Review Reports - Unstructured Modflow Model for Numerical Simulations of Groundwater Flow in Three-Dimensional Quaternary Aquifer of Beijing Plain, China

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors utilized the GMS software to establish a three-dimensional numerical simulation model of groundwater flow in the Beijing Plain area. This model is of significant value for predicting the evolution of the groundwater environment and for supporting the development, utilization, and protection of water resources in this region. However, as an international professional academic journal, it may not be suitable to publish a regional groundwater numerical simulation report. It is recommended that the authors use this groundwater flow numerical model as a foundation to investigate the impacts of climate change and human activities on the groundwater environment in the study area. For example, what effects have several extreme rainstorm events in Beijing and the South-to-North Water Diversion Project had on the groundwater environment of the Beijing Plain?

The labels (a), (b), and (c) in Figure 1 are incorrect.
The description of the upper boundary condition is missing in the section concerning the hydrogeological conceptual model.
Regarding the model's source and sink terms, the method for obtaining the river infiltration volume(seepage volume of river) is unclear. Furthermore, the explanation of how the model handles river-related processes is insufficient.
Lines 205 and 206 are repetitive and should be merged.
During the calibration and validation of the groundwater flow model, in addition to the fitting of the groundwater flow field and water levels, hydrogeological parameters and groundwater balance issues should also be considered

Author Response

what effects have several extreme rainstorm events in Beijing and the South-to-North Water Diversion Project had on the groundwater environment of the Beijing Plain?

In addition to establishing the groundwater flow numerical model, the impact of human activities, particularly restoration through ecological water supply projects from Mihuaishun and water plants, has been considered but not sufficiently highlighted in the article. This aspect was highlighted following your recommendations.

The labels (a), (b), and (c) in Figure 1 are incorrect.

The labels in Figure 1 have been corrected

The description of the upper boundary condition is missing in the section concerning the hydrogeological conceptual model.

The upper boundary condition has been described in the hydrogeological conceptual model paragraph.

Regarding the model's source and sink terms, the method for obtaining the river infiltration volume (seepage volume of river) is unclear. Furthermore, the explanation of how the model handles river-related processes is insufficient.

A paragraph has been added to explain how the river infiltration volume is handled and estimated in the model.

Lines 205 and 206 are repetitive and should be merged.

Lines 205 and 206 have been merged and sentences corrected.

During the calibration and validation of the groundwater flow model, in addition to the fitting of the groundwater flow field and water levels, hydrogeological parameters and groundwater balance issues should also be considered

Hydrogeological parameters and groundwater balance have been considered as suggested

Reviewer 2 Report

Comments and Suggestions for Authors

The authors used MODFLOW USG within the GMS platform to create and calibrate a groundwater flow model for Beijing, China. While the model development and future simulations are valuable for reporting, the study lacks the novelty required for a research paper. Despite the commendable effort in building the simulation, the study is a standard application that neither introduces new methodologies nor advances groundwater modeling. Although the authors highlighted the use of unstructured grids as innovative, study did not effectively demonstrate their benefits over traditional grids, particularly given the modeling uncertainties and limited data. No details were given about the calibration method and the parameters that were calibrated and their final values. It is not even clear to which of the three aquifers the plotted contours and timeseries comparing observed and simulated head values belong to. Moreover, details are missing on the temporal and spatial distributions of the observation wells and piezometers and their depths, making it difficult to judge the quality of the calibration.

The Introduction dedicates much space citing research on MODFLOW USG without articulating the need for unstructured grids in modeling groundwater in the Beijin plain as opposed to using traditional MODFLOW. I have no doubt that the latter can still be applied after doing away with the inactive cells.
The model calibration strategy lacks details on how unknown parameters were calibrated, their uniqueness, and which aquifers were simulated. Information on the aquifers' measured parameters, locations, and depths is absent, even in supplementary materials. The model's robustness can be questioned due to the short two-year observation period for groundwater heads, which fails to capture interannual variations, especially in the shallow aquifer. The stationary observed heads raise doubts about the calibrated storage coefficients. Important parameters like those related to inflow boundaries, riverbed conductance, and evapotranspiration are not mentioned, which makes it difficult to judge if the calibrated parameters are realistic and that this is not a curve fitting exercise.
Figures are too small for effective visual assessment, and results are presented collectively, ignoring the distinct aquifers, complicating the evaluation of model performance across different depths. This oversight is crucial since groundwater is likely pumped from different depths and aquifers, each with unique drawdown characteristics.
Model prediction for next 15 years seems to ignore annual variability in the climate, population growths, and potential land-use changes in the area. A critical statement on these limitations, confounded by the inevitable modeling errors and predictive uncertainty, should be made.
Geology (Section 2.2)
1. What is the predominant sediment characteristic of each of the three aquifers?
2. Should state here that the shallow aquifer is an unconfined or phreatic aquifer. What about the medium and deep aquifers? Are they confined or semi-confined?
3. Is the bedrock at the bottom of the deep aquifer?
4. What is Upper or Bottom plate depth?
5. Column Recharge mode in Table 1: Is “Atmospheric precipitation (western part)” for Shallow Aquifer or Medium Aquifer? What does “western part” refer to.
6. Figure 2: Too small to read the legends and understand the lithology. Why not show the hydrostratigraphic grouping into the three aquifers instead?

