Estimating the Impacts of Ungauged Reservoirs Using Publicly Available Streamflow Simulations and Satellite Remote Sensing

Round 1

Reviewer 1 Report

Dear Authors,

The paper is well-written, the subject is interesting, within the journal's scope, and has wide-range applicability. 

These are my comments for you to address:

Minor:

Table 2: Correlation coefficient (Pearson's); in both relative errors, use %, as you do in the results and discussion.

Major:

Figure 4: Please expand/include in the discussion what the reader should understand by observing LandSat 8 03/25 and 04/10, especially considering that 03/24 in Sentinel 2 differs significantly.

You deal too little with CC, and this is more relevant as in the shorter time scale (1 day), this value is low (0.41), which makes it unreliable for continuous modeling or for considering or simulating the effect of daily (or even weekly) reservoir regulation. Expand on this (about its meaning, is it because of maxima, minima, seasonality?), and I expect to find something about this in the abstract and conclusions (the parts of the document they would likely read at first). Yes, the method and results are promising, but you need to clearly warn your readers about limitations, nothing under the rug.

Best regards,

The Reviewer

Author Response

Dear Reviewer,

We want to express our sincere gratitude for your valuable comments and suggestions. We appreciate it.

Enclosed, please find our comprehensive response to your comments.

Thank you once again for your time and expertise in reviewing our work.

Warm regards,

Ngoc Thi Nguyen, Tien Le Thuy Du, Hyunkyu Park, Chi-Hung Chang, Sunghwa Choi, Hyosok Chae,E. James Nelson, Faisal Hossain,Donghwan Kim, Hyongki Lee

Author Response File: Author Response.docx

Reviewer 2 Report

The study presents an interesting approach to estimating the impacts of ungauged reservoirs using publicly available data and a global hydrological model platform. It is very useful for data-scarce regions. However, there are a few issues that have to be addressed:

About Figures

-         Figure 1, Remove the high and low in the elevation legend and could you put the catchment boundary for the Hwanggam dam and Pilseung bridge

-    Figure 3, the unit of the x-axis should be superscript.

-    Figure 4 , since a general legend is given at the bottom please remove the smaller legends in all the figures

-     Figure 5, what does it mean "climate climatology of precipitation and temperature" ?  Better to say "Monthly average precipitation and temperature"

-      The colors of the axis headings in Figure 5 (right) are insufficient as a legend for temperature and precipitation. Additionally, why the continuous dotted lines for temperature aren’t they discrete for every month? and maximum values of the charts are displayed above the axis range's maximum value.

-     Figure 6 and 7, the y-axis of both should be "Streamflow (m3 /s)".

-     Figure 8, the y-axis should be "Reservoir storage change (%)".

-     Figure S1, should be placed as a separate material from the main manuscript

 

The abstract section shall be rephrased by incorporating the findings in numeric forms, and better to be free from undefined acronyms.

 

What does GEOloWS ECMWF stands for? It is not stated elsewhere

In line 210 a simple linear interpolation was fitted to the lower E-A relationships, please put these equations in the supplementary material

In line 251-253 time series of daily surface areas were created by linearly interpolating the remote sensing based reservoir areas for the complete period from January 2016 to December 2021. Could you put the linear equation used in the manuscript?

In line 255-256 the associated reservoir elevations and storage volumes were determined by linking the reservoir AEV relationship with the time series of reservoir areas. Please provide the reservoir AEV relationship functions for reproducibility.

Line 310 states about coefficient of coefficient (CC) which I have never read from previous literature as a metrics and also not available in the cited reference papers. Please make sure where you get it or correct it. The cc in Kling Gupta’s efficiency is Pearson’s correlation coefficient. May be you need to correct to “correlation coefficient (CC)”

In line 314 it says “To assess the hydrological impacts of Hwanggang dam “What impacts? What is the reason the reservoir is built? Except for being stated as a multi-purpose dam, its purpose is not well elaborated.  The impact of the ungauged reservoir should be also associated with its intended purpose be it irrigation, flood control or hydropower, and the hydrological analysis should be in line with the purpose and show its impact. It is stated in the results section about floods and low flows but it should be well articulated in the methods section what kind of analysis will be done based on the purpose

In the introduction section, the authors should include more information on global hydrological models, the reliability and accuracy of the streamflow simulations and satellite remote sensing data to confirm the validity of their findings.

I suggest the authors to reword the research questions into concise and quantifiable question than a YES/NO kind of answer [line 124-129].

