Interactive discussion
Status: closed
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RC1: 'Comment on nhess-2021-129', Anonymous Referee #1, 10 May 2021
This paper by Wendt et al. investigates the effect of different drought policies on hydrological drought characteristics and is a relevant contribution for the community and water and drought management. The study uses a virtual catchment and embeds different hydrogeological settings in a model experiment. Different drought strategies are then implemented as different scenarios to simulate streamflow and groundwater storages under different conditions. The scenarios are well-chosen and help to evaluate different challenges in future drought management. I recommend publication after moderate revisions.
Title: For me the paper title would be more informative/stronger if it states what it is found rather than what is done in the study. I suggest to go into the direction of “Integrated drought policies on hydrological drought led to …” However, the readers might be more interested in the “implications” of the study than in a “demonstration of the impact”.
Introduction: I liked the condensed format of the introduction and a lot of recent publications are embedded there. However, it remains unclear for me whether the focus of this study is on hydrological droughts, groundwater (L35-36) or on both, or human-modified droughts (L31) – this should be clarified (although it gets clearer later on in the manuscript). “Hydrogeological conditions” are mentioned the first time in L64 in the introduction (in the Study Aims paragraph). How is it justified to consider different hydrogeological settings? This might be a logical approach for a drought researcher, but should be referenced and/or better introduced for a broader readership. There is also a mixture of objectives and method description (L63-71) in the introduction. Clear, tailored objectives or research questions could help to explain what exactly the science is in this paper? The key elements of drought policies (L39-41) are given but no further used in the introduction. Why is the list of those six elements important?
Major comments
- High, medium, low groundwater storage systems (L70, L143) are crucial definitions to understand the analysis. I wonder if the authors described/used here large, medium and small (or shallow) groundwater storage systems, i.e., characterizing the ability of the system to store more (large) or less (small) water in the subsurface. High/low is rather confusing here as high/low is often used for high or low permeability of the aquifer, i.e., the degree of infiltration of water into the aquifer. Has a high groundwater system (also) a large storage? This should be clarified (or changed) and a distinct definition of the three systems is needed in a prominent way in the manuscript. Wording should be revised also to other sections in the manuscript, e.g., “companies with access to principal aquifers might depend more on groundwater compared to companies with access to shallow, less productive aquifers” (L83-84).
- Furthermore, I found the only linkage between hydrogeology and groundwater systems in L139-149. Is the linkage only for some kind of justification for different GW model boxes in HBV or was the aim really to quantify the impact of integrated drought policies on hydrological droughts in different hydrogeological settings? For me it is not clear why the hydrogeological features of the virtual catchment (karstic, porous and fractured) are linked 1:1 to high, medium and low groundwater systems? I guess this could be clarified, however, the hydrogeological context could be better integrated in the study. To be honest, my first concern reading the manuscript was on the added value of the hydrogeological settings (i.e., karstic, porous, fractured). See also point (4) below.
- I suggest to have a separated section “virtual catchment” where the modelling approach is explained (in single steps) using HBV (with a specific model structure) a set of average forcing data for England. Do I understand it correctly that no calibration was done in HBV as fixed parameter values were derived from literature etc. and there is no observed runoff? This should be mentioned more clearly! Consistent terms would be beneficial (at the moment idealized, simplified and virtual catchment is used). Perhaps this could also be done with an extension of Fig.1 showing (a) the HBV model structure (+ extension with three different GW boxes) and forcing data and (b) the socio-hydrological model approach next to each other.
- What is the advantage to have three different GW model representations for the three different groundwater systems if also variation of the parameterization of the same model structure could do this job? Stoelzle et al. (2015) showed that the FLEX GW structure outperforms the three structures POW, 1LBY and 2PA used in this study and the hydrogeological clustering of catchments is also better/clearer for FLEX than for other structures (Fig. 4 in Stoelzle et al., 2015). Wouldn’t it be easier to compare the effects of larger and smaller groundwater systems on different drought policies if the model structures across those different GW systems stay the same?
- I like the section 5.3 model limitations as this discussion is really important to understand the results of the study. Beside the fact that this section could be incorporated into other sections of the manuscript or at least should not be placed at the end of the discussion section (to gain a more positive ending of the paper), I asked myself what would have been happened if another forcing than an England’s average was used? Are average conditions a good starting point for such drought analysis like here? Additional analysis could shed light on this, however, at least more discussion is needed to evaluate how representative the average approach is for the different regions in England or different water companies.
- Regarding the publication in NHESS my first impression was that a more hydrological journal such as HESS would be a better choice for this manuscript. Terms like risk, hazard, vulnerability are not or seldom embedded in this manuscript. But as the paper is planned to be published in the SI: “Drought vulnerability, risk, and impact assessments: bridging the science-policy gap” it is definitely a valuable contribution for this journal. It is in the scope of NHESS and the SI as the paper focuses on drought impact assessment. However, as the readership of NHESS (compared to HESS) is certainly less familiar with hydrological modelling approaches and differences in groundwater model structures, more explanation on model setup, role of model structures and e.g., response times of aquifers is needed (see other comments for more details).
