A Soil Moisture Profile Conceptual Framework to Identify Water Availability and Recovery in Green Stormwater Infrastructure

Round 1

Reviewer 1 Report

Paper Summary: This manuscript describes the development of a method (i.e., framework) to assess soil moisture profile data for the purpose of identifying saturation, infiltration, and evapotranspiration (ET) including the range of field capacity. The framework can be used to calculate infiltration rates and ET rates from soil moisture data.

General Comments:

S1.        Overall, I think the manuscript is well written and organized. The authors provide an assessment of soil moisture data that is easy to follow, apply to other data, and interpret.

S2.        Identification of Points A, B, C, F”, and F are straightforward, assuming the readers’ soil moisture data reflects similar shape to the authors’. The shape of soil moisture data I have analyzed is similar enough that I could locate these points. Points D and E, however, may be more challenging because the authors’ method for identifying these points appears to be limited to lines 166-170: “From observation of soil moisture recession curves, there are two inflection points that define a change in slope in this curve (Figure 4). The first, relatively fast recession decreases to the inflection point D (e.g., 0.401 V/V in Figure 4) and then decreases more slowly to the inflection point E (e.g., 0.365 V/V in Figure 4), and finally decreases at a relatively slow and constant rate until the start of the next storm event (point F, e.g., 0.289 V/V in Figure 4).” Only Figure 4 shows soil moisture data with Points D and E located. The readers may benefit from 1) more description of how to locate these points (D and E), 2) more examples of soil moisture data with these points located, or 3) both.

S3.        I noted a few typos that could revised before publication, which are listed below as specific comments.

Specific Comments:

S1.        Line 20: “The outputs the framework provides was validated and compared to the values that obtained…” may be improved as, “The outputs the framework provides were validated and compared to values obtained…” Please consider revising.

S2.        Line 202: “For specific-system behavior with soils moisture sensors…” I think ‘soils’ can be singular here. Please consider revising.

S3.        Line 356 (labeled Figure 8): This figure needs a vertical axis label. Please consider revising.

S4.        Line 356 (labeled Figure 8): Starting with this figure, the figure numbers in the captions are non-sequential. The text references appear to be correct, but the captions are misnumbered. Please consider revising. Once corrected, please double check the text references.

S5.        Line 428: “…enough for the site recovered between tests similar…” Is this intended to read, “…enough for the site to recover between tests similar…” ? Please consider revising.

S6.        Line 445 and beyond: “…by McKane (2020) these framework…” Several of the internal citations beyond line 445 are conventional citations with first author last name and year. Please consider revising with [ ] and numbered citations for consistency.

S7.        Line 491: This paragraph does not appear to be indented. Please consider revising.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Overall, this is an interesting conceptual framework derived from a robust field data set, which the author makes available by request, an added benefit of their work and paper publication. The applications introduced regarding saturation and de-saturation times regarding GSI functionality carry practical application. Using sensor technology to inform performance, management and maintenance of GSI is broadly applicable. The distinction of infiltration and ET dominated de-saturation defined in the contextual framework is also interesting.

Comments as follows: 

Abstract

Line 11 – Rephrase suggestion. The recovery of soil void space to sustain through infiltration and evapotranspiration processes within green stormwater infrastructure (GSI) is key to continued hydrologic function.

 ET is not supported by recovered soil void space it actually decreases…

Line 16 - I think it would be good to add commas between the points to clearly distinguish between them i.e. A, B, C, D, E, F, F”

Line 20 - I believe there needs to be some change in the description of the work in terms of development at one site and validation at the other site or further substantiation (in methods/results/discussion) of how one was used to develop and the other for validation. I'm not convinced that anything is “validated” at least in the conventional sense that comes to my mind in terms of model validation. Here there seems to be a conceptual framework that is observed at two locations, which is interesting. However, calling it “validation” seems strange in this context (at least as currently presented). 

1.       Introduction:

Overall – Would benefit from clear statement of objectives/hypotheses. Also, I wonder if there are other conceptual frameworks from soil sciences or otherwise and how those relate to this framework. Would you add a few references to substantiate the novelty/innovativeness of this work. 

Line 44 – full – Remove the word full, to me this implies reduction to hygroscopic point which rarely, if ever occurs.

Line 60 – Add saturating, during a saturating storm event

Line 61 – For clarity, add comma after, “After the storm event,”

 

2.       Site Description:  

79 – 81 – Justify why bioswale was used for "development" and the bioinfiltration used for "validation"

Line 96 – Explain why, “the upstream 60 cm depth sensor data was not able to be used for this work.”

