Decadal Changes in Soil Water Storage Characteristics Linked to Forest Management in a Steep Watershed
, Katsuhide Yokoyama 1,*, Hiroshi Sakai 1, Akira Koizumi 1 and Kenji Sakai 2Round 1
Reviewer 1 Report
Review of "Decadal changes in soil water storage characteristics attributed to forest management in a steep watershed", by Charles JC Gunay, Katsuhide Yokoyama, Hiroshi Sakai, Akira Koizumi, and Kenji Sakai.
In this work, Gunay et al. evaluate decadal changes in runoff and soil water storage as a function of rainfall and of indices linked to soil characteristics, for the Oguchi Dam watershed, Japan. They used both observational (rainfall and runoff) and modelling (SWAT hydrological model) approaches aiming at investigating whether forest management practices, such as regeneration cutting, fencing for deer control or for landslide retaining, had an impact on the soil water storage characteristics.
The work is generally well written and logically constructed, even if some of the assertions seem somewhat outlandish (I will come back to it later), and perhaps some re-structuration and clarifications are needed. My assessment is that the work has quality and potential to be published in Water.
I ask the authors to consider the following points:
General remarks:
My feeling is that the word "attribution" in the title is rather strong. Nowhere in the manuscrip it was shown maps or charts that depict the decadal changes in forest density, or charts that show the monitoring of forest management in the watershed. No direct measurements of soil characteristics were taken for different locations in the watershed (for example, areas with forest thinning vs "pristine" areas") to establish a cause-and-effect liason between the observed decadal changes in runoff and soil water holding capacity. Thus, the conclusions are based on inferences, and I would rather use the work "link" instead of "attribution".
The above is rather a "semantic" discussion, but it comes attached to a deeper question that is underlined above: would it be possible to include maps or charts showing the decades-long transformation? This reviewer (and potential readers) does not have an idea of the spatial scale of the forest changes, nor the distribution of the management actions. Were they concentrated in some areas? what are the frequency of these interventions in space (timeline is described in the Intro). Without this information, the discussion around the issue of forest management sounds sometimes speculative. In addition, no statistical analysis on the significance of the differences found between the decades have been discussed, nor ancillary data (rate of sedimentation in the water reservoir for instance) have been presented to corroborate the conclusions.
Specifics:
(i) Please explain when and how Equation (4) was used.
(ii) I think that Section 4.1 contains results mixed with discussion, and its placement and/or presentation should be reconsidered. Besides this structural point, I've had a great deal of difficulty trying to get the discussion on Figure 7. First, nowhere in the paper is described how was SWSC estimated. I suspect that it is related to equation 3, but a clear explanation is missing. Then, the authors relate SWSC to rainfall. I don't get it. SWSC should be a function of the soil properties, not rainfall thresholds. One would not analyse the dependency of a 200 ml-cup holding capacity on the water flux from a tap - that is, it is irrelevant. A 200 ml cup will hold 200 ml max, period. Perhaps it was simply the "soil water content" (but again, how was it estimated?)? Or, the infiltration capacity?? Another point, were the SWSC points in Figure 7 normalized with respect to the total rainfall (above or below 70 mm) or to mean rainfall intensity, for each decade? Finally, the infiltration capacity or water storage capacity should decrease the longer the rainfall period as the soil becomes saturated, was this taken into account?
(iii) Still in page 10 section 4.1, the authors present Pearson coefficient and p-values - I don't understand where these came from. What are the variables being correlated?
(iv) Figure 9 is very interesting, and it is at the heart of your claim that forest management explain the changes in soil available water capacity.
(iv.1) Presentation of the figure: could you please explain the ordinate values for Sol_Awc factor? I am puzzled that the values get above 100% - what does it mean that the soil available water capacity is at 150%? Also, I disagree that you show trends of Sol_AWc - we can infer some tendencies from the curves, but what is being shown are the values of this parameter per decade, not trends (otherwise, the ordinate would be a trend value).
