|1) This is an important transboundary issue in Eastern and Southeastern Asia that is suited for the special issue. However, I do not recommend the authors publishing the paper in the present form. The hydrological analysis, while likely time consuming, lacks hydrological rigour and ignores the findings of much past work showing that new insights of this complex system can only be gained by greater consideration of the 3 dimensional hydrological balance (e.g., Kummu et al., 2014), which was recently reiterated at by Kallio and Kummu (2021) in pointing out the limitations of the recent Wang et al (2020) analysis (and the reply of Wang et al (2021).|
2) The study considered is arguably limited too, as it only considers very few spatial measurements of water depth and (sometimes potentially flawed) discharge. Further it includes rainfall from only locations. The calculations based on this limited set of information likely do not allow a full interpretation of the myriad flow processes affecting this complicated hydrological setting. In the end, the main findings are basically the same from those published by Kummu et al (2014) that uncertainty in the hydrology of the tributaries of the Tonle Sap largely prevent closing the water balance of this complex system. The other aspect preventing closure of the budget is uncertainty in the groundwater dynamics.
3) Further, the findings do not go farther than echoing the findings in the most recent works (Wang et al., 2020; 2021; Kallio an Kummu, 2021, Ng and Park, 2021). Thus, the bulk of the conclusions are largely the information that is already alluded to in the introduction (or should be alluded to with careful inclusion of the 2021 papers). I am struggling therefore to see what the main contribution of this new analysis is other than "contributing to the discussion", but in a quality that is not quite at the level expected for HESS. Better would be if the authors invested much effort in addressing the theme of the special issue by using their "estimates" to re-enforce the issue of the complexity in determining the lake water balance and possible drivers, which are both local and transboundary in nature.
4) Also, to be publishable in this journal, much greater care is needed in telling the comprehensive story, as well as addressing the limitations of the calculations at hand--and perhaps the uncertainties in those of other studies. Currently, the message that the reader is left with is that too much attention has been focused on the role of upstream dams in the past, but again, even this focused topic is not addressed in a meaningful discourse that aligns with the specific theme of the special issue. Largely, incomplete explanations of processes related to the other important drivers are given (e.g, how exactly downstream sand mining affects upstream hydrology).
5) Finally, I have questions regarding some of the methods, which are addressed in the points below. Importantly, one question relates to uncorrecting the corrected stream rating curve at a critical location that informs on the changes in discharge to the area in question, and on the role of dams on reducing flows. In practice one adjusts a rating curve when the old one is no longer valid. By adopting the old curve (ignoring the new curve) for the new calculations in this paper, one wonders if the authors are corrupting their calculations. Even if explained elsewhere, sufficient details are needed here for the reader to have confidence in the calculations. Nevertheless, this issue of measurement uncertainty relates to prior calls (e.g., Kummu et al 2014 and likely others) for better and more comprehensive measurements of hydrological phenomena needed to study the Tonle Sap water balance with accuracy. In conclusion, I am hoping this is a very rough draft submitted hurriedly to make the initial deadline of the special issue and that the authors are already undergoing a much more comprehensive assessment to provide an engaging, objective story regarding the issue of Tonle Sap, which is increasingly tragic.
Ng and Park (2021). Shrinking Tonlé Sap and the recent intensification of sand mining in the Cambodian Mekong River. Science of The Total Environment, 777, 10 July 2021, 146180
Wang et al (2021). Reply to Comment on 'Changes of inundation area and water turbidity of Tonle Sap Lake: responses to climate changes or upstream dam construction?' Environmental Research Letters, Volume 16, Number 5. https://iopscience.iop.org/article/10.1088/1748-9326/abf3d
6) The introduction seems a bit dated, not really discussing the issue based totally on what is known (or debated): a) Mekong flows are reduced and certainly have an effect on the the water levels in Tonle Sap Lake; (b) Climate has had some influence on the hydrology of the entire region (especially the recent "dry" conditions in the region); (c) intense sand mining downstream of the lake is likely a culprit in changing the flow regime of the Mekong, potentially contributing to lake level changes; (d) anthropogenic changes in tributaries above the lake affect inflow to the lake (e) dams and diversion on other mid and lower stream tributaries of the Mekong affect flows in the river as well as other rivers on the Cambodian floodplain; and (f) and agriculture intensification in the floodplains may also affect flows. If these things are "known" how can they be conclusions to the paper? What is not known are the contributions and their combined effects. Importantly, the authors need to emphasize the novelty of their findings.
7) I think once the finalized message of the paper is determined, the title and abstract can be tuned to reflect that story. Is it simply the decline in the flood pulse (two words not one) or is there more to it? The decline in the flood pulse is already known.
8) The estimate of the "water withdrawal rate" from the floodplain is not believable with the simple, indirect methodology.
9) Lines 35 to 40. The brief inclusion of the studies elsewhere are not needed here as they distract from information that is needed to explain the Tonle Sap issue.
10) Line 46. The spatial (and temporal) extent of the Cambodian floodplain area, above and below the lake, should be defined; and an explanation of the hydrological processes operating on this area is needed (what are the boundary conditions?). Make sure to refer to the map.
11) Line 48. A map is needed showing the Lancang dams, as well as any other dams and features referred to in the paper. I was unable to follow the story without opening Google Maps.
12) Line 53: The "surface hydrology" of the floodplain system was studie, but only through flow on two rivers and one lake depth. This is not comprehensive.
13) Line 55: The "synthesis" amounts to a cursory description, but lacking support data and critical consideration.
14) Figure 1 Caption. Please provide more details and descriptions of important information.
15) Figure 1. Where are the areas of intense sandming and irrigation (See Ng and Park, 2021).