A subsection (e.g., 3.3) should be dedicated to observed data used for model calibration, number of wells/piezometers, at what depth or the aquifer they tap, and over what period and frequency of measurements.
Most of the areal figures and contour lines are hardly legible.

Additional comments:

Suggest replacing “identification” with “calibration” and “verification” with “validation” throughout.
L24: What is the significance of reporting groundwater levels exceeding “10 meters”? It is more informative to state, e.g., “Model predicts a stabilized and significantly increasing groundwater levels at the center of the Beijing area.”
L33-L35: Rewrite this sentence.
L35-37: I doubt this is a universally held classification. There is no such a thing “identification models”. Indeed, models built within an optimization framework with a predefined objective function can be used for groundwater management, but I am not sure this is a recognized class of models.
L 43: Rewrite to “For water flow, ????? (MODFLOW USG)…”. Spell out MODFLOW USG.
L59: Delete “Mohsen M”. When citing a reference, the last name of the main author should be mentioned only.
L67-69: Suggest deleting this sentence (redundant).
L75-76: Can this be elaborated. Superiority in what sense. I doubt that traditional MODFLOW with a sufficiently fine but regular mesh would not produce equally accurate results for larger scale applications. A regular mesh can accommodate irregular boundaries and become irregular grid using inactive cells.
L81-83: Still, in what sense?
L83-85: Ok, but also this can be modeled using traditional MODFLOW.
L103: Rewrite to “…North China plain within the Hebei province (Figure 1a).” Do you agree?
L104-105: Rewrite to “The total area …16,410 km² is 60% mountainous and 40% a plain area.”
L106: Couldn’t locate Taihang on the map.
L108-109: Rewrite to “…distinct seasons, a relatively concentrated rainfall and a climate characterized by rain and heat…”.
L113: Rewrite to “…in the plain of lengths 448 km and 1,248 km in 2021, respectively.”
L116: Rewrite to “…billion m3 of which 639 million m³ of water is available for use [29].”
Figure 1: The labels are off. Figure 1(b) and legend is hardly legible. Please enlarge.
L136: “groundwater burial conditions” has no meaning. Is this in reference to the vadose soil?
L146-148: Incoherent. Is subsection 3.1 G.I. generated?
L165-167: Not clear what was implied. How can borehole data describe boundary conditions? What is “bottom plate”? Isn’t the shallow aquifer bounded by the water table from above? The medium and deep aquifers separated by impervious or semipervious (leaky) layers? What about the bedrock and if it is at the very bottom as an impervious boundary?
L170-171: Rewrite to “In groundwater modeling, the aquifers can be divided into homogenous zones of constant parameters.”
L173-175: Rewrite to “…, vertical hydraulic conductivity of the aquifer (K_H, K_V), specific storage coefficient (S_s), and specific yield (S_y).”
L177-179: Rewrite to “The vertical hydraulic conductivity parameter is generally 1/10 the value of the horizontal hydraulic conductivity parameter and then adjusted during model calibration.”
Table 2: What are the bases of the values of infiltration coefficient?
L187-188: delete this sentence.
L191: Delete “runoff” and replace “evaporation” with “evapotranspiration”. Runoff is water running off the surface due to rainfall excess.
L192: Rewrite to “…of groundwater recharge and discharge elements.”.
Table 3:
1. use yr as the unit for year instead of a throughout.
2. Replace “Qc -lateral runoff…” with “Q_c – lateral recharge, m³/yr, inflow is positive,…”.
3. Use Q_g to replace Q_c for Irrigation water infiltration.
4. In the last column of Table 3, what is “Check the results of previous information”?
5. How was Evapotranspiration calculated? “Depth of burial > 4 m, not considered” has no meaning.
Section 3.3.2: Replace identification with calibrations and verification with identification throughout.
Figure 5: Why is “Water level diagram comparison” needed?
L239: Couldn’t locate Hebei Province on the map.
Figure 7: For what aquifer? Shallow aquifer? Not legible, too difficult to compare (a) and (b).
L248: What is observation hole? Was “well” or “piezometer” implied?
L249: Not clear what “2-3 typical observation points were selected in each district and county” means. Why 2-3 are typical? Is this in reference number of wells. Makes no sense as written.
Table 5: Should provide a similar table for the calibration.
Figure 8: Which aquifer? With such small changes in groundwater levels, I doubt the aquifers’ storage coefficients S_s and/or S_y were determined accurately.
L261-264: Incoherent. Rewrite to “The scatter of observed and simulated groundwater levels reveal a near linear relationship clustered around the 1:1 line with R² =0.98, indicating good model accuracy.”
L267-268: Rewrite to “From the comparison between the observed and simulated values of groundwater levels and their error statistics (Table 5), it can be seen…”.
L272: Replace “curve” with “timeseries”
L279-281: Rewrite to “…balance Table 6, it can be seen that the difference in balance between total recharge and total discharge in the study area is 463 million m³.”.
Table 6 title does not reflect the table entries! Report the units, Inflows better than Supply items and Outflows better than Excretory item. Replace “Equilibrium” with “Balance”.
10: In which of the three aquifers?
L298-299 and 304-305: Any explanation for the declining groundwater levels in the western and northwestern parts?
L341: Incoherent sentence.

