the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Apr 2021
Hydrometeorological drivers of flood characteristics in the Brahmaputra river basin in Bangladesh
Abstract. While flooding is an annual occurrence in the Brahmaputra basin during the South Asian summer monsoon, there is large variability in the flood characteristics that drive risk: flood duration, rate of water level rise and peak water level. The aim of this study is to understand the key hydrometeorological drivers influencing these flood characteristics. We analyse hydrometeorological time series of the last 33 years to understand flood dynamics focusing on three extraordinary floods in 1998 (long duration), 2017 (rapid rise) and 2019 (high water level). We find that long duration floods in the basin have been driven by basin-wide seasonal rainfall extremes associated with the development phase of strong La Niña events, whereas floods with a rapid rate of rise have been driven by more localized rainfall falling in a hydrological ‘sweet spot’ that leads to a concurrent contribution from the tributaries into the main stem of the river. We find that recent record high water levels are not coincident with extreme river flows, hinting that sedimentation and morphological changes are also important drivers of flood risk that should be further investigated. Understanding these drivers is essential for flood forecasting and early warning and also to study the impact of future climate change on flood.
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RC1: 'Comment on hess-2021-97: Hydrometeorological drivers of flood characteristics in the Brahmaputra river basin in Bangladesh by Hossain et al.', Anonymous Referee #1, 28 Jun 2021
The manuscript analyses the hydrometeorological factors that contribute to the occurrence of devastating floods in the Brahmaputra river (Bangladesh). The authors focus on three exceptional events characterised by either long duration, rapid rise or high water level. They use observational and reanalysis data, together with climate indices to determine the most relevant hydrometeorological drivers leading to each flood type. The authors find that strong La Niña favours long duration floods, whereas localised precipitation supports rapid rise floods. Conversely, other drivers are identified as more influential for high water level cases.
The authors tackle a relevant topic which fits within the scope of the journal and may be of great interest for readers. However, the structure of some parts of the manuscript needs to be revised in order to provide an enhanced description of the objectives of this study, its background and its conclusions. Therefore, I recommend a major revision to improve the quality of the manuscript before its publication.
General comments
Some parts of the manuscript require a revision to avoid repetitive ideas and improve discourse flow, as some parts of the text are rather disconnected. In addition, the authors should explain better the rationale behind each of the analysis described in the manuscript.
Besides the structure of the manuscript, I missed a more detailed analysis of the meteorological situation leading to localised short-duration rainfall events, which are key for rapid rise floods according to the manuscript's conclusions. Have the authors considered which mesoscale or local-scale atmospheric factors contribute to these intense rainfall events? If not, are they aware of any study that analyses this aspect for the events considered here (or others of similar nature)?
Moreover, it gives the impression that the 33-year data is not fully exploited. Some of the analyses shown in the manuscript could be extended to other years to better highlight the differences between the selected years, when exceptional floods occurred, and the rest. This is only done for some drivers, but not for all, which can be confusing for readers.
Specific comments
Abstract (L26-27): Does the last sentence refer to the drivers analysed in this study or to sedimentation and morphological changes? If it is the latter, I suggest removing this sentence, as this aspect is not investigated in this study nor anything related to climate change. In this case, I would replace this sentence by a new one that summarises the main conclusions of the manuscript.
Introduction: I recommend modifying its structure. I think it would be more appropriate to start with more general aspects of flooding in the area before introducing the details regarding the most devastating cases. In addition, the whole section should be carefully revised to avoid repetitive ideas (e.g., L37-38 and L46-47) and provide a coherent text that focuses on the most relevant aspects that will be discussed in the following sections.
L36: I suggest moving Fig. S1 to the main manuscript.
L59-61: Why are these factors chosen?
L62: Why are these cases analysed in more detail?
Section 3: The beginning of this section is too abrupt. I suggest adding an introductory sentence indicating the value of each data source for this study.
L212: Please, add a map illustrating the Niño 3.4 region or provide some geographical references or the area.
L236-243: The current wording of this part of the manuscript is confusing. Please, clarify what is intended to be said.