(iv.2) It would be really nice if you could show in parallel an index, or parameter, that would reflect the effects of forest management, for example, tree density, or spatial changes in tree distribution, number or extention of fences, etc., and compare it specially with pannel (f).
Minor:
= pp2 second full paragraph: why "logically"?
= pp3 section 2.1 - 185 mm3 instead of Mm3; remove "being" between "and is" and "subjected" (one line before last).
= pp4 : what is a "Goryorin forest"?
= pp5, last paragraph: "it was confirmed that the watershed is 91,3% forested" - how was it confirmed?
= pp11 section 4.2, first paragraph: "Although proven valid and logical..." ?? Please revise this - what proof did you present? Moreover, "proof" is in contradiction with "likely" used just after, and I am not sure what "logical" means here. Second paragraph: replace "ideal" by "idealized"
Author Response
12 December 2022
Dr. Sara Roje
Assistant Editor
Water Editorial Office
Re: Response to Reviewer 1 Comments
Dear Dr. Roje and Reviewer 1,
We are submitting the revised manuscript (water-2063554) for consideration of publication in Water. The revised manuscript is entitled “Decadal Changes in Soil Water Storage Characteristics Linked to Forest Management in a Steep Watershed”.
We thank you for the major revisions suggested for our paper. The comments of Reviewer 1 have been carefully addressed by all the co-authors. The effort and time allotted for the review of this research paper are highly valued.
Please see in the next pages the point-by-point response to each comment from Reviewer 1.
Thank you very much for your consideration.
Yours sincerely,
Katsuhide YOKOYAMA, Ph.D.
Professor, Department of Civil and Environmental Engineering
Tokyo Metropolitan University
Response to Reviewer 1 Comments
General remarks 1: My feeling is that the word "attribution" in the title is rather strong. Nowhere in the manuscrip it was shown maps or charts that depict the decadal changes in forest density, or charts that show the monitoring of forest management in the watershed. No direct measurements of soil characteristics were taken for different locations in the watershed (for example, areas with forest thinning vs "pristine" areas") to establish a cause-and-effect liason between the observed decadal changes in runoff and soil water holding capacity. Thus, the conclusions are based on inferences, and I would rather use the work "link" instead of "attribution".
Response: Thank you for your valuable recommendation. We edited the manuscript title as suggested. The article is now entitled “Decadal Changes in Soil Water Storage Characteristics Linked to Forest Management in a Steep Watershed”.
General remarks 2: The above is rather a "semantic" discussion, but it comes attached to a deeper question that is underlined above: would it be possible to include maps or charts showing the decades-long transformation? This reviewer (and potential readers) does not have an idea of the spatial scale of the forest changes, nor the distribution of the management actions. Were they concentrated in some areas? what are the frequency of these interventions in space (timeline is described in the Intro). Without this information, the discussion around the issue of forest management sounds sometimes speculative. In addition, no statistical analysis on the significance of the differences found between the decades have been discussed, nor ancillary data (rate of sedimentation in the water reservoir for instance) have been presented to corroborate the conclusions.
Response: Unfortunately, it may not be possible to include maps showing the decades-long transformation of forest management. As of writing, the spatial data of forest management that we possess is only reflective of the characteristics of the most recent decade. We thank you for your consideration and understanding.
Regarding the forest cover changes, we emphasized in the 4th paragraph of section 2.3 that the area covered by forests remain unchanged since 1976 and there are no documented reports about land use change in the area: “Other land cover maps created by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) reported an average forest coverage of 96.0% in the study area as early as 1976, the year when the first land cover map of the area was developed and released publicly, up to 2016. This reveals that the watershed remained dominantly forested, and there are no documented reports or maps of land use changes in the area for a very long period.”. The catchment area had undergone more of “ownership and management changes” rather than “land cover/land use changes”.
