Impact of Dense Networks of Reservoirs on Streamflows at Dryland Catchments
, Jörg Dietrich 3, Elahe Fallah-Mehdipour 3, Pieter Van Oel 4 and Iran Eduardo Lima Neto 1
Round 1
Reviewer 1 Report
This is an interesting modelling study of the hydrological impact of small dams on semiarid catchments. The question is an interesting one, as the construction of small dams on minor trbutaries is largely unregulated in most countries, yet there is the potential for them to have significant impacts on water availability, with both positive effects and many potential negative impacts. This study is nicely designed and well executed. It has some minor limitations (I'm not sure the hypothesis that dams will be evenly distributed across the catchment is completely reasonable but as a starting point for modelling it is acceptable - a more nuanced, realistic modelling approach might be preferable in future studies). The results are clear and unambiguous, with reasonable consideration of their implications. Overall, I have no issues with the paper and recommend acceptance.
Author Response
Dear Reviewer,
We are grateful for the constructive comments which helped us improve our manuscript. We have addressed your concerns in the response letter and in the revised version of the paper, as detailed as follows. All the changes are highlighted in the track-changes version of the manuscript.
Comments and Suggestions for Authors
This is an interesting modelling study of the hydrological impact of small dams on semiarid catchments. The question is an interesting one, as the construction of small dams on minor trbutaries is largely unregulated in most countries, yet there is the potential for them to have significant impacts on water availability, with both positive effects and many potential negative impacts. This study is nicely designed and well executed. It has some minor limitations (I'm not sure the hypothesis that dams will be evenly distributed across the catchment is completely reasonable but as a starting point for modelling it is acceptable - a more nuanced, realistic modelling approach might be preferable in future studies). The results are clear and unambiguous, with reasonable consideration of their implications. Overall, I have no issues with the paper and recommend acceptance.
Answer: Thank you for the constructive comments. We agree that the hypothesis that the construction of new reservoirs in a uniformly distributed way along the catchment is a limitation of the methodology. However, as a starting point for modeling high-density reservoir network in SWAT, it sounds reasonable to provide some insights about the impacts in streamflows. In this sense, we added a paragraph in the conclusion section to clarify that in future studies a more realistic scenario approach of increasing the number of small reservoirs should be adopted.
“This research provides insights about the influence of the increase in the number of small reservoirs at dryland catchments. However, as a starting point for the scenario approach, the increase in small reservoirs was evenly distributed across the catchment. For future studies, a more realistic scenario approach should be adopted, with a higher increase of small reservoirs in regions close to large strategic reservoirs and higher population densities.” (L580-585)
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Manuscript ID: sustainability-1961622
Article Title: Impact of Dense Networks of Reservoirs on Streamflows at Dryland Catchments
Comments
This study presents and focus on Impact of Dense Networks of Reservoirs on Streamflows at Dryland Catchments. The article has the scope to publish in the journal but not in the present form. The introduction needs refinements. Methodology is poorly written. Many sentences are unclear in the manuscript. Figures and Tables need to be re-formulated. Results need lot of improvements. This is the general research work. In the whole manuscript, I don’t see any novelty. So, I would recommend for major correction.
Comment 1: The abstract doesn’t show the accurate content and the main findings of the study area. Please add the main findings of the research work. In the abastract section, line no. 21-22, What is the meaning of this sentence. “The study area was a large-scale watershed in the semiarid of Brazil, modeled in SWAT”.
Comment 2: The first paragraph should explain more about the importance of Dense Networks of Reservoirs on Streamflows at Dryland Catchments.
Comment 3: Quantitative results should be provided in the abstract to make it more comprehensive. Results of model Should be added in the abstract section. Also, The main aim of the study should be clearly mentioned in the abstract.
Comments 4: I recommend the authors to write in the Introduction more explicitly based on existing literature what is missing in previous studies, what is the added value of this new study. In the Introdution section, line no. 125 to 127, Please correct the sentsnce, I am unable to understand.
Comment 5: Please proofread the article carefully; there are many linguistic errors in the manuscript. The manuscript's English need to be significantly improved.
Comment 6: There are lots of articles published in the similar research area in the last 3-5 years in the same journal. Please refer to it in this manuscript.
https://doi.org/10.1016/j.jhydrol.2021.127103
https://doi.org/10.1002/hyp.1350
https://doi.org/10.1061/JSWBAY.0000881
https://doi.org/10.1016/j.ejrh.2017.01.004
https://doi.org/10.1080/13241583.2005.11465259
https://doi.org/10.1061/(ASCE)HZ.2153-5515.0000508
Comment 7: The methodology section is very unclear and needs updates.
