Is drought protection possible without compromising flood protection? Estimating the maximum dual-use benefit of small flood reservoirs in Southern Germany

Ho, Sarah Quỳnh Giang; Ehret, Uwe

doi:https://doi.org/10.5194/egusphere-2024-2167

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https://doi.org/10.5194/egusphere-2024-2167

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23 Jul 2024

| 23 Jul 2024

Status: this preprint is open for discussion.

Is drought protection possible without compromising flood protection? Estimating the maximum dual-use benefit of small flood reservoirs in Southern Germany

Sarah Quỳnh Giang Ho and Uwe Ehret

Abstract. As climate change drives intensification and increased frequency of hydrological extremes, the need to balance drought resilience and flood protection becomes critical for proper water resources management. Recent extreme droughts in the last decade in Germany have caused significant damages to ecosystems and human society, prompting renewed interest in sustainable water resources management. At the same time, protection from floods such as the catastrophic 2021 event in the Ahr Valley remain heavy in the public conscience. In the state of Baden-Württemberg in Southwestern Germany alone, over 600 small (< 1 million m³) to medium-sized (1–10 million m³) reservoirs are currently operated for flood protection. In this study, we investigate optimal reservoir operating (storage and release) rules for water supply downstream in a dual flood-drought protection scheme for 30 selected modeled flood reservoirs in Baden-Württemberg. Daily target releases for drought protection are proposed based on modeled inflows from the calibrated hydrological model LARSIM. Modified operation rules are optimized in a scenario of perfect knowledge of the future by using meteorological observations as artificial weather forecasts in LARSIM. The results of different operating rules are then evaluated based on their adherence to the target releases and flood protection performance. Reservoirs were required to maintain the same level of flood protection under these modified rules. Optimized reservoirs were able to release up to 80 times their volume or improve up to 95 % of existing drought conditions (penalty and volume deficit) over a 24-year period, though never simultaneously—there seems to be a trade-off between relative water availability to the reservoir and ability to alleviate drought conditions. Certain reservoirs were near-optimal, others could be improved further, and still others were not very effective at reducing drought conditions. We find that relative water availability at the reservoir (expressed as the number of times the reservoir can be filled by the difference between the mean inflow and mean low flow) has a strong relation to the amount of water a reservoir can release for drought protection, but fails to summarily describe the reservoir’s potential impact on drought conditions downstream.

Received: 12 Jul 2024 – Discussion started: 23 Jul 2024

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Sarah Quỳnh Giang Ho and Uwe Ehret

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RC1:
'Comment on egusphere-2024-2167', Anonymous Referee #1, 15 Aug 2024 reply
The manuscript by Ho and Ehret investigates the potential of small, medium and large reservoirs designed for flood control (flood protection or multipurpose reservoirs) to mitigate drought. They select 30 examples from more than 600 existing reservoirs in Baden-Württenberg (Southwest Germany) and apply a “perfect runoff prediction” into these remaining reservoirs by using the conceptual hydrological model LARSIM. These inflows to the reservoirs are labelled “semi-natural”. Based on this inflow over a 24 year daily time series, the authors tried different operating rules for flood retention and drought release. The prerequisite for this was that flood protection should not be impaired by the new operating rules. The operating rules are based on two-point hedging rules (where hedging storage begins at one point and ends at another).

The idea using flood retention basins to mitigate droughts is worth exploring. However, the manuscript has significant flaws. I would recommend rejecting the paper and resubmitting it after a complete revison.
My main concerns are the following (see more details below):
The connection between drought mitigation and water release is solely based on Q70. This is too simple to draw conclusions about drought defense.

The spatial and hydrological context of the (arbitrary chosen) reservoirs is missing. Drought protection requires a model for river basin management.

The parameter SF is not helpful (as the authors admit). Why didn’t they choose a different parameter?

The conclusion “reservoirs can release up to 80 times their capacity and reduce drought penalties and water deficits by almost 95% over a 24-year simulation period” is mentioned twice (abstract and conclusions). That is the exception and not the rule and therefore misleading.