We are working on the analysis of the distribution of the management actions in the area. For now, we can say that the management actions have no pattern, i.e., unevenly distributed, and are considered random in space. We are planning to report it in detail in another manuscript. More importantly, the present work is more centered on temporal changes, rather than spatial changes. We believe that the current dataset and analyses presented are sufficient to attain our current objectives.
We added some details about the sedimentation rate in the Ogouchi Reservoir in the last sentence of the second paragraph in section 2.1: “The continuous efforts to expand the area of well-managed forests in the upstream catchment can be considered the most significant cause on why the Ogouchi Reservoir has maintained its low sedimentation rate at 350 m3 km-2 per year in the past two decades [39].”
Finally, we presented the results of the statistical analysis for the decadal changes in Figure 10 (formerly Figure 9). Pearson r values with respect to time and the respective p-values were added in the discussion in the 6th paragraph of section 4.2 for Figures 10a, 10b, 10e, and 10f where there is a generally increasing trend.
Specific point 1: Please explain when and how Equation (4) was used.
Response: Thank you for this clarification. We used the commonly employed recursive digital filter, which is Eckhardt’s (2005), for the separation of the hydrograph components. This was performed to verify the reproducibility of not just the total flow, but both the surface and sub-surface flow components. Although the results of the tank model can be utilized for such verification, we intended to use a completely independent method from the first approach to filter the flow components.
Specific point 2: I think that Section 4.1 contains results mixed with discussion, and its placement and/or presentation should be reconsidered. Besides this structural point, I've had a great deal of difficulty trying to get the discussion on Figure 7. First, nowhere in the paper is described how was SWSC estimated. I suspect that it is related to equation 3, but a clear explanation is missing. Then, the authors relate SWSC to rainfall. I don't get it. SWSC should be a function of the soil properties, not rainfall thresholds. One would not analyse the dependency of a 200 ml-cup holding capacity on the water flux from a tap - that is, it is irrelevant. A 200 ml cup will hold 200 ml max, period. Perhaps it was simply the "soil water content" (but again, how was it estimated?)? Or, the infiltration capacity?? Another point, were the SWSC points in Figure 7 normalized with respect to the total rainfall (above or below 70 mm) or to mean rainfall intensity, for each decade? Finally, the infiltration capacity or water storage capacity should decrease the longer the rainfall period as the soil becomes saturated, was this taken into account?
Response: Thank you for pointing out some issues with the clarity and presentation of Section 4.1. Upon reviewing the first paragraph, we believe that the first two sentences should be retained and are necessary to justify the separation between weaker and stronger rainfall events, as well as to provide a smoother flow of the discussion from the correlation plots (in the Results section) to the trends of SWSC (in the Discussion).
In the previously submitted manuscript, we discussed the method of SWSC estimation in the last paragraph of section 2.2. Values in Figure 8 (formerly Figure 7 Decadal changes in SWSC) were estimated by computing the area bounded by the 45° line in the correlation plots, originating at (0,0) and theoretically signifies the complete conversion of rainfall into surface runoff, and the actual linear fit of the established correlation plot. It is not related to equation 3. To prevent confusion, we divided the definition of SWSC and its estimation procedure into two separate sentences in the last paragraph of section 2.2: “The soil water storage capacity (SWSC) is defined as the total amount of water stored within the plant’s root zone which controls some of the major hydrological processes [46,47]. It was estimated by computing the area bounded by the 45° line, originating at (0,0), and theoretically signifies the complete conversion of rainfall into surface runoff and the actual linear fit of the established correlation plot. With this technique, the amount of rainfall that was not converted to surface runoff was assumed to be stored in the subsurface layers of the soil.”
We would also like to clarify that we did not relate SWSC to rainfall alone. SWSC is estimated from the rainfall-runoff correlation plots in Figure 6 (formerly Figure 5). The current analysis in the manuscript is a fundamental representation of the water balance where rainfall is the main input and runoff is the output. The balance indicates the amount of water stored in the soil. Generally, SWSC has a direct relationship with the infiltration capacity, hence we included it in our discussions.
