the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 May 2023
The physics behind groundwater recession and hydrologically passive mixing volumes
Abstract. Transit time and water age characteristics are fundamental descriptors of catchment response, and their determination is vital for the implementation of sustainable strategies for managing nutrients and other contaminants in water environments – especially for groundwater where the deeper stores take decades to flush the dissolved solutes. The deterministic transit time models can be broadly categorized into 2 sorts – lumped models based on conceptual parameters and distributed models based on physical and quantifiable hydrodynamic parameters. Due to their simplicity, applicability and flexibility, lumped conceptual models are thus far widely and successfully used in modelling the groundwater flow, transport, and transit time of solutes. Usually, a bunch of parallel hydrological response units work in harmony to model the desired hydrological and solute concentration time-series. But sole reliance on calibration, non-scalability, leveraging on hydrologically passive mixing volumes, lack of forward modelling potential and ineffective scrutiny of the physical basis of the parameters of these conceptual models often generate skepticism in the research community. To address this issue, we devised a technique to determine the physical basis of these conceptual reservoirs, and to establish a mathematical connection between physical hydrodynamic parameters and lumped conceptual parameters. A lumped groundwater nitrate transit time model composed of two parallel stores (slow and fast) was previously calibrated (using GLUE) to generate the time series of baseflow and nitrate concentration time series in a groundwater dominated agricultural catchment in France. In this study, we generated synthetic 2D Dupuit-Forchheimer unconfined aquifers using a standard finite element code (FEFLOW 7.5) to replicate outputs of the lumped model. Furthermore, sensitivity tests were performed on these synthetic catchments and overall, a clear mathematical connection between physical and conceptual parameters was demonstrated. It was further observed that the difference between fast and slow stores can be explained using dual porosity – with drainable porosity affecting recession, and immobile porosity affecting the size of hydrologically passive mixing volumes. The spatial mean of the age distributions, the mean transit time and the half nitrate recovery time agreed with each other for both stores. Further sensitivity tests showed that lumped conceptual stores individually cannot acknowledge dispersivity – the difference in attenuation of different stores, in unison, produce a pseudo-dispersive behavior. Also, being purely depth-based, there is a scale issue in lumped models – an erroneous input of catchment dimension can yield the identical results for a completely different set of hydrodynamic parameters leading to equifinality. Therefore, transit times should always be normalized by catchment scale while cross-comparing catchments using lumped models. These finding can help reduce calibration reliance of lumped models, providing options to investigate parameter effectiveness, and offers these models a forward modelling potential which can be used to calculate the flow and transport behavior of catchments that lack long term observed time series but have proper measurements of hydrodynamic properties.
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Interactive discussion
Status: closed
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RC1: 'Comment on hess-2023-112', Anonymous Referee #1, 26 Jun 2023
This paper focuses on nitrate transit time modeling and proposes a technique to establish a mathematical connection between the parameters of a physically based and a lumped conceptual model. The authors utilize a lumped groundwater nitrate transit time model with parallel slow and fast stores, as well as the finite element model FEFLOW 7.5, to replicate the outputs of the lumped model. The paper aims to reduce calibration reliance and suggests potential applications for calculating flow and transport behavior in catchments with limited observed time series but proper measurements of hydrodynamic properties.
The paper can be classified as a synthetic study since it utilizes modeled time series. Specifically, the finite element model FEFLOW 7.5 is calibrated to the output of the lumped ETNA model, which itself was calibrated to observed data in a previous study (Fovet et al., 2015). Additionally, the nitrate input used in the model is derived from another model, namely the Burns model for predicting the leaching of surface-applied nitrate.
While the paper demonstrates a complex analysis that indicates a significant amount of work, it suffers from several limitations, which are enumerated below.