Further comments in detail:
Title: “…Estimating the maximum dual-use benefit of small flood reservoirs in Southern Germany” à That is not correct. The authors also analyzed medium and large reservoirs.

Introduction: The sections on small reservoirs in Africa and worldwide do not fit the main content of the manuscript. Why do the authors write so much about agricultural (rainwater harvesting) reservoirs?

The hypothesis “that the reservoirs providing the most benefit in drought conditions will be those that have high inflow relative to the reservoir capacity” is not justified. Why did the authors formulate this hypothesis?

Line 127 until 135: The “slot” definition is unclear. The final selection of the reservoirs is not explained.

The storage factor SF should be renamed as “availability factor AF”. That would also avoid confusion with the variable designation SF already assigned for “small flood protection” (see Table 1). However, it is unclear why the storage factor can support drought mitigation. It is not logical that smaller C value (i.e. a higher SF value ) will “reduce drought conditions more effectively”.

Table 2: the volume of C should be added.

Equation 3: Why “-5” as a penalty factor?

Equation 3 and 4: Create a diagram for a better interpretation of the penalty functions.

Table 3: Move it to the appendix.

Figure 4: Why is the penalty benefit of SMALL multipurpose reservoirs so much lower than flood-only reservoirs? Please explain and discuss!

Figure 5 until 9: The flood penalty can be skipped because the precondition was that flood protection must not be impaired. Furthermore, the graphs related to outflow and drought penalties are not clearly recognizable.

Figure 10: Can be skipped (flood statistics are not changed due to the authors’ precondition)

Line 440 ff: The argumentation with the IWD is very weak. Either omit or elaborate in detail (seasonal influence, PET, soil conditions etc).

Line 473: “Changing the model so that the timing of reservoir releases such that water is given at the drought peaks could improve the penalty benefit further, though at the cost of complicating the model and the release rules.” This is recommended for the resubmission of the paper.

Minor comments:
Line 253: “is used only used to”--> “is only used to”

Equation 11 “V = C >=0” --> “V-C >=0”

Line 330: “former” --> “latter”

Line 338: “flood droughts” --> “flood”

Figure 11: Are the x- and y-axis swapped? “combined operation model” should be on the y-axis?

Better to use Bp (Benefit for Drought) instead of Vd,nor (y-axis, left figure)

