar ... there was no significant difference." I think it's the use of the word "Though" - it implies that there's a something discordant about the finding, when it is in fact perfectly normal for there to be no significant difference between things that are similar to one another.

The paragraph beginning on line 264 refers to Figure 5 throughout. I think it means Figure 6. On Figure 6, the meaning of the grey shaded areas is not indicated (it's a significance or error margin, but which, and of what value, is not made clear).

Line 357: the argument made here is weak. To suggest that the study by Nanson & Beach (1977) came to a different conclusion because of "vastly different hydrology" doesn't stack up, and sounds like a dismissal without evdience. Once water is on the floodplain, sediment should settle out, irrespective of anything else. The difference must be either methodological (Nanson & Beach were measuring something different), or related to differences in floodplain topography or vegetation structure. One difference that may be relevant is that Nanson & Beach were more interested in floodplain age, and their key finding was that sedimentation is rapid on "new" floodplains and drops off as the floodplain ages. This will be at least partly due to changes in topography, and the fact that vegetation is denser on the older floodplains (as a function of vegetation having more time to grow) may be largely coincidental (or at least, less relevant).

Line 371: there are some words missing here.

The standard of English is fine. One issue I had, which may relate to an American English usage I'm unfamilar with was the absence of a preposition when the authors use the word contrast or variations of it (contrasts, contrasted). For instance, on line 356 the authors say, "Forest inventory correlation results contrast that of Nanson and Beach". Reading this without a preposition (...contrast with that of Nanson...) makes it sound awkward and incomplete. A contrast is a comparison: an object has to be compared with (or to) something else. I can't find any grammatical guidance that suggests using the word contrast in this way without a preposition is acceptable in any form of standard written English. In total, the authors use contrast or its variants eight times, so by the end of the paper this was a recurrent and noticable error.

Author Response

Thank you for your feedback. See our response in the attached document.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The revisions address the issues I raised and the paper is now satisfactory.

One (very) minor issue is that in correcting the English, a new error was introduced on lines 389 and 406. In both locations, instead of saying "contrasted with" it says "contrast withed".

Author Response

Thank you for catching those mistakes! We greatly appreciate it. We have fixed both errors.