1. Clarity: First and foremost, the paper lacks clarity, which can be attributed to several issues:
a. The heavy reliance on previous work, such as the ETNA, FEFLOW 7.5, and Burns models, without adequate explanation. This lack of detail hampers understanding and leads to confusion for readers unfamiliar with these specific models. For example, when reading the first sentence of the methodology, "The main target is to use FEFLOW 7.5 to generate synthetic homogeneous box catchments hydrologically equivalent to ETNA stores," readers without knowledge of a "synthetic homogeneous box catchment" may feel confused. The same confusion arises with statements such as "the aquifer was taken to be unconfined because both stores of ETNA receive Burns recharge from top" (what is the connection?) or "The parabolic head distribution along X from upper to lower boundary is just a Hydraulic Grade Line (HGL) and not Z-dimension, and flow and transport are non-existent along Z, but the HGL height can be considered as the unconfined aquifer thickness that aids dilution/mixing" (too many unexplained definitions).
b. The paper often describes how things are done without providing sufficient explanation as to why they are done. For example, Section 2.4 starts with "We perform a sensitivity study" and then explains how it was done. However, to understand why this study was performed, readers have to wait for the second paragraph, which states, "With the knowledge of this sensitivity, we then adjust the parameters to match the hydrological recession and the nitrate breakthrough concentration time series." While this partially answers the why question, it raises the question of how the sensitivity helped in adjusting the model parameters.
c. The paragraphs are often information-dense. For instance, in Section 2.5, the following sentence contains multiple condensed pieces of information: "To determine the nitrate transit time using ETNA, unit pulses of nitrate were sent on 1st August 1968, 1974, and 1980 representative of dry, average, and wet climatic sequences, for the entire behavioral parameter set obtained from GLUE (Fovet et al., 2015), and the time required to recover half the input nitrate was calculated as Half Nitrate Recovery Time (HNRT), which is supposed to be slightly lower than Mean Transit Time (MTT) for long-tailed distributions." Each piece of information in this sentence deserves a dedicated paragraph to ensure clarity.
d. The lack of proper definitions further contributes to the overall lack of clarity. Some parameters are not explained at all in the main text. For example, the line "There are 20 isolines between 40m and 20m DBC heads, so at 7.5 isolines away from 40m DBC, we have the isolinear centroid where h = 32.5m" assumes that the parameter DBC is defined and explained somewhere earlier, but no definitions are found in the main text.2. Complexity: The paper relies on a complex architecture involving three different models (ETNA, FEFLOW 7.5, and Burns). It is unclear to what extent such complexity is necessary or directly relevant to the study objectives. Simplifying the setup and targeting the analysis more directly toward the study objectives could improve the overall coherence of the paper.
3. Generalizability: The results of the study appear to be specific to the chosen models and catchment conditions. It remains unclear to what extent these findings can be generalized to other models and diverse catchment scenarios. Addressing the potential limitations of generalizability and discussing the applicability of the proposed technique to different contexts would strengthen the paper.
4. Methodological Soundness: The methodology of the study raises some concerns regarding its solidity. For example, the sentence "We use trial and error to settle for the final set of parameters because a) FEFLOW 7.5 parameter estimation program (FEPEST) is highly time-consuming and buggy" does not provide a scientifically satisfactory justification for the underlying methodological choices. Instead, it raises concerns about the validity of the approach.
5. Focus and Replicability: It is suggested that the authors take a more focused approach by selecting a specific research question and utilizing simple and easily replicable methods. For example, exploring the linkage between Darcy flow and linear reservoir theory appears promising. Adopting a simpler and more targeted approach would facilitate effective communication to a wider audience.
In conclusion, the paper presents a detailed analysis of nitrate transit time modeling but suffers from several limitations that do not allow to capture its essence. Addressing these concerns through improved explanations, a more focused approach, and a discussion on the broader applicability of the findings would significantly enhance the quality and impact of the paper.
Considering the extent of the issues raised and the need for substantial modifications to the paper, I do not feel that the current version is adequate for continuing the review process. Instead, I strongly encourage the authors to carefully address the comments and suggestions and submit a new version of the paper once the necessary revisions have been made.
