the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jan 2024
Assessment of Hydrology Estimates from ERA5 Reanalyses in Benin (West Africa)
Abstract. In West Africa, the validation of distributed models is limited by the quality and availability of point station data measured in-situ. ERA5 is a climate reanalysis produced by European Centre for Medium-range Weather Forecasts (ECMWF) and suggested to overcome this constraint. This study assessed and compared over the Benin basins at spatial and monthly time scale, the quality of ERA5 and its variant ERA5-Land (namely LAND). ERA5 relies on the single-levels version with 0.25° x 0.25° resolution while LAND is the land surface version with 0.1° x 0.1° resolution. Four variables were collected including runoff, evapotranspiration (ETR), water table depth (WTD), and soil water content (SWC). Point station data were analyzed using the correlation performance evaluators, Mean Absolute Error (m) and Relative Mean Absolute Error (r). The results showed that LAND simulates well the peaks of mean runoff. It showed the best runoff performance in terms of correlation (~0.61) compared with ERA5 (correlation ~0.49). Both reanalysis showed high correlations (generally > 0.80) for SWC, but the correlations obtained from ETR are slightly lower (ERA5~0.58 vs. ERA5-Land~0.54). Correlations were below 0.5 on both reanalyses for WTD with slight overestimation (m=4.73 m for ERA5 vs. m=3.13 m for LAND). This study does not identify any reanalysis that is better than another, both spatially and monthly scale. Nevertheless, this study indicated that the choice of reanalyses must rely on their performance and the given water cycle element. Correcting the variables of these reanalysis could also improve their performance.
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Interactive discussion
Status: closed
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RC1: 'Comment on hess-2023-311', Anonymous Referee #1, 05 Mar 2024
The study titled "Assessment of Hydrology Estimates from ERA5 Reanalyses in Benin (West Africa)" aimed to evaluate and compare the quality of ERA5 and ERA5-Land in the Benin basins on spatial and monthly time scales, focusing on four key variables: runoff, evapotranspiration, water table depth, and soil water content. While I acknowledge the implication and efforts to verify the estimation qualities of reanalyses on hydrological cycle variables, after thorough consideration, I find that the paper may not meet the publication criteria of HESS for the following two main reasons:
1) The first concern is the novelty of the research. The primary approach of comparing ERA5 and ERA5-Land data at a regional scale with point station data is a method that has been extensively and comprehensively explored in the literature (e.g., https://doi.org/10.5194/hess-25-17-2021, https://doi.org/10.1016/j.jhydrol.2024.130649). The manuscript does not provide a sufficiently novel contribution to the body of knowledge that would justify its publication. The study follows a very straightforward assessment methodology without introducing new insights into the assessment process, analytical techniques, or applications that could significantly benefit the scientific community or advance the field of hydrological modeling in West Africa. For example, the main conclusion "This study does not identify any reanalysis that is better than another, both spatially and monthly scale. Nevertheless, this study indicated that the choice of reanalyses must rely on their performance and the given water cycle element. Correcting the variables of these reanalysis could also improve their performance." in the abstract is somehow well-known principle in hydrology science.
2) My second concern or curiosity is about the method of directly comparing ERA5(-LAND) runoff data, which does not include routing, with observed discharge data. The conclusion such as "Analysis of the spatial variability of runoff over the whole country gives the impression that there are no rivers in Benin" is weird because fundamentally, the runoff products are unrouted data that do not consider the lateral connectivity of grid cells through the river channel. To make a more meaningful comparison between the routed runoff and observed "discharge/streamflow," I recommend the authors refer to https://doi.org/10.5194/essd-12-2043-2020.
Some other parts also leave me unconvinced. For example, in Section 3.5 Checking the Hydrological Cycle Closing, it is mentioned that "neither ERA5 nor LAND has a closed balance," based solely on an annual balance check. Table 2 also leads me to question the nature of the "storage" component discussed. In the context of a water balance, changes in storage over time make more sense than a static notion of "storage" to assess whether the hydrologic cycle is closing. In addition, I am not sure if the analysis overlooks the role of 'Volumetric Soil Water Layer 4', which is excluded from other parts of the analysis.
Given the above concerns, I do not see a path forward for publication in HESS without substantial revisions and a re-evaluation of the study's methodology and conclusions.