16) One issue regarding understandability of the paper is the tendency of using "upstream" in reference to tributaries to the lake, tributaries to the Mekong in the vicinity, locations far above. Please be exact and descriptive and provide reference on maps.
17) Lines 79-82. Where exactly is the Chi River with respect to the lake and what are the known effects (use data)? What is meant near the floodplain and how is that more relevant than being far away when river discharge is considered? Please show the Chi and the other S3 dams on a map. What are the details regarding these dams?
18) Lines 83-92. It is important to show where this area is in relation to the lake and rivers, as up to 31% of the low season flow of the Bassac and Mekong Rivers could be consumed. How can this estimate simply be glossed over and not explored?
19) Lines 95-100. Check out the new Ng and Park (2021) paper and rewrite accordingly.
20) Section 3.1 Rewrite for clarity.
21) Line 110. Regarding rainfall from only 2 stations: How is this representative of a huge area? Others have estimated rainfall for the region using much more data (e.g, Wang et al 2020).
22) Lines 115. Please check the accuracy of the equations. Also, HESS uses numbering on equations correct? Better check. Why are the R2 values so high? What is the timing of the data (daily, monthly, yearly)?
23) Lines 124-130. Steung Treng is the primary station used to judge flows as affected by the dams, yet the authors alter the rating curve, which makes the most recent flows higher. Why assume the curve wasn't adjusted by the operators because it was wrong and predicting too high of values for a long time?. This "adjustment" affects the validity of the assessment. THIS IS A MAJOR ISSUE that must be addressed in this issue. I didn't read the prior paper, but discussion is needed here to ensure the reader that this is not egregious data manipulation that just so happens to support the story.
24) Figure 2 is useful, but all the mentioned locations need to be shown on a map; and here, something is needed to identify what water body they affect and where.
25) Section 3.2.1. Give the equations numbers please. Are both THRESHOLD and FT needed? Put the units in "( )" not with "/" to make it more clear to the reader.
26) Figure 3. Rise rate and fall rate are not accurately displayed. What is shown are the depths. You need to show the slope of the line from the start date to the max (for rise rate) for example (e.g a 45 degree angle on the figure).
27) Line 170. Regarding STATISTICS. Is the use of "validated" a misprint? Have a read again on statistical inference testing for wordiing. I assume it is meant that the Welch test was used because it doesn't assume equal variances; and it was used to assess if the two samples had statistically different means.
28) Line 173. Right after the PARAMETRIC Welch test is mentioned, the NONPARAMETRIC Mann-Whitney test is introduced. Why not simply only use nonparametric tests for everything?
29) Lines 176-177. Equation numbers again, and put the units in (). Actually check what HESS recommends.
30) Section 3.2.3. I would redo this section very carefully and look at net discharge for more combinations than only 2 months. When I look at the various calculations of discharges and water depths over time I am not sure I see a consistent pattern. I would spend some time on thinking this analysis through carefully. Problematic is that this analysis only considers surface flow in three large channels, but I am guessing water is moving with more complexity. THIS IS A MAJOR ISSUE TO INVESTIGATE FOR ACCURACY.
31) Line 207. Where exactly are the water infrastructure developments located?
32) Figure 3. Why only show water level and not discharge, if even in the supplement? Does one arrive at a different result if Q is used?
33) Line 208-210. Reference someone who showed the extent of changes -- or make a new map.
34) Lines 225 -230. Provide more details and show on maps if possible.
35) Line 235. This "hint" has been demonstrated by others and they should be cited.
36) Line 250. Figure 4b (maybe 5b, I have two versions of this draft). Maybe make the shaded area here light blue, as it is not the same period as the light gray one in plot a. Improves readability.
37) Lines 265 to 271. This information needs to be carefully presented. How do dams that are "being developed" affect the past flows analyzed in this paper? The dam at Stung Prasat was only 80-90% finished in May 2021. Where is Stung Sreng? All these places need to be shown on the maps. This paragraph is highly speculative.
38) Figure 6. Schematic of wet season discharge on the Cambodian floodplains during the pre-dam and mega-dam era. Across all stations, there is a reduction of discharge during the mega-dam era of 2010-2019. This is interesting, but the black arrows "admit" to the uncertainty of the flows and the difficulty in interpreting a few stage and Q measurements. I don't think the Q values listed agree entirely with your Qdiff values from before. Do they tell the same story? Please take a look and comment if needed.
39) Figure 6. What does the annual flux look like (put in SI)?
40) Section 5.2. To fully understand this potential impact the reader needs to know the location of the rice fields, and they need to know more about the rainfall: the droughts of 2015 to 2018 were prolonged and extensive. Your rainfall analysis is lacking in rigorousness as it includes only two stations and two months considered. IMPORTANT LIMITATION.
41) Section 5.3. Check out the Ng and Park (2021) paper, then rewrite this section. You will need to explain in better detail the processes by which sand mining below the lake is affecting flows, then relating this to observed discharges of water into the lake.
42) Section 5.4. Unless you are willing to go into detail the reference to the other systems is not informative. The attempt to look at wider implications is ok, though somewhat speculative. I think it is important to compare your finding(s) with the work of others, but it seems to me they basically reinforce what is largely known. You should discuss limitations of your approach with respect to errors and limitations.
43) Regarding the nature of the special issue on transboundary issues, you may want to make a small section on this aspect to add value to the paper. In particular, insights regarding social aspects of this transboundary issue is greatly missing, given the expertise on the Tonle Sap in Singapore, both present and past.
44) Conclusion. Make sure to highlight new findings (explain novelty), not just summarize results. I like the attempt in the last paragraph to emphasize the importance of the loss of the flood pulse, regardless of the reason(s). I am not sure what "water harvesting" means; is it in reference to irrigation?
46) Good luck.