L342-343: Didn’t see a table listing the calibrated parameters, their initial values, and final calibrated values.

Comments on the Quality of English Language

The English can be improved. I pointed to areas where edits are needed and made suggestions

Author Response

The Introduction dedicates much space citing research on MODFLOW USG without articulating the need for unstructured grids in modeling groundwater in the Beijing plain as opposed to using traditional MODFLOW. I have no doubt that the latter can still be applied after doing away with the inactive cells.

This comment has been considered and the introduction rewritten according to.

The model calibration strategy lacks details on how unknown parameters were calibrated, their uniqueness, and which aquifers were simulated. Information on the aquifers' measured parameters, locations, and depths is absent, even in supplementary materials. The model's robustness can be questioned due to the short two-year observation period for groundwater heads, which fails to capture interannual variations, especially in the shallow aquifer. The stationary observed heads raise doubts about the calibrated storage coefficients. Important parameters like those related to inflow boundaries, riverbed conductance, and evapotranspiration are not mentioned, which makes it difficult to judge if the calibrated parameters are realistic and that this is not a curve fitting exercise.

Details have been added throughout the article for a better understanding of the methods used to estimate and calculate the parameters considered in the study.

Figures are too small for effective visual assessment, and results are presented collectively, ignoring the distinct aquifers, complicating the evaluation of model performance across different depths. This oversight is crucial since groundwater is likely pumped from different depths and aquifers, each with unique drawdown characteristics.

The figures have been enlarged and the aquifers specified for the different results.

Model prediction for next 15 years seems to ignore annual variability in the climate, population growths, and potential land-use changes in the area. A critical statement on these limitations, confounded by the inevitable modeling errors and predictive uncertainty, should be made.

In this study, only restoration through ecological water supply projects from Mihuaishun and water plants has been considered. However, we agree that this could be an avenue for future research projects to improve the accuracy of the model.

Geology (Section 2.2)
1. What is the predominant sediment characteristic of each of the three aquifers?

The precision has been made in the article. As mentioned in the paragraph, predominant elements are pebbles, sand, gravel, clay elements

Should state here that the shallow aquifer is an unconfined or phreatic aquifer. What about the medium and deep aquifers? Are they confined or semi-confined?

The precision has been made in the article. For the model building, the shallow aquifer is unconfined and the medium and deep aquifers confined

Is the bedrock at the bottom of the deep aquifer?

It depends on the location.

What is Upper or Bottom plate depth?

Upper or Bottom plate depth refers to the upper and lower limits of each aquifer.

Column Recharge mode in Table 1: Is “Atmospheric precipitation (western part)” for Shallow Aquifer or Medium Aquifer? What does “western part” refer to.

In the western part of the region, the depth of groundwater in this layer is relatively shallow and recharge is mainly provided by atmospheric precipitation gradually changing to recharge by lateral runoff in the eastern part of the region.

Figure 2: Too small to read the legends and understand the lithology. Why not show the hydrostratigraphic grouping into the three aquifers instead?

Figure 2 has been enlarged.

A subsection (e.g., 3.3) should be dedicated to observed data used for model calibration, number of wells/piezometers, at what depth or the aquifer they tap, and over what period and frequency of measurements.

Data has been added and other parameters are described in section 4.

Most of the areal figures and contour lines are hardly legible.

The figures have been enlarged for better visibility.

Additional comments:

Suggest replacing “identification” with “calibration” and “verification” with “validation” throughout.

Replacement has been made throughout the article

L24: What is the significance of reporting groundwater levels exceeding “10 meters”? It is more informative to state, e.g., “Model predicts a stabilized and significantly increasing groundwater levels at the center of the Beijing area.”

Suggestion has been considered

L33-L35: Rewrite this sentence.

The sentence has been rewritten

L35-37: I doubt this is a universally held classification. There is no such a thing “identification models”. Indeed, models built within an optimization framework with a predefined objective function can be used for groundwater management, but I am not sure this is a recognized class of models.

This part has been rewritten to avoid any confusion

L 43: Rewrite to “For water flow, ????? (MODFLOW USG)…”. Spell out MODFLOW USG.

The sentence has been rewritten

L59: Delete “Mohsen M”. When citing a reference, the last name of the main author should be mentioned only.

The citation has been corrected.

L67-69: Suggest deleting this sentence (redundant).

The sentence has been deleted as suggested

L75-76: Can this be elaborated. Superiority in what sense. I doubt that traditional MODFLOW with a sufficiently fine but regular mesh would not produce equally accurate results for larger scale applications. A regular mesh can accommodate irregular boundaries and become irregular grid using inactive cells.

The paragraph has been rewritten

L81-83: Still, in what sense?

New reference has been added

L83-85: Ok, but also this can be modeled using traditional MODFLOW.

Additional details have been added for a better understanding of the use of Modflow USG compared to traditional Modflow in the study cited.