Citation: https://doi.org/10.5194/hess-2023-311-RC1 - AC1: 'Reply on RC1', René Bodjrènou, 09 Mar 2024
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CC1: 'Comment on hess-2023-311', JEAN-MARTIAL COHARD, 11 Mar 2024
I read extensively the paper of René Bodjrenou. I think the subject could have an interest but the content doesn't meet the scientific standards of the HESS journal.
My main concerns are :
1) a lack of references to back up numerous assertions throughout the text, use of unadapted citations, and a strong tendency to self-citation.
2) Easy assertions, like this product "is the best analysis ..." without giving any criteria or metric to justify.
3) The methodology of the comparison is not introduced nor discussed. It seems that authors directly compared runoff at the river station (m/h) with the co-located pixel runoff variable from the ERA5 product, which is supposed to be given in mm, and which doesn't receive the runoff from the all drainage area because ERA5 is not routed. No information is given about the way this comparison has been processed.
Moreover, the runoff comparison concerns 4 main rivers in the Benin region which are all transboundary rivers. Incoming runoff from side countries is never evoked or discussed as authors just focus on the Benin territory.
This is the same for the water table depth which is compared with the ERA5 product (I guess it concerns the sub-surface runoff variable expressed in m but it is not precised). To my knowledge, ERA5 doesn't provide any water table depth. The way the data has been processed to make this comparison possible is not detailed.
Last, the authors seem to confuse PET with AET. The observations in figure 9 are clearly PET which are compared with AET from ERA5. By the way, the AMMA-CATCH observatory hasn't provided yet AET for such a long period and I can say that the existing AET data from the AMMA-CATCH observatory, for which I'm responsible, are full of gaps and will never look like it is in figure 9.
4) The evaluation only comments on metrics without describing time series. This makes the analysis often inconsistent.In conclusion, I believe this work has huge methodological issues and needs to be thoroughly reconsidered before resubmission.
Citation: https://doi.org/10.5194/hess-2023-311-CC1 - AC2: 'Reply on CC1', René Bodjrènou, 14 Mar 2024
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RC2: 'Comment on hess-2023-311', Anonymous Referee #2, 19 Mar 2024
In the study “Assessment of Hydrology Estimates from ERA5 Reanalyses in Benin (West Africa)” Bodjrènou et al. aim to compare data of runoff, evapotranspiration, water table depth and soil water content of gridded climate reanalysis data, namely ERA5 and ERA5-LAND, with local measurements of stations. Unfortunately, the major concept of gridded data seems not to be fully understood. The data of one grid point refers to the total grid box. This is also described in the information of ERA5 products, e.g. for mean runoff rate: “It is the rate the runoff would have if it were spread evenly over the grid box. Care should be taken when comparing model parameters with observations, because observations are often local to a particular point rather than averaged over a grid box.” (https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels?tab=overview). In consequence, the measurement station should be located in the grid box. Taking the data of the grid point closest to the measurement station (lines 158/159 of the manuscript) for the comparison with the station’s data, may not use the best estimate of the grid data for the location of the measurement stations. A thorough explanation is required if there is any reason to assume that the data of a neighboured grid box represents a point (station) better than the data of the grid box in which the station is located. This is crucial for the total work of the study.
In addition to this, the manuscript lacks a fully comprehensible description of the study approach and suffers from unclear wording. Basic information is missing. To name a few important, it is not clear at which temporal resolution the different datasets are compared and why two different ERA5 datasets are used. It may appear in the results or discussion section, but this is then too late in the manuscript. A few further examples for incomplete or unclear descriptions are listed below.
- Lines 41 – 43: The spatial distribution of measurement stations instead ‘of the data’ might be meant.
- Lines 70 – 72: How can a coarse data resolution restrict the application of the data to small basins?
- Lines 89 – 103: It becomes not fully clear to the reader which open question the study aims to answer.
- Lines 90/91: “best reanalysis” in terms of what?
- Line 111: The thousand separator should not be a point.
- Line 120: “Battance” seems to be the French term.
- Lines 143 – 146: What is the temporal resolution?
- Lines 154/155: Water table depth is usually measured relative to the surface level and therefore, it is expected to be zero or negative.