The SWSC points in Figure 8 (formerly Figure 7) were normalized based on the total rainfall for each decade as such values were more comparable. Finally, we had taken into account that infiltration capacity and SWSC vary based on the duration of the wet period and the saturation of the soil.
Specific point 3: Still in page 10 section 4.1, the authors present Pearson coefficient and p-values - I don't understand where these came from. What are the variables being correlated?
Response: Thank you for this clarification. We correlated the values with respect to time, with the background and consideration that the increase in time reflects a kind of better management. In addition, such Pearson coefficient and p-values were approved by the former reviewers and are already published (Reference [44]).
Specific points 4, 4.1, and 4.2:
(iv) Figure 9 is very interesting, and it is at the heart of your claim that forest management explain the changes in soil available water capacity.
(iv.1) Presentation of the figure: could you please explain the ordinate values for Sol_Awc factor? I am puzzled that the values get above 100% - what does it mean that the soil available water capacity is at 150%? Also, I disagree that you show trends of Sol_AWc - we can infer some tendencies from the curves, but what is being shown are the values of this parameter per decade, not trends (otherwise, the ordinate would be a trend value).
(iv.2) It would be really nice if you could show in parallel an index, or parameter, that would reflect the effects of forest management, for example, tree density, or spatial changes in tree distribution, number or extention of fences, etc., and compare it specially with pannel (f).
Response: We understand the concern of the reviewer about the presentation of Figure 10 (formerly Figure 9 Trends of Sol_Awc factors). The ordinate values show the percentage from the initial soil available water capacity set by the SWAT model for each soil type which ranges from 0.16 mm water/mm soil to 0.21 mm water/mm soil. To simplify, a Sol_Awc factor of 100% indicates the originally SWAT-assigned Sol_Awc value, 75% is considered a decrease from such initial value, and 125% is an increase. For each decade, we assigned the “most optimal Sol_Awc factor”, defined as the Sol_Awc input that will best predict the monthly discharge for the decade to achieve high values of NSE and R2 and low values of RMSE, MAE, and PBIAS. We did this to reflect and represent the value of soil available water capacity that will result in the best reproducibility of discharge for each decade. It becomes quite complicated because the results in reference to each individual performance index were different. We added this sentence in the 3rd paragraph of section 4.2 to clarify the meaning of the ordinate values: “Specifically, a Sol_Awc factor of 100% reflects the original value assigned by SWAT, a factor of less than 100% indicates a decrease in soil water capacity, while a factor of greater than 100% represents an increase and improvement in the soil water condition.”.
We understand your point in the last sentence. We had shown the values of Sol_Awc per decade. In this case, we revised the title of Figure 10 (formerly Figure 9) to “Figure 10. Decadal changes in soil available water capacity (Sol_Awc) factor…”.
For specific point 4.2, we mentioned earlier that it may not be possible for us to present decades-long transformation of forest management in the watershed. We thank you for your kind understanding.
Minor point 1: pp2 second full paragraph: why "logically"?
Response: Thank you for this minor comment. By definition, “logically” is an adverb depicting a sound, convincing, and reasonable argument. Regardless of the differences in the methods employed, we believe that the frameworks of the studies cited before such paragraph were reasonably constructed, hence we add “logically”. However, since it caused slight confusion and questioning from the reviewer, we opt to omit it: “Although most of the aforementioned papers ascertained…”
Minor point 2: pp3 section 2.1 - 185 mm3 instead of Mm3; remove "being" between "and is" and "subjected" (one line before last).
Response: Thank you for this minor comment. It is actually 185 million cubic meters and not cubic millimeters. To prevent confusion, we wrote “185 million m3” instead of 185 Mm3.
Minor point 3: pp4 : what is a "Goryorin forest"?