- Around the half of all Figures are placed in the Appendix. This is a nice approach to have additional information for the reader. However, reading the manuscript a lot of references were made to Fig. in the appendix even for major outcomes of the results and discussion section.
- L232-241: Is the also a presentation of results according to the different storages models (Eq.4-6)? Different models will have more or less ability to buffer mild droughts. L284-L303: Again, more elaboration is needed here on the effect of the different groundwater models on the found drought characteristics. Is this included in the discussion section? L324-325 also suggests different effects of the same scenario for different groundwater systems (high, low etc).
- As there are major differences between high and low groundwater systems (suggested by Fig. 5) a comparison of both systems across the different drought metrics might be helpful.
- Water import is identified as important component in a future water management. Would be nice to have more discussion on potential ways to store redundant water (e.g., during winter high flows) to increase summerly low flows.
Minor/technical comments:
- First sentence in the abstract is rather long, I suggest to split into two.
- L24: ‘sustained’ means ‘prolonged’ here?
- L27: what is meant with “is available longer”?
- I suggest do move the sentence “In this study,…” (L31-33) to the end of the paragraph or to integrate it in a better way at the end of the introduction (L63-71) as here the aims/objectives of the study could be summarized.
- More elaboration on the term “conjunctive use of water” (L55) might be helpful here.
- “large impact on streamflow droughts” (L59). Have Jaeger et al. (2019) performed a comparison of all the above-mentioned drought policy components (L53-57)? If so, that should be clarified. If not, I suggest a rephrasing “reservoir regulations and timely interventions have a larger impact on streamflow droughts than X, Y, Z...”.
- What is the share of surface water/groundwater use for the 13 of 18 companies (L79)? Could be summarized from Table A1 here.
- Trigger levels are communicated as percentages (e.g., L200-202) or as kind of return periods (e.g., L198). Return periods could be understand as probability. I wonder why SPI has then to be used? Of course, SPI as a trigger level can also be converted to a probability, but is there an additional value to communicate SPI (as proxy for drought severity) instead of the probability (or percentiles?). Are there references or experiences from stakeholders that justify the SPI as a more valuable metric in drought assessment? Compared to that the reservoir triggers are nor communicated as a deviation from the mean and that reduces the clarity in trigger level communication here.
- Code availability: I suggest to enable open access to the code with at least a simple example code file (and data set) to reproduce the major parts of the analysis.
- L210: 80th percentage? Is this 80th percentile (exceedance level)? Is this percentile calculated on monthly basis or for the entire series?
- L219-L223: Would be helpful to have periods of reduced recharge etc. and the model spin-up also as color-coded information in Fig A2.
- “the overall hydrological drought intensity and duration reduce for most scenarios” (L327) – here is missing something.
- What are examples of high costs for providers and users (L421) ?
- “larger inter-annual storage” (L449), here is missing something or it should be a comparison with larger than?
Figures/Tables
Many Figures have a nice formatting and I especially like the different point shapes (e.g. Fig 6). However, please remove the grey background in the Fig. to increase the readability (e.g. Fig A6).
Table A1: Please add numbers (1-13) to Table A1 to increase the connection to the numbers in the columns 3+6 in Table 1. I do not understand what “headroom” means here.
Table 1: Why are some drought plan numbers (#) like 2 or 11 not mentioned in this table? Add “yr” to the numbers in square brackets (e.g., average 8.5 yr, range 5 yr - 20 yr) for clarification or add return period in this column description.
Fig. 6: Try out a 2x2 panel instead of 4x1 and consider to add a regression line to evaluate the deviation of specific points from the “average”. At the moment this Fig. is to wide compared to its height. Same is partly an issue in Fig A7.
Fig. 3: Stacked barcharts are critical here. Please move to a dodged version with 4 single bars for each category on the y-axis. Baseline, Scenarios and Combination could be placed in facets (4 facets in one column) to highlight the different groups here. 0%-label is missing and I cannot find the explanation for the dotted vertical line. The colored categories could also be placed into facets (if the dodged version doesn’t work out).
FigA6: Is it 12000 days in panel 2? Removing the outlier would improve the data representation (outlier comment could be added in the caption). Drought duration could be transformed to months (easier to read, but this is just a recommendation).
Citation: https://doi.org/10.5194/nhess-2021-129-RC1 -
AC1: 'Comment on nhess-2021-129', Doris Wendt, 04 Jun 2021
%% Please use the following documentclass and journal abbreviations for discussion papers and final revised papers.