Reply
Citation: https://doi.org/10.5194/egusphere-2024-2167-RC1
- AC1:
  'Reply on RC1', Sarah Ho, 03 Sep 2024 reply
  Dear Editor, dear Reviewer 1,
  Please find our point-by-point reply to the reviewer comments below. For easier reading, the reviewer comments are in italics and our replies are in standard font.
  The manuscript by Ho and Ehret investigates the potential of small, medium and large reservoirs designed for flood control (flood protection or multipurpose reservoirs) to mitigate drought. They select 30 examples from more than 600 existing reservoirs in Baden-Württenberg (Southwest Germany) and apply a “perfect runoff prediction” into these remaining reservoirs by using the conceptual hydrological model LARSIM. These inflows to the reservoirs are labelled “semi-natural”. Based on this inflow over a 24 year daily time series, the authors tried different operating rules for flood retention and drought release. The prerequisite for this was that flood protection should not be impaired by the new operating rules. The operating rules are based on two-point hedging rules (where hedging storage begins at one point and ends at another).
  The idea using flood retention basins to mitigate droughts is worth exploring. However, the manuscript has significant flaws. I would recommend rejecting the paper and resubmitting it after a complete revison.
  We thank the reviewer for their time in writing these comments. However, we feel that the reviewer’s rejection of the manuscript is based in a fundamental misunderstanding of our work. It seems that the reviewer is under the impression that our work aims to provide a comprehensive plan to mitigate drought in the study area using a network of flood retention basins. This is not the case. Rather, we seek to demonstrate that pre-existing individual flood basins can be repurposed for alleviation of drought conditions downstream. We will make this point clear in a revised version of the manuscript. Please see our related detail reply to referee comment 2 further below, as well as our point-by-point responses to the reviewer’s comments.
  My main concerns are the following (see more details below):
  The connection between drought mitigation and water release is solely based on Q70. This is too simple to draw conclusions about drought defense.While we agree that drought is an inherently complex and multivariable phenomena that, in general, cannot be defined by a single observed variable, we would like to maintain that Q70 is a simple yet comprehensive method to define drought. We can use Q70 to draw conclusions about drought defense for two reasons: First, we do not claim to have a holistic drought protection strategy; rather, in the introduction, we state that we are looking specifically at streamflow drought. Second, use of streamflow thresholds such as the Q70 are a long-standing and commonly accepted way to evaluate a reservoir’s impact on drought conditions (Shih and Revelle, 1995; Wu et al., 2022; Brunner, 2021; Padiyedath Gopalan et al., 2020; Chang et al., 2019) and in general for streamflow drought, though often at higher percentiles (Hisdal et al., 2004; Knight et al., 2011; Knight et al., 2013; Vigiak et al., 2018; Yarnell et al., 2020; Lubw, 2024; Van Loon et al., 2010; Van Loon et al., 2012). Moreover, in our analysis the Q70 is a stand-in for a demand curve, where any failure to meet the demand is a shortage. Because we do not explicitly know the demands downstream of the river, we assume that if there is a general shortage of water—indicated here by the Q70—there is a user downstream who is lacking water. In a revised version of the manuscript, we will clarify these points more explicitly.
  
  The spatial and hydrological context of the (arbitrary chosen) reservoirs is missing. Drought protection requires a model for river basin management.
  
  Based on this and the previous comments, we are under the impression that there is a misunderstanding about the purpose of these reservoirs and of our work: this study is meant to assess, from a water supply perspective, how much drought mitigation benefit each existing (flood retention) reservoir, working independently, can produce for its local surroundings. It is not to present a plan for complete water resources management in the area. We use the Q70 as a reference for drought conditions downstream and as a stand-in for water demand so that we can assess a reservoir’s ability to supplement low flows; further water resources management plans are beyond the scope of this paper. We will seek to stress this better in the abstract of a revised version of our manuscript.
  
  The main spatial and hydrological context of our work is the German state of Baden-Württemberg, which provides a common hydroclimatic background that is discussed in the paper. The spatial contexts of these reservoirs are, for the purposes of downstream discharge regulation, largely irrelevant because they operate independently. Therefore, their spatial context only matters insofar as their impacts (due to geography, etc.) on the hydrology, which—for the purposes of comparing some several hundred reservoirs—can be summarized by their inflows. The hydrological context is expressed via the AF (formerly called SF, which we have adapted due to the reviewer's suggestion)—because the AF is based on the mean annual flow and the average low flow (i.e. the Q70), the AF aims to summarize the local hydrological context and relate it to the reservoir’s capacity.
  
  Also, we would like to emphasize that the reservoirs were chosen non-arbitrarily. We aimed to choose reservoirs with a range of physical (volume) and hydrological (summarized by the AF) characteristics that would represent the existing reservoirs in Baden-Württemberg and documented this in the manuscript in section 2.2.
  
  The parameter SF is not helpful (as the authors admit). Why didn’t they choose a different parameter?
  