The authors used publicly available data from various sources, and providing clear information on the data sources, including their availability and any necessary preprocessing steps, would strengthen the study's methodology.

Despite the fact that there are a number of bias correction methods, the authors chose Quintile mapping for their study. Why is this method selected? I wonder if the authors show how this method captures the underlying biases. It can be in the supplemental material if it increases the manuscript pages.

The study discusses a comparative analysis of different satellite images such as Landsat-8 and Sentinels to map reservoir surface extents as illustrated in figure 4. It is difficult to understand which images source is most accurate compared to certain benchmarks/ground-truth data.

Additionally, the temporal resolutions (figure 4) are different for each satellite product to make visual comparisons on the reservoir storage changes. These require further explanation. If you are saying for example a one day inflow doesn’t have any impact on the area of the reservoir.

 

The authors concluded the importance of using publicly accessible global hydrological model, such as GESS platform to provide acceptable streamflow simulation at ungauged reservoirs and more. The conclusion section still lacks to put remarks on how the estimated impacts can inform decision-making processes, and the need to appropriate mitigation measures for the findings.

 

 

The quality of English is good.

 

Author Response

Dear Reviewer,

We want to express our sincere gratitude for your valuable comments and suggestions. We appreciate it.

Enclosed, please find our comprehensive response to your comments.

Thank you once again for your time and expertise in reviewing our work.

Warm regards,

Ngoc Thi Nguyen, Tien Le Thuy Du, Hyunkyu Park, Chi-Hung Chang, Sunghwa Choi, Hyosok Chae,E. James Nelson, Faisal Hossain,Donghwan Kim, Hyongki Lee

Author Response File: Author Response.docx

Reviewer 3 Report

[General Comments]

 

1) If possible, please describe the expected accuracy of estimated reservoir storage volume V (considering the observation error of satellite-observed water surface area and topography data).

# basd on your past research and other example of applications

 

2) In this study, Quantile Mapping Approach by Farmer et al. (2018) was used for bias correction of simulated streamflow, however the basic idea and concept of bias correction method is not well described in the section 3.2. So, please add a little more explanation about bias correction for GESS streamflow data at ungauged location (Hwanggang dam).

 

[Specific Comments]

 

1) P5, Table 1, Estimated streamflow, Temporal resolution

 Hourly --> Daily (for Historical Data (?))

 

2) P6, Line 213

 "... each elevation interval (n) ..."

-->

 "... each elevation interval (i) ..."  (?)

 

3) P6, Eq.(1)

 Sigma  --> Sigma_i=1 ^ n

# Sigma (summation) from i=1 to n (?)

# n: the number of elevation band (?)

 

4) P7, L225

 MNDWI = (X_green - ... --> Please explain what X represents.

# X_green: reflectance for the green wavelength band (?)

 

5) P8, Line 284

 "The drainage ratio ..." --> How was the "ratio" value?

# This is the question to find out how different these two catchment areas are.

 

6) P8, Line 310 and P9, Table 2

 "Coefficient of coefficient"  -->  "Coefficient of correlation" (?)

 

7) P10, Line 357

 What does "reservoir patterns" means?

# Does this imply a seasonal pattern of dam storage/release?

 

8) P11, Line 375-

 "Figure 5 illustrates that ..."

-->

 You have discussed the pattern of seasonal changes in water surface area in Figure 5, however how about the discussion about inter-annual variations?

For example, water surface area in winter season (Feb.-Apr.) of 2021 & 2022 are larger than other years (2017-2020). Is it caused by more precipitation from previous fall season?

 

9) P15, Figure 8

 Very high value of "median monthly percentage of reservoir storages over monthly natural streamflow" was found for Hwanggang site in May (more than 90%?). I wonder whether such high value is possible (realistic) or not. If it occurs, what would be the possible causes? 

Author Response

Dear Reviewer,

We want to express our sincere gratitude for your valuable comments and suggestions. We appreciate it.

Enclosed, please find our comprehensive response to your comments.

Thank you once again for your time and expertise in reviewing our work.

Warm regards,

Ngoc Thi Nguyen, Tien Le Thuy Du, Hyunkyu Park, Chi-Hung Chang, Sunghwa Choi, Hyosok Chae, E. James Nelson, Faisal Hossain, Donghwan Kim, Hyongki Lee

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Authors, 

Besides some editing of figures and tables for them to be on the same page as their captions, I think the changes are enough to justify my recommendation to publish this manuscript.