%% 2-column papers and discussion papers
\documentclass[hessd, manuscript]{copernicus}\begin{document}
\section*{General response to Reviewer 1}
We thank the Reviewer for their careful reading and constructive comments to improve the clarity of the submitted manuscript.
We will address each comment in detail in the rebuttal, but we have addressed the first five major comments in this Author's response to clarify important aspects of the paper and show how we will include the provided suggestions in the revised manuscript.
\newline\textbf{Title and Introduction:}\\
We agree with the Reviewer that the title could be stronger once reflecting the findings and implications, rather than the impact of drought policies. We will re-evaluate our choice of title and consider rephrasing. \par
Regarding the introduction, we will rephrase the scope to clarify the reasoning behind the focus on hydrological droughts, including both baseflow and groundwater droughts. Because of the specific focus on base flow and groundwater, we considered different hydrogeological settings that are associated with different types of drought characteristics.
\newline\textbf{Comment 1: `High, medium, low groundwater storage systems (L70, L143) are crucial definitions to understand the analysis. I wonder if the authors described/used here large, medium and small (or shallow) groundwater storage systems, i.e., characterizing the ability of the system to store more (large) or less (small) water in the subsurface. High/low is rather confusing here as high/low is often used for high or low permeability of the aquifer, i.e., the degree of infiltration of water into the aquifer. Has a high groundwater system (also) a large storage? This should be clarified (or changed) and a distinct definition of the three systems is needed in a prominent way in the manuscript. Wording should be revised also to other sections in the manuscript, e.g., “companies with access to principal aquifers might depend more on groundwater compared to companies with access to shallow, less productive aquifers” (L83-84). '}\\
We acknowledge the confusion caused by the naming of the three types of groundwater storage systems. We thank Reviewer 1 for highlighting this and we will rename these groundwater systems as suggested. The reasoning behind the high, medium, low naming of groundwater systems is indeed characterising the overall availability of groundwater storage given the modelled groundwater storage-outflow equations. We will include a detailed description in the suggested `virtual catchment' section to define the large, medium and small groundwater storage systems.
\newline\textbf{Comment 2: `Furthermore, I found the only linkage between hydrogeology and groundwater systems in L139-149. Is the linkage only for some kind of justification for different GW model boxes in HBV or was the aim really to quantify the impact of integrated drought policies on hydrological droughts in different hydrogeological settings? For me it is not clear why the hydrogeological features of the virtual catchment (karstic, porous and fractured) are linked 1:1 to high, medium and low groundwater systems? I guess this could be clarified, however, the hydrogeological context could be better integrated in the study. To be honest, my first concern reading the manuscript was on the added value of the hydrogeological settings (i.e., karstic, porous, fractured). See also point (4) below.'}\\
We agree with Reviewer 1 that this should be highlighted earlier, i.e. in the revised introduction. The reason for representing different aquifer types is driven by 1) the aquifer-dependent delay in groundwater storage-outflow (L23-28), 2) the increased dependency on groundwater during droughts (L28-36) and 3) the absent groundwater component in recent drought policy modelling (L52-60). We will summarise these arguments in the last paragraph of the introduction to emphasise the need for different groundwater systems in this study. \\
Previous hydrological drought modelling by \citet{Lanen2013} applied a modified the standard HBV model to simulate hydrological droughts globally and changed the response time (in days) to represent different groundwater systems, finding that the responsiveness of groundwater systems has a large impact on drought characteristics. \citet{stoelzleBaseflowmodelling} extended the representation of different aquifer structures for a lumped model approach by identifying superior groundwater simulation structures. They tested and recommended a range of alternative model structures for five aquifer types (see our response to Comment 4 for details). Out of these five aquifer types, we modelled three aiming to show the impact of drought management strategies for different groundwater systems. \\
The three hydrogeological settings (karstic, porous and fractured aquifers) were selected to broadly representative for catchments in England and therefore excluded the `mixed' and `combined' aquifer types of \citet{stoelzleBaseflowmodelling}. The first hydrogeological setting is generally associated with a large groundwater storage availability and non-linear drainage as found in karstic aquifers \citep{Bloomfield2013,hartmann2014}.
Medium groundwater storage availability was found when modelling the porous aquifer type with slow drainage and possible leakage \citep{Shepley425,allen1997}. The last setting represented smaller groundwater storage availability with short response times for fractured or weathered aquifers \citep{allen1997}. In modelling these three types of aquifers, none of the groundwater storage potential was constrained and different baseflow groundwater storage resulted from the different groundwater storage-outflow equations. In the revised manuscript, we will rephrase the introduction of these three groundwater systems and highlight the link to these different hydrogeological settings.