L103: Rewrite to “…North China plain within the Hebei province (Figure 1a).” Do you agree?

I do not agree, Beijing city is not located in the Hebei province. The sentence has been rewritten for a better understanding.

L104-105: Rewrite to “The total area …16,410 km²is 60% mountainous and 40% a plain area.”

The sentence has been rewritten

L106: Couldn’t locate Taihang on the map.

Correction has been made in the sentence for a better correlation with the figure.

L108-109: Rewrite to “…distinct seasons, a relatively concentrated rainfall and a climate characterized by rain and heat…”

Sentence has been rewritten as suggested

L113: Rewrite to “…in the plain of lengths 448 km and 1,248 km in 2021, respectively.”

Sentence has been rewritten as suggested

L116: Rewrite to “…billion m3 of which 639 million m³of water is available for use [29].”

Sentence has been rewritten as suggested

Figure 1: The labels are off. Figure 1(b) and legend is hardly legible. Please enlarge.

Labels have been arranged and figure 1(b) enlarged

L136: “groundwater burial conditions” has no meaning. Is this in reference to the vadose soil?

The paragraph has been rewritten for a better understanding.

L146-148: Incoherent. Is subsection 3.1 G.I. generated?

Lines 146 – 48 in subsection 3.1 have been rewritten.

L165-167: Not clear what was implied. How can borehole data describe boundary conditions? What is “bottom plate”? Isn’t the shallow aquifer bounded by the water table from above? The medium and deep aquifers separated by impervious or semipervious (leaky) layers? What about the bedrock and if it is at the very bottom as an impervious boundary?

Paragraph has been rewritten for a better understanding

L170-171: Rewrite to “In groundwater modeling, the aquifers can be divided into homogenous zones of constant parameters.”

Sentence has been rewritten as suggested

L173-175: Rewrite to “…, vertical hydraulic conductivity of the aquifer (K_H, K_V), specific storage coefficient (S_s), and specific yield (S_y).”

Sentence has been rewritten as suggested

L177-179: Rewrite to “The vertical hydraulic conductivity parameter is generally 1/10 the value of the horizontal hydraulic conductivity parameter and then adjusted during model calibration.”

Sentence has been rewritten as suggested

Table 2: What are the bases of the values of infiltration coefficient?

The infiltration coefficient values were determined based on the lithology and data collected in previous studies on the Beijing plain area.

L187-188: delete this sentence.

Sentence has been deleted

L191: Delete “runoff” and replace “evaporation” with “evapotranspiration”. Runoff is water running off the surface due to rainfall excess.

The correction has been made

L192: Rewrite to “…of groundwater recharge and discharge elements.”.

Sentence has been rewritten as suggested

Table 3:
1. use yr as the unit for year instead of a throughout.
2. Replace “Qc -lateral runoff…” with “Q_c– lateral recharge, m³/yr, inflow is positive,…”.
3. Use Q_gto replace Q_c for Irrigation water infiltration.
4. In the last column of Table 3, what is “Check the results of previous information”?
5. How was Evapotranspiration calculated? “Depth of burial > 4 m, not considered” has no meaning.

Changes have been made as suggested, and explanations provided regarding groundwater extraction and evapotranspiration.

Section 3.3.2: Replace identification with calibrations and verification with identification throughout.

Replacement has been made

Figure 5: Why is “Water level diagram comparison” needed?

Figure 5 has been modified; water level diagram is included in the section "real and simulated values comparison"

L239: Couldn’t locate Hebei Province on the map.

Hebei province label has been added on the figure for a better understanding

Figure 7: For what aquifer? Shallow aquifer? Not legible, too difficult to compare (a) and (b).

The type of aquifer has been added (shallow aquifer) and the figure quality enhanced

L248: What is observation hole? Was “well” or “piezometer” implied?

Correction has been made in the text

L249: Not clear what “2-3 typical observation points were selected in each district and county” means. Why 2-3 are typical? Is this in reference number of wells. Makes no sense as written.

The paragraph has been rewritten

Table 5: Should provide a similar table for the calibration.

More data has been provided

Figure 8: Which aquifer? With such small changes in groundwater levels, I doubt the aquifers’ storage coefficients S_sand/or S_y were determined accurately.The aquifer type has been precised. While respecting the expert's opinion, this figure was produced with the aim of comparing the actual data collected at the observation wells with the data calculated during the validation period. The small difference between the actual and calculated time series reflects the accuracy of the coefficients considered in the model.
L261-264: Incoherent. Rewrite to “The scatter of observed and simulated groundwater levels reveal a near linear relationship clustered around the 1:1 line with R²=0.98, indicating good model accuracy.”

Sentence has been rewritten as suggested

L267-268: Rewrite to “From the comparison between the observed and simulated values of groundwater levels and their error statistics (Table 5), it can be seen…”.

Sentence has been rewritten as suggested

L272: Replace “curve” with “timeseries”

Replacement has been made

L279-281: Rewrite to “…balance Table 6, it can be seen that the difference in balance between total recharge and total discharge in the study area is 463 million m³.”.

Sentence has been rewritten as suggested

Table 6 title does not reflect the table entries! Report the units, Inflows better than Supply items and Outflows better than Excretory item. Replace “Equilibrium” with “Balance”.