  The AF is a combined indicator representing the number of times the reservoir can be filled throughout a typical year, based on the difference between the average flow volume and the average low flow volume. This is a slightly modified version of a storage ratio (volume of inflow to capacity) used in the state of Baden-Württemberg to categorize flood retention basins. For this purpose, it has proven very useful, and therefore using a modified version of it for reservoir categorization for flood/drought was a natural first choice and a reasonable hypothesis. Our modified AF estimates how much water exists above the streamflow drought threshold in a typical year. When conceptualizing a study to determine relevant characteristics that would indicate suitability for storage of flood volume for drought usage, we hypothesized that a reservoir that had more available water to store—in other words, a higher AF—would have a greater impact on the downstream drought conditions. This is based on the assumption that more water available would also mean more water to release in drought conditions.
  Testing this hypothesis also gave us another criteria to refine our reservoir selection without having to study all 600 reservoirs: by summarizing the hydrology of each reservoir by its inflow and normalizing it by the capacity, we gain an indicator that describes the hydrological conditions in a way that is comparable across different reservoir sizes. Our study showed, however, that this was not entirely the case (please see related test in the conclusions). While reservoirs with high AF were indeed able to release significant volumes of water during drought conditions, they were ultimately unable to significantly reduce total drought deficits (due to the total deficit being quite high) or drought penalty (due to shortages in summer being penalized more heavily than those in winter). Thus, while AF was ultimately not helpful as an indicator for potential drought penalty, it remains useful for characterizing water availability.
  
  The conclusion “reservoirs can release up to 80 times their capacity and reduce drought penalties and water deficits by almost 95% over a 24-year simulation period” is mentioned twice (abstract and conclusions). That is the exception and not the rule and therefore misleading.
  
  The statement about releases and penalty / deficit reductions is not meant to be a general conclusion valid for each reservoir, but rather a summary of the range of results. The actual conclusions follow these statements in the abstract (“there seems to be a trade-off between relative water availability to the reservoir and ability to alleviate drought conditions”, lines 23-24) and conclusions (“… the relative water ability… did not have a strong relationship to a reservoir’s ability to curtail drought conditions”, lines 491-492). We also qualify that there are other reservoirs with different results in the abstract (“Certain reservoirs were near-optimal, others could be improved further, and still others were not very effective at reducing drought conditions”, line 24-25). However, to avoid misunderstandings, we will further stress that this is a range of results rather than a generally valid value in a revised version of the manuscript.
  
  Further comments in detail:
  Title: “…Estimating the maximum dual-use benefit of small flood reservoirs in Southern Germany” à That is not correct. The authors also analyzed medium and large reservoirs.
  
  We concede that the wording in our title can create a misunderstanding, as we do study reservoirs that are technically “large”. However, even the “large” reservoirs here are, in comparison to the typical reservoir studied for this sort of usage, quite small—in the manuscript we note that the size descriptors large, medium, and small follow the German reference standard DIN19700 categories (Line 113-116). According to common definitions of small reservoirs in the literature which we introduced in the introduction ( <= 15 m dam height, capacity of 1-2 million m³; line 45), the reservoirs studied here are indeed primarily small reservoirs—the capacity and dam heights for our DIN19700 “medium” reservoirs shown in Table 1 are well within this definition. The largest reservoir in this study has a capacity of 4.3 million m³, whereas typical reservoirs for this research have billions of m³. In other words, the “typical” reservoir in this field are several orders of magnitude greater than ours. We do, however, note this size definition discrepancy in our abstract (line 15), our introduction (line 72), our reservoir selection section (line 113-115), and our conclusion (line 483). However, in the interest of clarity, we suggest removing "small” from the title and further emphasize the working definitions in the introduction.
  
  Introduction: The sections on small reservoirs in Africa and worldwide do not fit the main content of the manuscript. Why do the authors write so much about agricultural (rainwater harvesting) reservoirs?
  
  The section on small reservoirs worldwide serve to illustrate that reservoirs of that capacity are used for water resources. This is to show that there is precedent for this kind of work, as well as to introduce the challenges associated with reservoirs of this size. The references to agricultural reservoirs serve to demonstrate potential uses for the water and examples of existing reservoirs. We therefore propose keeping this section in the manuscript.
  
  The hypothesis “that the reservoirs providing the most benefit in drought conditions will be those that have high inflow relative to the reservoir capacity” is not justified. Why did the authors formulate this hypothesis?
  