\newline\textbf{Comment 3: `I suggest to have a separated section “virtual catchment” where the modelling approach is explained (in single steps) using HBV (with a specific model structure) a set of average forcing data for England. Do I understand it correctly that no calibration was done in HBV as fixed parameter values were derived from literature etc. and there is no observed runoff? This should be mentioned more clearly! Consistent terms would be
beneficial (at the moment idealized, simplified and virtual catchment is used). Perhaps this could also be done with an extension of Fig.1 showing (a) the HBV model structure (+ extension with three different GW boxes) and forcing data and (b) the sociohydrological model approach next to each other.' }\\We appreciate the suggested section with the heading `virtual catchment', as this will indeed clarify the scope of the paper and associated modelling assumptions related to the three hydrogeological settings. As stated in L144-149, modelled groundwater storage-outflow parameters were based on the tested parameter range by \citet{stoelzleBaseflowmodelling} and mean aquifer characteristics in England \citep{allen1997}. A wider range of parameters was tested in the sensitivity analysis, which results can indeed be better included in earlier sections, as suggested in Comment 5. We will rephrase this in the Results section and include findings of the sensitivity analysis were relevant. \\
As noted by Reviewer 1, there is no comparison to observed discharge or groundwater storage in this idealised hydrological system (L64-66). We used the term `idealised' referring to the simplified hydrological system that can be seen as a stand-alone simplified example used for analysis, hence the term `virtual catchment'. We will review the use of these terms for consistency. Given the stand-alone example and modelling exercise, there is no validation of the performance of the different aquifer structures in this virtual modelling study that is merely building on from the validated model structure to assess hydrological droughts \citep{Lanen2013}. \\
Lastly, we have revised Fig. 1 to reflect both validated model structure, different groundwater modules and environmental and anthropogenic water demand. All three components have been included in the revised Figure \ref{Revised_Fig01} (see below).
\newline\begin{figure}[h]
\centering
\includegraphics[width=0.73\textwidth]{revisedFig_AuthorResponse.png}
}
\caption{Socio-hydrological model consisting of a soil moisture balance driven by precipitation (P in mm d$^{-1}$) and potential evaporation (PET in mm d$^{-1}$), a surface water reservoir storing runoff (Qr mm d$^{-1}$), and a groundwater module that consists of three groundwater system options (large, medium, small groundwater availability) driven by groundwater recharge (Rch in mm d$^{-1}$). Anthropogenic water demand is met by reservoir abstractions (Ares in mm d$^{-1}$ ) and groundwater abstractions (Agw in mm d$^{-1}$), both in striped dark red arrows. Natural water demand is represented by ecological flow requirements (Qeco in mm d$^{-1}$; dotted green arrow) and abstracted as part of the baseflow (Qb in mm d$^{-1}$). Remaining baseflow is routed to the reservoir. Additional water is imported in the model when reservoir or groundwater storage is insufficient (Qimp and GSimp both in mm d$^{-1}$). Drought management scenarios apply to the surface water reservoir, groundwater module, and environmental and anthropogenic water demand (all model components in the thick black box). }
\label{Revised_Fig01}
\end{figure}\textbf{Comment 4: `What is the advantage to have three different GW model representations for the three
different groundwater systems if also variation of the parameterization of the same model structure could do this job? Stoelzle et al. (2015) showed that the FLEX GW structure outperforms the three structures POW, 1LBY and 2PA used in this study and the hydrogeological clustering of catchments is also better/clearer for FLEX than for other structures (Fig. 4 in Stoelzle et al., 2015). Wouldn’t it be easier to compare the effects of larger and smaller groundwater systems on different drought policies if the model structures across those different GW systems stay the same?'}\\It is a valid point raised by Reviewer 1 that when model validation is possible, all three groundwater systems can be well-simulated using the FLEX GW structure.
However, in this virtual catchment modelling we aimed to represent different groundwater systems without the need for validation to find a representative share of fast and slow responding groundwater discharge. Therefore, the FLEX GW structure is not ideal given the dependency on the threshold-controlled storage outflow (h) to determine either primarily fast or slow groundwater discharge response. As an alternative, we applied suggested conceptual model structures to represent different kinds of groundwater storage-outflow characteristics that typically result in different drought characteristics for karstic, porous and fractured aquifer types. The non-linear groundwater discharge release, as observed in karstic aquifers, is best represented by a power law (POW) \citep{Wittenberg03}. Slow porous flow could be represented by a number of structures, but given the overall high performance by 1LBY (Figure 5 in \citet{stoelzleBaseflowmodelling}) and the reported slow flow and possibly leakage in English Permo-Triassic sandstone aquifers, we selected the 1LBY. For fractured aquifers, the overall recommendation to apply parallel reservoirs and the overall high performance of 2PA (with groundwater recharge input) were the reasons for selecting 2PA for the last groundwater system. \\
By selecting these three conceptual model structures, we aimed to represent three different kinds of groundwater storage-outflow release whilst testing a range of representative parameters for English catchments. In the revised manuscript, we will include the reasoning behind the selection of model structures in a virtual catchment. \newline\textbf{Comment 5: `I like the section 5.3 model limitations as this discussion is really important to
understand the results of the study. Beside the fact that this section could be incorporated into other sections of the manuscript or at least should not be placed at the end of the discussion section (to gain a more positive ending of the paper), I asked myself what would have been happened if another forcing than an England’s average
was used? Are average conditions a good starting point for such drought analysis like here? Additional analysis could shed light on this, however, at least more discussion is needed to evaluate how representative the average approach is for the different regions in England or different water companies.'}\\We thank Reviewer 1 for these two suggestions. We will include relevant results of the sensitivity analysis in earlier sections if relevant (also see response to comment 2). Regarding the second suggestion, our intention was to select a representative precipitation record for England. The HadUKP data consists of weighted observations, including extremely wet and dry periods within its record \citep{alexander2001updated}. However, we will verify if other forcing English data, i.e. a representative location in the CEH-GEAR dataset \citep{GEAR-PDataCEH} changes the overall water balance and outcomes. We will include these additional analyses in the full rebuttal and amend the results in revised manuscript if necessary.