All in all, the manuscript needs a fundamental revision regarding the arguments stated above. Due to the generally high value of comparing grid datasets from reanalysis, remote sensing and derived products with locally measured data, I encourage the authors to go on working on their manuscript.Citation: https://doi.org/10.5194/hess-2023-311-RC2 -
AC3: 'Reply on RC2', René Bodjrènou, 20 Mar 2024
Dear Reviewer
We would like to thank you for taking the time to review and comment on our manuscript.
We have carefully read the comments on the article. Without exception, we consider all of them relevant. They will therefore be considered in the revised version. Any major changes will also be indicated.
Thank you again for taking the time to review our manuscript.Citation: https://doi.org/10.5194/hess-2023-311-AC3
-
AC3: 'Reply on RC2', René Bodjrènou, 20 Mar 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on hess-2023-311', Anonymous Referee #1, 05 Mar 2024
The study titled "Assessment of Hydrology Estimates from ERA5 Reanalyses in Benin (West Africa)" aimed to evaluate and compare the quality of ERA5 and ERA5-Land in the Benin basins on spatial and monthly time scales, focusing on four key variables: runoff, evapotranspiration, water table depth, and soil water content. While I acknowledge the implication and efforts to verify the estimation qualities of reanalyses on hydrological cycle variables, after thorough consideration, I find that the paper may not meet the publication criteria of HESS for the following two main reasons:
1) The first concern is the novelty of the research. The primary approach of comparing ERA5 and ERA5-Land data at a regional scale with point station data is a method that has been extensively and comprehensively explored in the literature (e.g., https://doi.org/10.5194/hess-25-17-2021, https://doi.org/10.1016/j.jhydrol.2024.130649). The manuscript does not provide a sufficiently novel contribution to the body of knowledge that would justify its publication. The study follows a very straightforward assessment methodology without introducing new insights into the assessment process, analytical techniques, or applications that could significantly benefit the scientific community or advance the field of hydrological modeling in West Africa. For example, the main conclusion "This study does not identify any reanalysis that is better than another, both spatially and monthly scale. Nevertheless, this study indicated that the choice of reanalyses must rely on their performance and the given water cycle element. Correcting the variables of these reanalysis could also improve their performance." in the abstract is somehow well-known principle in hydrology science.
2) My second concern or curiosity is about the method of directly comparing ERA5(-LAND) runoff data, which does not include routing, with observed discharge data. The conclusion such as "Analysis of the spatial variability of runoff over the whole country gives the impression that there are no rivers in Benin" is weird because fundamentally, the runoff products are unrouted data that do not consider the lateral connectivity of grid cells through the river channel. To make a more meaningful comparison between the routed runoff and observed "discharge/streamflow," I recommend the authors refer to https://doi.org/10.5194/essd-12-2043-2020.
Some other parts also leave me unconvinced. For example, in Section 3.5 Checking the Hydrological Cycle Closing, it is mentioned that "neither ERA5 nor LAND has a closed balance," based solely on an annual balance check. Table 2 also leads me to question the nature of the "storage" component discussed. In the context of a water balance, changes in storage over time make more sense than a static notion of "storage" to assess whether the hydrologic cycle is closing. In addition, I am not sure if the analysis overlooks the role of 'Volumetric Soil Water Layer 4', which is excluded from other parts of the analysis.
Given the above concerns, I do not see a path forward for publication in HESS without substantial revisions and a re-evaluation of the study's methodology and conclusions.
Citation: https://doi.org/10.5194/hess-2023-311-RC1 - AC1: 'Reply on RC1', René Bodjrènou, 09 Mar 2024
-
CC1: 'Comment on hess-2023-311', JEAN-MARTIAL COHARD, 11 Mar 2024
I read extensively the paper of René Bodjrenou. I think the subject could have an interest but the content doesn't meet the scientific standards of the HESS journal.
My main concerns are :
1) a lack of references to back up numerous assertions throughout the text, use of unadapted citations, and a strong tendency to self-citation.
2) Easy assertions, like this product "is the best analysis ..." without giving any criteria or metric to justify.