Comment 8: Please improve the Figure 2 and 4 with high resolutions. I am unable to see the values of the figures and legibility is very poor.
Comment 9: Recheck all the notations and use the same notations. Some unnecessary spaces, full stop (.) and comma (,) were used in the whole manuscript. Please rectify those errors.
Comment 10: The applications of the study should be highlighted at the end of the results section.
Comments 11: Please add more discussion material. What were perhaps different results from other studies and why?
Comments 12: The conclusion should be specific. It is recommended to just highlight the key findings of the work. In the section Research gaps and future scope, it should be specific not in general form.
The article has the scope to publish in the journal but not in the present form. The introduction needs refinements. The methodology is poorly written. Many sentences are unclear in the manuscript. Figures and Tables need to be re-formulated. The discussion section needs a lot of improvements. I would recommend it for a major correction.
Comments for author File: Comments.pdf
Author Response
Dear Reviewer,
We are grateful for the constructive comments and suggestions, which helped us improve our manuscript. We have addressed all concerns in the response letter and in the revised version of the paper, as detailed as follows. All the changes are highlighted in the track-changes version of the manuscript.
Comments and Suggestions for Authors
This study presents and focus on Impact of Dense Networks of Reservoirs on Streamflows at Dryland Catchments. The article has the scope to publish in the journal but not in the present form. The introduction needs refinements. Methodology is poorly written. Many sentences are unclear in the manuscript. Figures and Tables need to be re-formulated. Results need lot of improvements. This is the general research work. In the whole manuscript, I don’t see any novelty. So, I would recommend for major correction.
Answer: All sections of the manuscript have been completely revised, and all suggestions were considered in the new version. A new paragraph has been added to the introduction section to specify the gaps from previous studies and confusing sentences have been rewritten for clarification. In the methodology, a better explanation was given to the parameterization of the rainfall-runoff processes, as well as tables with detailed parameters were inserted in the supplementary material. We added a figure with a flowchart showing the steps of the methodology. Tables and graphs have been changed for better quality. New sentences have also been added to clarify the results, discussion, and conclusions sections. We thank you for your contributions.
Comment 1: The abstract doesn’t show the accurate content and the main findings of the study area. Please add the main findings of the research work. In the abastract section, line no. 21-22, What is the meaning of this sentence. “The study area was a large-scale watershed in the semiarid of Brazil, modeled in SWAT”.
Answer: The abstract was rewritten in accordance with the changes suggested in Comment 1.
The main findings of our research were included, as follows:
“The results showed that the influence of the HdRN on runoff reduction mostly occurs for a probability of exceedance between 1% and 10% of month flows and is very small for months with very high peaks of flow. The reduction in the outlet flow due to the increase in the number of small reservoirs is stronger during dry years (up to 30%) than during wet years (up to 8%) and tends to increase in years with consecutive lack of rains (from about 7% in the first year to about 20% in the last year and in the worst scenario), which may intensify the period of extended droughts. This research provides insights about the impact of the increase in the number of small reservoirs in the interannual variability of flow retention and the understanding of the influence of small reservoirs on runoff reduction may help water resources agencies to better prepare for hydrologic extremes (droughts and floods).” (L28-37)
The previous sentence in the L21-22 has been rewritten:
“The study area was the Conceição river catchment (3,347 km²) in the semiarid of Brazil. The hydrological model of the study area was developed in SWAT.” (L23-24)
Comment 2: The first paragraph should explain more about the importance of Dense Networks of Reservoirs on Streamflows at Dryland Catchments.
Answer: The first paragraph has been improved.
“Small reservoirs play an important role in providing water to rural communities. Increased construction of small reservoirs to mitigate the effects of droughts leads to a High-density Reservoirs Network (HdRN) of small reservoirs, which can potentially modify the streamflows both in dry and wet periods. However, there is a lack of understanding the interannual behavior of flow retention and the impact of future increases in the number of small reservoirs, mainly for HdRN in dryland catchments.” (L15-20)
Comment 3: Quantitative results should be provided in the abstract to make it more comprehensive. Results of model Should be added in the abstract section. Also, The main aim of the study should be clearly mentioned in the abstract.