Line 100 – Describe how saturated hydraulic conductivity was measured. Also, if that work is part of this study, results i.e. the actual rates of hydraulic conductivity would best be included in the results section.

General comment: Please report the catchment area size and catchment:GSI ratio for each site.

 

 

3.       Soil Moisture Conceptual Framework Development:

Please report a percentage of storms that met the criteria for the framework development.

Figure 3 – Very nice graphic which clearly exemplifies the selection criteria.

Line 151 – Explain the 0.304 V/V, is this volumetric or voltage from sensor?

General – Explain more about how does this translate to tool or otherwise usable broadly, in discussion? Line 177 is a start…though should be in discussion.

General – Were points averaged? How is data analyzed between the two sensors. Why two sensors?

Figure 5 – Please correct figure. Field capacity seems to be pointing to two locations on the chart.

Line 228 – Good statement of assumption, “The root zone 228 was considered to reach the deepest soil moisture depth.” Would be further corroborated with statement of typical rooting depth of plant species in the sites around the soil moisture sensors.

Figure 6 - Not understanding what’s going on with Pe in this graph. Why the dip and return? Please provide more explicit explanation in text.

 

4.       Results and Discussion

Figure 8 - Was suction measured? Or is the “measured” an estimate of soil suction based on the volumetric water content and soil type….? Please define more explicitly.

Line 364 – Elaborate further here. Include details of drainage area size, depths to native soil (and characteristic of that soil if available).

Table 1 – The “all seasons” average seems high. Please report the number of events per season that were analyzed and use a weighted average, if appropriate. I wonder if there were more events during the higher ET months skewing the average or if this is “real” and a result of the established vegetation in the site as mentioned elsewhere in the paper.  

Line 459 – Literature should be introduced in the introduction section and referenced here. Additional reference: DiGiovanni (2013) Evapotranspiration from Urban Green Spaces in a Northeast United States City - Found rates of ET measured by weighing lysimeter from two bioinfiltration sites in NYC for over three years to be 0.3 and 0.18 cm/day on average, with ranges of 0 – 0.97 cm/day and 0 – 0.93 cm/day, respectively.

Conclusion

Line 482 – I think it would be good to add commas between the points to clearly distinguish between them i.e. A, B, C, D, E, F, F”

Line 484 – Suggestion, try to incorporate these terms, “pre-storm (point A), post-storm (point F), satura-484 tion (point B and C), field capacity (point D and E) and soil moisture recession (F’ and F”).” I find this quite interesting and the use throughout more may lead to wider adoption.

Line 491 – Please explain or re-word, I don’t understand how the framework estimated? Do you mean you observed these based on the framework definitions?

Line 501 forward – I think this is a really nice way to wrap up this practical paper. Please elaborate further on how exactly the framework might be used for practical application to inform performance, management and maintenance of GSI. This is really interesting and key part of your work.

Overall, very well written and clearly presented. 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Hi, you did a good job and wrote an interesting paper. I am a bit missing more information about the swales (dimensions) and connected area to estimate the inflow. Did you measure the inflow separately?

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 4 Report

Thank you for your submission to the journal, I enjoyed reading this as I am really interested in urban GI. I have a few comments and recommendations. I have also attached a PDF with some yellow highlights and one comment in the later sections. The first few pages did not have any major issues.

1. Your conceptual model of soil moisture which underpins this work (Fig 5) is fairly sound but I am not sure why you have the final point labelled  as F but the point before as F", why?. If the post-storm soil moisture is going to fall back down to the pre-event value then would something like A' be better than F as the last point? In one instance you also refer to a point labelled F', which I have highlighted in yellow in the attached PDF.

2. One problem with the conceptual model may be the ET calculations, if there is still some drainage from the media then you may be overestimating ET? (Eq. 3) You are this relying on field capacity (see below) to mean what it is defined as, i.e. the water content where soil water is held in the matrix by gravity, so below this the water has to be forcibly removed from the pores by ET.

3. In general you do not seem very aware of the classical literature on soil moisture analysis or the literature on soil moisture as a measured variable (often remote sensing related). Soil moisture is measured by a "Probe" essentially by conversion of an electrical signal of some form into a moisture content value? These are often called TDR or WCR probes. perhaps (L85) you could explain what type a Hydroprobe is?

