Thank you for inviting me to review the paper: “Historical and Projected Future Runoff over the Mekong River Basic” by Wang et al.

This is a useful and important paper. It is useful because it builds links between the detailed hydrological modelling community and those developing GCMs. Too often, climate researchers consider a relatively basic land surface model in a GCM as sufficient – but in reality, something much better is needed to help understand future flooding impacts. The paper is important because, as the authors state, 65 million rely on the Mekong River for access to water.

The analysis is well-considered and thorough. My only concern with the paper is that there needs to be better wording and removing ambiguity in places. All of this can be easily rectified in the generation of the new paper version (and I am happy to re-review any revisions).

Below are illustrative examples, but please check through the entire manuscript.

P2, Line 48. “However, these studies do not systematically analyse….”. Is this suggesting that substantial errors could occur with good GHMs, should there be major biases in GCMs (so a GCM+GHM combination fails, even if the GHM is good).

P2, Line 49. The wording here is clumsy. Maybe something like: “and the simulated years at the beginning of the RCP scenarios, which are now pre-2023 and for which runoff data exists.”

P3, Line 58. Please state what ISI-MIP is. Are the GCM-GHM combinations already calculated in ISI-MIP, or is that database just GCM output? Are any of the outputs from ISI-MIP already bias corrected?

P3, Line 72. Please clarify why you would use the MK test, and not the standard statistical test of whether a regression line is statistically significant.

P5, Line 91. Please state where the GHMs come from. Is it a database such as ISI-MIP. State here that information, and also in the caption of Table 2 (Also, please check the current Caption of Table 2 – it looks wrong, referring to eight hydrological stations).

There is one technical issue. Could the authors describe if there is any bias correction undertaken e.g. of the ESMs? To my knowledge, some ESMs in the “MIPs” are corrected. If so, are these used – because they should reduce climate uncertainty/errors in any GCM+GHM projections of the contemporary period?

P6, Table 2 – as noted elsewhere, captions appear overly succinct. I often see people give talks where diagrams and tables are extracted from papers, so if they can be more complete (i.e. with essential details in captions), then this is very helpful.

P7, Figure 2 – One possibility to avoid the repeated words “insignificant change” is to give the p-value for the regression. Then where it is significant (e.g. p < 0.05), mark with a star symbol. Something like that… Also, again, expand the caption slightly. For instance, state, “Eight hydrological stations are numbered N1-N8, with their locations presented in the map of Figure 1”.

P7, Figure 2. Although very obvious, please put the word “Year” under panels (f), (g) and (h).

P8. Table 3. The table is nice, but isn’t all the information in Figures 1 and 2?

Figure 3. Again, please make the caption much more informative. Something like “Figure 3. Performance of all combinations of GCMs and GHMs. The three rows correspond to three performance matrices……. In each row, each panel is for a different GCM, as annotated. Then in each panel, the different colours are for each GHM, as marked under each row……”

Figure 3. Although it is important to make captions informative, placing a result there is not usual practice. So please reconsider the words “WaterGap2 has the best performance compared to other models”, and should they be in a caption?

Figure 3. And on the same point above, statements such as “…best performance compared to other models” requires very careful quantification. Is it the best performance when WaterGap2 is driven by a specific GCM? Is it the best for one statistical metric or many?

In the Conclusions, critical is consistency between direct drivers of runoff change and changes imposed by raised GHGs and related altered rainfall patterns. Here, the impression is that the former is small (i.e. “However, the impact of the reservoir on the annual runoff after the completion of water storage is small”). Elsewhere in the paper, there is the suggestion that humans have impacted – directly – runoff strongly. I still think it would be useful to have a summary statistic that is some sort of ratio between historical direct change and the impact of raised GHGs on runoff. This should be easy to do, as all the numerical values to build such a single comparison statistic are calculated at different points within the manuscript.

Small things – here, for Abstract but may be representative elsewhere

 

Abstract: These need to avoid ambiguity, as often read in isolation by a reader in a hurry. Hence please: 

(1)    tighten line 7 and explain the difference between how “four GCMs” and “five GHMs” are used. Are they operated independently e.g. raw runoff output from the GCMs are used – while the GHMs are forced with known near-surface meteorological drivers? The next sentence, however, talks about “best simulation combination”, so state this is 4 x 5 simulations – all combinations of GCMs and GHMs (I realise the main body of the paper makes this clearer, but such very basic information should be in an Abstract).

(2)    Line 11. State what “WaterGap2” is (i.e. GHM).

(3)    The Abstract presents two lines of investigation but does not bring them together in a coherent way. One direction is that for the contemporary period, it is dams and reservoirs that have had the biggest effect on runoff. However, when describing the future based on RCPs, runoff is described as “projected to increase significantly”. The question is then whether future changes caused by climate change are bigger than current changes caused by dams/reservoirs? (See similar comment above)

(4)    Line 13. Is “increase significantly” a formal statistical statement, and should there be a p-value?