Answer: The abstract was rewritten in accordance with the changes suggested in Comment 3. Quantitative results were included (please see L28-34).
The results of statistical parameters of the model were presented too:
“The model obtained appropriate results for daily streamflows, with values of 0.63, 0.81 and 0.53% for NSE, KGE and PBIAS, respectively.” (L24-26)
Also, to make clearer the main aim of our study we changed some sentences in the abstract, as follows:
“Increased construction of small reservoirs to mitigate the effects of droughts leads to a High-density Reservoirs Network (HdRN) of small reservoirs, which can potentially modify the streamflows both in dry and wet periods. However, there is a lack of understanding the interannual behavior of flow retention and the impact of future increases in the number of small reservoirs, mainly for HdRN in dryland catchments. This research aims to determine the possible impact of the increase in the number of small reservoirs on dry hydrological networks, evaluating the annual flows generated at the outlet of a dryland watershed for scenarios with different densities of small reservoirs (number of reservoirs per area).” (L15-23)
Comments 4: I recommend the authors to write in the Introduction more explicitly based on existing literature what is missing in previous studies, what is the added value of this new study. In the Introdution section, line no. 125 to 127, Please correct the sentsnce, I am unable to understand.
Answer: We agree that in the previous version of the manuscript the gaps in the previous studies were not clear for the reader. We added a paragraph after the presentation of the literature about dense networks of reservoirs in drylands to explain the novelty of this research.
“A system of small reservoirs can influence hydrological processes at catchment scale [1,18]. However, most studies use highly simplified models to represent small reservoirs in hydrological networks. The storage capacities of small reservoirs, their horizontal connectivity, and their interaction with large strategic reservoirs are not represented in detail, which may lead to misinterpretation of the role of small reser-voirs in simulating interannual variability of runoff. There is a gap in understanding the interannual variability of runoff retention, particularly during hydrologic extremes (floods and droughts), which complicates modeling the effects of future increases in the number of small reservoirs in drylands.” (L100-108)
Also, the sentence in the previous L125-127 was rewritten as follows:
“To achieve this objective a detailed representation of a watershed, including large and small reservoirs, was modeled in SWAT. The study area was in the Brazilian semiarid region.” (L136-138)
Comment 5: Please proofread the article carefully; there are many linguistic errors in the manuscript. The manuscript's English need to be significantly improved.
Answer: Based on the constructive comment, the manuscript has been revised.
Comment 6: There are lots of articles published in the similar research area in the last 3-5 years in the same journal. Please refer to it in this manuscript.
https://doi.org/10.1016/j.jhydrol.2021.127103 (1)
https://doi.org/10.1002/hyp.1350 (2)
https://doi.org/10.1061/JSWBAY.0000881 (3)
https://doi.org/10.1016/j.ejrh.2017.01.004 (4)
https://doi.org/10.1080/13241583.2005.11465259 (5)
https://doi.org/10.1061/(ASCE)HZ.2153-5515.0000508 (6)
Answer: Thanks for the suggestion of references.
Reference (1) is important for our study since this is a previous research work that we conducted in the same study area. Thus, the present manuscript advances in the knowledge of the impact of HdRNs on streamflows in dryland catchments. Please, note that the above-mentioned study refers to Rabelo et al. [1] or [1] in the manuscript. Additionally, the number (5) refers to number [12] in the manuscript. Observe that we also added the references (3), (4) and (6) in the manuscript as the numbers [35], [54] and [36], respectively.
Comment 7: The methodology section is very unclear and needs updates.
Answer: Thanks for pointing that out. We have revised the methodology section.
The section 2.1 (Study Area) only included minor changes.
In section 2.2 (Model Parameterization), a new paragraph was added to better explain the model input data.
“The SWAT model was used to model hydrological processes and reservoirs, and to simulate streamflows in the outlet of the catchment. The delineation of the watershed and its river network (Figure 1) were carried out based on a digital elevation model (DEM) with 90 m resolution. The climate data was made available by FUNCEME. All analyzed scenarios of small reservoirs were based on the SWAT model calibrated and validated for the Conceição River catchment available at Rabelo et al. [1]. To clarify the modeling processes carried out in SWAT, in the following sections the parameterization of large and small reservoirs and the calibration of the model are briefly presented.” (L188-195)
In section 2.2.1 (Reservoir system), a new sentence has been added to show the criteria for distinguishing “reservoirs” and “ponds” in the SWAT model. Also, we have added to the supplementary material a brief description of the aggregation of small reservoirs into “ponds”.