Citation: https://doi.org/10.5194/hess-2023-112-RC1 -
RC2: 'Comment on hess-2023-112', Anonymous Referee #2, 05 Jul 2023
This manuscript establishes a mathematical connection between physically based and lumped conceptual models. The work is based on a (2-store) lumped groundwater transit time model alongside the more complex finite element model (FEFLOW) to reproduce the outputs of the simpler lumped model. The (very relevant) main objective seems to find way to decrease the need for model calibration reliance in calculating flow and transport behavior in catchments. Despite the significant amount of work demonstrated in the paper, I have several primary concerns that appear to be in line with Reviewer #1.
The results appear to be specific to the chosen models and catchment conditions. This makes it unclear to the extent to which findings can be generalized to other models and diverse catchment scenarios.
The paper relies on a complex architecture involving three different models, which raises questions about the necessity and direct relevance of such complexity to the study objectives.
The paper is not always clear. A major example is, there is heavy reliance on previous work, such as the ETNA, FEFLOW 7.5, and Burns models, without sufficient explanation. This omission of info makes it challenging for readers unfamiliar with these specific models (which will be almost any HESS reader) to fully grasp the details.
This scientific justification is not always clear. For example, as stated by R1 the implications for the use of trial and error for settling on the final set of parameters are unclear.
Citation: https://doi.org/10.5194/hess-2023-112-RC2 -
EC1: 'Editor Comment on hess-2023-112', Hubert H.G. Savenije, 05 Jul 2023
Dear Authors,
Both referees are very clear in their reviews. The fact that you base your study on the comparison of different model simulations only make your paper less interesting for the hydrological community. Unfortunately there is not much innovation in comparing models without additional data or theoretical innovation. In addition, the referees #1 has pointed out a list of shortcomings that are not easily addressed in a revised paper. I fully agree with the comments made by the referees and I will have to reject the article.
Citation: https://doi.org/10.5194/hess-2023-112-EC1 -
AC1: 'Reply on EC1', Baibaswata Bhaduri, 16 Aug 2023
Dear editor,
We understand the reasons motivating your decision. We thank both reviewers for their detailed feedback and their appreciation of the relevance of our work.
We realize that our narrative was lacking clarity especially in explaining the broader implications of the study and the methodology. In fact, rather than “a comparison of models”, our ambition was to conduct an analysis showing that conceptual catchment modelling, at least when developed for simulating solute transport, can rely on physical basis and therefore lead to catchment residence time estimates that are reliable.
We still think we could have addressed the review comments satisfactorily. Nevertheless, we humbly accept your decision, and we thank you for your input.
Citation: https://doi.org/10.5194/hess-2023-112-AC1 -
EC2: 'Reply on AC1', Hubert H.G. Savenije, 16 Aug 2023
Dear Baibaswata Bhaduri,
Thank you for your reply. I would encourage you to still provide replies to the reviewer comments in the Discussion. In that way your reaction will go on record.
Sibncerely,
Hubert Savenije
Citation: https://doi.org/10.5194/hess-2023-112-EC2
-
EC2: 'Reply on AC1', Hubert H.G. Savenije, 16 Aug 2023
-
AC1: 'Reply on EC1', Baibaswata Bhaduri, 16 Aug 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on hess-2023-112', Anonymous Referee #1, 26 Jun 2023
This paper focuses on nitrate transit time modeling and proposes a technique to establish a mathematical connection between the parameters of a physically based and a lumped conceptual model. The authors utilize a lumped groundwater nitrate transit time model with parallel slow and fast stores, as well as the finite element model FEFLOW 7.5, to replicate the outputs of the lumped model. The paper aims to reduce calibration reliance and suggests potential applications for calculating flow and transport behavior in catchments with limited observed time series but proper measurements of hydrodynamic properties.