Response: Thank you for this minor clarification. Goryorin is the Japanese term for the type of forest owned and managed by Japan’s Imperial family. We made it clear in the second paragraph of section 2.1.
Minor point 4: pp5, last paragraph: "it was confirmed that the watershed is 91,3% forested" - how was it confirmed?
Response: Thank you for this minor clarification. The total percentage of land covered by forests was confirmed by adding the areas covered by deciduous, evergreen, and mixed forests. The land areas were estimated by using a GIS software.
Minor point 5: pp11 section 4.2, first paragraph: "Although proven valid and logical..." ?? Please revise this - what proof did you present? Moreover, "proof" is in contradiction with "likely" used just after, and I am not sure what "logical" means here. Second paragraph: replace "ideal" by "idealized"
Response: Thank you for this minor comment. Since the results from the first approach were already peer-reviewed and briefly discussed in an earlier publication (Reference [44]), we wrote strong adjectives like “valid” and “logical”. We revised the first sentence of section 4.2 as follows: “Although the analysis method had already been reported and affirmed by some experts [44], the general conclusion from the first approach that sustainable forest management likely improved the water capacity of the soil needs to be further justified.”. We also replaced “ideal” by “idealized” as suggested by the reviewer.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The impact of effective forest management on soil moisture is factual and critical. This paper specifically aims to observe the decadal changes in the soil water storage condition of the Ogouchi Dam watershed in Japan, which has been subjected to various forest management schemes through the years, using two different approaches. The long time series of rainfall runoff data are valuable, but the authors did not analyze and utilize them well. It is suggested that the authors can further use the data effectively, and it is likely to achieve many unexpected results.
1. Soil moisture storage is discussed throughout the paper, but there are no observed data.
2. Th e overall characteristics of the 50-year runoff dataset need to be shown to the reader.
3. How is a rainfall event defined and what is the basis for using 70 as a split.
Author Response
12 December 2022
Dr. Sara Roje
Assistant Editor
Water Editorial Office
Re: Response to Reviewer 2 Comments
Dear Dr. Roje and Reviewer 2,
We are submitting the revised manuscript (water-2063554) for consideration of publication in Water. The revised manuscript is entitled “Decadal Changes in Soil Water Storage Characteristics Linked to Forest Management in a Steep Watershed”.
We thank you for the major revisions suggested for our paper. The comments of Reviewer 2 have been carefully addressed by all the co-authors. The effort and time allotted for the review of this research paper are highly valued.
Please see in the next pages the point-by-point response to each comment from Reviewer 2.
Thank you very much for your consideration.
Yours sincerely,
Katsuhide YOKOYAMA, Ph.D.
Professor, Department of Civil and Environmental Engineering
Tokyo Metropolitan University
Response to Reviewer 2 Comments
Point 1: Soil moisture storage is discussed throughout the paper, but there are no observed data.
Response: We appreciate the reviewer for pointing out the importance of observed data to improve the discussion in our paper. However, it may be impossible to get observed data of soil moisture storage from previous decades. Thus, our study intends to highlight the use of observational (rainfall and runoff) and physical modeling (SWAT hydrological model) approaches to investigate whether the forest management practices in the watershed had an impact on the soil water storage characteristics. Specifically, since we intended to deal more with the decadal scale rather than short-term changes, our work is centered on the long-term analysis of available hydroclimatic records and calibration of flow-related parameters that can directly be associated with the soil water condition. We thank you for your kind consideration.
Point 2: The overall characteristics of the 50-year runoff dataset need to be shown to the reader.