“… water impoundments caused by dam constructions built across the main river reach and with estimated storage capacity larger than 0.01 hm³ were modeled in the “reservoir” structure of the SWAT model (strategic reservoirs and main private reservoirs); the remaining reservoirs were modeled in the “pond” structure of the SWAT model (small reservoirs); due to the large number of small reservoirs and the limitation of SWAT2012 in allowing only one “pond” per sub-basin, they were aggregated into a single “pond” for each sub-basin, by the cascade or parallel arrangement. A more de-tailed description of the methodology for aggregation of small reservoirs into ponds by cascade and parallel arrangement is available in the section S1 of the supplementary material.” (L201-210)
In section 2.2.2 (Calibrated Model), a new paragraph has been added to make a brief explanation of the calibration of the rainfall-runoff processes. Also, we added in the supplementary material some tables to show the parameters calibrated for strategic reservoirs, main private reservoirs, small reservoirs, and sub-basins.
“The calibration of the reference model was based on the available data, literature, and experience of the modelers. The following methods were used by applying the curve number method, plant evaporation method and Muskingum method for the calculation of infiltration, evapotranspiration, and channel routing, respectively. The parameters to describe the rainfall-runoff relationship were calibrated with an iterative trial and error procedure, by keeping parameter values in a physically meaningful range. Initial values for the model parameters were derived from field data as much as possible. When field data were not available, dryland-based literature values were chosen for them. Tables S3, S4, and S5 presented in supplementary material show parameters set to the entire catchment, parameters set for specific sub-basins of the catchment, and parameters set for specific soil zones, respectively. More detailed information about model parameterization and calibration can be found in Rabelo et al. [1].” (L244-255)
Finally, in section 2.3 (Scenarios Approach for Increase in the Number of Small Reservoirs), we added a flowchart to make a visual representation for all steps of the methodology (please see L326).
Comment 8: Please improve the Figure 2 and 4 with high resolutions. I am unable to see the values of the figures and legibility is very poor.
Answer: We have changed the legend font size to make the figures clearer. We also sent these figures in original format separately from the manuscript. It is important to note that with the addition of the flowchart (new Figure 3) the previous Figure 4 becomes Figure 5.
Comment 9: Recheck all the notations and use the same notations. Some unnecessary spaces, full stop (.) and comma (,) were used in the whole manuscript. Please rectify those errors.
Answer: The manuscript has been revised accordingly.
Comment 10: The applications of the study should be highlighted at the end of the results section.
Answer: We added a new paragraph in the end of the conclusion section to highlight the applications of the study, as follows:
“Since small reservoirs on minor tributaries are largely unregulated in most coun-tries, especially in drylands due to water scarcity, yet there is the potential for them to have significant impacts on water availability, with both positive effects and many potential negative impacts. In this sense, the methodology proposed in this study is highly transferable for different catchments worldwide. Moreover, as the population growth coupled with climate change trends may intensify the construction of small reservoirs to meet water demand of rural communities, the future scenarios of growth in the number of small reservoirs and the understanding of their influence on streamflow reduction may help water resources agencies to better prepare for future periods of droughts and extended droughts.” (L586-595)
Comments 11: Please add more discussion material. What were perhaps different results from other studies and why?
Answer: We have extended the discussion and added new paragraphs to improve this section.
In the first paragraph, only minor changes were done, as some sentences were moved from other paragraphs.
In the second and third paragraphs, new sentences were added to relate our findings on average water retention to other studies, and to improve the discussion based on hydrological processes and particularities of small reservoirs in different dryland catchments.
