Estimating Reservoir Evaporation Under Mediterranean Climate Using Indirect Methods: A Case Study in Southern Portugal

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript addresses reservoir evaporation in Southern Portugal, a region characterized by a typical Mediterranean climate where evaporation plays a key role in water budgets. The study is well-structured and relevant, with clear objectives to (i) assess reservoir evaporation patterns based on offshore meteorological measurements, (ii) benchmark various indirect methods for evaluating reservoir evaporation at a monthly scale, and (iii) provide recommendations on the most suitable methods for operational practices. The topic is timely and within the scope of the journal. However, some aspects of the manuscript would benefit from further clarification and refinement before it can be recommended for publication.

I would like to suggest a slight revision to the title for improved clarity and alignment with scientific writing style. My proposed version is: “Estimating reservoir evaporation under Mediterranean climate using indirect methods: a case study in Southern Portugal.”

A major methodological concern arises from the choice of the Bowen Ratio Energy Balance (BREB) method as the reference against which other evapotranspiration (ET) models are compared. The BREB approach provides estimates of actual evapotranspiration (ET), as it partitions the surface energy balance into sensible and latent heat fluxes based on local gradients of temperature and humidity. While this is a robust micrometeorological technique for quantifying energy fluxes at a specific site, it is not directly comparable to models such as Penman or other mass-transfer formulations, which were originally designed to estimate reference evapotranspiration (ETo) under standardized conditions (e.g., FAO-56 reference surface).

By comparing actual ET from BREB with reference ET from empirical or semi-empirical models, the study may conflate two distinct physical concepts: (i) ETo, which represents the atmospheric evaporative demand over a reference surface, and (ii) ET, which reflects the combined influence of climate and surface-specific factors (soil moisture, vegetation, canopy resistance). This mismatch can introduce bias and misinterpretation of the relative performance of the tested methods.

For consistency with international standards in agricultural and hydrological studies, I strongly recommend that the FAO-56 Penman–Monteith equation be adopted as the benchmark method. This model is widely recognized for its physical basis, robustness across climates, and suitability for evaluating alternative approaches to ETo estimation. The Bowen Ratio method may still be presented as an independent measurement of actual ET to discuss surface energy partitioning, but it should not be used as the baseline for assessing the performance of reference evapotranspiration models.

With this consideration, the results should be rewritten, as the comparison framework needs to be adjusted. I firmly believe that by adopting the FAO-56 Penman–Monteith equation as the reference, the manuscript will achieve significant improvements in the evaluation of other ETo estimation methods, such as Thornthwaite, Penman, mass transfer approaches, and the pan evaporation method.

Author Response

General comments: "This manuscript addresses reservoir evaporation in Southern Portugal, a region characterized by a typical Mediterranean climate where evaporation plays a key role in water budgets. The study is well-structured and relevant, with clear objectives to (i) assess reservoir evaporation patterns based on offshore meteorological measurements, (ii) benchmark various indirect methods for evaluating reservoir evaporation at a monthly scale, and (iii) provide recommendations on the most suitable methods for operational practices. The topic is timely and within the scope of the journal. However, some aspects of the manuscript would benefit from further clarification and refinement before it can be recommended for publication.”

Response: We sincerely thank the reviewer for the thorough and constructive assessment of our manuscript. We appreciate the recognition of the study’s relevance, structure, and objectives, as well as the acknowledgment that the topic is timely and aligned with the journal’s scope. We acknowledge the reviewer’s observation that some aspects require further clarification and refinement. A new version of the paper is being prepared, incorporating all reviewers’ comments and suggestions, including yours, to improve the overall clarity, consistency, and scientific robustness of the study. Specific responses to each of these comments are provided in detail below:

Comment 1: “I would like to suggest a slight revision to the title for improved clarity and alignment with scientific writing style. My proposed version is: “Estimating reservoir evaporation under Mediterranean climate using indirect methods: a case study in Southern Portugal.”

Response 1: “We thank the reviewer for the suggestion. We agree that the proposed title improves clarity and aligns well with scientific writing style. The title has been revised accordingly to: “Estimating reservoir evaporation under Mediterranean climate using indirect methods: a case study in Southern Portugal.”