  Please also see our related reply to reviewer main comment 3. The hypothesis is built on the assumption that high inflow relative to reservoir capacity means that there is a lot of water that can be retained and released throughout the year. This is especially important in our reservoirs, as they must be completely empty before flood events in order to guarantee flood protection and since not every flood event will completely refill the reservoir. More water available implies that more water can be stored—in other words, potentially more water can be delivered in drought conditions, thus reducing streamflow drought. We will state this more clearly in a revised version of the manuscript.
  
  Line 127 until 135: The “slot” definition is unclear. The final selection of the reservoirs is not explained.
  
  The “slots” simply refer to number of allocated positions for the final selection. Selecting numbers of reservoirs based on proportion to the whole would result in either an overrepresentation of rather similar reservoirs or an unwieldy number of reservoirs; hence, we assigned each of the highly populated categories several “slots” or positions in the final selection. We further adjusted the number based on stakeholder interest (increasing the number of “large” reservoirs) and proportionality (increasing the number of MF, MM, and SF reservoirs). The final selection is, per line 146, based on the AF—we aimed to select reservoirs across the spectrum of AF. This is related to the use of AF as a hypothesizing factor: in order to test the AF equally among varying size and use categories, we selected reservoirs of varying AF within their category. If the editor agrees, we will seek to emphasize this more clearly in the next revision.
  
  The storage factor SF should be renamed as “availability factor AF”. That would also avoid confusion with the variable designation SF already assigned for “small flood protection” (see Table 1). However, it is unclear why the storage factor can support drought mitigation. It is not logical that smaller C value (i.e. a higher SF value ) will “reduce drought conditions more effectively”.
  
  Thank you for this suggestion for a change of naming, we will adopt it in a revised version of the manuscript and have adopted it for these responses.
  
  The storage factor is the number of times per year that the reservoir can be filled to capacity—in other words, there are more chances to save water for drought conditions. Please also see our related reply to the previous comment. A smaller C in and of itself does not reduce conditions more effectively. Rather, that it can store and release more water relative to its inflow is its hypothesized basis for drought mitigation. A reservoir that can be filled 50 times in a year, for example, was hypothesized be able to affect more relative change on streamflow than one that is filled only 10 times in a year because it can store and release more water. This is again important due to the complete drawdown of the reservoir before a flood event without guarantee that it will be entirely refilled: by looking for reservoirs with higher water available for storage, we increase the chances of water being stored for delivery in drought. We will seek to emphasize this more clearly in the next revision.
  
  Table 2: the volume of C should be added.
  
  Thank you for the suggestion. We will move the capacity from Table 3 to Table 2 and move Table 3 to the appendix, as suggested in a later comment.
  
  Equation 3: Why “-5” as a penalty factor?
  
  The only constraint for the penalty factor is that it should be negative; the exact number is arbitrary for its functionality, as it is a simple linear transformation. We will add this information to the manuscript in a future revision.
  
  Equation 3 and 4: Create a diagram for a better interpretation of the penalty functions.
  
  We feel that the equations already provide all necessary information, but if the editor also agrees that this would significantly improve the interpretation of the penalty functions, we will add a supporting diagram.
  
  Table 3: Move it to the appendix.
  
  Will do; please also see our response to comment 6.
  
  Figure 4: Why is the penalty benefit of SMALL multipurpose reservoirs so much lower than flood-only reservoirs? Please explain and discuss!
  
  Thank you for this question. This is due to the limitations of the reservoirs in this category. Nonnenbach falls into the same trap as other high AF reservoirs in the high AF, low improvement grouping (i.e. Gottswald)—namely, not having a capacity that is able to cope with the high water variability, thus limiting the range (and reducing the median) of the small multipurpose category. Similarly, Lennach and Hoelzern are limited in the same way as Wollenberg—they are unable to fill often enough to supply the water necessary for drought deficit reduction. Moreover, the capacities of these reservoirs are smaller than the typical capacity of the DIN 19700 “small” category (but remain in that category due to their dam height), resulting in reduced water stored for drought conditions. We have mentioned these points briefly in the manuscript, but not apparently not in enough detail. We will add a more detailed discussion to a revised version of the manuscript.
  