“The impact of small reservoirs in streamflows is currently small compared to strategic reservoirs. For Conceição River catchment Rabelo et al. [1] found on average a 2% of annual flow retention with the density of reservoirs equaling 0.068 res/km². However, the increase in the number of small reservoirs may increase the effects in the streamflows of the hydrological network. In this sense, scenarios with higher densities of small reservoirs can lead to cumulative impacts, increasing the flow retention to values close to those found for large reservoirs impacts [18,47]. This study obtained, on average, streamflow reductions from 1% to 14% in semiarid Brazil for densities of reservoirs from 0.1 res/km² to 3.0 res/km², while studies in semiarid West Africa obtained flow reductions of 14% in mean annual streamflow for scenarios with 0.08 res/km² [30].” (L495-504)
“By an analysis of around 30 references, Habets et al. [18] show that similar densities of small reservoirs can lead to different flow retentions (from 5.4% to 21.4%), as we see for semiarid Brazil and semiarid West Africa. The definition of a single indicator, as the density of small reservoirs in the area, to provide a first guess for the flow retention of small reservoirs has limitations due to the hydro-climatic conditions. The distribution of the reservoir network in dryland catchments and the hydrological processes in these regions, such as transmission losses and increased evaporation by small reservoirs should be evaluated to understand better the streamflow reduction caused by small reservoirs [1,30].” (L505-512)
The fourth and fifth paragraph were improved to bring a discussion about the major impact of the small reservoirs on water retention, that occurs in dry years or consecutive dry years. Furthermore, the fifth paragraph was improved to show that the impact of small reservoirs in the transition from meteorological droughts to hydrological droughts is not an easy task due to the complexity of hydrological processes in these regions
“The impact of the increase in the number of small reservoirs on streamflow reduction occurs strongly during dry years, with low flows. The decrease in low flows also has a large range, between 0.3 to 60% in Australia, Brazil, New Zealand, South Africa, and the USA [1,18,30]. In addition, this research found values of up to 30% of stream-flow retention for scenarios with densities of reservoirs until 3.0 res/km², while during extended drought years the values of retention ranged from 0.3% to 18.6%. These results lead to the intensification of droughts by the increase in the number of small reservoirs.” (L513-519)
“When a meteorological drought starts it is common an increase in well digging for water supply. In a region with low availability of underground water the construction of small reservoirs becomes a possibility for the local population to cope with droughts. The spatial distribution of reservoirs has a great impact on the occurrence of the hydrological drought. As streamflow drought responds more quickly than reservoir drought to a meteorological drought, the presence of water stored in small reservoirs can cause a delay between the beginning of the meteorological drought and the beginning of the hydrological drought, mainly in upstream regions. These small reservoirs dry up quickly in the dry season, consequently downstream reservoirs suffer the effects of droughts more quickly [23,48-51]. In dense networks of reservoirs this problem may be aggravated. As the presence of a dense network of reservoirs can lead to a 30% increase in the duration of hydrological droughts, the greater the number of small reservoirs, the greater the impact on water availability in the region [3]. Despite those results, it is still hard to evaluate individually the impact of small reservoirs on the transition from meteorological drought to hydrological drought due to the complexity of the hydrological processes in the catchment. Many authors observed a clear nonlinear relationship of hydrological drought and meteorological drought in different regions, with nonlinear functions modeled to propagation threshold from meteorological drought to hydrological drought [52-54].” (L520-537)
The sixth and seventh paragraph were added to show that the impact of small reservoirs on streamflows reduction is smaller during wet years, and that the storage of water during rainy periods has an important social and economic function for rural communities.
“The impact of small reservoirs on streamflow reduction is smaller during rainy years. In these years, as the precipitations are high and the small reservoirs usually fill quickly, the overflow of the spillways occurs in most of the small reservoirs also quickly. In this sense, for rainy years the hydraulic connectivity is achieved and the potential for water held in small reservoirs decreases, as they are already full [1,3].” (L538-542)
“The accumulation of water in small reservoirs during rainy years has an important social and economic function for rural communities. Not only in the Conceição River catchment, but also in several other watersheds in regions with a semiarid climate, small reservoirs act as an important structure to increase water access to the population [1,30].” (L543-546)
The seventh paragraph only had minor changes.
Comments 12: The conclusion should be specific. It is recommended to just highlight the key findings of the work. In the section Research gaps and future scope, it should be specific not in general form.
Answer: We formulated the conclusion section to be more especifically (please see L557-581).
Also, we added a paragraph explaining the limitations of the model and the suggestions for future works.
“This research provides insights about the influence of the increase in the number of small reservoirs at dryland catchments. However, as a starting point for the scenario approach, the increase in small reservoirs was evenly distributed across the catchment. For future studies, a more realistic scenario approach should be adopted, with a higher increase of small reservoirs in regions close to large strategic reservoirs and higher population densities.” (L580-585)
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The article has the scope to publish in the journal in the present form. The authors did sufficient revision in the revised manuscript. I would recommend it for acceptance.