%\bibliography{PhD_Bibliography}
\bibliographystyle{copernicus}
\begin{thebibliography}{9}
\providecommand{\natexlab}[1]{#1}
\providecommand{\url}[1]{{\tt #1}}
\providecommand{\urlprefix}{URL }
\expandafter\ifx\csname urlstyle\endcsname\relax
\providecommand{\doi}[1]{https://doi.org/\discretionary{}{}{}#1}\else
\providecommand{\doi}{https://doi.org/\discretionary{}{}{}\begingroup
\urlstyle{rm}\Url}\fi\bibitem[{Alexander and Jones(2001)}]{alexander2001updated}
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discussion of recent extremes, Atmospheric science letters, 1, 142--150,
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Allen, D., Bloomfield, J., and Robinson, V., eds.: The physical properties of
major aquifers in England and Wales, British Geological Survey (WD/97/034),
1997.\bibitem[{Bloomfield and Marchant(2013)}]{Bloomfield2013}
Bloomfield, J.~P. and Marchant, B.~P.: {Analysis of groundwater drought
building on the standardised precipitation index approach}, Hydrology and
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Weiler}]{hartmann2014}
Hartmann, A., Goldscheider, N., Wagener, T., Lange, J., and Weiler, M.: Karst
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\end{document}
Citation: https://doi.org/10.5194/nhess-2021-129-AC1
-
AC1: 'Comment on nhess-2021-129', Doris Wendt, 04 Jun 2021
%% Please use the following documentclass and journal abbreviations for discussion papers and final revised papers.
%% 2-column papers and discussion papers
\documentclass[hessd, manuscript]{copernicus}\begin{document}
\section*{General response to Reviewer 1}
We thank the Reviewer for their careful reading and constructive comments to improve the clarity of the submitted manuscript.
We will address each comment in detail in the rebuttal, but we have addressed the first five major comments in this Author's response to clarify important aspects of the paper and show how we will include the provided suggestions in the revised manuscript.
\newline\textbf{Title and Introduction:}\\
We agree with the Reviewer that the title could be stronger once reflecting the findings and implications, rather than the impact of drought policies. We will re-evaluate our choice of title and consider rephrasing. \par
Regarding the introduction, we will rephrase the scope to clarify the reasoning behind the focus on hydrological droughts, including both baseflow and groundwater droughts. Because of the specific focus on base flow and groundwater, we considered different hydrogeological settings that are associated with different types of drought characteristics.
\newline\textbf{Comment 1: `High, medium, low groundwater storage systems (L70, L143) are crucial definitions to understand the analysis. I wonder if the authors described/used here large, medium and small (or shallow) groundwater storage systems, i.e., characterizing the ability of the system to store more (large) or less (small) water in the subsurface. High/low is rather confusing here as high/low is often used for high or low permeability of the aquifer, i.e., the degree of infiltration of water into the aquifer. Has a high groundwater system (also) a large storage? This should be clarified (or changed) and a distinct definition of the three systems is needed in a prominent way in the manuscript. Wording should be revised also to other sections in the manuscript, e.g., “companies with access to principal aquifers might depend more on groundwater compared to companies with access to shallow, less productive aquifers” (L83-84). '}\\
We acknowledge the confusion caused by the naming of the three types of groundwater storage systems. We thank Reviewer 1 for highlighting this and we will rename these groundwater systems as suggested. The reasoning behind the high, medium, low naming of groundwater systems is indeed characterising the overall availability of groundwater storage given the modelled groundwater storage-outflow equations. We will include a detailed description in the suggested `virtual catchment' section to define the large, medium and small groundwater storage systems.