The paper can be classified as a synthetic study since it utilizes modeled time series. Specifically, the finite element model FEFLOW 7.5 is calibrated to the output of the lumped ETNA model, which itself was calibrated to observed data in a previous study (Fovet et al., 2015). Additionally, the nitrate input used in the model is derived from another model, namely the Burns model for predicting the leaching of surface-applied nitrate.
While the paper demonstrates a complex analysis that indicates a significant amount of work, it suffers from several limitations, which are enumerated below.
1. Clarity: First and foremost, the paper lacks clarity, which can be attributed to several issues:
a. The heavy reliance on previous work, such as the ETNA, FEFLOW 7.5, and Burns models, without adequate explanation. This lack of detail hampers understanding and leads to confusion for readers unfamiliar with these specific models. For example, when reading the first sentence of the methodology, "The main target is to use FEFLOW 7.5 to generate synthetic homogeneous box catchments hydrologically equivalent to ETNA stores," readers without knowledge of a "synthetic homogeneous box catchment" may feel confused. The same confusion arises with statements such as "the aquifer was taken to be unconfined because both stores of ETNA receive Burns recharge from top" (what is the connection?) or "The parabolic head distribution along X from upper to lower boundary is just a Hydraulic Grade Line (HGL) and not Z-dimension, and flow and transport are non-existent along Z, but the HGL height can be considered as the unconfined aquifer thickness that aids dilution/mixing" (too many unexplained definitions).
b. The paper often describes how things are done without providing sufficient explanation as to why they are done. For example, Section 2.4 starts with "We perform a sensitivity study" and then explains how it was done. However, to understand why this study was performed, readers have to wait for the second paragraph, which states, "With the knowledge of this sensitivity, we then adjust the parameters to match the hydrological recession and the nitrate breakthrough concentration time series." While this partially answers the why question, it raises the question of how the sensitivity helped in adjusting the model parameters.
c. The paragraphs are often information-dense. For instance, in Section 2.5, the following sentence contains multiple condensed pieces of information: "To determine the nitrate transit time using ETNA, unit pulses of nitrate were sent on 1st August 1968, 1974, and 1980 representative of dry, average, and wet climatic sequences, for the entire behavioral parameter set obtained from GLUE (Fovet et al., 2015), and the time required to recover half the input nitrate was calculated as Half Nitrate Recovery Time (HNRT), which is supposed to be slightly lower than Mean Transit Time (MTT) for long-tailed distributions." Each piece of information in this sentence deserves a dedicated paragraph to ensure clarity.
d. The lack of proper definitions further contributes to the overall lack of clarity. Some parameters are not explained at all in the main text. For example, the line "There are 20 isolines between 40m and 20m DBC heads, so at 7.5 isolines away from 40m DBC, we have the isolinear centroid where h = 32.5m" assumes that the parameter DBC is defined and explained somewhere earlier, but no definitions are found in the main text.2. Complexity: The paper relies on a complex architecture involving three different models (ETNA, FEFLOW 7.5, and Burns). It is unclear to what extent such complexity is necessary or directly relevant to the study objectives. Simplifying the setup and targeting the analysis more directly toward the study objectives could improve the overall coherence of the paper.
3. Generalizability: The results of the study appear to be specific to the chosen models and catchment conditions. It remains unclear to what extent these findings can be generalized to other models and diverse catchment scenarios. Addressing the potential limitations of generalizability and discussing the applicability of the proposed technique to different contexts would strengthen the paper.
4. Methodological Soundness: The methodology of the study raises some concerns regarding its solidity. For example, the sentence "We use trial and error to settle for the final set of parameters because a) FEFLOW 7.5 parameter estimation program (FEPEST) is highly time-consuming and buggy" does not provide a scientifically satisfactory justification for the underlying methodological choices. Instead, it raises concerns about the validity of the approach.
5. Focus and Replicability: It is suggested that the authors take a more focused approach by selecting a specific research question and utilizing simple and easily replicable methods. For example, exploring the linkage between Darcy flow and linear reservoir theory appears promising. Adopting a simpler and more targeted approach would facilitate effective communication to a wider audience.