Comment 2: “A major methodological concern arises from the choice of the Bowen Ratio Energy Balance (BREB) method as the reference against which other evapotranspiration (ET) models are compared. The BREB approach provides estimates of actual evapotranspiration (ET), as it partitions the surface energy balance into sensible and latent heat fluxes based on local gradients of temperature and humidity. While this is a robust micrometeorological technique for quantifying energy fluxes at a specific site, it is not directly comparable to models such as Penman or other mass-transfer formulations, which were originally designed to estimate reference evapotranspiration (ETo) under standardized conditions (e.g., FAO-56 reference surface). By comparing actual ET from BREB with reference ET from empirical or semi-empirical models, the study may conflate two distinct physical concepts: (i) ETo, which  represents the atmospheric evaporative demand over a reference surface, and (ii) ET, which reflects the combined influence of climate and surface-specific factors (soil moisture, vegetation, canopy resistance). This mismatch can introduce bias and misinterpretation of the relative performance of the tested methods. For consistency with international standards in agricultural and hydrological studies, I strongly recommend that the FAO-56 Penman–Monteith equation be adopted as the benchmark method. This model is widely recognized for its physical basis, robustness across climates, and suitability for evaluating alternative approaches to ETo estimation. The Bowen Ratio method may still be presented as an independent measurement of actual ET to discuss surface energy partitioning, but it should not be used as the baseline for assessing the performance of reference evapotranspiration models. With this consideration, the results should be rewritten, as the comparison framework needs to be adjusted. I firmly believe that by adopting the FAO-56 Penman–Monteith equation as the reference, the manuscript will achieve significant improvements in the evaluation of other ETo estimation methods, such as Thornthwaite, Penman, mass transfer approaches, and the pan evaporation method.”

Response 2: We thank the reviewer for the thorough and constructive comment regarding the use of the Bowen Ratio Energy Balance (BREB) method as the reference. The FAO Penman-Monteith (PM) equation is widely used to estimate reference evapotranspiration (ETo) from vegetated surfaces and includes several resistance parameters that are specific to plants. However, the authors acknowledge that some of these parameters (such as heat transport resistances, albedo, etc.) could be adapted to the conditions of open water surfaces. The use of the energy balance approach for reservoirs is justified because we employ temperature measurements to characterize the energy profile of the water bodies, while atmospheric variables are measured offshore, directly above the water surface, using floating stations. All the indirect methods considered in this study estimate evaporation or have been adapted to estimate evaporation from the reservoir water surface specifically.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The goal of the paper is to present a comparative analysis of different evaporation models using in situ sensor measurements. The final result is to identify he most appropriate model for assessing evaporative losses from reservoirs The work is well written and fails within the scope of the journal. I recommend a major revisions and my comments are presented below:

Abstract: the abstract is insufficient in its current form. Please amend it to provide an insightful overview of the paper. 

Introduction: This section needs expansion with a literature review of similar studies (lines 39-46). Please refer to the papers presented below.

Could you please provide an explanation how you conclude the selection of the 5 models based on the varying complexity?

Why do you use BREB model as a base to assess the reference evaporation and not Penman model? Could you please discuss this further?

Figure 9: The spatial analysis IDW refers also to the areas outside the reservoirs. If yes it this correct as the catchment losses are influenced by real evapotranspiration which is outside of the scope of the paper.

Im confused with the results and the conclusion of this work.  See below parts of paper which may lead to confusions:

Lines 345-346 The PEN method consistently overestimates annual evaporation, with the most pronounced positive bias occurring during dry season months

Lines 363- 364 The THOR method, based on the Thornthwaite formula using air temperature measurements only, showed surprisingly good agreement with the standard BREB method,  considering its simplicity.

Lines 455-456 The study concludes that the PEN and, PT methods provide accurate  estimations of reservoir evaporation, with slight variations depending on the specific reservoir characteristics and micro climatic conditions

Some parts of the results discussed in the chapter are moved in the conclusion section (diagram etc). Could you please rewrite the chapter 3.3.

A discussion section will be beneficial which can be summarized limitations of the study carried out, future research etc.