L250: I propose including other years in the supplement.
Section 5.1: Can mesoscale or local-scale mechanisms be analysed? (see general comments)
Fig. 4b: I suggest including mean JJAS rainfall for each year in this panel.
L356: Please, define "sweet spot".
Section 5.1.3: Please, add a brief introductory sentence.
L451: The authors should mention some of these "other drivers".
L483: The relevance of these "short-duration rainfall events" motivates an analysis of them (see general comments).
Section 5.2.3: Please, add an introductory sentence indicating why is soil moisture relevant.
L488: Can the data confirm that "basin-wide and frequent rainfall events maintained soil moisture" and thus, avoid saying "it is likely"?
Section 5.2.4: Again, add some introduction to this section.
L529: I suggest including these values in a table.
L534: I recommend adding Fig. S6 to Fig. 10.
L537-538: The authors should give some recommendations to perform this task.
L563-565: I suggest moving this sentence to the beginning of the subsection. This kind of sentences can be used as introduction for other subsections (see previous comments).
L586-588: Please, move this sentence to the beginning of the subsection.
Section 6: As it is currently written this section is more like a summary than a discussion. In addition, the two subsections include various repetitive ideas. I suggest summarising the whole section in order to succinctly provide the most important take-home messages for readers.
L671-672: The selected cases fall in different categories. This classification should be better explained to avoid confusion.
L674 onwards: Strong La Niña seems to be a necessary but not sufficient condition for long duration floods. This idea should be emphasised.
L686-688: Accurate forecast of heavy rainfall is extremely challenging for small scales and long lead times. The authors should clarify the spatial and temporal scales of interest and include some references indicating the feasibility of this proposal and/or its associated challenges.
Table 2: I recommend moving this table to the supplement.
Section 6.2: Climate change and disaster management perspectives: Since climate change is not considered in this study, I suggest moving this part to the conclusions as a subject to be considered for future work.
L737-738: The simultaneity of the two factors seems crucial for the development of long duration floods. This idea should be highlighted in the discussion, rather than focusing only on strong La Niña.
L746-747: All climate change discussions should be moved here (see previous comment).
Technical corrections
L45: Fig. 3 is cited before Fig. 2 (L115).
L56: I suggest "(e.g. 2017 floods). Indeed, the Bangladesh Flood Forecasting and warning Centre (FFWC) is keen to improve early warnings for these events".
Caption Fig. 3: Consider a slight modification of the caption, for instance: "Dates on which water level (WL) exceeded the danger level at the Bahadurabad station on the Brahmaputra river (see Fig. 1b for location of the gauge) indicated by a coloured dot. The colour represents WL value as indicated in the legend". Please, also indicate the threshold for severe flooding.
L210: El Ninõ (La Ninã) -> El Niño (La Niña).
L215: Please, define IPRC/SOEST.
L238: MJO -> (MJO).
L246: Remove the parenthesis from the reference to Wheeler and Hendon.
L298: Please, define ECMWF.
L328: "floods in August 2017 was" -> flood...was or floods...were.
Caption Fig. 4: Is 2010 missing in the caption?
L487: Please, add a coma between monsoon and soil.
Caption Fig. 12: Please, mention the different vertical scales for each panel.