In conclusion, the paper presents a detailed analysis of nitrate transit time modeling but suffers from several limitations that do not allow to capture its essence. Addressing these concerns through improved explanations, a more focused approach, and a discussion on the broader applicability of the findings would significantly enhance the quality and impact of the paper.
Considering the extent of the issues raised and the need for substantial modifications to the paper, I do not feel that the current version is adequate for continuing the review process. Instead, I strongly encourage the authors to carefully address the comments and suggestions and submit a new version of the paper once the necessary revisions have been made.
Citation: https://doi.org/10.5194/hess-2023-112-RC1 -
RC2: 'Comment on hess-2023-112', Anonymous Referee #2, 05 Jul 2023
This manuscript establishes a mathematical connection between physically based and lumped conceptual models. The work is based on a (2-store) lumped groundwater transit time model alongside the more complex finite element model (FEFLOW) to reproduce the outputs of the simpler lumped model. The (very relevant) main objective seems to find way to decrease the need for model calibration reliance in calculating flow and transport behavior in catchments. Despite the significant amount of work demonstrated in the paper, I have several primary concerns that appear to be in line with Reviewer #1.
The results appear to be specific to the chosen models and catchment conditions. This makes it unclear to the extent to which findings can be generalized to other models and diverse catchment scenarios.
The paper relies on a complex architecture involving three different models, which raises questions about the necessity and direct relevance of such complexity to the study objectives.
The paper is not always clear. A major example is, there is heavy reliance on previous work, such as the ETNA, FEFLOW 7.5, and Burns models, without sufficient explanation. This omission of info makes it challenging for readers unfamiliar with these specific models (which will be almost any HESS reader) to fully grasp the details.
This scientific justification is not always clear. For example, as stated by R1 the implications for the use of trial and error for settling on the final set of parameters are unclear.
Citation: https://doi.org/10.5194/hess-2023-112-RC2 -
EC1: 'Editor Comment on hess-2023-112', Hubert H.G. Savenije, 05 Jul 2023
Dear Authors,
Both referees are very clear in their reviews. The fact that you base your study on the comparison of different model simulations only make your paper less interesting for the hydrological community. Unfortunately there is not much innovation in comparing models without additional data or theoretical innovation. In addition, the referees #1 has pointed out a list of shortcomings that are not easily addressed in a revised paper. I fully agree with the comments made by the referees and I will have to reject the article.
Citation: https://doi.org/10.5194/hess-2023-112-EC1 -
AC1: 'Reply on EC1', Baibaswata Bhaduri, 16 Aug 2023
Dear editor,
We understand the reasons motivating your decision. We thank both reviewers for their detailed feedback and their appreciation of the relevance of our work.
We realize that our narrative was lacking clarity especially in explaining the broader implications of the study and the methodology. In fact, rather than “a comparison of models”, our ambition was to conduct an analysis showing that conceptual catchment modelling, at least when developed for simulating solute transport, can rely on physical basis and therefore lead to catchment residence time estimates that are reliable.
We still think we could have addressed the review comments satisfactorily. Nevertheless, we humbly accept your decision, and we thank you for your input.
Citation: https://doi.org/10.5194/hess-2023-112-AC1 -
EC2: 'Reply on AC1', Hubert H.G. Savenije, 16 Aug 2023
Dear Baibaswata Bhaduri,
Thank you for your reply. I would encourage you to still provide replies to the reviewer comments in the Discussion. In that way your reaction will go on record.
Sibncerely,
Hubert Savenije
Citation: https://doi.org/10.5194/hess-2023-112-EC2
-
EC2: 'Reply on AC1', Hubert H.G. Savenije, 16 Aug 2023
-
AC1: 'Reply on EC1', Baibaswata Bhaduri, 16 Aug 2023
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Baibaswata Bhaduri
Ophelie Fovet
Sekhar Muddu
Laurent Ruiz
This preprint has been withdrawn.
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