The manuscript presents a thorough investigation into the seasonal dynamics of water vapor transport and its influence on precipitation isotopes within the Dongting Lake Basin. The study’s findings contribute valuable insights into the complex interactions between atmospheric circulation and the hydrological cycle, particularly in the context of a monsoon-influenced region. The research is well-articulated, and the methodology is robust, providing a solid foundation for the conclusions drawn. The implications of this study for understanding regional climate patterns and their potential impact on water resource management are significant. The manuscript is well-written and engaging, but there are areas where further refinement could enhance its clarity and academic rigor. Here are some specific comments:

Figs. 3, 5, 7, and 9 feature a red box representing the Dongting Lake Basin that is disproportionately small compared to the scale of Region V in Fig. 11. The scale inconsistency across these figures necessitates a redrawing to ensure clarity and consistency. The visual presentation of data through figures and tables is crucial. Ensuring that all figures are clear, well-labeled, and effectively supporting the text is essential.

Regarding the figures depicting water vapor transport pathways, what is the representativeness of the factors along these paths? In other words, what are the characteristics of these factors’ variations, the features at different grid points, and the attributes during the water vapor transport process?

For the Significance Statement, I believe the narrative logic should be adjusted to align with the main research content and results of the paper.

Figures 3, 5, 7, and 9 did not have labels, the authors need to add “abcdef” to them. Although each figure was annotated within the sb-plots to indicate the corresponding conference logo, only by making this change can we prevent confusion among readers.

Lines 652-666: This section includes numerous comparative statistical values. What is the purpose of these comparisons, and what useful conclusions can be drawn from them?

Lines 175-176: The main cause of the East Asian monsoon should be attributed to the changes in the average atmospheric circulation patterns due to the thermal differences between land and sea.

Lines 178 and 181: The reference should be to the lower layer, that is, the lower troposphere, rather than “near the surface” or “near-surface”, correct?

Line 188: The term “precipitation isotope dynamics” would be better off without “dynamics”.

Line 189: It is suggested to change “analysis” to “measurement”.

Line 263: This section pertains to the “calculation of the vertical integral of water vapor flux Q”, not “Model Analysis”.

Line 322: There is no content in this manuscript comparing the “spatial distribution” of simulated and observed data. Is it necessary to include such a comparison? Or, the phrase “spatial distribution” could be directly removed.

There are minor grammatical errors and awkward phrasings that should be corrected to improve the manuscript's readability:

Line 161: The term used in the text to express “moist and cold” is more accurately described as “wet and cold”. Are there any alternative terms that could be used?

Line 276: The gravitational acceleration “g” should be in italics.

Line 304: The term “RA5” should be “ERA5”.

Line 411: “The East Asian mid-to-high latitude regions”, please check the accuracy of this expression. I suspect the original intent might have been to say “The mid-to-high latitude regions of East Asia...”. Please ensure that the same terms are used consistently throughout the manuscript to avoid confusion.

Lines 353, 599, and two other instances: The expressions “regarding the Dongting Lake Basin as the endpoint” and “regarding the Changsha site as the endpoint” should be consistent. Alternatively, use “According to the method introduced in section 3.2.1, ...”.

Review of Water Vapor Transport and Its Influence on Water Stable Isotopes in Dongting Lake Basin

This manuscript presents an analysis of seasonally varying moisture transport paths that influence the water isotopic composition of 1668 days’ worth of precipitation samples collected in the Dongting Lake Basin area of China. Transport paths are estimated using a “vector interpolation method” that is applied to vertically integrated moisture fluxes within IsoGSM, a General Circulation Model equipped with water isotopic tracers. The results largely follow expectation. Given that the moisture flux is a function of both wind field and moisture supply, water vapor transport paths do not always align strictly with dominant wind flows. Transport from higher latitudes tends to advect more isotopically depleted water than transport from lower latitudes. And precipitation processes en route to Dongting Lake Basin also influence precipitation isotope ratios. The manuscript is fairly easy to read but lengthy.

I have three overarching comments:

First, the study focuses almost entirely on seasonal climatological output from IsoGSM, which feels a bit disappointing given the incredibly large number of daily precipitation samples collected in the Dongting Lake Basin. None of the major findings require the precipitation isotope data, and the paper does not clearly link the IsoGSM interpretation back to the collected samples.

Second, the study is largely descriptive in nature. Seasonal maps of moisture, wind, and isotopic output are shown, but the reader is required to eyeball small spatial variations in otherwise large latitudinal gradients. One way to address this might be to plot composite differences (e.g. mapping one season as an anomaly from the annual mean, or plotting the difference between one season and another). Differences between the representative source regions could also be tested statistically, and transport paths could be shown on a single plot—again, to facilitate the reader’s ability to compare. The text states that isotopic variations along transport paths are consistent with Rayleigh distillation; however, it does not appear that this was ever tested, which would be quite easy to do in a quantitative manner.

