Revisiting SWAT as a Saturation-Excess Runoff Model

Round 1

Reviewer 1 Report

This study presents an improvement of SWAT for surface runoff simulation in hilly and mountainous regions. This approach shows better performance in the study domain and is easy to apply. However, there are some points to be clarified clearer and are listed below:

- Too many keywords, pick the most relevant ones, no more than 5.

- Provide the full name of SCS curve, will be useful for readers who are not familiar with SWAT.

- Also a full name for SWAT-HS.

- Please re-organize the introduction part. For example, paragraph of line 68-78, do not list the statements from other references, but only present information that have been used in the study in a more organized way. The same for the last three paragraphs in introduction.

- I would suggest to move section 3.2 and 3.3 to section 2 as it is part of methodology.

- In Figure 4, the authors demonstrate that the spatial distribution of surface runoff by SWAT-wil is very similar with SWAT-HS. However, it can only been seen that the surface runoff along the river network in two simulations are similar; runoff outside the river seem have some magnitude discrepancies (one is 0-250, one is 250-300). Please explain. Also it is suggested to have some access index to quantify the spatial similarity.

- Line 330-334, it would be intuitive to plot the time series of different flow parts, e.g., water content in surface aquifer.

- Please recheck the reference format of the journal. I'm not sure if citation in Line 47 "shown by [12] and acknowledged by [2]" is proper. Similar citations can be found in other parts of the manuscript.

- Line 413, a comma at the end of a sentence.

Author Response

Dear Reviewer

Thank you for your comments. Our detailed response is added as a pdf file

Thank you again

For all authors

Tammo Steenhuis

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript “Revisiting SWAT as a saturation-excess runoff model for humid temperate (and monsoonal) climates” presents a new method for simulating saturation-excess runoff in SWAT without making major changes to the code. I like the approach that the authors are suggesting and believe that it will be useful for the SWAT community. Nevertheless, I have a few comments that should be addressed before the manuscript is accepted for publication.

There are a lot of typos and careless mistakes in the manuscript that need to be fixed. Also, the wording,  the structure of several sentences, and the use of commas are a little odd in several places throughout the manuscript.

The study watershed is spelled in three different ways throughout the manuscript: Townbrook, Town Brook, Town brook. Please be consistent. Also, sometimes you write “wil” and sometimes “will”. SWAT2012 should be written without the hyphen.

Title: I suggest delete “(and monsoonal)” as this paper only deals with a humid temperate watershed.

L56: Delete “Hoang et al”.

L57: Explain what SWAT-HS stands for.

Chapter 2.2: The HRU definition process needs to be described better. I am not sure what you mean when you say that the HRUs are ranked according to the topographic wetness index. Does this happen after HRU definition or is the wetness class used as an additional variable for defining HRUs (as described by Hoang et al. 2017)?  Or were the HRUs defined based on wetness class only?

Figure 1 has already been published by Hoang et al. (2017), so please make sure that you reference their work correctly and don’t violate any Water or Hydrological Processes policies.

Figure 2: Remove white rectangle behind watershed outline. The percentages of land use classes don’t match the values given in the paragraph above the figure.

Chapter 3.3: Was wind speed generated?

Results: Why do you compare the SWAT-wil results to SWAT-HS results and not directly to the observed data that Hoang et al. (2017) used? You talk about the differences in the spatial distribution of lateral flow simulated by SWAT-HS and SWAT-wil. It would be very useful for the readers if you included maps in the paper that show the lateral flow.  

Figure 3: In my opinion, the hydrographs for the three models should be combined in one graph as that will make it a lot easier to compare them.

L266: It should be “Wetness class 10 in SWAT-wil was divided…”, shouldn’t it?

L383: Delete “Brooks et al”

L384: Delete “Greyson et al”

Author Response

Dear Reviewer

The detailed response is included as a pdf file

Thank you for your comments

For all authors

Tammo Steenhuis

Author Response File: Author Response.pdf

Reviewer 3 Report

This study created a new model SWAT-wil which can be used to simulate saturation excess overland flow in hilly and mountainous regions with an impermeable layer at shallow depth. It overcomes the limitation of SWAT-2012 with no changes in the original SWAT code. The model development significantly improved the ability of SWAT model and potentially has a wide range of applications. Moreover, the experiment was decently designed and the manuscript is well written. I strongly support it to be published with only some tiny revisions.

L22: Though the SCS method is well known by hydrologists, please show the full phrase of "Soil Conservation Service" when it first appears in the text before any acronym (SCS) is used.

