The HESSD paper has triggered three referee reports which I consider to be thorough and constructive and which happen to complement each other. The general tenor of the reviews are cautiously positive.

I am generally happy with the way the authors have now responded to the issues raised by the three reviewers, and the details provided.

I therefore invite the authors to submit a revised manuscript, consistent with the authors' response. If further changes have been made, kindly report these in detail. I will then scrutinise the revised manuscript and formulate a final recommendation.

Success!

Dear authors,

I am satisfied with the revisions made to the document. However, I have still one issue that I need to understand better. It only relates to the Chiang Sean (CS) station, and Table 3.

You write: “Results show high hydrological alteration factors (> -0.7) and statistically significant (p ≤ 0.001) increases in water levels during the dry season months (January to May)…” for the CS station (lines 246-248).

You attribute this increase later in the paper to upstream dam development in the Langcang river in China, when stating: “The development of the four mainstream hydropower dams in the upper Mekong in China was observed to have an impact on seasonal water level changes, resulting in a large increase in dry season water levels in the stations closer to the dams, but with diminishing effects further downstream.” (lines 338-341).

In the conclusions the statement is more cautiously formulated as: “Significant increases in water levels during the dry season (March, April and May) of 80% to 20% post-1991 in Chiang Sean downstream to StungTreng were documented, and such alterations are most likely caused by water infrastructure development in the basin.” (lines 453-456).

I tend to agree with Reviewer #1 when he observed that “Monthly precipitation assessment for the pre-and-post 1991 period is inevitable to explain the results given in Table 3 and Figure 3.” Because indeed it is thinkable that the increase in water levels is due to increase in rainfall (and/or increase of snowmelt and/or significant change in land use and land cover) over the period. I am just wondering how the Chinese dams can cause the dry season discharge at CS to increase, especially if the information on average discharge at CS (Table 1) and the Chinese dams in the Upper Mekong/Langcang (Table 2) are correct.

According to Table 1 only two dams were operational in the Langcang before 2008, with a total active volume of 624x106 m3 (total volume 1.85 x109 m3). With an average flow of 2,700 m3/s (equivalent to some 85x106 m3/yr), the average residence time in the active part of the dams is thus less than 3 days (8 days if you take the total volume). I don’t see how these dams can increase the flow during the dry season as they cannot store wet season water for the dry season: they are far too small.

The commissioning of two recent dams (with active storage of 249x106 m3 and 9.9x109 m3 in 2008 and 2010 respectively) certainly would have had some additional impact. However that impact is only felt in the last 3 or so years over the 20 year period, so would have a limited impact on the mean values. Even then total residence time remains fairly short (in the order of 50 days for active storage, 80 days total storage); still marginally sufficient for inter-seasonal storage (and filling the newest dam would take some time and certainly REDUCE (not increase) the flow coming to CS for some time).

So the inferred causality that the dams have caused the increase in dry season flow at CS is not warranted based on the data presented. You either need to substantiate this claim of causality (e.g. by showing that average rainfall in the upper Mekong/Langcang were identical for both periods considered, i.e. 1960-1990 and 1991-2010; but then you still need to explain where the additional water observed during the dry season in the recent period has come from!), or you carefully qualify or re-phrase the statements that I quoted above.

I kindly invite you to address my concern.

Pieter van der Zaag

Editor

Dear authors,

I am happy with the changes made. I am now recommending to publish the latest amended manuscript as submitted.

Pieter van der Zaag