The correction has been made

10: In which of the three aquifers?

In the shallow aquifer, the precision has been made in the study

L298-299 and 304-305: Any explanation for the declining groundwater levels in the western and northwestern parts?

Although this is the outflow limit of our study area, the observation of these groundwater declines can be explained by the lack of data on supplies and extractions in Hebei Province. This limitation will be studied and addressed in our future studies.

L341: Incoherent sentence.

Sentence has been corrected

L342-343: Didn’t see a table listing the calibrated parameters, their initial values, and final calibrated values.

Tables listing the calibrated parameters have been added

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors made careful revisions to the manuscript based on the expert review comments, which enhanced its scientificity and readability and met the publication requirements of this journal. Therefore, it is recommended to accept for publication.

Author Response

There is no additional comments by reviewer 1

Reviewer 2 Report

Comments and Suggestions for Authors

While the introduction is improved, the revised paper lacks the novelty and new insights required for a technical journal. Key methodology and results-related comments were not addressed. The revision is primarily textual, and missing critical elements undermine the model's suitability for future predictions.

MODFLOW modeling approach: The manuscript should be clear if the true layer approach was implemented to model three-dimensional flow or if the quasi-three-dimensional approach was used to model essentially two-dimensional flow in each of the three major aquifers. In the latter, aquitards separating the aquifers are accounted for by conductance parameter/leakage factor.

Model calibration is too vague and replete with missing critical information:
1. The manuscript is mute on the calibration approach; is it manual or automated?
2. Based on what criterion the calibration ended and was deemed successful?
3. How were the inflow (blue line Fig. 1(c)) and outflow (redline Fig. 1(c)) calculated as flux boundary conditions independently as input to the model? Where the boundary fluxes assumed as constants or time variable? The accuracy of these boundary conditions can be consequential on the hydraulic head distribution within the modeled area.
4. Are the parameter values in Table 2 and Fig. 3 default values to initiate the calibration, or the final calibrated values? Please be specific.
5. Were leakage factor/conductance of the aquitards separating the aquifers and the conductance parameter of the riverbeds treated as calibration coefficients or assigned values by default? Or are these irrelevant because the true-layer approach (brute 3-D modeling) was implemented?
6. S_s values in Table 2 for the two lower confined aquifers are more typical of the storage coefficient (or aquifer storativity) [LL^-1], which is the specific storage coefficient S_s [L^-1] multiplied by the layer thickness. The latter (S_s) typically ranges from 10^-4 to 10^-5 [m^-1] for unconsolidated material. From the values (of order 10^-3), I believe the storage coefficient was implied on L180 and in Table 2, not the specific storage coefficient.

Quality of the calibration cannot be judged:
1. why limit the validation results to only the top unconfined aquifer (Figs 7-9)? What about the two lower confined aquifers? Isn’t groundwater pumped from the deeper aquifers? In accurate computation of the hydraulic heads in the lower confined aquifer could impact the accuracy of computed long-term heads in the shallow aquifer.
2. What about the calibrated values of leakage factors of the aquitards separating the aquifers and the leakance coefficient of the riverbeds? Or, true layer (3-D) approach was implemented?
3. To which of the three aquifers, the wells in Table 6 belong to?

Minor comments:

L167-168: This is an incoherent sentence.
L169-170: This addition is welcomed, but details are missing as to how the conductance parameter values of the low-conductivity layers (aquitards) separating the confined aquifers were determined. Also, indicate if the River Package was used to parametrize the river parameters.
L180: don’t see µ in Table 2. Also, as per major comment 2c above, delete “specific” and refer to S_s as storage coefficient.
3 (a-c) and caption and throughout the document: replace “coeffiecient of permeability” with “hydraulic conductivity”. The two have different meanings, with the latter an intrinsic property of the media and the fluid.
3(b): rewrite to “Shallow aquifer hydraulic conductivity (add the unit).
Table 4: At the very bottom: What is “the diving surface”? Did you mean the water table?
Apology on my previous Additional comments 29: I meant this “Replace identification with calibration and verification with validation” So, 3.3.2 Model Calibration, Validation and Application. Kind of surprised you didn’t challenge my comment!
Figure 5: What do you mean by “Initial conditions comparison”? Comparing what?
L261-263: Incoherent. What does “submerged water aquifer” mean?
Figure 6: The caption should be “Contour map of groundwater level in aquifer (?) at the beginning of simulation period.” What about the initial hydraulic head in the other two aquifers?
L280: As per my original Additional comments 31, I still cannot locate Hebei Province on the map and Fig. 7(a). Your response was that Hebei province label has been added on the figure.
Figure 7(a): Still, too small to read the labels and contour values for comparison with Fig. 7(b). Cannot judge the calibration.
Table 6: Indicate in an added column which of the three aquifers each of the listed wells tap.
L345-347: Incoherent. What does Please rewrite. For example, more concisely “Figure 8 compares observed with simulated groundwater level values in selected observation wells tapping the shallow unconfined aquifer”. Also, should add “well labels marked with * denotes simulated values” in the caption of Fig. 8, not on L347.
Figure 9: Please be specific this scatter applies to which aquifer? What about the other two aquifers?