Response: Thank you for this valuable recommendation. We revised as suggested and added the annual time series of discharge ratio (Figure 5) to provide the overall characteristics of the 50-year runoff dataset. The second paragraph of section 3.1 explains the general trend of the dataset and its implications on soil water storage: “The annual discharge ratio, estimated by dividing the total discharge by the total precipitation in the watershed each year, is illustrated in Figure 5. Its values ranged from 0.47 (in 1978) to 0.76 (in 1989), with an average value of 0.63. This indicates that about 63% of precipitation is converted into surface and subsurface flows. With the time series of the discharge ratio showing a significantly decreasing trend (at α = 0.001 level of significance) at a rate of -0.0625 per year, it is highly plausible that a larger portion of water is getting stored in the soil. Moreover, accounting for the surface flows only, the discharge ratio will be about 0.25, which falls in the typical range for forests underlain by moderately permeable soils [54].”
Point 3: How is a rainfall event defined and what is the basis for using 70 as a split?
Response: Thank you for these questions. We defined one rainfall event as the cumulative amount of rainfall measured from continuous wet days that generated daily amounts of runoff greater than 0.50 mm. The last sentence of the second paragraph of section 2.2 was revised as follows: “To further reduce the large number of correlation points that may potentially cause significant differences among the sample sizes for each decade, the cumulative amount of rainfall collected from continuous wet days that generated daily amounts of runoff greater than 0.50 mm was considered as one event, or in the context of the correlation plot, one point.”.
To answer the second question, when we zoomed in and magnified the correlation plots in Figure 6 (formerly Figure 5 Rainfall-runoff correlation plots in each decade), we observed apparent changes in linear slopes at continuous-event rainfall (x-axis) of 70 mm. This is the basis of the separation of the SWSC trends for weaker (R < 70) and stronger events (R > 70).
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Second review of "Decadal Changes in Soil Water Storage Characteristics Linked to Forest Management in a Steep Watershed", by Charles John C. Gunay, Katsuhide Yokoyama, Hiroshi Sakai, Akira Koizumi and Kenji Sakai.
I thank the authors for their thorough responses to my comments in the first review.
I agree and am satisfied with most responses, but I ask the authors to consider a few remaining questions.
First, a very minor comment that I forgot to include in my previous report: it seems that there is a problem in the legend in Figure 1. In my pdf version, the river gauging stations (red squares) appear as a quote sign ("), and the weather stations black symbols appear as a dollar sign ($).
Regarding the main body, there are some parts that I still have difficulty following:
(1) Regarding Equation 4:
Authors: "We used the commonly employed recursive digital filter, which is Eckhardt’s (2005), for the separation of the hydrograph components. This was performed to verify the reproducibility of not just the total flow, but both the surface and sub-surface flow components. Although the results of the tank model can be utilized for such verification, we intended to use a completely independent method from the first approach to filter the flow components."
The authors are correct the above was/is clearly stated in the text, but it was not clear to me that equation (4) was intended to "check" on the results of the tank model (is this correct?). I see no direct or indirect connection between (4) and equations (1)-(3) that are part of the SWAT model, and equation 4 appears in the context of the SWAT model description. What am I missing?
(2) The method of estimation of SWSC remains obscure in the main document.
Authors: "In the previously submitted manuscript, we discussed the method of SWSC estimation in the last paragraph of section 2.2. Values in Figure 8 (formerly Figure 7 Decadal changes in SWSC) were estimated by computing the area bounded by the 45° line in the correlation plots, originating at (0,0) and theoretically signifies the complete conversion of rainfall into surface runoff, and the actual linear fit of the established correlation plot."
I kindly point that "in the correlation plots" was not -and is not- part of the original text. I could not figure out what was the 45° and the origin (0,0) were about, and did not find useful information regarding this particular point (SWSC calculation) in the references.
Based on the authors' explanation, I understand that the SWSC correspond to the area under a 45° line with origin at (0,0) up to a certain continuous rainfall amount, that is, the area of a plain isosceles right triangle, is that correct? Or is it the area between the correlation curves and the 45° line?
(3) Figure 10 (previous figure 9): Could you please add in the Caption as well that the Sol_Awc factor of 100% in the ordinate indicates the originally SWAT-assigned Sol_Awc value?