References

Melišová E, Vizina A, Hanel M, Pavlík P, Šuhájková P. Evaluation of Evaporation from Water Reservoirs in Local Conditions at Czech Republic. Hydrology. 2021; 8(4):153. https://doi.org/10.3390/hydrology8040153.

McMahon, T. A., Finlayson, B. L., & Peel, M. C. (2016). Historical developments of models for estimating evaporation using standard meteorological data. Wiley Interdisciplinary Reviews: Water, 3(6), 788-818.

McMahon, T. A., Peel, M. C., Lowe, L., Srikanthan, R., & McVicar, T. R. (2013). Estimating actual, potential, reference crop and pan evaporation using standard meteorological data: a pragmatic synthesis. Hydrology and Earth System Sciences, 17(4), 1331-1363.

Malamos N, Tegos A. Advances in Evaporation and Evaporative Demand. Hydrology. 2022; 9(5):78. https://doi.org/10.3390/hydrology9050078

Author Response

Reviewer 2 (28/09)

 

General comments: The goal of the paper is to present a comparative analysis of different evaporation models using in situ sensor measurements. The final result is to identify he most appropriate model for assessing evaporative losses from reservoirs The work is well written and fails within the scope of the journal. I recommend a major revisions and my comments are presented below:

 

Response: We sincerely thank the reviewer for the positive assessment of our manuscript and for recognizing its relevance within the scope of the journal. We appreciate the careful evaluation and the constructive feedback provided. Detailed responses to each of the reviewer’s comments are provided below.

 

Comment 1: Abstract: the abstract is insufficient in its current form. Please amend it to provide an insightful overview of the paper.

 

Response 1: We thank the reviewer for the comment regarding the abstract. A new version of the paper is being prepared, incorporating all reviewers’ comments and suggestions, and the abstract will be improved to provide a more comprehensive and insightful overview of the paper.

 

 

Comment 2: Introduction: This section needs expansion with a literature review of similar studies (lines 39-46). Please refer to the papers presented below.

 

References

Melišová E, Vizina A, Hanel M, Pavlík P, Šuhájková P. Evaluation of Evaporation from Water Reservoirs in Local Conditions at Czech Republic. Hydrology. 2021; 8(4):153. https://doi.org/10.3390/hydrology8040153.

 

McMahon, T. A., Finlayson, B. L., & Peel, M. C. (2016). Historical developments of models for estimating evaporation using standard meteorological data. Wiley Interdisciplinary Reviews: Water, 3(6), 788-818.

 

McMahon, T. A., Peel, M. C., Lowe, L., Srikanthan, R., & McVicar, T. R. (2013). Estimating actual, potential, reference crop and pan evaporation using standard meteorological data: a pragmatic synthesis. Hydrology and Earth System Sciences, 17(4), 1331-1363.

 

Malamos N, Tegos A. Advances in Evaporation and Evaporative Demand. Hydrology. 2022; 9(5):78. https://doi.org/10.3390/hydrology9050078

 

Response 2:  We agree with the reviewer. The Introduction section has been expanded to include a more comprehensive literature review of similar studies, including the papers suggested.

 

 

Comment 3: Could you please provide an explanation how you conclude the selection of the 5 models based on the varying complexity?

 

Response 3: Based on the monitoring, data, and information collected, these five models were intended to be the easiest to use for the reservoir managers. Furthermore, the collected information enabled the calibration of the mass transfer coefficient at the sites.

 

 

Comment 4: Why do you use BREB model as a base to assess the reference evaporation and not Penman model? Could you please discuss this further?

 

Response 4: We thank the reviewer for this valuable comment. An additional explanation has been included in the manuscript to clarify the choice of the BREB method as the reference for assessing evaporation. Further references have also been added to support and justify this choice.

 

Comment 5: Figure 9: The spatial analysis IDW refers also to the areas outside the reservoirs. If yes it this correct as the catchment losses are influenced by real evapotranspiration which is outside of the scope of the paper.

 

Response 5: The IDW spatial representation was based solely on the evaporation results from the nine reservoirs under study. Its purpose was only to illustrate the spatial variation of evaporation within the study area, without any intention of adding additional spatial information on evaporation, which was not directly measured.