Author Response

MODFLOW modeling approach: The manuscript should be clear if the true layer approach was implemented to model three-dimensional flow or if the quasi-three-dimensional approach was used to model essentially two-dimensional flow in each of the three major aquifers. In the latter, aquitards separating the aquifers are accounted for by conductance parameter/leakage factor.

A better understanding of the approach is provided by the responses to the reviewer's questions that followed this comment.

Model calibration is too vague and replete with missing critical information:

The manuscript is mute on the calibration approach; is it manual or automated?

In the process of calibration and validation, the model adopts the trial and error correction method which continuously adjusts parameters to achieve the optimal fit between simulated and observed values. The information is missing in the text and has been added.

Based on what criterion the calibration ended and was deemed successful?

As mentioned in section 4.3.1, the calibration ended and was deemed successful for this study when results indicated that for the majority of the observations point the relative error between observed and computed values is under 20%.

How were the inflow (blue line Fig. 1(c)) and outflow (redline Fig. 1(c)) calculated as flux boundary conditions independently as input to the model? Where the boundary fluxes assumed as constants or time variable?

The accuracy of these boundary conditions can be consequential on the hydraulic head distribution within the modeled area.

The mountain front boundary in the west and north were defined as lateral inflow boundary while the administrative boundary in the south and east was simulated with the general head boundary in the model. The boundary fluxes were assumed constants for the model simulation.

Are the parameter values in Table 2 and Fig. 3 default values to initiate the calibration, or the final calibrated values? Please be specific.

It has been specified that parameter values for Table 2 and Figure 3 are default values to initiate the calibration

Were leakage factor/conductance of the aquitards separating the aquifers and the conductance parameter of the riverbeds treated as calibration coefficients or assigned values by default? Or are these irrelevant because the true-layer approach (brute 3-D modeling) was implemented?

Leakage factor/conductance of the aquitards separating the aquifers has not been considered in the model. As mentioned in section 3.2, the conductance parameter of the riverbeds has been estimated and input in the model.

S_svalues in Table 2 for the two lower confined aquifers are more typical of the storage coefficient (or aquifer storativity) [LL^-1], which is the specific storage coefficient S_s[L^-1] multiplied by the layer thickness. The latter (S_s) typically ranges from 10^-4 to 10^-5 [m^-1] for unconsolidated material. From the values (of order 10^-3), I believe the storage coefficient was implied on L180 and in Table 2, not the specific storage coefficient.

During data collection, the specific storage values ranged from 10^-5 to 10^-6. To fit the parameters established for the conceptual model, certain adjustments were made. We agree with the reviewer's comment.

Quality of the calibration cannot be judged:

why limit the validation results to only the top unconfined aquifer (Figs 7-9)? What about the two lower confined aquifers? Isn’t groundwater pumped from the deeper aquifers? In accurate computation of the hydraulic heads in the lower confined aquifer could impact the accuracy of computed long-term heads in the shallow aquifer.

Due to lack of comprehensive long-term continuous observation well data and as mentioned in the conclusion and future perspectives, this study can be further developed and refined by examining the evolution of groundwater in the deeper layers. As you mentioned, this limits the accuracy of computed long-term heads in shallow aquifers. We would like to share these initial findings and get an outside, critical perspective from the scientific community for the implementation of a project that will focus on these confined aquifers.

What about the calibrated values of leakage factors of the aquitards separating the aquifers and the leakance coefficient of the riverbeds? Or, true layer (3-D) approach was implemented?

To which of the three aquifers, the wells in Table 6 belong to?

The attributes of the aquifer have been added in Table 6.

Minor comments:

L167-168: This is an incoherent sentence.

The sentence has been rewritten.

L169-170: This addition is welcomed, but details are missing as to how the conductance parameter values of the low-conductivity layers (aquitards) separating the confined aquifers were determined. Also, indicate if the River Package was used to parametrize the river parameters.

Details have been provided in sections 3.1.2 and 3.2 on the determination and calculation method for hydraulic conductivities in shallow and deep aquifers and conductance, an essential input parameter for estimating river water volume using the Modflow RIVER package.

L180: don’t see µ in Table 2. Also, as per major comment 2c above, delete “specific” and refer to S_sas storage coefficient.

We apologize, this point had not been addressed previously. During data collection, attention was focused on the storage coefficient Ss and µ was not determined. The term “specific” has been removed.

3 (a-c) and caption and throughout the document: replace “coefficient of permeability” with “hydraulic conductivity”. The two have different meanings, with the latter an intrinsic property of the media and the fluid.

Replacement has been made.

Figure 3. Zoning maps. (a) Groundwater rainfall infiltration coefficient. (b) Shallow groundwater hydraulic conductivity. (c) Deep groundwater hydraulic conductivity.

3(b): rewrite to “Shallow aquifer hydraulic conductivity (add the unit).

Figure label has been rewritten

Table 4: At the very bottom: What is “the diving surface”? Did you mean the water table?