Citation: https://doi.org/10.5194/hess-2021-97-RC1 -
AC1: 'Reply on RC1', Sazzad Hossain, 24 Sep 2021
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2021-97/hess-2021-97-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Sazzad Hossain, 24 Sep 2021
-
RC2: 'Comment on hess-2021-97', Anonymous Referee #2, 18 Jul 2021
Although this study would be worthy of publication in HESS, I think its current version does not meet the overall quality of HESS journal. The paper is not well written, and the structure (“key story”) is not provided in a logical sequence. The idea that hydrogeological drivers (e.g., monsoon rainfall and antecedent soil moisture) determines the characteristics of floods Brahmaputra and Ganges rivers is not novel. I believe that the authors have done a lot of work analyzing the data, but authors failed to convince the readers why they are doing so. The style of this manuscript is more like “this is what we did”. As a reader, I am not convinced by the authors that their findings are novel and interesting. Here I provide three general (personal) suggestions for authors’ review which may help improve the overall quality of this manuscript:
- State the key objective or research question clearly and revise the introduction accordingly. The current version of this manuscript covers, at least, two main topics: attribution of flood characteristics into hydrometeorological drivers and comprehensive analysis of three historical flood events in Brahmaputra basin. However, none of them is well defined and presented in a logical structure. If the first case is the objective, authors should increase their sample size (i.e., over 30 years flood data) to make a robust conclusion. If the second case is the goal, authors should focus contrasting three flood events (i.e., types) and highlighting the key features that cause the difference. A good example of analyzing single flood event can be found: Smith, James A., et al. "Extreme flood response: The June 2008 flooding in Iowa." Journal of Hydrometeorology 14.6 (2013): 1810-1825.
- Based on the determined research objective, authors should consider remove some unnecessary analyses which fail to directly support the main conclusion. The current study used GEV distribution, trend analysis, correlation between climate indices with floods, wavelet transform etc. However, some of the analysis does not directly support the conclusion. For example, authors show there is a trend in water level but failed to attribute this trend to any of the hydrometeorological drivers and to explain how this trend affects the flood characteristics in general. If authors want to include a conclusion or result, defend it in detail. Otherwise, drop it.
- Go through the paper and make every sentence convincing and logical. Also, delete the sentences conveying the same idea. As a reader, some sentence sound vague and does not provide the information I am expected to understand. Here are two examples:
- Line 556: “However, due to spatial variation of rainfall there can also be cases of a more rapid rise in water levels.” I am expected to understand the reason why rainfall heterogeneity causes the rapid rise in water levels at the gage. Is it because rainfall hit the region where the watershed slope is high? Are these rainfall have similar temporal distribution?
- Line 534: “The estimated annual maximum discharge in 2019 and 2017 was lower than the one in 1998, despite higher water levels.” In most gages, the discharge is estimated using rating curve and water level. So I am expected to understand why high water level is linked to a low discharge, which authors failed to provide.
Citation: https://doi.org/10.5194/hess-2021-97-RC2 -
AC2: 'Reply on RC2', Sazzad Hossain, 25 Sep 2021
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2021-97/hess-2021-97-AC2-supplement.pdf
Status: closed
-
RC1: 'Comment on hess-2021-97: Hydrometeorological drivers of flood characteristics in the Brahmaputra river basin in Bangladesh by Hossain et al.', Anonymous Referee #1, 28 Jun 2021
The manuscript analyses the hydrometeorological factors that contribute to the occurrence of devastating floods in the Brahmaputra river (Bangladesh). The authors focus on three exceptional events characterised by either long duration, rapid rise or high water level. They use observational and reanalysis data, together with climate indices to determine the most relevant hydrometeorological drivers leading to each flood type. The authors find that strong La Niña favours long duration floods, whereas localised precipitation supports rapid rise floods. Conversely, other drivers are identified as more influential for high water level cases.
The authors tackle a relevant topic which fits within the scope of the journal and may be of great interest for readers. However, the structure of some parts of the manuscript needs to be revised in order to provide an enhanced description of the objectives of this study, its background and its conclusions. Therefore, I recommend a major revision to improve the quality of the manuscript before its publication.
General comments
Some parts of the manuscript require a revision to avoid repetitive ideas and improve discourse flow, as some parts of the text are rather disconnected. In addition, the authors should explain better the rationale behind each of the analysis described in the manuscript.
Besides the structure of the manuscript, I missed a more detailed analysis of the meteorological situation leading to localised short-duration rainfall events, which are key for rapid rise floods according to the manuscript's conclusions. Have the authors considered which mesoscale or local-scale atmospheric factors contribute to these intense rainfall events? If not, are they aware of any study that analyses this aspect for the events considered here (or others of similar nature)?