 

Comment 6: I’m confused with the results and the conclusion of this work.  See below parts of paper which may lead to confusions:

• Lines 345-346 The PEN method consistently overestimates annual evaporation, with the most pronounced positive bias occurring during dry season months
• Lines 363- 364 The THOR method, based on the Thornthwaite formula using air temperature measurements only, showed surprisingly good agreement with the standard BREB method, considering its simplicity.
• Lines 455-456 The study concludes that the PEN and, PT methods provide accurate estimations of reservoir evaporation, with slight variations depending on the specific reservoir characteristics and micro climatic conditions

 

Response 6: We thank the reviewer for the comment. The Conclusions section has been thoroughly reformulated to improve clarity and ensure consistency with the Results, reducing any potential confusion.

 

 

Comment 7: Some parts of the results discussed in the chapter are moved in the conclusion section (diagram etc). Could you please rewrite the chapter 3.3.

 

Response 7: We agree with the reviewer. The manuscript has been fully restructured to improve coherence between sections.

 

Comment 8: A discussion section will be beneficial which can be summarized limitations of the study carried out, future research etc.

 

Response 8: We thank the reviewer for the suggestion. While we agree that a dedicated Discussion section summarizing the study limitations and potential directions for future research would be beneficial, due to time constraints it was not possible to include this in the current revision.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript is based on extensive observational data and employs multiple evaluation methods to investigate the characteristics of reservoir evaporation in Southern Portugal. The study has important implications for accurately characterizing evaporation processes in the region. However, the paper has several issues in terms of organization and writing. Specific revision suggestions are as follows:

1)    In the title, the term “indirect methods” should be replaced with the specific method names.

2)    The abstract lacks essential information. It should clearly present the research objectives, methods, and key findings, so as to attract a broader readership.

3)    L28: The section discussing current understanding and research progress on reservoir evaporation is not comprehensive. Please supplement it with additional case studies.

4)    L44: This section should be merged with the preceding one.

5)    L52: The research content, methods, and objectives should be described in more detail here.

6)    L65–68: Since it is already 2025, more recent climate characteristics should be used. The 1981–2010 dataset is outdated.

7)    Figures 3 and 4 currently present raw observational records. It is recommended to process and refine the data before displaying them, and preferably highlight spatial patterns to better capture hydrological variability at the regional scale.

8)    L180–235: The introduction of evaporation research methods could be moved to the Introduction section. Here, the focus should be on explaining the principles of the methods you actually applied.

9)    Table 3 lists commonly used methods. A brief textual explanation would suffice, accompanied by references for the formulas.

10)  In Table 4, does the “d value” apply to all indices? What is the basis for categorizing different classes?

11)  The text in all figures is too small and not clearly legible.

12)  The results section lacks a proper discussion. The findings should be compared with existing studies, and the reasons for similarities or differences should be explained.

13)  The results section is overly long. It should be streamlined to highlight the main findings rather than presenting excessive detail.

Author Response

General comments: This manuscript is based on extensive observational data and employs multiple evaluation methods to investigate the characteristics of reservoir evaporation in Southern Portugal. The study has important implications for accurately characterizing evaporation processes in the region. However, the paper has several issues in terms of organization and writing. Specific revision suggestions are as follows:

Response: We sincerely thank the reviewer for the careful evaluation of our manuscript and for recognizing the importance of the study and the value of the observational data. We appreciate the constructive feedback regarding the organization and writing of the paper. We are currently preparing a revised version of the manuscript in which these issues, along with the detailed suggestions provided, will be carefully addressed to improve clarity, structure, and overall readability.

Comment 1: In the title, the term “indirect methods” should be replaced with the specific method names.

Response 1: We thank the reviewer for the suggestion regarding the title. We believe that including all method names in the title would make it excessively long and less clear. Instead, we have added the specific methods used to the abstract, ensuring that this information is clearly presented to the reader.

Comment 2: The abstract lacks essential information. It should clearly present the research objectives, methods, and key findings, so as to attract a broader readership.

Response 2: We thank the reviewer for the comment. The abstract has been fully revised and improved to clearly present the research objectives, methods, and key findings, ensuring it provides a comprehensive and informative overview of the study.