Yes, the expression “the diving surface” is used for the water table

Apology on my previous Additional comments 29: I meant this “Replace identification with calibration and verification with validation” So, 3.3.2 Model Calibration, Validation and Application. Kind of surprised you didn’t challenge my comment!

The various expressions have been corrected in the text.

Figure 5: What do you mean by “Initial conditions comparison”? Comparing what?

Figure 5 has been modified. It is not a question of comparing the initial conditions, but rather of inserting and interpolating them into the model.

L261-263: Incoherent. What does “submerged water aquifer” mean?

The expression “submerged water aquifer” has been replaced by “shallow unconfined aquifer”

Figure 6: The caption should be “Contour map of groundwater level in aquifer (?) at the beginning of simulation period.” What about the initial hydraulic head in the other two aquifers?

Aquifer has been precised. As mentioned in the section 4.1.1, due to the lack of comprehensive long-term continuous observation well data on the confined water level, the initial water head of the confined water was not provided.

L280: As per my original Additional comments 31, I still cannot locate Hebei Province on the map and Fig. 7(a). Your response was that Hebei province label has been added on the figure.

Figure 7(a) is taken from the official report of the Beijing Water Affairs Office for the period in question. Given its official nature, it was deemed appropriate to modify it as little as possible. In section 2.1 of this article, the study area was clearly located in relation to Hebei Province, hence the decision to add this label to the figure for better understanding and consistency between the figure and the preceding text.

Figure 7(a): Still, too small to read the labels and contour values for comparison with Fig. 7(b). Cannot judge the calibration.

Figure 7(a) has been enlarged

Table 6: Indicate in an added column which of the three aquifers each of the listed wells tap.

Precision has been made

L345-347: Incoherent. What does Please rewrite. For example, more concisely “Figure 8 compares observed with simulated groundwater level values in selected observation wells tapping the shallow unconfined aquifer”. Also, should add “well labels marked with * denotes simulated values” in the caption of Fig. 8, not on L347.

Sentence has been rewritten and a note added to Figure 8.

Figure 9: Please be specific this scatter applies to which aquifer? What about the other two aquifers?

Aquifer has been precised. As mentioned in the section 4.1.1,there is a lack of comprehensive long-term continuous observation well data on the confined water level, which affects the accuracy of the results for other aquifers. We will present these in our future studies.

All the paper revisions have been marked with a yellow background.

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have adequately addressed most of my comments on the revised version. However, before finalizing the review process, one additional but minor revision is requested addressing specifically some lingering issues:
1- It is highly recommended that a small paragraph or subsection be added under Section 3 describing the availability of the groundwater hydraulic head data spatially, most importantly from which aquifer, and the monitoring period. It makes no sense to talk about model calibration and validation without being specific on the observed spatiotemporal data that were used.
2- Calibration criteria L275-279: The phrase “the model adopts trial and error correction method” is rather an uncommon phrase in the parlance of hydrologic modeling. Was manual calibration (trial and error) implemented by the authors or some automated calibration of GMS-MODFLOW-USG was implemented? For example, PEST calibration package? Please be specific.
3- It is rather awkward not to state the criterion used for model calibration under Subsection 4.1.2 Model Calibration as was done for the validation model run (L352 in Subsection 4.1.3 Model Validation). Please state the criterion used for model calibration in Subsection 4.1.2.
4- Just to be sure, parameter values are kept at their calibrated values during the validation period September 2022 to February 2023. The statement on line 349-350 “…, the numerical fitting of the vast majority of points is good.” In a validation run, calibrated parameters can no longer be tweaked to force a “fit”, for this is no longer model validation.
5- Need to be specific on how the boundary lateral inflow and outflow rate values were selected. Just to sate these where imposed is not sufficient. Also, a general head boundary is a mixed head-flux boundary condition, whereby the flux at that boundary is unknown. The hydraulic head at that boundary rather is tweaked during a calibration. Yet the statement on L171 implies that flux assumed a constant value at the outlet boundary, which contradicts the idea of using the general head boundary! Please clarify this confusion.
6- A statement should be provided as to how the values of m and k of riverbed were selected. That is, based on what?

Minor Edits:
1- Caption of Fig. 3: Please rewrite to “… (b) Shallow aquifer hydraulic conductivity. (c) Deep aquifer hydraulic conductivity. Groundwater has no hydraulic conductivity!
2- L249: Please rewrite to “Model Calibration, Validation and Application.
3- L251: replace “verification” with “validation”.
4- Caption of Figure 5: Model Calibration.
5- The last box in Figure 5 should read “Calibrated and validated model”.
6- Suggested title for Section 4.1: Groundwater Flow Model for the Study Area
7- L261-262: Rewrite to “…data on groundwater levels in the confined aquifers, the initial hydraulic heads of the confined aquifers were not available. The simulation period initial groundwater level map of the shallow unconfined aquifer is shown in Figure 6.”
8- Caption of Figure 6: Rewrite to “Groundwater level contour map of the shallow aquifer at the beginning of the simulation period.”
9- L294: Rewrite to “After completing the model calibration, the model was validated using the observed data for the period September 2022 to February 2023.”
10- Suggest deleting the sentence on L295-296: “The values of each …for validation”.
11- L296: Rewrite to “Of the 122 long-term groundwater level observation wells, 34 typical observation wells were selected…”.
12- Title of Table 6: Suggest this title: “Validation results for typical observation wells in the study area.”
13- Table 6, third column: should be Validation value (m), second column title Observed value (m).
14- Referring to comment 13, my concern is that the calibrated model was recalibrated on the validation period. If this was the case, this is no longer a validation!
15- Figure 3: Should add labels (a), (b), and (c) inside or on top of the figures. The legend box for (b) and (c) are hardly legible. Please enlarge.