\newline\textbf{Comment 2: `Furthermore, I found the only linkage between hydrogeology and groundwater systems in L139-149. Is the linkage only for some kind of justification for different GW model boxes in HBV or was the aim really to quantify the impact of integrated drought policies on hydrological droughts in different hydrogeological settings? For me it is not clear why the hydrogeological features of the virtual catchment (karstic, porous and fractured) are linked 1:1 to high, medium and low groundwater systems? I guess this could be clarified, however, the hydrogeological context could be better integrated in the study. To be honest, my first concern reading the manuscript was on the added value of the hydrogeological settings (i.e., karstic, porous, fractured). See also point (4) below.'}\\
We agree with Reviewer 1 that this should be highlighted earlier, i.e. in the revised introduction. The reason for representing different aquifer types is driven by 1) the aquifer-dependent delay in groundwater storage-outflow (L23-28), 2) the increased dependency on groundwater during droughts (L28-36) and 3) the absent groundwater component in recent drought policy modelling (L52-60). We will summarise these arguments in the last paragraph of the introduction to emphasise the need for different groundwater systems in this study. \\
Previous hydrological drought modelling by \citet{Lanen2013} applied a modified the standard HBV model to simulate hydrological droughts globally and changed the response time (in days) to represent different groundwater systems, finding that the responsiveness of groundwater systems has a large impact on drought characteristics. \citet{stoelzleBaseflowmodelling} extended the representation of different aquifer structures for a lumped model approach by identifying superior groundwater simulation structures. They tested and recommended a range of alternative model structures for five aquifer types (see our response to Comment 4 for details). Out of these five aquifer types, we modelled three aiming to show the impact of drought management strategies for different groundwater systems. \\
The three hydrogeological settings (karstic, porous and fractured aquifers) were selected to broadly representative for catchments in England and therefore excluded the `mixed' and `combined' aquifer types of \citet{stoelzleBaseflowmodelling}. The first hydrogeological setting is generally associated with a large groundwater storage availability and non-linear drainage as found in karstic aquifers \citep{Bloomfield2013,hartmann2014}.
Medium groundwater storage availability was found when modelling the porous aquifer type with slow drainage and possible leakage \citep{Shepley425,allen1997}. The last setting represented smaller groundwater storage availability with short response times for fractured or weathered aquifers \citep{allen1997}. In modelling these three types of aquifers, none of the groundwater storage potential was constrained and different baseflow groundwater storage resulted from the different groundwater storage-outflow equations. In the revised manuscript, we will rephrase the introduction of these three groundwater systems and highlight the link to these different hydrogeological settings.
\newline\textbf{Comment 3: `I suggest to have a separated section “virtual catchment” where the modelling approach is explained (in single steps) using HBV (with a specific model structure) a set of average forcing data for England. Do I understand it correctly that no calibration was done in HBV as fixed parameter values were derived from literature etc. and there is no observed runoff? This should be mentioned more clearly! Consistent terms would be
beneficial (at the moment idealized, simplified and virtual catchment is used). Perhaps this could also be done with an extension of Fig.1 showing (a) the HBV model structure (+ extension with three different GW boxes) and forcing data and (b) the sociohydrological model approach next to each other.' }\\We appreciate the suggested section with the heading `virtual catchment', as this will indeed clarify the scope of the paper and associated modelling assumptions related to the three hydrogeological settings. As stated in L144-149, modelled groundwater storage-outflow parameters were based on the tested parameter range by \citet{stoelzleBaseflowmodelling} and mean aquifer characteristics in England \citep{allen1997}. A wider range of parameters was tested in the sensitivity analysis, which results can indeed be better included in earlier sections, as suggested in Comment 5. We will rephrase this in the Results section and include findings of the sensitivity analysis were relevant. \\
As noted by Reviewer 1, there is no comparison to observed discharge or groundwater storage in this idealised hydrological system (L64-66). We used the term `idealised' referring to the simplified hydrological system that can be seen as a stand-alone simplified example used for analysis, hence the term `virtual catchment'. We will review the use of these terms for consistency. Given the stand-alone example and modelling exercise, there is no validation of the performance of the different aquifer structures in this virtual modelling study that is merely building on from the validated model structure to assess hydrological droughts \citep{Lanen2013}. \\
Lastly, we have revised Fig. 1 to reflect both validated model structure, different groundwater modules and environmental and anthropogenic water demand. All three components have been included in the revised Figure \ref{Revised_Fig01} (see below).
