Robust, Long-term Lake Level Change from Multiple Satellite Altimeters in Tibet: Observing the Rapid Rise of Ngangzi Co over a New Wetland

Round 1

Reviewer 1 Report

This paper is mainly about using an appropriate method to generate Tibetan lake level time series. Paper is well-organized and well-written, however, it still needs more details before final publication.

Provide more details about your new findings in abstract section.

You have generated high-quality results, however, you need to compare them with other similar studies with more details.

You need to discuss the results in more details. For example, in figure 7, differences do not show the same pattern for each of the figures. You need to discuss the reasons and provide more details.

Bias adjustment is a very complicated process. I think you should provide more details for those readers who are not familiar with this subject. You need to provide some fundamentals.

Author Response

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This paper is mainly about using an appropriate method to generate Tibetan lake level time series. Paper is well-organized and well-written, however, it still needs more details before final publication.

Response: Thank you for the comments.

1. Provide more details about your new findings in abstract section.

Response: We added quantitative description of our results in abstract as following:

“……Finally, a 25-year-long lake level time series of Ngangzi Co are constructed using the TOPEX/Poseidon-family altimeter data from October 1992 to December 2017, with the accuracy of ~17 cm for TOPEX/Poseidon and ~10 cm for Jason-1/2/3”.

 “……The time series indicates that the lake level of Ngangzi Co increased by ~8 m over 1998-2017 and changed with different rates in the past 25 years (-0.39 m/yr in 1992-1997, 1.03 m/yr in 1998-2002 and 0.32 m/yr in 2003-2014). These findings will enhance the understanding of water budget and effect of climate change.”

2. You have generated high-quality results, however, you need to compare them with other similar studies with more details.

Response: We added a table (Table 4) in discussion section to show the detailed result of comparison with G-REALM and Hydroweb.

3.You need to discuss the results in more details. For example, in figure 7, differences do not show the same pattern for each of the figures. You need to discuss the reasons and provide more details.

Response: Figure 7 shows only the inter-satellite differences of lake level against the point number (can be thought of as sampling time) similar to Figure 8, not the statistical histogram. We don’t think they should show the same pattern. Detailed statistical information is provided in Table 2.   

4. Bias adjustment is a very complicated process. I think you should provide more details for those readers who are not familiar with this subject. You need to provide some fundamentals.

Response: We added a new section (Section 2.5) to explain the process of bias adjustment, and provided three references about typical methods for bias adjustment.

Author Response File: Author Response.docx

Reviewer 2 Report

Dear authors,

I reviewed the paper entitled “Robust, Long-term lake level change from multiple satellite altimeters in Tibet: observing the rapid rise of Ngangzi Co over a New Wetland” by Haihong Want et al. The paper analyzes 25 years of T/P altimeter data to construct a time series of Ngangzi Co level change from 1992 to 2017. ICESat and SARAL data are used for validation with good agreement, once no gauge in situ data are available. The authors also analyze their results comparing them with different available public databases for inland water bodies trying to explain the reasons for the different biases found. The method checks the consistence of the different geophysical corrections to the data from different time period including the application of different retracking algorithms. The authors also points out the advantages of their results with respect to previous studies in Ngangzi Co.

The study is very interesting. The paper is well written, including the structure, enough number of references to focus and analyze the study, and the language is correct. The figures are of good quality.

Just few comments:

Line 39. Check “rertacker”

Line 62. Check “DARIn”

Line 111. Check “senor”

Line 120. Add a reference to formula (1)

Line 380. Replace “data was used” with “data were used”

Line 399. Check “Combing”

Reference [20] seems no cited in the text

 

Author Response

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I reviewed the paper entitled “Robust, Long-term lake level change from multiple satellite altimeters in Tibet: observing the rapid rise of Ngangzi Co over a New Wetland” by Haihong Want et al. The paper analyzes 25 years of T/P altimeter data to construct a time series of Ngangzi Co level change from 1992 to 2017. ICESat and SARAL data are used for validation with good agreement, once no gauge in situ data are available. The authors also analyze their results comparing them with different available public databases for inland water bodies trying to explain the reasons for the different biases found. The method checks the consistence of the different geophysical corrections to the data from different time period including the application of different retracking algorithms. The authors also points out the advantages of their results with respect to previous studies in Ngangzi Co.

The study is very interesting. The paper is well written, including the structure, enough number of references to focus and analyze the study, and the language is correct. The figures are of good quality.

Response: Thank you for the comments.

Just few comments:

Line 39. Check “rertacker”

Response: Corrected accordingly.

Line 62. Check “DARIn”

Response: Corrected accordingly.

Line 111. Check “senor”

Response: Corrected accordingly.

Line 120. Add a reference to formula (1)

Response: Corrected accordingly.

Line 380. Replace “data was used” with “data were used”

Response: Corrected accordingly.

Line 399. Check “Combing”

Response: Corrected accordingly.

Reference [20] seems no cited in the text

Response: Corrected accordingly.

Author Response File: Author Response.docx

Reviewer 3 Report

In order to obtain water level change of Lake Ngangzi Co in Tibet, this manuscript uses inland T/P-series satellite altimetry data. The noisy data required series of data processing and quality controls. The method has been validated by other altimeters with smaller footprint size (ICESat and SARAL) and public databases (G-REALM and Hydroweb). The determined time series of the lake level show significant O(8m) long-term changes with O(1m) seasonal variations, although their causes were not investigated in this manuscript.

I personally appreciate challenging studies like this manuscript, so I would like to eventually accept this manuscript. However, I’m afraid that this manuscript has significant problems at the present form. These problems can be summarized as the lack of consideration of horizontal boundary (lake shore and area). 