Third, it is not clear to me what new information about water cycle processes (including transport) this study provides, other than to provide a thorough description of seasonal mean wind flows, moisture transport paths, and vapor and precipitation isotope ratios around a single location. It motivates me to ask: was the Dongting Lake Basin chosen for a particular reason as a scientifically important location? Or was this a study of opportunity based on the large number of event-based precipitation samples?

I feel that this study would benefit from some additional context for why this work was conducted, how the IsoGSM output guides us in understanding the precipitation data, and the inclusion of quantitative analyses.

I also have two additional comments specific to methodology:

First, the study needs to provide more information about the “vector interpolation method.” The introduction criticizes other moisture transport evaluation approaches, such as those based on Langrangian back trajectory techniques, because these cannot definitively determine what moisture becomes rain. However, it is not clear to me how interpreting vertically integrated moisture flux fields in IsoGSM would allow one to do this either. In fact, there is a possibility that the vertically integrated transport could be quite distinct from the transport vectors of moist layers that generate precipitation (e.g. if free tropospheric moisture convergence is more critical than low-level moisture convergence for producing rain).

Second, it appears that the paper weights all days equally in its analysis, regardless of whether rain occurred or not. Thus it is hard to draw conclusions about whether the descriptive analysis accurately describes the conditions in which precipitation samples were collected.

Other line-by-line comments:

Line 53 - This mention of isotope paleoclimate applications seems out of context. Could more information be provided or this sentence eliminated?

Line 89 - Here and in all other instances, I believe “southwesterly” is meant, rather than southwestward.

Line 94 - I do not understand what this concluding sentence is trying to say.

Line 116 - Rain forms from water vapor that condenses, so I’m not sure how water vapor source regions are not relevant for or the same as precipitation source regions, unless the source regions are being defined climatologically (irrespective of whether there is rain). Clarification is needed.

Line 129 - I’m not sure what is meant by “by simple deduction.” What follows is not obvious to me.

Line 139 - This seems to be the problem statement for the paper: what the analysis will address. This should appear sooner in the introduction, and the analyses should test these ideas (e.g. test consistency with Rayleigh distillation).

Line 155 - Sub “influence” for “influencing.”

Line 223 - I’m not sure what is meant by the “fractionation process…cannot be directly observed.” We cannot observe individual molecules evaporating and condensing, but we do observe the partitioning of isotopes between distinct water phases.

Line 243 - What are “the driving factors” referring to?

Line 285 - Here and elsewhere, variables should be defined. Deuterium excess is never explicitly defined in the paper, and the notation is irregular. Typically, we would write d for deuterium excess.

Figure 2 Caption is missing an “E” before “RA5.”

Line 359 - “Lied” is not the correct word.

Line 360 - it would help to point the vortex out if it is significant for the interpretation.

Line 381 - It is a bit misleading to state that deuterium excess is generally affected by condensation. It is often conserved when condensation occurs under thermodynamic equilibrium (at saturation). It is not when condensation occurs under supersaturation. There is also a dependence on temperature conditions, but this is more detail than required for this sentence.

Line 393 - It is very hard to see the January wind vectors to verify this statement.

Line 401 - here and elsewhere, “relatively positive” is misleading, since these values are always negative. “Less negative” or “higher” could work instead.

Figure 5 - It should be made more clear here and in the text that there are two dominant transport paths for this particular month. At first, I thought the January path was being copied over from the preceding plots. This leads me also to ask: how are two dominant paths selected? How does the method permit the identification of more than one average path?

Line 440 - I’m not sure one can make such a broad conclusion based solely on the fact that there are similar deuterium excess values between the lake basin and the ocean. Also, does the large bin size on the color scale of the maps hide small-scale spatial variability?

Line 487 - What is the “variation rule?” I am unfamiliar with this concept. A brief description would help.

Line 557 - I’m not sure I agree without knowing more. Similar advective paths is one reason deuterium excess may be similar between sites. They could also be influenced by similar degrees of sub-cloud evaporation or other processes that produce similar signals.

Line 638 - Should the representative regions be interpreted as the vapor source regions? This is not clear to me.

Line 654 - I don’t think “seasonal differences” is the right term. I believe the text is describing the difference between the max and min values for each season, not differences between seasons, which is what the former implies. Perhaps “seasonal range?” It’s also not clear to me why this particular metric is chosen.

Table 1 - I think some of the Discussion length could be cut down by removing parts of the text that simply repeat what is already in Table 1.

Line 683 - Sub “Furthermore” for “Furtherly”

Line 744 - I’m not sure what a “modificatory” air mass is or why it should become more negative. Condensation and precipitation cause air masses to lose heavy isotopes, as does mixing.

Line 749 - That air masses are distinct in various ways can be taken for granted. Hopefully we can conclude more than this from this original work? A more specific conclusion sentence would be welcome.

Line 783 - Point B in the schematic is where mixing between cold and warm advected air masses should occur, and yet mixing is not discussed in the paper as a possible process shaping the water vapor and precipitation isotope ratios. Some discussion of the contribution of mixing might be a worthwhile addition depending on what a revised, more quantitative analysis yields.