\newline\begin{figure}[h]
\centering
\includegraphics[width=0.73\textwidth]{revisedFig_AuthorResponse.png}}
\caption{Socio-hydrological model consisting of a soil moisture balance driven by precipitation (P in mm d$^{-1}$) and potential evaporation (PET in mm d$^{-1}$), a surface water reservoir storing runoff (Qr mm d$^{-1}$), and a groundwater module that consists of three groundwater system options (large, medium, small groundwater availability) driven by groundwater recharge (Rch in mm d$^{-1}$). Anthropogenic water demand is met by reservoir abstractions (Ares in mm d$^{-1}$ ) and groundwater abstractions (Agw in mm d$^{-1}$), both in striped dark red arrows. Natural water demand is represented by ecological flow requirements (Qeco in mm d$^{-1}$; dotted green arrow) and abstracted as part of the baseflow (Qb in mm d$^{-1}$). Remaining baseflow is routed to the reservoir. Additional water is imported in the model when reservoir or groundwater storage is insufficient (Qimp and GSimp both in mm d$^{-1}$). Drought management scenarios apply to the surface water reservoir, groundwater module, and environmental and anthropogenic water demand (all model components in the thick black box). }
\label{Revised_Fig01}
\end{figure}\textbf{Comment 4: `What is the advantage to have three different GW model representations for the three
different groundwater systems if also variation of the parameterization of the same model structure could do this job? Stoelzle et al. (2015) showed that the FLEX GW structure outperforms the three structures POW, 1LBY and 2PA used in this study and the hydrogeological clustering of catchments is also better/clearer for FLEX than for other structures (Fig. 4 in Stoelzle et al., 2015). Wouldn’t it be easier to compare the effects of larger and smaller groundwater systems on different drought policies if the model structures across those different GW systems stay the same?'}\\It is a valid point raised by Reviewer 1 that when model validation is possible, all three groundwater systems can be well-simulated using the FLEX GW structure.
However, in this virtual catchment modelling we aimed to represent different groundwater systems without the need for validation to find a representative share of fast and slow responding groundwater discharge. Therefore, the FLEX GW structure is not ideal given the dependency on the threshold-controlled storage outflow (h) to determine either primarily fast or slow groundwater discharge response. As an alternative, we applied suggested conceptual model structures to represent different kinds of groundwater storage-outflow characteristics that typically result in different drought characteristics for karstic, porous and fractured aquifer types. The non-linear groundwater discharge release, as observed in karstic aquifers, is best represented by a power law (POW) \citep{Wittenberg03}. Slow porous flow could be represented by a number of structures, but given the overall high performance by 1LBY (Figure 5 in \citet{stoelzleBaseflowmodelling}) and the reported slow flow and possibly leakage in English Permo-Triassic sandstone aquifers, we selected the 1LBY. For fractured aquifers, the overall recommendation to apply parallel reservoirs and the overall high performance of 2PA (with groundwater recharge input) were the reasons for selecting 2PA for the last groundwater system. \\
By selecting these three conceptual model structures, we aimed to represent three different kinds of groundwater storage-outflow release whilst testing a range of representative parameters for English catchments. In the revised manuscript, we will include the reasoning behind the selection of model structures in a virtual catchment. \newline\textbf{Comment 5: `I like the section 5.3 model limitations as this discussion is really important to
understand the results of the study. Beside the fact that this section could be incorporated into other sections of the manuscript or at least should not be placed at the end of the discussion section (to gain a more positive ending of the paper), I asked myself what would have been happened if another forcing than an England’s average
was used? Are average conditions a good starting point for such drought analysis like here? Additional analysis could shed light on this, however, at least more discussion is needed to evaluate how representative the average approach is for the different regions in England or different water companies.'}\\We thank Reviewer 1 for these two suggestions. We will include relevant results of the sensitivity analysis in earlier sections if relevant (also see response to comment 2). Regarding the second suggestion, our intention was to select a representative precipitation record for England. The HadUKP data consists of weighted observations, including extremely wet and dry periods within its record \citep{alexander2001updated}. However, we will verify if other forcing English data, i.e. a representative location in the CEH-GEAR dataset \citep{GEAR-PDataCEH} changes the overall water balance and outcomes. We will include these additional analyses in the full rebuttal and amend the results in revised manuscript if necessary.
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\end{document}
Citation: https://doi.org/10.5194/nhess-2021-129-AC1 -
AC2: 'Reply on AC1', Doris Wendt, 04 Jun 2021
We thank the Reviewer for their careful reading and constructive comments to improve the clarity of the submitted manuscript.
We will address each comment in detail in the rebuttal, but we have addressed the first five major comments in this Author's response to clarify important aspects of the paper and show how we will include the provided suggestions in the revised manuscript.Please find the attached Author Response in PDF version.
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RC2: 'Reply on AC2 - Fig. 1', Anonymous Referee #1, 07 Jun 2021
The revised Fig. 1 is a great move forward. I suggest to replace the reservoir design of "Groundwater module" with a normal box (as it is a representation). Then, the three options below are a great explanation (and they are actually storage systems in the model!), please add "groundwater aquifer" to "Large", "medium" and "small" and perhaps something like power-law, by-pass and two-parallel to the storage boxes (and the hydrogeological representations). The two arrows in "medium" and "small" can be confused, is the by-pass flowing through the storage, is the outflow of the first parallel storage added to the second?
