General comments:
The paper presents an analysis of measures aimed at restoring groundwater ecosystem services in the Grand Bahamas. A technical assessment of the potential for Managed Aquifer Recharge (MAR) is combined with a cost-benefit analysis (CBA) that applies two different system boundaries (referred to as a financial and an extended CBA). The underlying problem and the aim of combining a technical assessment and CBA to enable a more holistic analysis to identify sustainable mitigation measures are relevant and fit the journal's scope. An interesting case study is presented, and relevant aspects related to the applied methods and environmental challenges relevant to Grand Bahamas and similar areas are presented. The current structure (e.g., the problem description and aim in relation to how the results are presented) makes it unclear what the manuscript's primary aim and novelty are. For example, according to the title the focus is on groundwater ecosystem service restoration, but since one of the analysed measures is reforestation, which has no (or rather no analysed) effects on the groundwater ecosystem services, it is a bit confusing for the reader. It is of course possible to compare measures with different purposes, but since much of the manuscript focuses on MAR and the drinking water supply, it becomes unclear. Furthermore, it should be more clearly demonstrated how the proposed method enables the extended CBA, i.e. how it overcomes the perceived weaknesses of CBA.
Below, more detailed comments are provided related to the above mentioned aspects and some other details.
Specific comments:
Abstract: I miss a comment on the generic results and novelty of the paper, i.e. not only the case study results but also the conclusions and contributions related to the methods or similar.
Lines 21-25: MAR only provides 10% of the water demand, but reforestation provides 0% even though the extended CBA show that it is profitable. So, it is important to remember what decision criteria are used. Is it to find the most profitable measure that mitigate any of the damage caused by the hurricane or is it specifically to improve the drinking water supply/restore groundwater ecosystem services?
Line 36: forest, is it not more appropriate to use the term terrestrial ecosystems?
Line 65: The RO's high energy consumption is highlighted here. Should the social cost of carbon emissions due to energy consumption be included in the CBA, or is the energy production not causing any CO2 emissions?
Line 64-66: You refer to potential future hurricanes and the potential consequences to the existing RO system and other infrastructure. This is used as a motive for additional measures, but to fully consider this a probabilistic and risk-based approach could have been needed where the probability of future hurricanes and the consequences of them could have been included in the CBA (i.e., the consequences differ between the different alternatives depending on if GB is fully dependent on RO or not). I understand this has not been done and cannot be added now, so this is just a comment on how such aspects typically are included in risk-based CBA.
Lines 72-74: You state that “NBS are considered cost-effective and viable solutions”. Is it not more appropriate to say that they typically are since all types of NBS are not always cost-effective?
Lines 75-76 (this also relates to several other parts of the manuscript): It is stated that two NBS measures to mitigate the impacts of Dorina on groundwater ecosystems are analysed. Since a technical feasibility assessment is performed and the focus is on drinking water supply, it is natural to think that the measures aim to improve access to drinking water. However, reforestation has no effect on water supply, so as a reader, I wonder if the aim is to evaluate any measure that may mitigate the impacts of Dorina. Earlier in the manuscript (line 51), you mentioned that measures to mitigate both groundwater and forest ecosystems were implemented. According to the title, the focus is on groundwater. I suggest you clarify the focus of the paper an why the analysed measures are included.
Lines 81-91: You state that CBA “falls short of adequately monetarize ES”. There are numerous examples of how CBA has been applied to consider effects on ES and is it not rather so that the problem is not the method as such but how it is applied and that there may be a lack of valuation data to properly value economically the ES? You correctly state that all benefits and costs should be included in a CBA. This is, of course, typically not possible, and limitations are done. However, all relevant benefits and costs should be identified, but some may be excluded from the analysis due to different reasons. Based on this, I have two comments: (1) you state that you propose a method combining a technical feasibility assessment with CBA to (as I understand it) overcome the problem of overlooking ES. However, based on the presented results, it is not clear how the feasibility assessment improves the identification of relevant ES; (2) You do not present a thorough identification ES and related costs and benefits. The analysed ES are presented (lines 221-224), but how was this done, did you use CICCES or any other classification system to ensure no ES were overlooked?