First, changes of the area of the lake are not discussed in the manuscript. Since the lake is not small, 8-m increase of the lake level require significant amount of the water volume. The authors should suggest consistent sources of these water volume, e.g. water flux by precipitation minus evaporation. In addition, please discuss whether the flux is consistent with the seasonal variation. 

Note also that the change of the area would alter the distance of the sub-satellite tracks from the shore line; the distance would be shortest in T/P period (1992-2002) since the lake level is low, which could also explain worse performance of T/P. In general, altimetry data close to shore lines should not be used even retracking algorithms were applied, since the contaminated part of the waveform becomes close to the leading edge. Therefore, I guess the southern part of Path 079 should not be included in the analysis in order to keep p in Equation (2) high as the paths of the other altimeters. 

Therefore, as my conclusion, I recommend moderate revision before the manuscript will be finally accepted. 

--------Minor Comments

L113) Are these data in 1Hz or 20Hz? 

Fig.1) Since temporal changes of the lake area is concerned, it would better to describe when the map data were obtained. The authors may use historical visible satellite imagery to show temporal changes of the lake area.

Equation 1) The authors may mention that other geophysical terms (e.g. lake tides, hydrostatic variations, thermal expansion, wind piling-up effect) are negligible.

Author Response

Responses to the comments of Reviewer 3

In order to obtain water level change of Lake Ngangzi Co in Tibet, this manuscript uses inland T/P-series satellite altimetry data. The noisy data required series of data processing and quality controls. The method has been validated by other altimeters with smaller footprint size (ICESat and SARAL) and public databases (G-REALM and Hydroweb). The determined time series of the lake level show significant O(8m) long-term changes with O(1m) seasonal variations, although their causes were not investigated in this manuscript.

1. I personally appreciate challenging studies like this manuscript, so I would like to eventually accept this manuscript. However, I’m afraid that this manuscript has significant problems at the present form. These problems can be summarized as the lack of consideration of horizontal boundary (lake shore and area). 

Response: Thank you for the comments. We agree that the lake area will affect the altimeter data. We added a discussion in the revised version. (see Response 3 for more details)

2. First, changes of the area of the lake are not discussed in the manuscript. Since the lake is not small, 8-m increase of the lake level require significant amount of the water volume. The authors should suggest consistent sources of these water volume, e.g. water flux by precipitation minus evaporation. In addition, please discuss whether the flux is consistent with the seasonal variation. 

Response: We added two passages and two figures (Figure 12 and Figure 13) in discussion section. 

The increase of the lake level is manly due to the enhanced precipitation. It shows high consistency between the lake level change and precipitation. The lake area change derived from satellite images and the rise of water storage in 2002-2016 also confirm our result.

 

3. Note also that the change of the area would alter the distance of the sub-satellite tracks from the shore line; the distance would be shortest in T/P period (1992-2002) since the lake level is low, which could also explain worse performance of T/P. In general, altimetry data close to shore lines should not be used even retracking algorithms were applied, since the contaminated part of the waveform becomes close to the leading edge. Therefore, I guess the southern part of Path 079 should not be included in the analysis in order to keep p in Equation (2) high as the paths of the other altimeters. 

Response: We agree with reviewer’s point about the change of lake area. We used a static lake boundary to select data points instead of time-varying boundary. One of the reason is it is difficult to obtain the accurate time-varying boundary for each cycle. Another reason is that the estimation of mean lake levels would be less affected by the lack of consideration of horizontal boundary, although some data on shore might be included. It is because we can remove these data in the procedure of outlier removal. Indeed, this would reduce the p value in Equation (2), resulting in a small PSR. But the sigma will not be changed. And it should be emphasized that PSR is only proposed to assess the performance of retrackers.

     We added a discussion in the revise manuscript as follows: “It should be mentioned that the change of lake area was not considered in data processing. A static shoreline was used to extract altimeter data for all missions. This will cause some data on the shore may be extracted, especially when the lake level is low (e.g. in T/P period). The shrink of lake area will also make the lake level observation noisier. It can explain why the PSR value of T/P is so small. Modelling accurate time-varying shorelines is a difficult and time-consuming work and need other data source, such as satellite imagery. So a procedure for outlier removal is often performed, instead of the consideration of dynamical lake boundary. Figure 5 has shown that the estimation of mean lake levels would be less affected by the lack of consideration of horizontal boundary, benefitted from the good performance of outlier detection procedure proposed in this study.”

--------Minor Comments

L113) Are these data in 1Hz or 20Hz? 

Response: The data resolutions of these products are 10 Hz for T/P, 20 Hz for Jason-series, 18 Hz for Envisat and 40 Hz for SARAL, respectively. We added this description in revised manuscript.

Fig.1) Since temporal changes of the lake area is concerned, it would better to describe when the map data were obtained. The authors may use historical visible satellite imagery to show temporal changes of the lake area.

Response: The background of Fig. 1 is the topography from the SRTM DEM. The temporal changes of lake area by satellite imagery have been supplemented in Figure 13a.

Equation 1) The authors may mention that other geophysical terms (e.g. lake tides, hydrostatic variations, thermal expansion, wind piling-up effect) are negligible.

Response: We added a sentence in Section 2.1 to mention this as follows:

“With respect to the above-mentioned corrections, other geophysical terms (e.g. lake tides, hydrostatic variations, thermal expansion, wind piling-up effect) are negligible and hence ignored in this study.”

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

I am very satisfied with the revisions. I agree to accept this manuscript in the present form.

Here are just a few very-minor comments;

Fig.12 "surface temperature": Is this air temperature? Some readers may confuse with water or land temperature.

L431-432: You may plot the T/P subsatellite track in Fig.13(a).