Moreover, it gives the impression that the 33-year data is not fully exploited. Some of the analyses shown in the manuscript could be extended to other years to better highlight the differences between the selected years, when exceptional floods occurred, and the rest. This is only done for some drivers, but not for all, which can be confusing for readers.
Specific comments
Abstract (L26-27): Does the last sentence refer to the drivers analysed in this study or to sedimentation and morphological changes? If it is the latter, I suggest removing this sentence, as this aspect is not investigated in this study nor anything related to climate change. In this case, I would replace this sentence by a new one that summarises the main conclusions of the manuscript.
Introduction: I recommend modifying its structure. I think it would be more appropriate to start with more general aspects of flooding in the area before introducing the details regarding the most devastating cases. In addition, the whole section should be carefully revised to avoid repetitive ideas (e.g., L37-38 and L46-47) and provide a coherent text that focuses on the most relevant aspects that will be discussed in the following sections.
L36: I suggest moving Fig. S1 to the main manuscript.
L59-61: Why are these factors chosen?
L62: Why are these cases analysed in more detail?
Section 3: The beginning of this section is too abrupt. I suggest adding an introductory sentence indicating the value of each data source for this study.
L212: Please, add a map illustrating the Niño 3.4 region or provide some geographical references or the area.
L236-243: The current wording of this part of the manuscript is confusing. Please, clarify what is intended to be said.
L250: I propose including other years in the supplement.
Section 5.1: Can mesoscale or local-scale mechanisms be analysed? (see general comments)
Fig. 4b: I suggest including mean JJAS rainfall for each year in this panel.
L356: Please, define "sweet spot".
Section 5.1.3: Please, add a brief introductory sentence.
L451: The authors should mention some of these "other drivers".
L483: The relevance of these "short-duration rainfall events" motivates an analysis of them (see general comments).
Section 5.2.3: Please, add an introductory sentence indicating why is soil moisture relevant.
L488: Can the data confirm that "basin-wide and frequent rainfall events maintained soil moisture" and thus, avoid saying "it is likely"?
Section 5.2.4: Again, add some introduction to this section.
L529: I suggest including these values in a table.
L534: I recommend adding Fig. S6 to Fig. 10.
L537-538: The authors should give some recommendations to perform this task.
L563-565: I suggest moving this sentence to the beginning of the subsection. This kind of sentences can be used as introduction for other subsections (see previous comments).
L586-588: Please, move this sentence to the beginning of the subsection.
Section 6: As it is currently written this section is more like a summary than a discussion. In addition, the two subsections include various repetitive ideas. I suggest summarising the whole section in order to succinctly provide the most important take-home messages for readers.
L671-672: The selected cases fall in different categories. This classification should be better explained to avoid confusion.
L674 onwards: Strong La Niña seems to be a necessary but not sufficient condition for long duration floods. This idea should be emphasised.
L686-688: Accurate forecast of heavy rainfall is extremely challenging for small scales and long lead times. The authors should clarify the spatial and temporal scales of interest and include some references indicating the feasibility of this proposal and/or its associated challenges.
Table 2: I recommend moving this table to the supplement.
Section 6.2: Climate change and disaster management perspectives: Since climate change is not considered in this study, I suggest moving this part to the conclusions as a subject to be considered for future work.
L737-738: The simultaneity of the two factors seems crucial for the development of long duration floods. This idea should be highlighted in the discussion, rather than focusing only on strong La Niña.
L746-747: All climate change discussions should be moved here (see previous comment).
Technical corrections
L45: Fig. 3 is cited before Fig. 2 (L115).
L56: I suggest "(e.g. 2017 floods). Indeed, the Bangladesh Flood Forecasting and warning Centre (FFWC) is keen to improve early warnings for these events".
Caption Fig. 3: Consider a slight modification of the caption, for instance: "Dates on which water level (WL) exceeded the danger level at the Bahadurabad station on the Brahmaputra river (see Fig. 1b for location of the gauge) indicated by a coloured dot. The colour represents WL value as indicated in the legend". Please, also indicate the threshold for severe flooding.
