the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jul 2026
Assessing and reducing dry-season water-scarcity risk on a tourism-exposed small island: a multi-source water-balance and robust-optimisation framework for Si Chang Island, Thailand
Abstract. Dry-season freshwater scarcity is a recurrent hazard on small tropical islands, where seasonal rainfall, limited storage and tourism-driven demand coincide. We assess and reduce this risk for Si Chang Island, Thailand, with an integrated framework that resolves inter-month storage carry-over across coupled sources. Connected catchment is estimated from a land-use layer and satellite imagery, separating rooftop harvesting to household cisterns from watershed runoff to the reservoir–pond–bank system, while tourism demand enters through an equivalent-resident formulation. Supply reliability is quantified with reliability–resilience–vulnerability metrics under a downscaled CMIP5 ensemble, from which RCP4.5 is selected by bias screening. A mixed-integer optimisation, extended to a two-stage robust formulation, sizes capacity against stochastic dry years. Rooftop harvesting is the dominant source, and the present system is reliable but limited by small cistern capacity; under projected demand a structural annual deficit renders the island supply- rather than storage-limited, so additional storage alone cannot secure supply and harvestable catchment must expand. Critically, a design optimised on mean rainfall delivers markedly lower reliability under realistic variability, and drought-robust sizing requires substantially more catchment. The framework yields transferable, drought-robust design targets for small islands.
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Status: open (until 11 Oct 2026)
- RC1: 'Comment on egusphere-2026-4005', Anonymous Referee #1, 19 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-4005', Anonymous Referee #2, 16 Sep 2026
reply
This manuscript addresses a relevant problem for small-island water management and proposes an interesting framework combining a multi-source water balance, reliability–resilience–vulnerability metrics, optimization and stochastic analysis. The comparison between deterministic and stochastic design is particularly useful. However, I think that some methodological aspects require clarification and that the manuscript would substantially benefit from a clearer and more concise presentation. My main comments are provided below.
Structure and readability
The manuscript contains a large amount of information, numerical results and methodological detail, which makes the overall workflow and the main findings difficult to follow. The text is highly fragmented into sections and subsections, and several concepts and results are repeated across different parts of the manuscript. I strongly encourage the authors to revise the structure with the aim of streamlining the presentation. Related sections could be merged, repetitions removed, and secondary analyses and detailed numerical results moved to an Appendix or Supplementary Material. Overall, the main text would benefit from a clearer and more continuous narrative focused on the main methodological steps, key results and scientific contribution. The graphical quality and readability of some figures should also be improved (e.g. Figures 11 and 12).
Stochastic rainfall generation
If I understand the approach correctly, monthly variability is generated through independent multipliers, while the robust design is intended to account for persistent multi-year droughts. Could the authors clarify how temporal persistence across consecutive dry months and years is represented in the synthetic rainfall? This seems particularly relevant considering that the generator is parameterized from only ten years of observations.
Role of climate projections in robust design
If I understand correctly, the robust design is derived from stochastic rainfall generated around the historical 2012–2021 monthly means, while the downscaled RCP4.5 projections are not directly used to force the design runs. Could the authors clarify more explicitly the role of the climate-change analysis within the proposed framework and to what extent the resulting robust design accounts for projected climate change, rather than only for variability around the historical climate? This distinction should also be reflected consistently in the Abstract and Conclusions.
Per-capita water demand
The sensitivity analysis shows that the required catchment area is particularly sensitive to the assumed per-capita water demand. Given the large effect of relatively moderate changes in this parameter, could the authors provide further justification for the adopted per-capita demand values and discuss more explicitly how this uncertainty affects the resulting infrastructure requirements?
Open-water evaporation
The analysis in Section 4.6 shows that including open-water evaporation substantially reduces the reliability of the robust design and increases the required catchment area. Given the magnitude of this effect, I wonder whether evaporation should be included directly in the core water balance rather than treated only as an additional sensitivity analysis.
The evaporation calculation would also benefit from further clarification, particularly regarding the source and justification of the adopted net evaporation value, its temporal distribution within the monthly water balance, and the relationship used to account for variations in the evaporating surface with reservoir storage or water level. The corresponding statement in the Conclusions should also be revised, since evaporation is already evaluated in Section 4.6 and shown to have a substantial effect on the results.