Author Response
20 December 2022
Dr. Sara Roje
Assistant Editor
Water Editorial Office
Re: Response to Reviewer 1 Comments (Round 2 – Minor)
Dear Dr. Roje and Reviewer 1,
We are submitting the revised manuscript (water-2063554) for consideration of publication in Water. The revised manuscript is entitled “Decadal Changes in Soil Water Storage Characteristics Linked to Forest Management in a Steep Watershed”.
We thank you for the minor revisions suggested for our paper. The comments of Reviewer 1 have been carefully addressed by all the co-authors. The effort and time allotted for the review of this research paper are highly valued.
Please see in the next pages the point-by-point response to each comment from Reviewer 1.
Thank you very much for your consideration.
Yours sincerely,
Katsuhide YOKOYAMA, Ph.D.
Professor, Department of Civil and Environmental Engineering
Tokyo Metropolitan University
Response to Reviewer 1 Comments
Point 1: First, a very minor comment that I forgot to include in my previous report: it seems that there is a problem in the legend in Figure 1. In my pdf version, the river gauging stations (red squares) appear as a quote sign ("), and the weather stations black symbols appear as a dollar sign ($).
Response: We appreciate the time and effort allotted by the reviewer to improve our manuscript. In the PDF version of the original draft, the legend of river gauging stations appears as a red square icon, while the legend of weather stations appears as a black pentagon icon. It is probably because we have a GIS software installed on our PC, allowing Adobe or our PDF reader to read the original legend icons. In the revised manuscript, we edited Figure 1 by overlaying manually created icons, so now it should look like the actual legend icons plotted in the map of the study area.
Point 2: Regarding the main body, there are some parts that I still have difficulty following:
(1) Regarding Equation 4:
Authors: "We used the commonly employed recursive digital filter, which is Eckhardt’s (2005), for the separation of the hydrograph components. This was performed to verify the reproducibility of not just the total flow, but both the surface and sub-surface flow components. Although the results of the tank model can be utilized for such verification, we intended to use a completely independent method from the first approach to filter the flow components."
The authors are correct the above was/is clearly stated in the text, but it was not clear to me that equation (4) was intended to "check" on the results of the tank model (is this correct?). I see no direct or indirect connection between (4) and equations (1)-(3) that are part of the SWAT model, and equation 4 appears in the context of the SWAT model description. What am I missing?
Response: Thank you for this clarification. Equation (4) was intended to check the results of the SWAT model, and not the tank model. Specifically, we used Equation (4) to filter the surface and subsurface components of the total discharge from the measured dataset. While drafting and reviewing the manuscript, we decided to put such mathematical expression in section 2.3 (SWAT), rather than in the earlier sections since it will be used to evaluate the predictability of the subsurface flow component. To prevent confusion, we revised the last paragraph of section 2.3 as follows: “In addition, the recursive digital filter method [13,52,53], which estimates baseflow qb using parameter filters, recession constant α and maximum baseflow index BFImax, was used to ensure realistic and reliable predictions for both the surface and subsurface flow components of the SWAT discharge model.”
Point 3: (2) The method of estimation of SWSC remains obscure in the main document.
Authors: "In the previously submitted manuscript, we discussed the method of SWSC estimation in the last paragraph of section 2.2. Values in Figure 8 (formerly Figure 7 Decadal changes in SWSC) were estimated by computing the area bounded by the 45° line in the correlation plots, originating at (0,0) and theoretically signifies the complete conversion of rainfall into surface runoff, and the actual linear fit of the established correlation plot."
I kindly point that "in the correlation plots" was not -and is not- part of the original text. I could not figure out what was the 45° and the origin (0,0) were about, and did not find useful information regarding this particular point (SWSC calculation) in the references.
Based on the authors' explanation, I understand that the SWSC correspond to the area under a 45° line with origin at (0,0) up to a certain continuous rainfall amount, that is, the area of a plain isosceles right triangle, is that correct? Or is it the area between the correlation curves and the 45° line?