3) The methodology of the comparison is not introduced nor discussed. It seems that authors directly compared runoff at the river station (m/h) with the co-located pixel runoff variable from the ERA5 product, which is supposed to be given in mm, and which doesn't receive the runoff from the all drainage area because ERA5 is not routed. No information is given about the way this comparison has been processed.
Moreover, the runoff comparison concerns 4 main rivers in the Benin region which are all transboundary rivers. Incoming runoff from side countries is never evoked or discussed as authors just focus on the Benin territory.
This is the same for the water table depth which is compared with the ERA5 product (I guess it concerns the sub-surface runoff variable expressed in m but it is not precised). To my knowledge, ERA5 doesn't provide any water table depth. The way the data has been processed to make this comparison possible is not detailed.
Last, the authors seem to confuse PET with AET. The observations in figure 9 are clearly PET which are compared with AET from ERA5. By the way, the AMMA-CATCH observatory hasn't provided yet AET for such a long period and I can say that the existing AET data from the AMMA-CATCH observatory, for which I'm responsible, are full of gaps and will never look like it is in figure 9.
4) The evaluation only comments on metrics without describing time series. This makes the analysis often inconsistent.In conclusion, I believe this work has huge methodological issues and needs to be thoroughly reconsidered before resubmission.
Citation: https://doi.org/10.5194/hess-2023-311-CC1 - AC2: 'Reply on CC1', René Bodjrènou, 14 Mar 2024
-
RC2: 'Comment on hess-2023-311', Anonymous Referee #2, 19 Mar 2024
In the study “Assessment of Hydrology Estimates from ERA5 Reanalyses in Benin (West Africa)” Bodjrènou et al. aim to compare data of runoff, evapotranspiration, water table depth and soil water content of gridded climate reanalysis data, namely ERA5 and ERA5-LAND, with local measurements of stations. Unfortunately, the major concept of gridded data seems not to be fully understood. The data of one grid point refers to the total grid box. This is also described in the information of ERA5 products, e.g. for mean runoff rate: “It is the rate the runoff would have if it were spread evenly over the grid box. Care should be taken when comparing model parameters with observations, because observations are often local to a particular point rather than averaged over a grid box.” (https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels?tab=overview). In consequence, the measurement station should be located in the grid box. Taking the data of the grid point closest to the measurement station (lines 158/159 of the manuscript) for the comparison with the station’s data, may not use the best estimate of the grid data for the location of the measurement stations. A thorough explanation is required if there is any reason to assume that the data of a neighboured grid box represents a point (station) better than the data of the grid box in which the station is located. This is crucial for the total work of the study.
In addition to this, the manuscript lacks a fully comprehensible description of the study approach and suffers from unclear wording. Basic information is missing. To name a few important, it is not clear at which temporal resolution the different datasets are compared and why two different ERA5 datasets are used. It may appear in the results or discussion section, but this is then too late in the manuscript. A few further examples for incomplete or unclear descriptions are listed below.
- Lines 41 – 43: The spatial distribution of measurement stations instead ‘of the data’ might be meant.
- Lines 70 – 72: How can a coarse data resolution restrict the application of the data to small basins?
- Lines 89 – 103: It becomes not fully clear to the reader which open question the study aims to answer.
- Lines 90/91: “best reanalysis” in terms of what?
- Line 111: The thousand separator should not be a point.
- Line 120: “Battance” seems to be the French term.
- Lines 143 – 146: What is the temporal resolution?
- Lines 154/155: Water table depth is usually measured relative to the surface level and therefore, it is expected to be zero or negative.
All in all, the manuscript needs a fundamental revision regarding the arguments stated above. Due to the generally high value of comparing grid datasets from reanalysis, remote sensing and derived products with locally measured data, I encourage the authors to go on working on their manuscript.Citation: https://doi.org/10.5194/hess-2023-311-RC2 -
AC3: 'Reply on RC2', René Bodjrènou, 20 Mar 2024
Dear Reviewer
We would like to thank you for taking the time to review and comment on our manuscript.
We have carefully read the comments on the article. Without exception, we consider all of them relevant. They will therefore be considered in the revised version. Any major changes will also be indicated.
Thank you again for taking the time to review our manuscript.Citation: https://doi.org/10.5194/hess-2023-311-AC3
-
AC3: 'Reply on RC2', René Bodjrènou, 20 Mar 2024
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