Comment 3, 4 e 5: L28: The section discussing current understanding and research progress on reservoir evaporation is not comprehensive. Please supplement it with additional case studies; L44: This section should be merged with the preceding one, L52: The research content, methods, and objectives should be described in more detail here.

Response 3, 4 e 5: We thank the reviewer for the detailed comments. We agree with the suggestions regarding the structure and content of the chapter. The chapter has been reorganized to improve clarity, with the relevant sections merged as recommended. Additionally, the discussion on current understanding and research progress has been expanded to include additional case studies. The description of research content, methods, and objectives has also been elaborated, and the introduction of evaporation research methods has been appropriately moved to the Introduction section, focusing here on the principles of the methods actually applied.

Comment 6: L65–68: Since it is already 2025, more recent climate characteristics should be used. The 1981–2010 dataset is outdated.

Response 6: We agree with the reviewer. The climate dataset has been updated to include more recent data (1991-2020), ensuring that the analysis reflects current climate characteristics.

Comment 7: Figures 3 and 4 currently present raw observational records. It is recommended to process and refine the data before displaying them, and preferably highlight spatial patterns to better capture hydrological variability at the regional scale.

Response 7: We agree with the reviewer that capturing hydrological variability at the regional scale is important. To address this, Figure 3 presents a 14-day forward moving average calculated using data from all reservoirs, which highlights regional trends effectively. The main objective of Figure 4 is to represent the isothermal profile and to show the temporal evolution of temperature at different depths within the reservoirs. This representation allows readers to visualize the thermal structure and its changes over time in the water column.

Comment 8 and 9: L180–235: The introduction of evaporation research methods could be moved to the Introduction section. Here, the focus should be on explaining the principles of the methods you actually applied. Table 3 lists commonly used methods. A brief textual explanation would suffice, accompanied by references for the formulas.

Response 8 and 9: We thank the reviewer for the suggestion. Following your recommendation, the introduction of evaporation research methods has been moved to the Introduction section.

Comment 10: In Table 4, does the “d value” apply to all indices? What is the basis for categorizing different classes?

Response 10: We thank the reviewer for the comment. The “d value” in Table 4 corresponds to the product of the Index of Agreement (IOA) and the correlation coefficient (R) for each method. The classification of different categories is based directly on the value of d, as indicated in the table, allowing an objective comparison of the performance of the evaluated indices. To improve the clarity and comprehension of Table 4, we have revised the table header and added a reference.

Comment 11: The text in all figures is too small and not clearly legible.

Response 11: We agree that the text in the figures is currently too small and may not be clearly legible. We will revise all figures to increase text size and improve overall readability.

Comment 12 and 13: The results section lacks a proper discussion. The findings should be compared with existing studies, and the reasons for similarities or differences should be explained. The results section is overly long. It should be streamlined to highlight the main findings rather than presenting excessive detail.

Response 12 and 13: We agree with the reviewer. The Results section has been revised and streamlined to focus on the main findings, reducing unnecessary detail and improving overall readability.

 

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Dear authors,

I have reviewed you article and I found it very intersting, well structured and easy for reading . ITs the most important  value is possibility of its implementation in any other regions becuse problem of reservoir evaporation  can be very serious for reservoir water management.  My remarks and suggestions are give in the manuscript directly ( attached to this review) .

Comments for author File: Comments.pdf

Author Response

Comments: “I have reviewed you article and I found it very interesting, well structured and easy for reading . It’s the most important  value is possibility of its implementation in any other regions because problem of reservoir evaporation  can be very serious for reservoir water management.  My remarks and suggestions are given in the manuscript directly (attached to this review).”

Response: “We sincerely thank the reviewer for the positive and encouraging feedback. We are pleased that the reviewer found our manuscript interesting, well structured, and relevant for broader application. We highly appreciate the remarks and suggestions provided directly in the annotated manuscript.

We are currently preparing a revised version of the document that will incorporate the comments and suggestions from all reviewers, and your valuable observations will be fully taken into account in this updated version. We believe that these contributions will further improve the quality and clarity of our paper.”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors My notes were taken into account and explained and I consider that the manuscript meets the requirements for publication in this journal.

Reviewer 2 Report

Comments and Suggestions for Authors

Authors have been taken my comments on board and this is much appreciated. I have no further comments to add.