Density Slopes in Variable Density Flow Modeling

Round 1

Reviewer 1 Report

Review of the article “Density Slopes in Variable Density Flow Modeling”

This article arises a particular issue in groundwater as it is related to saltwater density, density slope relative to total dissolved solids and variable density groundwater flows in coastal aquifers.

I think that what is presented is good and appropriate, but looks more like a review paper rather than a particular research, about the saltwater density parameters to be used in variable density flow modeling.

It seems not clear, which are the contributions of the author (measured data, new relationships, better approach, etc) to what it looks like a state-of-the-art concerning the saltwater density parameters, in a way very useful for density-drive modeling codes. Whether it is a review analysis it should be noticed as it, given that any of the presented relationships, measured data and analyses, doesn’t seem to be original contributions of the author.

I think anyway that as a review, this paper is worth to be published. In general, the text is well presented, written and organized, but must be checked for what seems some flaws and inaccuracies. I have found the following problems along the outline:

-L28                 Reference [1] looks like not properly addressed.

-L39                 Bluid -> Fluid

-L99                 Sure references [3-4] ?.

-L156-161        Bold text ?.

-L171               Aa -> A.

-L206               ap25/19 = 1,3158,… sure ?,…….  What’s the meaning?

-L264               “This text” should be added to the Figure 4 caption.

- Reference [2] does not have the year of publication.

- Is Reference [3] similar to Reference [2] ?

- Formula (12), justify right

Author Response

The author thanks the reviewers for their careful review of the manuscript and insightful comments.

This paper is intended to be a technical note rather than a “review” or particular research.  Density slope plays a role in all variable-density modeling, but some of the hidden issues behind this seemingly simple input parameter have never been discussed.

Although the data presented in this technical note were not originally collected by the author, they were cited, analyzed, and used here to demonstrate the points the author intends to discuss.

The author revised the original data from Baxter and his colleagues 100 years ago and checked the validity of “linearity.” The author assessed the linearity of density slope for brines vs. measured fluid density up to TDS=200 g/L.

The author raised a technical issue when chloride concentration is used for fluid density calculation if chloride/TDS is not as it is in a typical seawater. The author showed the wide range of chloride/TDS ratios based on the water quality data collected by the U.S. Geological Survey. To my knowledge, that this issue has never been discussed in literature.

The author tabulated some commonly used density slope values for various water density and solute concentration are used in variable density modeling. the author has seen the cases when incorrect density slope values were used and hope this technical note may be helpful for people choose correct density slope values.

All the editorial comments are fixed.

Reviewer 2 Report

Paper: WATER 1425252

Title: Density Slopes in Variable Density Flow Modeling

Authors: Weixing Guo

GENERAL COMMENTS

The submitted paper is a technical note.  It uses the laboratory results of Baxter and his colleagues for the computation of the fluid density from salt concentration and other published laboratory and field data.  The author recalculates the density slopes for various solutes, concentrations and ratios of chloride and total dissolved solids.  He discussed the findings and other relative issues regarding groundwater seawater intrusion.

The paper is, in general, well-written and structured.  I have only some minor/moderate comments and suggestions aiming in the improvement of the paper.  My comments/suggestions are outlined in the next paragraphs. 

COMMENTS

1) Title of the paper. The title of the paper should be changed in order to indicate that the paper deals with the values of density flow for problems of groundwater seawater intrution.

2) Author’s affiliation should be more precise and detailed.

3) Keywords. They should be expanded including seawater intrusion. 

4) Abstract. The abstract of the paper needs re-writing.  The abstract should briefly present the issue treated in the paper, the approach/method used and the results found. 

5) Introduction. The objectives of the paper should be clearly presented at the end of the Introduction section. 

6)  Statistics should be presented about the linear regression equations (e.g. R², SEE).

7) It would be very interesting and important of how large is the error introduced in groundwater seawater intrusion modeling  by making the assumption of constant density slope made (e.g. in SEAWAT).  The author should discuss further this issue and provide references.

8) There are some paragraphs with just one sentence.  Paragraphs have more than two sentences.

Author Response

The author thanks the reviewers for their careful review of the manuscript and insightful comments.

Author’s responses are provided after each comment.

COMMENTS

• Title of the paper. The title of the paper should be changed in order to indicate that the paper deals with the values of density flow for problems of groundwater seawater intrution.

Response: I believe the original title is appropriate for the paper. Variable density flow is a general term including groundwater seawater intrusion. And I added “seawater intrusion” in “Keywords.” (See below).

• Author’s Affiliation should be more precise and detailed:

              Response: Revised

• They should be expanded including seawater intrusion.

Response: “Saltwater intrusion” is added

• The abstract of the paper needs re-writing. The abstract should briefly present the issue treated in the paper, the approach/method used, and the results found.

Response: Revised.

• The objectives of the paper should be clearly presented at the end of the Introduction section.

          Response: Revised.

6) Statistics should be presented about the linear regression equations (e.g. R, SEE).

       Response: R² value is added to Figure 1. Not sure about “SEE.”

 7) It would be very interesting and important of how large is the error introduced in groundwater seawater intrusion modeling by making the assumption of constant density slope made (e.g. in SEAWAT). The author should discuss further this issue and provide references.

Response: The relative error for constant density slope when TDS is 200 g/L is added. Calculated fluid density using chloride and TDS for the two samples shown are also added to demonstrate the difference.

Most people choose TDS, as the solute for fluid density calculation. However, there are some cases that people use chloride as the solute for density evaluation. Two examples were provided in the paper. When chloride is used, an implicit assumption was made the chloride is 55% of the TDS as in seawater. Field data show that ratios of chloride/TDS vary from 5% to 60%. Although the error introduced maybe seemingly small, but the issue should be worth of discussion.

The author has seen several cases when people messed up the concept and field data. To my best knowledge, however, I have not seen this issue being discussed in literature.

8) There are some paragraphs with just one sentence. Paragraphs have more than two sentences.

                             Response: Fixed.