Monitoring and Analysis of Drought Using Gravity Recovery and Climate Experiment (GRACE)

Round 1

Reviewer 1 Report

Review of Article Submission to the journal Hydrology

Article title: Monitoring and Analysis of Drought using Gravity Recovery and Climate Experiment (GRACE)

 

Summary: The article uses ground-monitor precipitation and temperature measurements and remotely sensed terrestrial water storage measurements from the GRACE satellites to study drought conditions at 3, 6 and 12 month time scales across central Iran over the period 2002 through 2016. Three metrics are intercompared: the Standardized Precipitation Index (SPI), the Modified Reconnaissance Drought Index (RDIe), and the Total Storage Deficit Index (TSDI). SPI is based on precipitation data only, RDIe utilizes precipitation and temperature, and TSDI based solely on terrestrial water storage from GRACE. A main focus is on assessing the maps and event detection based on TSDI, since it is based on remotely sensed data and thus offers potential advantages in wide catchments (as studied in this paper) where large spatial gaps between ground monitors may exist.  Results are analyzed with respect to zones of varying aridity within central Iran. Drought conditions were most prevalent in the semi-arid zones.

 

Critique: Overall I am very favorable about the paper and recommend publication after minor revisions. The paper is well organized overall, and does a good job of intercomparing results between SPI, RDIe, and TSDI. The broad conformity of the TSDI drought index maps those derived from the other two (Fig 6) provide an effective demonstration of the utility of TSDI for wide catchments: a central theme of the study. The tables in Section 3.5 that compare indices for different time periods in each aridity zone are also useful for readers in this field to understand results on a finer level.

 

However, before publication the paper needs to thoroughly be proofread, correcting English issues and several areas where wording is confusing and can be made more clear and concise, and omitting unnecessary phases and words. Also, Figures 2 and 6 need to be clarified to include colorbars.

 

 Here are some examples, however please go through entire manuscript to improve English …

 

Line 18: “present study aims to analyze …” (correct “to analysis …”)

Line 30: “The study results indicated … “ (correct “It indicated …”)

Line 106 (and throughout text)”: Change “the expansion of area” to “the catchment area” (or something else besides “expansion of area”

Line 127: Center for Space Research (capitalized)

Line 129: “ … to coincide with the first full month when GRACE data are available”

Lines 130 – 133: Does this detail related to SLR need to be mentioned? Did author perform adjustments of GRACE data for this study based on this detail? If not, omit this since it confuses the reader.

Lines 135 – 137: Omit this part. It confuses reader since on one hand author’s say data is insufficient to develop model, and then they go ahead and develop a model in equation (1). I would replace just say, “to evaluate the GRACE data, a water budget model is applied …”

Line 139: d/dT should be d/dt in equation

Lines 140 – 142: replace verbs “represents”, “regarded”, “shows” with “is”. Omit part on “dt” … this is generally understood to be time.

Line 146: change “in” to “over” the desired time period

Line 153: Add sentence referring to Figure 1 so reader can be reminded of station locations

Line 158 – 159: Sentence is confusing … reword to make more concise and easier to follow.  And what “other two sets” are author’s referring to?

Section 2.5 (Lines 160- 192): Overall this is confusing. I think there are too many unnecessary words and explanations. Make more concise to more clearly, and directly state what the author’s did, omitting other material. One paragraph should suffice. Also, it would be good to show the Thornwaite equation to reader can see more clearly the method.

 

- Section 2.6: Like Section 2.5, each section needs to be explained more clearly. I am confused by what was previously done and what the authors did.

 

A good template would be a first paragraph summarizing the method, including equations if needed, explicitly referencing the paper of the method in the first sentence. Then, in a second (or third if needed) paragraph explain how the authors of this paper implemented the method based on the data they used. Did they compute each metric for each individual station or GRACE pixel? Was any spatial aggregation used?

 

In summary, a consistent two or three paragraph format for each Section 2.6.1, 2.6.2 and 2.6.3 would help reader follow the method better.

 

- Section 2.6.3: The explanation of how p and q were developed is confusing. Here, a figure showing an example of the time series plotting described in 288 and 289 would be beneficial, similar to something Yirdow et al. show in their paper.

 

- Line 264: Equation 8. Numerator is confusing with big gap between 125 and .2 … is there a symbol missing?

 

- Line 278: Equation 10. Should MTS be MTSA in numerator?

 

- Figures 2 and Figure 6. What are color scales of figure? Same as Table 5? Mention in Figure caption is so, or preferably include on figure. Also, there is a yellow color on Figure 6 not on Table 5. Clarify meaning of color scale on these figures.

 

- Figure 6: How were these maps constructed? What spatial mapping technique was used? Bi-linear interpretation? Kriging? This is important since SPI and RDIe are based on station measurements (discrete in space) and TSDI is based on regular GRACE measurements (continuous in space). So method of constructing maps should be stated so reader can understand better how to intercompare them.

 

 

 

 

 

 

Author Response

Response to Reviewer 1 Comments

 

Dear Reviewer

Thanks to the accuracy of your review, the questions below are briefly answered. All of the answers are presented in the form of an edited manuscript. Please find the attachment.