L210: El Ninõ (La Ninã) -> El Niño (La Niña).
L215: Please, define IPRC/SOEST.
L238: MJO -> (MJO).
L246: Remove the parenthesis from the reference to Wheeler and Hendon.
L298: Please, define ECMWF.
L328: "floods in August 2017 was" -> flood...was or floods...were.
Caption Fig. 4: Is 2010 missing in the caption?
L487: Please, add a coma between monsoon and soil.
Caption Fig. 12: Please, mention the different vertical scales for each panel.
Citation: https://doi.org/10.5194/hess-2021-97-RC1 -
AC1: 'Reply on RC1', Sazzad Hossain, 24 Sep 2021
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2021-97/hess-2021-97-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Sazzad Hossain, 24 Sep 2021
-
RC2: 'Comment on hess-2021-97', Anonymous Referee #2, 18 Jul 2021
Although this study would be worthy of publication in HESS, I think its current version does not meet the overall quality of HESS journal. The paper is not well written, and the structure (“key story”) is not provided in a logical sequence. The idea that hydrogeological drivers (e.g., monsoon rainfall and antecedent soil moisture) determines the characteristics of floods Brahmaputra and Ganges rivers is not novel. I believe that the authors have done a lot of work analyzing the data, but authors failed to convince the readers why they are doing so. The style of this manuscript is more like “this is what we did”. As a reader, I am not convinced by the authors that their findings are novel and interesting. Here I provide three general (personal) suggestions for authors’ review which may help improve the overall quality of this manuscript:
- State the key objective or research question clearly and revise the introduction accordingly. The current version of this manuscript covers, at least, two main topics: attribution of flood characteristics into hydrometeorological drivers and comprehensive analysis of three historical flood events in Brahmaputra basin. However, none of them is well defined and presented in a logical structure. If the first case is the objective, authors should increase their sample size (i.e., over 30 years flood data) to make a robust conclusion. If the second case is the goal, authors should focus contrasting three flood events (i.e., types) and highlighting the key features that cause the difference. A good example of analyzing single flood event can be found: Smith, James A., et al. "Extreme flood response: The June 2008 flooding in Iowa." Journal of Hydrometeorology 14.6 (2013): 1810-1825.
- Based on the determined research objective, authors should consider remove some unnecessary analyses which fail to directly support the main conclusion. The current study used GEV distribution, trend analysis, correlation between climate indices with floods, wavelet transform etc. However, some of the analysis does not directly support the conclusion. For example, authors show there is a trend in water level but failed to attribute this trend to any of the hydrometeorological drivers and to explain how this trend affects the flood characteristics in general. If authors want to include a conclusion or result, defend it in detail. Otherwise, drop it.
- Go through the paper and make every sentence convincing and logical. Also, delete the sentences conveying the same idea. As a reader, some sentence sound vague and does not provide the information I am expected to understand. Here are two examples:
- Line 556: “However, due to spatial variation of rainfall there can also be cases of a more rapid rise in water levels.” I am expected to understand the reason why rainfall heterogeneity causes the rapid rise in water levels at the gage. Is it because rainfall hit the region where the watershed slope is high? Are these rainfall have similar temporal distribution?
- Line 534: “The estimated annual maximum discharge in 2019 and 2017 was lower than the one in 1998, despite higher water levels.” In most gages, the discharge is estimated using rating curve and water level. So I am expected to understand why high water level is linked to a low discharge, which authors failed to provide.
Citation: https://doi.org/10.5194/hess-2021-97-RC2 -
AC2: 'Reply on RC2', Sazzad Hossain, 25 Sep 2021
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2021-97/hess-2021-97-AC2-supplement.pdf
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Cited
2 citations as recorded by crossref.
- A multi-system comparison of forecast flooding extent using a scale-selective approach H. Hooker et al. 10.2166/nh.2023.025
- A decision‐led evaluation approach for flood forecasting system developments: An application to the Global Flood Awareness System in Bangladesh S. Hossain et al. 10.1111/jfr3.12959