|Review of manuscript “Investigating suspended sediment dynamics in contrasting agricultural catchments using ex situ turbidity-based suspended sediment monitoring” by Sherriff et al. for Hydrology and Earth System Sciences.|
This manuscript is a revision of a manuscript that I have previously reviewed. My comments have been adequately addressed in this revised manuscript and I appreciate the authors’ efforts in making the text more exacting with richer details on the methodology and analysis. This manuscript describes (1) a comparison of estimates of suspended sediment concentrations and yields from turbidity deployed in a stream channel and located in a stream-side kiosk where water and sediment samples are pumped from the stream for analysis and (2) differences in suspended sediment yields from agricultural catchments in Ireland of different soil drainage classes and agricultural land uses. This manuscript contributes to better understanding sediment transport from agricultural catchments in Ireland with a particular focus on informing landscape (sediment) management. This manuscript fits within the scope of Hydrology and Earth System Sciences and is generally well-written and easy to follow. I recommend the manuscript be accepted with a few minor comments for the authors to consider.
I mention my more major comments here first and then provide detailed manuscript, table, and figure comments below.
(1) In reading this paper (specifically in relation to L 353-357 and L 457-460) one might conclude that to reduce sediment loading from agricultural fields you can simply increase subsurface drainage. While this is accurate, the increased drainage may shift sediment generation from fields to river banks. In many agriculturally dominated catchments in the U.S., most of the sediment generation is now from stream banks and no longer from upland fields (see for example Belmont et al., 2011; Massoudieh et al., 2013). As a watershed manager, if the goal is to reduce sediment concentrations in the stream, then reducing agricultural field erosion at the expense of increased drainage and associated increased sediment generation from in-channel sources is counterproductive. This is an important issue of scale not considered in your study, but might be important to mention as your study has management implications and any management strategies that come out of your work should consider this scale issue.
(2) In the text and tables there are instances where you report more than 3 significant figures. For instance at line 23 you report 5 significant figures in your load numbers. It is easier to read 12,800 t than 12,828 t. But more importantly I suggest the authors consider how accurately they think they can estimate their reported values given all the uncertainties and assess whether the number of significant figures reported throughout is warranted.
Detailed manuscript comments (Line, L):
At several places in the text, words are bold-faced. I think this is unintentional. These include: L 24: “readings”, L 142: “01”, L 176: “Fig. 2”, L 218: Text in parenthesis, L 262: “Fig. 3”, L 296: end of the sentence, L 308: “Fig. 6”, L 313: “Fig. 7..”, L 715: “dashed lines”, and several other places throughout the text. I stop at line 323 but there are more occurrences beyond this line, please check.
L 234: The average change in SSC during transect sampling is reported in absolute SSC. In addition to or instead of absolute SSC, it would be nice to see the percentage change in SSC relative to the measured SSC. For instance, did SSC change by 82 mg/L (81.94 mg/L in text, think about significant figures) when the SSC was 100 mg/L or 1000 mg/L. This would tell the reader if the change in SSC was typically 1% of the reported SSC or 10%, a much more useful metric for characterizing uncertainty.
L 253: Elsewhere in the text the variable “n” is used for number of samples. Here “n” is used again to denote a turbidity threshold. To clarify notation I suggest changing “n” here to “T’” or “T*” and at line 250 add something like, “and split linear at a T value of T’…”. Also, typical mathematical notation is to report the equation first and then the condition, such as “SSC=aT, where T<T’…”.
L 264: The occurrence of local in-stream debris interference is not really a “trend” suggest changing to “occurrence” or “effect”.
L 265-269: Great characterization of the resulting effect of missing data on sediment loads!
L 313: The reference should be to Fig. 6 c and d not 7, based on context and that figure 7 only has 2 panels.
L 330: The beginning of this sentence suggests both sites have reported mean SSC up to 17 mg/L. The way it is currently written tries to do two things, say the SSC is highest in these two basins and that those values are upwards of 17 mg/L. But really Grassland B only goes up to 14 mg/L. Suggest changing to something like, “The highest mean SSCs were recorded at Grassland B (up to 14 mg/L) and Arable B (up to 17 mg/L) and …”.
L 345: Here you reference 4 studies with “existing data on SSY in Ireland” but in figure 8 I only see 2 data points for Ireland. Without looking in detail at these studies, I wonder why at least 4 data point are not shown in figure 8. Or if some of these studies report the same data then only 2 references are needed here.
L 365: Grassland B only exceeds its target value of 40 t/km2/yr in 2009, not in 2012 and Arable B only exceeds its target value of 40 t/km2/yr in 2012, not in 2011.
L 397: The average annual inter-catchment SSY of Grassland B is 24.7 t/km2/yr and for Arable B is 24.4 t/km2/yr (see line 340). It is listed here for both as 24.0 t/km2/yr. Here is where significant figures are important because 24.0 is not the same as 24.4 or 24.7. Maybe you can say 24 without the extra decimal place but then Grassland B has a yield of 25. I suggest here changing the 24.0 to “about 24” or be more exacting for both sites.
L 398: “catchment” should be “catchments”.
Table comments (Table, T):
T 1: The land use for Arable B is 24% arable and 48% (29+19) grazing. Should this then be called grassland D rather than arable B because it is predominantly grassland for grazing?
T 4: Vertical lines separating the columns for the different sites would be helpful. Particularly because some sites have 4 years of data and others have 3 years.
Figure comments (Figure, F):
F 9: Related to comment on table 1, Arable B is 24% arable and 48% grazing yet it is shown near 50/50 arable/grazing. This makes me wonder if some fraction of spring crops was not reported in table 1. Otherwise the dot for Arable B should be moved to be consistent with the values reported in table 1.
Belmont, P., et al. (2011), Large shift in source of fine sediment in the Upper Mississippi River, Environ. Sci. Technol., 45, 8804–8810, doi:10.1021/es2019109.
Massoudieh, A., A. Gellis, W. S. Banks, and M. E. Wieczorek (2013), Suspended sediment source apportionment in Chesapeake Bay watershed using Bayesian chemical mass balance receptor modeling, Hydrol. Process., 27, 3363–3374, doi: 10.1002/hyp.9429.