(5)    Line 13. Actual values are given here (units of m^3 s^-1 a^-1). Similar to the comments above, how large are the 3.81 – 16.36 numbers compared to the effects of dams/reservoirs? And how large are these numbers compared to background contemporary flows. Would a simple statistical value help? 

General Comments:

The paper by Wang et al. provides a comprehensive analysis of historical and projected future runoff in the Mekong River Basin (MRB). The authors examined the runoff using four Global Climate Models (GCMs) and five Global Hydrological Models (GHMs) sourced from the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) 2b. These models were applied to data from eight gauge stations across MRB, considering three Representative Concentration Pathways (RCP2.6, RCP6.0, RCP8.5). The results indicate that while the annual runoff in the basin has remained relatively stable since 1971, significant increases are projected under the various RCPs.

In general, the paper is a strong fit for the journal, and its subject matter aligns well with the journal's scope. The study's focus on runoff in the MRB is of paramount importance for the region, making the findings particularly valuable. Additionally, the methodology employed in the research is both standard and widely accepted within the field.

However, one aspect that requires improvement is the clarity regarding the paper's contribution or novelty. The authors should explicitly highlight what sets their work apart from the numerous previous studies on future runoff and streamflow projections in the MRB.

Another crucial aspect overlooked by the authors is the estimation of uncertainty associated with their future projections. In studies of this nature, accounting for uncertainty is of utmost importance and cannot be overlooked. Although the authors attempted to mitigate uncertainty by using the ensemble-average approach, it falls short in providing a comprehensive estimate of the associated uncertainty. Addressing and quantifying uncertainty in their projections would add significant value to the paper's findings and enhance the overall robustness of their research.

Specific comments:

1.    Does the paper address relevant scientific questions within the scope of ESD?

Yes. The paper provides a comprehensive analysis of historical and future runoff in MRB (hydrosphere) under climate change (global change). The paper aims to quantify the impact of climate change on runoff, and thus improving our understanding of hydrological system behavior to global changes. Thus, I believe the paper aligns well with the scope of Earth System Dynamics (ESD).

2.    Does the paper present novel concepts, ideas, tools, or data?

No. While the results presented in the paper are undoubtedly interesting and useful, it is worth noting that there have been several previous studies conducted in the Mekong River Basin (MRB) with similar objectives. For example, https://doi.org/10.3390/w12061556 and https://doi.org/10.1016/j.eng.2021.06.026. Additionally, the data and methodology adopted by the authors is also widely used. Therefore, it is difficult to understand the novelty of the work. 

3.    Are substantial conclusions reached?

Yes, based on the GHM simulation, the authors were able to reach at the conclusion that runoff in the MRB is projected to increase under climate change scenarios. However, the paper has not presented estimate of uncertainty associated with the projections. 

4.    Are the scientific methods and assumptions valid and clearly outlined?

Yes, the methodology adopted in the paper is widely accepted and clearly described. 

5.    Are the results sufficient to support the interpretations and conclusions?

Yes, but uncertainty associated with projections should also be highlighted in the conclusions to provide a clearer and more comprehensive understanding to the readers. It is allow readers or policymakers to interpret the results in a more informed manner.

6.    Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)?

Yes. The methods and data used in the study are sufficiently explained, thus enabling other researchers to reproduce the results. 

7.    Do the authors give proper credit to related work and clearly indicate their own new/original contribution?

Yes, the paper includes a substantial number of cited references. However, the authors fail to highlight how their results or approach is different from some of the similar studies conducted in the past. While they have conducted a comprehensive analysis of historical and future runoff in the Mekong River Basin, it is essential to clearly articulate the novel contributions of their work in relation to the existing literature.

8.    Does the title clearly reflect the contents of the paper?

Yes. 

9.    Does the abstract provide a concise and complete summary?

Yes. 

10.    Is the overall presentation well structured and clear?

Yes, the paper is well structured. However, the discussion section seems to redundantly summarize the results without delving into new insights. To enhance the paper's impact, the authors should utilize the discussion section to highlight how their results offer fresh perspectives and novel contributions concerning the future runoff of MRB beyond what has already been published. By focusing on these unique insights, the authors can provide a more substantial context for their findings. 

11.    Is the language fluent and precise?

Yes. 

12.    Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?

Yes. 

13.    Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?

Yes, as I mentioned in previous comments, the discussion section should be improved and elaborated. 

14.    Are the number and quality of references appropriate?

Yes. 

15.    Is the amount and quality of supplementary material appropriate?

N/A