Kind Regards

 

Point 1: Line 18: “present study aims to analyze …” (correct “to analysis …”)

Response 1: It is corrected.

Point 2: Line 30: “The study results indicated … “ (correct “It indicated …”)

Response 2: It is corrected.

Point 3: Line 106 (and throughout text)”: Change “the expansion of area” to “the catchment area” (or something else besides “expansion of area”

Response 3: All of them corrected.

Point 4: Line 127: Center for Space Research (capitalized)

Response 4: It is corrected.

Point 5: Line 129: “ … to coincide with the first full month when GRACE data are available”

Response 5: It is corrected.

Point 6: Lines 130 – 133: Does this detail related to SLR need to be mentioned? Did author perform adjustments of GRACE data for this study based on this detail? If not, omit this since it confuses the reader.

Response 6: It is unnecessary and omitted.

Point 7: Lines 135 – 137: Omit this part. It confuses reader since on one hand author’s say data is insufficient to develop model, and then they go ahead and develop a model in equation (1). I would replace just say, “to evaluate the GRACE data, a water budget model is applied …”

Response 7: It is corrected.

Point 8: Line 139: d/dT should be d/dt in equation

Response 8: It is corrected.

Point 9: Lines 140 – 142: replace verbs “represents”, “regarded”, “shows” with “is”. Omit part on “dt” … this is generally understood to be time.

Response 9: It is corrected.

Point 10: Line 146: change “in” to “over” the desired time period

Response 10: It is corrected.

Point 11: Line 153: Add sentence referring to Figure 1 so reader can be reminded of station locations

Response 11: It is added.

Point 12: Line 158 – 159: Sentence is confusing … reword to make more concise and easier to follow.  And what “other two sets” are author’s referring to?

Response 12: We rewrite this section.

Point 13: Section 2.5 (Lines 160- 192): Overall this is confusing. I think there are too many unnecessary words and explanations. Make more concise to more clearly, and directly state what the author’s did, omitting other material. One paragraph should suffice. Also, it would be good to show the Thornwaite equation to reader can see more clearly the method.

 Response 13: We rewrite this section and omit the redundant materials.

Point 14: Section 2.6: Like Section 2.5, each section needs to be explained more clearly. I am confused by what was previously done and what the authors did.

 A good template would be a first paragraph summarizing the method, including equations if needed, explicitly referencing the paper of the method in the first sentence. Then, in a second (or third if needed) paragraph explain how the authors of this paper implemented the method based on the data they used. Did they compute each metric for each individual station or GRACE pixel? Was any spatial aggregation used?

 In summary, a consistent two or three paragraph format for each Section 2.6.1, 2.6.2 and 2.6.3 would help reader follow the method better.

 Response 14: We rewrite these sections and omit the redundant materials.

Point 15: Section 2.6.3: The explanation of how p and q were developed is confusing. Here, a figure showing an example of the time series plotting described in 288 and 289 would be beneficial, similar to something Yirdow et al. show in their paper.

 Response 15: We add a figure and describe how one can calculate p and q.

Point 16:  Line 264: Equation 8. Numerator is confusing with big gap between 125 and .2 … is there a symbol missing?

 Response 16: This equation is omitted and the equivalent table replced.

Point 17:  Line 278: Equation 10. Should MTS be MTSA in numerator?

 Response 8: We checked it again and apparently it was correct.

Point 18:  Figures 2 and Figure 6. What are color scales of figure? Same as Table 5? Mention in Figure caption is so, or preferably include on figure. Also, there is a yellow color on Figure 6 not on Table 5. Clarify meaning of color scale on these figures.

 Response 18: It is corrected. A color scale is added to the drought spatial map.

Point 19:  Figure 6: How were these maps constructed? What spatial mapping technique was used? Bi-linear interpretation? Kriging? This is important since SPI and RDIe are based on station measurements (discrete in space) and TSDI is based on regular GRACE measurements (continuous in space). So method of constructing maps should be stated so reader can understand better how to intercompare them.

Response 19: The method is kriging. A short description is added before the maps.

 

