This paper reports on a synthesis analysis of EO altimetry data, surface water extent and in situ gauge data for the Congo Basin. It includes a large number of virtual stations derived from multiple missions (and adjusted to the same datum) and covers the whole basin spatially. The findings cover the surface water dynamics of the CRB in detail, including variations in WSE amplitudes and SWE extent, their seasonality and lag when compared to the gauged outlet of the basin.  

This is an important paper in a relatively poorly studied basin. It is impressive in its scope and thorough in its analysis. It is well written and illustrated carefully. It has a detailed and considerate literature context covering previous work. While some results importantly confirm previous studies, this paper also uncovers new findings and detail that will be incredibly valuable to the science community that study the basin.

A few suggested minor changes are listed below, but other than these small typos there is a lot to recommend publishing this paper and no doubt it will become an important reference point for future work.

Some Specific Comments:

1.    Line 39: something is missing in this sentence “ranging from 0-1 month in its vicinity downstream the basin up to 3 months in remote areas and small tributaries.“ – maybe it should be “ranging from 0-1 month in its vicinity in downstream parts of the basin and up to 3 months in remote areas and small tributaries.

2.    Line 49, “40.500” should be “40,500”

3.    Where “CRB” is used at the start of a sentence, it would be better to say “The CRB”

4.    Line 208: “uncertainties ranging between few centimeters” should be “uncertainties ranging between a few centimetres” (note spelling of units)

5.    Figure 2(d): end bracket on the key seem to be the wrong way around. Should be [xx-yy] instead of [xx-yy[.

6.    Line 243: this sentence seems like it is missing something “surveyed by a ten of static and cinematic GNSS campaigns” Where there 10 campaigns? If so remove “a” and “of”.

7.    Line 252: “we merged them in a same reference” would be better as “we merged them to the same reference”

8.    Line 335: “The variation of amplitude in the southern is similar” - should this say southern part?

9.    Figure 4(a) and Figure 5: end bracket on the key seem to be the wrong way around. Should be [xx-yy] instead of [xx-yy[. Please also check all other figures.

10.    Line 426: “sub-basin which hydrology might” should be “sub-basin where the hydrology might”

11.    Line 464: “complement each other individual result” should be “complement each other’s individual result”

12.    Line 520: “all over the various tributaries” should be “over all the various tributaries”

This paper presents a synthesis of multiple sources of altimetric water surface height and remotely sensed flood extent over the Congo basin and compares these to in-situ data.  The data are then analysed to better understand the spatial variability of surface water hydraulics over the basin.  This is a major piece of scholarship that will likely become to the ‘go to’ reference for such data for many years to come.  Altimetric height data show a good fit to ground observations over the Congo, corroborating the findings obtained elsewhere, and are then related to water extent data from the GIEMS-2 product.  I think the work will be suitable for publication in HESS with correction of the following issues:

1. I think the paper would benefit from more carefully identifying what we learn as a result of this work that we didn’t know before. Whilst assembling and quality controlling the various data sets is a major effort, they are somewhat under-exploited in the present work if I am being honest.  I think the paper will be more highly cited if it were to first carefully outline what we currently understand about Congo surface water variability.  In the results section and conclusions, the paper should then state where the presented results either corroborate existing knowledge or provide substantial new understanding.  There is some attempt at doing this in the paper, but it could be made much more systematic and effective.
2. A lot of the text on GIEMS-2 (e.g. Section 3.3) really just summarises previous work. What is new here is the correlation of GIEMS-2 basin total flood extent time series with various discharge measurements.  I think the text on GIEMS-2 could therefore be significantly shortened to only cover the new results presented in this paper.
3. I also think that a number of the statements about GIEMS-2 cannot be proven based on the research presented in the paper. For example, from line 359 onwards there is a statement that GIEMS-2 exhibits:

“very realistic spatial distributions of the major drainage systems, rivers and tributaries (Lualaba, Congo, Ubangui, Kasaï) of CRB. The dataset captures well the associated wetlands and inundated areas even in regions with complex floodplains, characterized by extensive flooding in the presence of dense vegetation cover, such as in the Cuvette Centrale”

There are two issues with this.  First, there are no objective tests in the paper of the accuracy of the GIEMS-2 inundation patterns, so it is impossible to say what is realistic or not: the statements above are a subjective evaluation and not repeatable science.  I think that at most you can only say that the GIEMS-2 patterns are plausible.  Second, at 0.25 degree resolution I think the above statement overstate the level of detail that can be seen in GIEMS-2.  I think these sections of the paper need correction to more accurately reflect what the paper is really able to show.

4. The paper would be significantly improved (and citations would be higher) if the data sets were made available to download from a data repository such as Zenodo instead of just saying that the data are “are available upon request to the authors”. This would also allow the data sets to have a DOI such that use of the data could be properly tracked by the authors.

A few more minor issues:

31. Line 31. “Shows a good behaviour”?  What exactly does this mean when there is no objective and repeatable test of the GIEMS-2 accuracy over the Congo?
32. Line 277. “GIEMS-2 uncertainties are quantified to be about 10 %”.  Could you explain exactly what this uncertainty refers to.  I’m assuming it is errors in total inundated area over a large domain, but please could you confirm.  Also, what was the ground truth data that was used to calculate this error?
33. Line 455. “The satisfactory behaviour of both SWH from radar altimetry and SWE from GIEMS-2””.  Again, I don’t think you can make this statement for GIEMS-2 at this site on the basis of the data you have presented.  The data are quite low resolution and their evaluation is only subjective.
34. The figures would be better as vector files rather than bitmaps.
35. The text still needs further a proof read to catch a number of grammatical and typographic errors.

I hope these comments are useful

Paul Bates

University of Bristol