There is a large amount of heterogeneity found in soil especially topsoils where there are vegetation planted (e.g. roots cause cracking and preferential flow pathways to form). Two soil moisture sensors close by can give very different readings due to this (L314 does elude to the fact that soil moisture sensor readings are very inexact!). Soil moisture probes at a deeper depth typically exhibit a flatter response as the drainable porosity is less (compare to storativity in an aquifer).  L286 also mentions this. Unless the plants have deep roots reaching down to 60/65cm then ET from this layer will be very small indeed.

Also, Fig 8, it's not uncommon to see a poor fit to the Van Genuchten equation from this type of data, as you have found for the bioswale. Perhaps you could explain more about how you fitted the curve? Was it automated or manual? When obtaining the wet range of measured points presumably the bioswale was holding the water for longer under high tensions (towards 10m suction head) than the VG curve predicted.

L399, agree, you may have a saturated wedge developing at the downstream site where the deeper soil is saturated for longer.  The only design option to improve this would be an underdrain (as fitted) L104 but these have maintenance issues (access to unblock them etc) so you are probably better off allowing natural drainage as with the biofiltration site. Perhaps add a sentence to contrast the behaviour of the deeper layers in both sites, if you haven't done so? I like the photos in Fig. 2 the bioinfiltration site looks quite well designed. The bioswale looks like it may have maintenance issues regarding the drain in the middle photo? Is that an overflow?

4. I think one of the key findings comes from Fig 10, which is the amount of time the soil moisture takes to drop back down after events. This is key for the design of green infrastructure if the climate has frequent rainfall events? We need GI that can drain before the next event starts to fill it again. The time taken between points A and B seems quite short for these features presumably due to their small size and tendency for ponding on the surface soil to form. How could you lengthen the time between A and B to improve the performance of the GI for really large storms? Could you find a way of increasing the surface drainage rate of the GI?. Also, in the study region (PA) do you get many low-intensity but very long storms when the GI may take longer to fill , and the drainage rate starts to become in equilibrium with the rainfall rate?

5. Instead of "cm of water" you really should refer to "cm suction" which is more correct and is used generally by the literature? The first occurrence on L342-343 is okay but after that please change it?

6. Again, my text book (Shaw) definition of "Field Capacity" is the water content of the soil after the saturated soil has drained to equilibrium by gravity usually after a few days. I am not sure where the range   61-337cm for the associated suction has come from? I did find it in this paper so presumably it is correct as Stovin's GI research work is usually sound.

Peng, Zhangjie, Colin Smith, and Virginia Stovin. "The importance of unsaturated hydraulic conductivity measurements for green roof detention modelling." Journal of Hydrology 590 (2020): 125273.

Comments for author File: Comments.pdf

It's mostly good quality English. The authors have written a clear and concise article, it was easy to read.

Author Response

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Author Response File: Author Response.docx

Reviewer 5 Report

The manuscript concerns the important issue of the soil moisture profile conceptual framework to identify water availability and recovery in green stormwater infrastructure. A fresh conceptual model for understanding soil moisture behavior was created to determine the availability of soil moisture before, during, and after storms. This model utilizes soil moisture measurements and identifies seven crucial points (ABCDEFF”) that explain how the void space in the soil, which holds water, recovers after a storm passes through a Green Stormwater Infrastructure (GSI). This model also calculates various aspects of subsurface hydrology in a GSI, such as the average soil moisture, duration of saturation, soil moisture reduction, time taken for desaturation, rates of water infiltration, and rates of evapotranspiration (ET). To confirm its accuracy, the model's results were compared to values obtained through conventional methods like spot field infiltration testing for infiltration, different approaches to quantify GSI ET, Soil Water Characteristics Curve for soil moisture measurements, and Simulated Runoff Test for duration of saturation/desaturation time. This comparison strongly validated the effectiveness of the conceptual framework. Remarks and comments: Lack of discussion about possible limitations of using the proposed approach. What is the added value  and novelty of the paper? Why these approaches were chosen for the analysis? This should be underlined in the general framework of the presented study. Why this approach is the best solution for performed analysis? Are there concrete steps that can be recommended and how generalizable are the findings? Can they be applied to other areas? How dependent are they to specific characteristics of the region under examination? The conclusion should be interpreted and deliver the meanings, this section should be more focused and based on the results. Please add some perspectives of future work. Furthermore, it is crucial to highlight the unique contributions and novelty of this paper. Exploring the added value of the research is necessary to emphasize its significance and distinctiveness in the field.

 

 

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 5 Report

Accept in the present form.