Response: Thank you for pointing out this unclear part in the main text. The area considered in the estimation of SWSC is the area bounded by the 45° line and the correlation curve. We computed it using integration technique, with y = x as the equation of the 45° line and a specific expression, y = f(x), as the equation of the correlation in each decade. The lower and upper bounds of the integral are based on the minimum and maximum continuous-rainfall amounts. We revised the last paragraph of section 2.2 as follows: “The soil water storage capacity (SWSC) is defined as the total amount of water stored within the plant’s root zone which controls some of the major hydrological processes [46,47]. It was estimated by computing the area bounded by the 45° line, originating at (0,0) and theoretically signifies the complete conversion of rainfall into surface runoff, and the actual linear fit of the established correlation plot. Such area was computed using integration technique, with y = x as the equation of the 45° line and a specific expression, y = f(x), as the equation of the linear correlation in each decade. The maximum and minimum continuous-rainfall amounts were assigned as the upper and lower bounds of the integral equation, respectively. With this technique, the amount of rainfall that was not converted to surface runoff was assumed to be stored in the subsurface layers of the soil.”
Point 4: (3) Figure 10 (previous figure 9): Could you please add in the Caption as well that the Sol_Awc factor of 100% in the ordinate indicates the originally SWAT-assigned Sol_Awc value?
Response: Thank you for this minor comment. We revised as suggested: “Figure 10. Decadal changes in soil available water capacity (Sol_Awc) factor based on a) NSE, b) RMSE (RSR), c) MAE, d) PBIAS, e) R2, and f) overall ranking scheme; Sol_Awc factor of 100% indicates the originally SWAT-assigned Sol_Awc value, which ranges from 0.16 to 0.21 mm water/mm soil.”
Author Response File: Author Response.pdf
Reviewer 2 Report
The author has revised the manuscript, but the author's explanation of using 70mm as the division point is still not enough to convince the reader.
Author Response
20 December 2022
Dr. Sara Roje
Assistant Editor
Water Editorial Office
Re: Response to Reviewer 2 Comments (Round 2 – Minor)
Dear Dr. Roje and Reviewer 2,
We are submitting the revised manuscript (water-2063554) for consideration of publication in Water. The revised manuscript is entitled “Decadal Changes in Soil Water Storage Characteristics Linked to Forest Management in a Steep Watershed”.
We thank you for the minor revisions suggested for our paper. The comment of Reviewer 2 had been carefully addressed by all the co-authors. The effort and time allotted for the review of this research paper are highly valued.
Please see on the next page the response to the comment from Reviewer 2.
Thank you very much for your consideration.
Yours sincerely,
Katsuhide YOKOYAMA, Ph.D.
Professor, Department of Civil and Environmental Engineering
Tokyo Metropolitan University
Response to Reviewer 2 Comment
Point 1: The author has revised the manuscript, but the author's explanation of using 70mm as the division point is still not enough to convince the reader.
Response: We appreciate the reviewer’s time and effort to improve our manuscript. In the analysis, we had taken into account that the soil water storage capacity (SWSC) varies based on the duration of the rainy (wet) period and the saturation of the soil. Since our analysis is highly dependent on the rainfall dataset, which is the main water input, and the results presented were correlation plots, we attempted to highlight the division of the weaker and stronger rainfall events based on the shifts on the linear slope of the correlation. In the revised manuscript, we added these sentences in the third paragraph of section 3.1 to justify the 70 mm as the division point: “When magnified, the decadal plots generally illustrated apparent shifts in linear slopes at rainfall of 70 mm. It specifically implies that the amount of water stored in the soil will be different for rainfall amounts of less than and greater than 70 mm for all the decades, hence SWSC was interpreted separately for weaker (R < 70 mm) and stronger events (R > 70 mm). This is also to take into consideration that the infiltration capacity and SWSC vary based on the duration of the continuous wet days and the saturation of the soil.”
Author Response File: Author Response.pdf