Citation: https://doi.org/10.5194/egusphere-2026-4005-RC2 -
RC3: 'Comment on egusphere-2026-4005', Anonymous Referee #3, 25 Sep 2026
reply
The manuscript entitled "Assessing and reducing dry-season water-scarcity risk on a tourism-exposed small island: a multi-source water-balance and robust-optimisation framework for Si Chang Island, Thailand" is relevant and present an analysis of small-island hydrology, which is rarely available in the Literature. Despite its merits, the text readability is poor and, in our opinion, it requires major revision before acceptance. Below, we list major and minor remarks to help improve the quality of the paper.
Major remarks.
1. The following sentence ("Connected catchment is estimated from a landuse layer and satellite imagery, separating rooftop harvesting to household cisterns from watershed runoff to the reservoir–pond–bank system, while tourism demand enters through an equivalent-resident formulation") is hard to follow: What do you mean by "connected catchment"; "landuse layer"; "rooftop... runoff"; "equivalent-resident formulation"? Further, the abstract brings several terms that have not been properly introduced, such as "mixed-integer optimisation" (optimisation of which function?); "stochastic dry years" (see also Line 85); "harvestable catchment"; "design optimised on mean rainfall".
2. In the chapter "Study area and input data". Chapters 2.4 and 2.5 should be inserted in Chapter 3 Methods.
3. In chapter "Climate, rainfall and runoff", is the main river perennial, intermittent, or ephemeral?
4. The text from Line 110 to 116 needs improvement: The main reservoir was not properly characterized (is it a dam; an artificial reservoir...?), except for its volume; Figure 3 does not show it. This is also relevant for the estimation of open-water evaporation (see Chapter 4.6 Effect of evaporation and seasonal tourism).
5. In Line 127, we read "Lk days": Where does Lk appear in Equation 1? What are the units of P, V, and EMV? What do the numbers '12' and '30' refer to? Months/year and days/month? Why not simply use 365 days per year in the denominator? The sentence "𝑃tot ≈ 4,522 + 2,511 + 1,256 ≈ 8,289 persons" is not appropriate for a scientific journal (see also Line 247). The statement "under projected population and tourism growth this rises to ≈10,173" lacks methodological justification. The same applies to Table 3: To which timely horizon was the population scenario designed? What was the method used to define the values for dry and wet months?
6. In the Methods chapter, there are several variables and parameters without a proper definition and without a reference to the units even in empirical equations (e.g., Iroof, Croof, eta, P, Aroof, Irun, Cws...). Furthermore, there are several parameters without references to the estimation method or to the citation (e.g., Croof = 0.85, eta = 0.80, Cws = 0.31... CV, see Line 193). Please enhance quality of Figure 6.
7. To which horizon were the global circulation models used? This is a relevant information missing in the manuscript.
8. Figure 7 and the text below Table 5 ("Future rainfall projections for Si Chang Island under the RCP 4.5 scenario using the HadGEM-AO model indicate that the month of January will become the most critical period for water availability... will increasingly become the bottleneck for year-round water security on small islands like Si Chang Island") belongs to results and discussion, not to methods.
Minor remarks.
1. Title "Assessing and reducing dry-season water-scarcity risk on a tourism-exposed small island: a multi-source water-balance and robust-optimisation framework for Si Chang Island, Thailand". Is "a multi-source water-balance and robust-optimisation framework for" necessary in the title?
2. The phrase "reducing dry-season water-scarcity risk" creates the expectation that a new water management system has actually been installed and its performance evaluated.
3. The statement "the present system is reliable" contradicts the first sentence "Dry-season freshwater scarcity is a recurrent hazard".
4. In the Introduction, please rephrase the sentences "The result is recurrent, often unplanned, shortage... resources are lacking" (Lines 31 - 33). What do you mean by "additional catchment can substitute for storage" (Line 60)? Please rephrase the excessively long sentence ("And although... remains uneven"; Lines 65 - 70). The important statement (Lines 77 - 80) should be rephrased for clarity sake (the sentence is too long and some terms have not been properly defined, e.g., "harvestable catchment" and "embedded transient-demand term"). Please check for the grammar in "this study contributes: (i) a measured estimate..." (Line 81).
5. In Chapter "2.1 Location and land use", the sentence "its building roofs fall predominantly within the City/Town/Commercial class (U1 = 0.964 km²)" is difficult to understand: What is "City/Town/Commercial class"? What does U1 mean?