Lines 120-123: Wellfield 6 constitutes approximately 42% of the total abstraction rate (26/11), but you state that it corresponds to 30% of the total demand. Does it have a higher capacity compared to the actual demand? Please clarify this. In the abstract, it is stated that Dorian caused 40% of the island’s water supply to become brackish.
Line 135: “sustainability measures (e.g., reforestation)”, but you only analyse reforestation and no other sustainability measures.
Section 2.2. (Figure 2, etc.): The CBA in both part 2 and part 3 shows what is most profitable; the only difference is that different system boundaries have been applied. In the first case, the focus is on the water supply, so e.g. what is most profitable with respect to water supply. Part 3 identifies the measure most profitable from a societal point of view. This does not make part 3 a novel type of CBA, it is a common type of CBA sometimes referred to as social CBA. The manuscript would benefit from a comment on system boundaries, i.e. which parts of society are included in part 2 and how does it differ from part 3 (global perspective or not?). Linked to a previous comment (lines 81-91), I also suggest you elaborate on how/if the feasibility assessment facilitates the CBA in step 3 or not.
Section 2.4 and supplementary material (Table S1): Water price is used to value drinking water supply as an ecosystem service. Since there is typically no real market for drinking water, this will most likely not represent the true value of the drinking water supply. Due to lack of useful valuation studies (WTP etc.), this might be a reasonable assumption, but must be discussed since this is a key ES in the case study.
Lines 193-194: You say all costs and benefits were identified, but this is with respect to the applied system boundaries.
Results section (but also the method): In a CBA, you typically present a reference scenario, and the measures are compared with this (i.e. what costs and benefits the measures cause). The current situation with RO implemented is presented in the manuscript, but RO is included as a measure. Therefore, the reference scenario/alternative is the current system without RO. The manuscript would benefit from a clear presentation of what is used as the reference scenario.
Lines 267: “the demand was calculated”, what calculations are done is it not only assumed equal to the abstraction rate for wellfield 6?
Line 268: “30% of the current brackish water supplied” is it not 100% of the supplied brackish water but 30% of the total demand today?
Line 285-287: Is the conclusion based on the risk assessment that the final total risk is acceptable or how are the results used?
Lines 314-315: An analysis of the distribution of costs and benefits in society is typically part of a CBA. Hence, this aspect can be considered in a CBA to make sure there is not an unreasonable distribution.
Line 325: has the option for recovering costs been considered in the analysis?
Line 334: Here, the results are presented for a 4% discount rate, but in the supplementary material a discount rate interval from 1 to 10% is presented (and the results are also presented for this interval). 10% is an extremely high discount rate for a project like the one presented here. More recent recommendations from national authorities recommend a lower rate and in the document referred to (Floy, 2013), it is mentioned that discount rates ranging from 1 to 10% are reported in the literature, but the author recommends “2.2% for benefit-cost analysis with low and high values of 1.5% and 3.5% for sensitivity analysis”. If keeping the range 1-10%, please comment on what range is commonly applied.
Lines 343-345: Since no benefits of reforestation are included, it seems to fall outside the scope of the analysis, and as a reader, you wonder why it is included.
Lines 350-352: It is quite a strange comment since you earlier stated that it has no benefits, so it will of course not perform well in a CBA with the system boundaries applied.
Section 3.2 and 3.3: As earlier commented, MAR and reforestation have different purposes, and I suggest this is included when the results are discussed.
Line 409-: You only consider uncertainties in the discount rate, but the uncertainties in the model assumptions should be discussed. To what extent can the applied data for valuing the ES affect the results etc.?
Lines 434-435: How do you ensure there is no double-counting?
Lines 468-470: If this is included, the results may look different, so it is an important uncertainty to comment on when discussing model uncertainties.
Line 506: You write two measures, but you include and analyse three measures since RO is considered a measure in the analysis. |