L176: "...above the unsaturated zone above the water table" should be "..in the unsaturated zone above the water table"?

L230: "8oC" to "8°C".

L277: "truns of" to "turns off".

L403: "hard pan" to "hardpan".

L406: "turning of" to "turning off".

Author Response

Dear Reviewer

Thank you for your positive evaluation and comments.  We addressed them below. The blue in the cited text indicates the changes made.

For all authors, thank you so much

Tammo

COMMENT:

This study created a new model SWAT-wil which can be used to simulate saturation excess overland flow in hilly and mountainous regions with an impermeable layer at shallow depth. It overcomes the limitation of SWAT-2012 with no changes in the original SWAT code. The model development significantly improved the ability of SWAT model and potentially has a wide range of applications. Moreover, the experiment was decently designed and the manuscript is well written. I strongly support it to be published with only some tiny revisions.

RESPONSE:

The comment is very much appreciated. It took us 2 plus years to get to this stage

COMMENT:

L22: Though the SCS method is well known by hydrologists, please show the full phrase of "Soil Conservation Service" when it first appears in the text before any acronym (SCS) is used.

RESPONSE:

We provided the full name in both the abstract and the text

ABSTRACT

“Finally, input parameters were chosen such that overland flow from variable saturated areas (VSAs) corresponds to the variable source interpretation of the Soil Conservation Service, (SCS) curve number runoff equation. We tested the model for the Townbrook watershed in Catskill Mountains.”

INTRODUCTION

“Over the past 25 years, the Soil and Water Assessment Tool (SWAT) [1] has become the most used hydrologic model in part due to its excellent support structure [2]. The SWAT model predicts overland flow based on either the Soil Conservation Service (SCS) curve number approach or Green and Ampt method. In the SCS method, runoff is calculated as a function of land use, soil, hydrological condition, and some form of antecedent rainfall [3] and, therefore, inherently generates overland flow when the saturated hydraulic conductivity is less than the rainfall intensity.”

COMMENT:

L176: "...above the unsaturated zone above the water table" should be "..in the unsaturated zone above the water table"?

RESPONSE:

Thanks, it is changed

COMMENT:

L230: "8oC" to "8°C".

RESPONSE:

Changed in the revised manuscript

COMMENT:

L277: "truns of" to "turns off".

RESPONSE:

Ennglish is not simple at times. The change is made.

COMMENT:

L403: "hard pan" to "hardpan".

RESPONSE:

Thanks we changed it

COMMENT:

L406: "turning of" to "turning off".

RESPONSE:

English is not an easy language, thanks

Round 2

Reviewer 1 Report

The authors have replies most of my comments and the manuscript has been improved significantly. I would suggest the paper to be accepted with minor revision.

There is no section 3.4 between 3.3 and 3.5. Please recheck through the whole manuscript about the format.

For Figure 4, the authors still didn't explain why the runoff outside the river are very different. In my understanding, the the values between group 0-250 and 250-300 are not distinct vastly. Will re-organize the group like 0-150, 150-300 help a bit?

Author Response

Dear Reviewer 1

Thank you for your time and effort in evaluating  the manuscript.

Your remarked the following:

Comment

There is no section 3.4 between 3.3 and 3.5. Please recheck through the whole manuscript about the format.

Response

There were two sections numbered 3.3. We renumbered the second section to 3.4. Thanks

Comment

For Figure 4, the authors still didn't explain why the runoff outside the river are very different. In my understanding, the the values between group 0-250 and 250-300 are not distinct vastly. Will re-organize the group like 0-150, 150-300 help a bit?

Response

The intent of Figure 4 is to compare the location of areas that produce the greatest amount of runoff  between the three models.  These are the variable source (VSA) areas in the SWAT-wil and SWAT-HS models.  These  areas  potentially  contribute most pollutants to the stream.  Showing the lower amounts of runoff will make it more difficult to visually compare the distribution the distribution of the  VSAs.    We thought for this reason to leave the figure as it is right now. We changed the title of the figure to make it more obvious whey we do not distinguish between the amounts of runoff less than 250 mm per year as follows:

" Figure 4. Simulated surface runoff contributing areas (VSAs) as indicated by average surface flows over 250 mm a^-1 component by SWAT-2012, SWAT-wil and SWAT-HS"

Thanks again for all the comments. It improved the manuscript greatly

Tammo

Reviewer 3 Report

The authors have addressed all my concerns. I have no more comments at this moment.

Author Response

Dear Reviewer 3

We  checked the spelling in the manuscript

Thank you for approving the manuscript for publication. 

Tammo