Citation: https://doi.org/10.5194/nhess-2021-129-RC2 -
AC4: 'Reply on RC2', Doris Wendt, 05 Jul 2021
We thank Reviewer 1 for these additional suggestions.
We have changed the groundwater module box as suggested in the revised figure 1 and completed the names of the three groundwater storage systems for consistency. In the caption, we linked the names of the groundwater systems to the power law, by-pass, and parallel storage-outflow equations and referred to a detailed description of the hydrogeological representations in the manuscript.
In response to the last question, the by-pass storage-outflow equation results in a small proportion of recharge (10% in L155-156 in submitted manuscript) that bypasses the groundwater storage and is discharged immediately, as indicated by the different shade of arrow in revised Figure 1. The two parallel storages in the small groundwater storage system are modelled in parallel, for which groundwater recharge and water demand is equally divided (L159-160 in submitted manuscript).
Attached is the last version of revised Figure 1.
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AC4: 'Reply on RC2', Doris Wendt, 05 Jul 2021
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RC2: 'Reply on AC2 - Fig. 1', Anonymous Referee #1, 07 Jun 2021
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AC2: 'Reply on AC1', Doris Wendt, 04 Jun 2021
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RC3: 'Comment on nhess-2021-129', Anonymous Referee #2, 22 Jun 2021
I have found this manuscript to be well written, well structured and well presented. The topic is timely and I am very glad to see joining up of groundwater and water resources modelling, it happens far less than it should! Although the case study model is synthetic, a great deal of thought has gone into the both the model setup and parameter choices; the authors clearly have a strong working understanding of how UK water resource systems operate - and their synthetic model well reflects that. With the synthetic nature of the case study in mind, the authors are careful in their discussion and wider recommendations, making only recommendations that their sensitivity analysis supports. I have only some minor recommendations around improving clarity (primarily around presentation of the demand model) and some wider thoughts that may be addressed before the paper is published.
Minor comments
As I mention, modelling water resources with groundwater in a joined up way is surprisingly rare - the authors may wish to include a slightly expanded review (even just a paragraph in the intro) on this subject to help justify their reasonably simplistic hydrological representation.
Can water demand be written as an equation, just to make it easier to follow.
L116/133 - Perhaps either move some of the text from S3.2 here, or at least reference that this is described in more detail in the Data section. On first reading it appeared that this line was essentially the only description of water demand in the model!
I was under the impression that ecological flows are typically met by reservoir releases, rather than groundwater pumping (though I suppose this is highly regional). No need to rerun the model, but might be interesting for UK readers.
The groundwater and reservoir levels in this model are often 0 (Fig 2). This is no problem since the case study is synthetic, but the authors should note in the text that UK groundwater/reservoir systems are not this stressed (even if effort has been taken to parameterise the model in a sensible and nationally reflective manner) - perhaps expand a little in Section 5.3 (water companies might be alarmed if you give the impression that this is portrayed as a 'nationally average' model).
Can the model/modelling setup be made publicly available so that results can be reproduced - it seems that the models/data are all openly available so I don't see why not?
Figure A2. soil moisture: can the green, light blue and dark blue horizontal lines be described in the caption.
L204/Table 3: The increase in surface/ground water demand is actually an increase in water availability no? If I understand the reference to table 1, then it seems the licences are being maximised. Increasing demand as a response to drought seems confusing phrasing to me (why would a water company do that!). Maybe say +X% surface/ground water abstraction capacity? I would probably also describe this in a bit more detail in the text because it seems not trivial to conceptualise!
L207 - I guess conjunctive use is in contrast to a fixed proportion of demand met by surface water : groundwater? If so, I would state that here to make it clear.
Figure 4: There is quite a lot to unpack here! I would rename the panels slightly to help the reader navigate the figure a bit more quickly:
-High groundwater storage system groundwater level (GWL)
-High groundwater storage system baseline GWL minus scenario GWL
-Low groundwater storage system GWL
-Low groundwater storage system baseline GWL minus scenario GWLL368: transferred, not traded!
Finally, and feel free to ignore this as I find terminology to mainly be a subjective choice, the use of 'socio-hydrological model' in this context has left me a little confused. In terms of how sophisticated a representation of hydrological process and sociological processes - this model is very asymmetric, with a minimal social representation. The feedback between hydrological and social is only the pre-defined water company drought response to lowering groundwater/reservoir levels (which I wouldn't class as a social process). By this definition, any water resources modelling application that includes a feedback between physical state variables and water consumption (and this is surely many/most - at least from my experience in water supply modelling) may be classified as socio-hydrological (and if this is the case, then socio-hydrology is surely not an emerging field, as stated in L52!).
Citation: https://doi.org/10.5194/nhess-2021-129-RC3 - AC3: 'Reply on RC3', Doris Wendt, 02 Jul 2021
Peer review completion