Author Response File: Author Response.docx

Reviewer 2 Report

This study is not well organized. Datasets and methods are mixed in the material and mathods section. Present the data and then the methods. The comparison between dS/dt from GRACE and P-ET as R=0, is not relayy convincing. After 2007, there is strong decrease in dS/dt from GRACE that is not observed on the in-situ data. What are the reason of these discrpancies. I strongly disagree with what is said by the authors. dS/dt = P-ET dS/dt containing all the water variations in the soil column including sub-surface storage (see Frappart and Ramillien, 2018 recent reviw on GRACE and the aquifer water content). In my opinion, two thingsthree reasons can account for these differences: i) the study area is not an endhoreic basin, but just part of it, so the previous relationship is not valid, ii) the GRACE signal is affected by leakage from a surrounding region (which is subject to recent changes in agricultural practices, e.G., groundwater pumping), ii) the estimate of ET is wrong. By the way, how it has been calculated. If the authors used PET instead of ET, this could explain the discrepancies. I would recommend the authors to use GRACE mascons solutions and compare the results with the current ones. Many references are missing in the introduction concerning: - the GRACE mission: Tapley, B.D.; Bettadpur, S.; Ries, J.C.; Thompson, P.F.; Watkins, M.M. GRACE measurements of mass variability in the Earth system. Science 2004, 305, 503–505. - the droughts detected using the GRACE data: Andersen, O.B.; Seneviratne, S.I.; Hinderer, J.; Viterbo, P. GRACE-derived terrestrial water storage depletion associated with the 2003 European heat wave. Geophys. Res. Lett. 2005, 32. Castle, S.L.; Thomas, B.F.; Reager, J.T.; Rodell, M.; Swenson, S.C.; Famiglietti, J.S. Groundwater depletion during drought threatens future water security of the Colorado River Basin. Geophys. Res. Lett. 2014, 41, 5904–5911. Chen, J.L.; Wilson, C.R.; Tapley, B.D.; Yang, Z.L.; Niu, G.Y. 2005 drought event in the Amazon River basin as measured by GRACE and estimated by climate models. J. Geophys. Res. 2009, 114. Chen, J. L., Wilson, C. R., Tapley, B. D., Longuevergne, L., Yang, Z. L., & Scanlon, B. R. (2010). Recent La Plata basin drought conditions observed by satellite gravimetry. Journal of Geophysical Research: Atmospheres, 115(D22). Frappart, F., Ramillien, G., & Ronchail, J. (2013). Changes in terrestrial water storage versus rainfall and discharges in the Amazon basin. International Journal of Climatology, 33(14), 3029-3046. Frappart, F., Papa, F., Güntner, A., Tomasella, J., Pfeffer, J., Ramillien, G., ... & Moreira, D. M. (2019). The spatio-temporal variability of groundwater storage in the Amazon River Basin. Advances in water resources, 124, 41-52. Leblanc, M.J.; Tregoning, P.; Ramillien, G.; Tweed, S.O.; Fakes, A. Basin-scale, integrated observations of the early 21st century multiyear drought in southeast Australia. Water Resour. Res. 2009, 45. Previous drought index computed using GRACE data should be mentionned: Thomas, B.F.; Famiglietti, J.S.; Landerer, F.W.; Wiese, D.N.; Molotch, N.P.; Argus, D.F. GRACE Groundwater Drought Index: Evaluation of California Central Valley groundwater drought. Remote Sens. Environ. 2017, 198, 384–392. As well as that GRACE is commonly used to monitor depletion of the aquifers (see the recent review from Frappart and Ramillien, 2018), especially as surface water is negligible in your study area: Frappart, F., & Ramillien, G. (2018). Monitoring groundwater storage changes using the Gravity Recovery and Climate Experiment (GRACE) satellite mission: A review. Remote Sensing, 10(6), 829.

Author Response

Response to Reviewer 2 Comments

 

Dear Reviewer

Thanks to the accuracy of your review, the questions below are briefly answered. All of the answers are presented in the form of an edited manuscript. Please find the attachment.

Kind Regards

 

Point 1: This study is not well organized. Datasets and methods are mixed in the material and mathods section. Present the data and then the methods.

Response 1: The structure of these sections is corrected.

Point 2: The comparison between dS/dt from GRACE and P-ET as R=0, is not relayy convincing. After 2007, there is strong decrease in dS/dt from GRACE that is not observed on the in-situ data. What are the reason of these discrpancies. I strongly disagree with what is said by the authors. dS/dt = P-ET dS/dt containing all the water variations in the soil column including sub-surface storage (see Frappart and Ramillien, 2018 recent reviw on GRACE and the aquifer water content). In my opinion, two thingsthree reasons can account for these differences: i) the study area is not an endhoreic basin, but just part of it, so the previous relationship is not valid, ii) the GRACE signal is affected by leakage from a surrounding region (which is subject to recent changes in agricultural practices, e.G., groundwater pumping), ii) the estimate of ET is wrong.

Response 2: According to some recent studies (Castle, S.L. et al., 2014) the monthly storage changes, dS/dt, is equal to P-ET-Q. But as illustrated in the map below, central cachment of Iran is an endhoreic basin. So we can suppose that there are no stream in or out of the basin. As you mentioned correctly, strong decrease in dS/dt is due to human contribution to groundwater depletion (Joodaki,  et al., 2014)which we omit this trend from TWSC for calculating of TSDI. Also, since the field data of evapotranspiration were not available in this area, this parameter was extracted as time series from Global Land Data Assimilation System Version 2.1 (GLDAS_NOAH10_M v2.1).

Point 3: I would recommend the authors to use GRACE mascons solutions and compare the results with the current ones.

Response 3: The mascon solutions for central catchment of Iran which downloaded from http://ccar.colorado.edu/grace/ is added to the graph which it’s trend is well in line with the previous graph.

Point 4: Many references are missing in the introduction concerning: - the GRACE mission

Response 4: All of the references are reviewed and added to the introduction and conclusion sections.

 

 

 

 

 

 

Author Response File: Author Response.docx