  Figure 5 until 9: The flood penalty can be skipped because the precondition was that flood protection must not be impaired. Furthermore, the graphs related to outflow and drought penalties are not clearly recognizable.
  
  We would like to maintain that the flood penalty carries relevant information for the reader about the complete interaction of the system (how flooding affects volume, which affects penalty), e.g. in Figure 8. However, if the editor agrees, we can remove the flood penalty. We understand the concern with the outflow and drought penalties and will explore options to more improve readability.
  
  Figure 10: Can be skipped (flood statistics are not changed due to the authors’ precondition)
  
  We would like to maintain that the flood penalty statistics provide context for high flood pre-releases and differences in performances between reservoirs. In a future revision, we will seek to make this context more explicit. However, if the editor agrees, we can move this figure to the appendix.
  
  Line 440 ff: The argumentation with the IWD is very weak. Either omit or elaborate in detail (seasonal influence, PET, soil conditions etc).
  
  The discussion with the IWD is intended to contextualize how much water is made available through our strategy and what real world impacts it could have with a callback to the agricultural small reservoirs in the introduction. We think this has value for the reader and therefore prefer to keep it in the manuscript.
  
  Line 473: “Changing the model so that the timing of reservoir releases such that water is given at the drought peaks could improve the penalty benefit further, though at the cost of complicating the model and the release rules.” This is recommended for the resubmission of the paper.
  
  While we agree that this would be useful and interesting, we would like to argue that this is beyond the current scope of the paper. In this paper, we aim to a) determine whether or not the idea of reusing flood basins for drought protection is viable; and to b) determine whether or not water availability (via AF) is a suitable indicator for a reservoir’s potential impact under these schemes. We do, however, plan to explore this potential change in releases—as well as more realistic demand targets tailored to local conditions—in a further paper. We will add this information to a revised version of the manuscript.
  
  Minor comments:
  Line 253: “is used only used to”--> “is only used to”
  
  Thank you; this was an oversight. We will correct this in the next revision.
  
  Equation 11 “V = C >=0” --> “V-C >=0”
  
  Thank you for noting the error; the condition should be “V = C”. We will correct this in the next revision.
  
  Line 330: “former” --> “latter”
  
  Thank you; this was an oversight. We will correct this in the next revision.
  
  Line 338: “flood droughts” --> “flood”
  
  Thank you; this was an oversight. We will correct this in the next revision.
  
  Figure 11: Are the x- and y-axis swapped? “combined operation model” should be on the y-axis?
  
  Thank you; this was an oversight. We will correct this in the next revision.
  
  Better to use Bp (Benefit for Drought) instead of Vd,nor (y-axis, left figure)
  
  If this is in reference to Figure 14, we respectfully disagree—a plot of Bp against AF already exists in Figure 4. Moreover, this pair of figures serves to demonstrate that, while increasing water availability is strongly correlated with a higher Vd,nor, the total volume benefit remains variable among different reservoirs. We therefore will keep the figure as is.
  