6. Figure 1 numbers, including topographical isolines and scale, are not readable (at least in the version we had access to). Please show where the north is in Fig. 1.
7. In Table 1, the column "Area (m²)" is redundant and, thus, it is not necessary.
8. "Watershed runoff to the reservoir" (Lines 104 - 105): which reservoir? "Aws = 268,878 m²": Please provide area in km². "weighted runoff coefficient": Either indicate how the weighting was calculated or present simpler metrics (mean, median...). Figure 2 captions: Please refer to the monitoring period.
9. The text from Line 110 to 116 needs improvement: The sentence "The main-system capacity used in the balance is 83,000 + 29,744 + 3,400 = 116,144 m³" is not necessary. The cistern volume (5,650 m³) is presented twice.
10. Table 6 first column reads "Scenarios: current... future". Scenario is a term normally used for future possibilities, thus, we recommend the use of a different word. The meaning of the term "robust" in Figure 8 is not clear. Please explain the meaning of both lines either in the captions or in the legend.
Citation: https://doi.org/10.5194/egusphere-2026-4005-RC3
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The paper presents a strong methodology that could be widely applicable or adaptable for similar islands or water management systems. I see a need for improvement in the paper's presentation, especially in the methods section. I also think the paper would benefit from adding an SI section, where some of the tables and figures should be moved. Code and data availability should also be shared during the review process so the reviewers can check it as well. I recommend publishing this paper but it could benefit from some substantial editing.
Figure 1 is called location of the Si Chang Island but I think you cannot read the coordinates well not the legends with the scale, there is no north arrow and no way to understand where in regards to the rest of Thailands land mass it is located.
Table 1 and line 97: City/Town/Commercial class for land and named Land Development Department classification from which country? Is this information necessary maybe remove the Code. Table 1 should be moved to SI and also the area in m2 and km2 conversion is unnecesary. Table 1 you calculate a sum but where do you use this value and what for?
Line105, 165 : where did you take the runoff coeffcient from?
Figure 2 includes the demand and demand future which is only calcuted in the 2.4 and there Figure 2 is not cited anymore.
Figure3 and 4: move to SI or combine Figure 1 and Figure 4 for example?
Line 131: how is the future population growth calculation justified?
Line 142-145: you give two methods here but the units between the two methods to compare the values are not the same on is m2 and the other ha. In the end which value did you choose and why?
Figure 5 and chapter 3 methods. I would rather you describe the method in written form and have less information in the figure. It is hard for me to follow the methodology in this way. The subheadings don’t match with the wording in the Figure 5 so it is hard for me to follow which chapter explains which part of the methodology
Line 175: whats the self suficiency criterion is it included in the equations (9) and three equations are given but only one index.
Chapter 5.3 is maybe better to redistrubute along the other chatpers and add to the introduciton.
Line 182: do not understand why this comment on physical units in necessary. Why did you add it?
Line 190: Please add a graphical representation of the probability density function used for the Monte Carlo sampling to the Supplementary Information.
Line 201: comment of data availability should be mentioned in the end of the manuscript. Consider removing.
Figure 6 and text 3.6, similar ot text Figure 5 I think the figure describes the method better to me than the written text. The section tries to do too many jobs at once, method, selection criterion, results, a tie-break, a robustness check, and a CMIP5-vs-CMIP6 defense, all in one dense block. There's also a criterion-switching problem. The text says selection is "made on annual-rainfall bias... not day-to-day correlation," and screens out MPI-ESM-LR on that basis. But to choose between the two survivors, it switches to monthly dry-season fidelity without acknowledging the switch or explaining why that criterion wasn't part of the original screen.
Section 3.6: is it climate forcing? Just say forcing makes me wonder what you’re doing.
Table 4, 5 and 8 should be moved to the SI. As well as Table 9 and 10 because the data can be already seen from Figure 11. Same for Table 11.
Explain Figure 8 only in chapter 4.4. why need to add chapter 4.3 then?
Section 4.3: why did you only try to add 20,000m3 of added storage how do you justify this number (only explained in chapter 5.3) and can you add two or three other scenarios to the figure 8 to compare different outcomes and influences. You already did all the work to make these calculations so adding more scenarios would be a great way to extend on your current work.
Code and data availability should be shared also during the review process so the reviewers can check it as well.
Author contributions: Tanachaichoksirikun, P. developed the optimization model, performed simulations, and wrote the original manuscript why is ne not the first author?