  References:
  Brunner, M. I.: Reservoir regulation affects droughts and floods at local and regional scales, Environmental Research Letters, 16, 10.1088/1748-9326/ac36f6, 2021.
  Chang, J., Guo, A., Wang, Y., Ha, Y., Zhang, R., Xue, L., and Tu, Z.: Reservoir Operations to Mitigate Drought Effects With a Hedging Policy Triggered by the Drought Prevention Limiting Water Level, Water Resources Research, 55, 904-922, 10.1029/2017wr022090, 2019.
  Hashimoto, T., Loucks, D. P., and Stedinger, J. R.: Reliability, resiliency, robustness, and vulnerability criteria for water resource systems, Water Resources Research, 18, 1982.
  Hisdal, H., Tallaksen, L. M., Gauster, T., Bloomfield, J. P., Parry, S., Prudhomme, C., and Wanders, N.: Hydrological drought characteristics, in: Hydrological Drought, Elsevier, 157-231, 2004.
  Karamouz, M. and Araghinejad, S.: Drought Mitigation through Long-Term Operation of Reservoirs: Case Study, Journal of Irrigation and Drainage Engineering - ASCE, 134, 2008.
  Knight, R. R., Gain, W. S., and Wolfe, W. J.: Modelling ecological flow regime: an example from the Tennessee and Cumberland River basins, Ecohydrology, 5, 613-627, 10.1002/eco.246, 2011.
  Knight, R. R., Murphy, J. C., Wolfe, W. J., Saylor, C. F., and Wales, A. K.: Ecological limit functions relating fish community response to hydrologic departures of the ecological flow regime in the Tennessee River basin, United States, Ecohydrology, 7, 1262-1280, 10.1002/eco.1460, 2013.
  LUBW: NIZ Pegel Klassifizierung, 2024.
  Padiyedath Gopalan, S., Hanasaki, N., Champathong, A., and Tebakari, T.: Impact assessment of reservoir operation in the context of climate change adaptation in the Chao Phraya River basin, Hydrological Processes, 35, 10.1002/hyp.14005, 2020.
  Shih, J.-S. and ReVelle, C.: Water supply operations during drought: A discrete hedging rule, European Journal of Operational Research, 82, 1995.
  Van Loon, A. F., Van Huijgevoort, M. H. J., and Van Lanen, H. A. J.: Evaluation of drought propagation in an ensemble mean of large-scale hydrological models, Hydrology and Earth System Sciences, 16, 4057-4078, 10.5194/hess-16-4057-2012, 2012.
  Van Loon, A. F., Van Lanen, H. A., Hisdal, H., Tallaksen, L. M., Fendeková, M., Oosterwijk, J., Horvát, O., and Machlica, A.: Understanding hydrological winter drought in Europe, Global Change: Facing Risks and Threats to Water Resources, IAHS Publ, 340, 189-197, 2010.
  Vigiak, O., Lutz, S., Mentzafou, A., Chiogna, G., Tuo, Y., Majone, B., Beck, H., de Roo, A., Malago, A., Bouraoui, F., Kumar, R., Samaniego, L., Merz, R., Gamvroudis, C., Skoulikidis, N., Nikolaidis, N. P., Bellin, A., Acuna, V., Mori, N., Ludwig, R., and Pistocchi, A.: Uncertainty of modelled flow regime for flow-ecological assessment in Southern Europe, Sci Total Environ, 615, 1028-1047, 10.1016/j.scitotenv.2017.09.295, 2018.
  Wu, Y., Sun, J., Hu, B., Xu, Y. J., Rousseau, A. N., and Zhang, G.: Can the combining of wetlands with reservoir operation largely reduce the risk of future flood and droughts?, EGUsphere [preprint], 10.5194/egusphere-2022-1103, 2022.
  Yarnell, S. M., Stein, E. D., Webb, J. A., Grantham, T., Lusardi, R. A., Zimmerman, J., Peek, R. A., Lane, B. A., Howard, J., and Sandoval‐Solis, S.: A functional flows approach to selecting ecologically relevant flow metrics for environmental flow applications, River Research and Applications, 36, 318-324, 10.1002/rra.3575, 2020.
  
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  Citation: https://doi.org/10.5194/egusphere-2024-2167-AC1

Sarah Quỳnh Giang Ho and Uwe Ehret

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Short summary

In this paper, we use models to demonstrate that even small flood reservoirs – which capture water to avoid floods downstream – can be repurposed to release water in drier conditions without affecting their ability to protect against floods. By capturing water and releasing once water levels are low, we show that reservoirs can greatly increase water available in drought. Having more water coming in, however, is not necessarily better for drought protection.


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