Author Response

1- It is highly recommended that a small paragraph or subsection be added under Section 3 describing the availability of the groundwater hydraulic head data spatially, most importantly from which aquifer, and the monitoring period. It makes no sense to talk about model calibration and validation without being specific on the observed spatiotemporal data that were used.

A small paragraph has been added to section 3.3.2 describing the availability of the groundwater hydraulic head data spatially, the aquifer they belong to and the monitoring period. A figure of the spatial distribution of typical observations wells in the study area has been added.

2- Calibration criteria L275-279: The phrase “the model adopts trial and error correction method” is rather an uncommon phrase in the parlance of hydrologic modeling. Was manual calibration (trial and error) implemented by the authors or some automated calibration of GMS-MODFLOW-USG was implemented? For example, PEST calibration package? Please be specific.

The sentence has been rewritten for a better understanding. In this study, authors have not MODFLOW-USG. It was a manual Trial-and-Error Calibration by changing the hydraulic conductivity and recharge values assigned to the polygonal zones.

“A Trial-and-Error Calibration is applied in this study by changing the hydraulic conductivity and recharge values assigned to the polygonal zones and achieve the optimal fit between simulated and observed values.”

3- It is rather awkward not to state the criterion used for model calibration under Subsection 4.1.2 Model Calibration as was done for the validation model run (L352 in Subsection 4.1.3 Model Validation). Please state the criterion used for model calibration in Subsection 4.1.2.

The criterion used for model calibration in Subsection 4.1.2 has been precised.

“Observation Head interval which represents the estimated error (±) in the observed value is set to 1.5. This confidence value represents the confidence in the error estimate is set to 95%. The interval can be used as a calibration target. Calibration is achieved when the error is within the estimated error interval (± 1.5 m, in this case) of the observed value.”

4- Just to be sure, parameter values are kept at their calibrated values during the validation period September 2022 to February 2023. The statement on line 349-350 “…, the numerical fitting of the vast majority of points is good.” In a validation run, calibrated parameters can no longer be tweaked to force a “fit”, for this is no longer model validation.

This section has been rewritten

“From the comparison between the observed and simulated values of groundwater levels and their error statistics (Table 6), some observation points are greatly affected by lateral inflow resulting in relatively large values and local errors.”

5- Need to be specific on how the boundary lateral inflow and outflow rate values were selected. Just to sate these where imposed is not sufficient. Also, a general head boundary is a mixed head-flux boundary condition, whereby the flux at that boundary is unknown. The hydraulic head at that boundary rather is tweaked during a calibration. Yet the statement on L171 implies that flux assumed a constant value at the outlet boundary, which contradicts the idea of using the general head boundary! Please clarify this confusion.

The sentence has been deleted for more clarity. The input values in the model were determined based on data from the Beijing Water Resources Bulletin for the study period.

6- A statement should be provided as to how the values of m and k of riverbed were selected. That is, based on what?

The values of m and k for the riverbed were selected taking into account data from previous studies and articles. The drilling logs from piezometers installed along the rivers to monitor their evolution were also considered.

Minor Edits:
1- Caption of Fig. 3: Please rewrite to “… (b) Shallow aquifer hydraulic conductivity. (c) Deep aquifer hydraulic conductivity. Groundwater has no hydraulic conductivity!

The captions have been rewritten as suggested “(b) Shallow aquifer hydraulic conductivity. (c) Deep aquifer hydraulic conductivity.”

2- L249: Please rewrite to “Model Calibration, Validation and Application.

Title has been rewritten as suggested

3- L251: replace “verification” with “validation”.

Replacement has been made

4- Caption of Figure 5: Model Calibration.

Caption has been changed

5- The last box in Figure 5 should read “Calibrated and validated model”.

Text in the last box has been rewritten according reviewer suggestions

6- Suggested title for Section 4.1: Groundwater Flow Model for the Study Area

Section title has been changed

7- L261-262: Rewrite to “…data on groundwater levels in the confined aquifers, the initial hydraulic heads of the confined aquifers were not available. The simulation period initial groundwater level map of the shallow unconfined aquifer is shown in Figure 6.”

Section has been rewritten as suggested

8- Caption of Figure 6: Rewrite to “Groundwater level contour map of the shallow aquifer at the beginning of the simulation period.”

Caption has been rewritten as suggested

9- L294: Rewrite to “After completing the model calibration, the model was validated using the observed data for the period September 2022 to February 2023.”

Sentence has been rewritten

10- Suggest deleting the sentence on L295-296: “The values of each …for validation”.

Sentence has been deleted

11- L296: Rewrite to “Of the 122 long-term groundwater level observation wells, 34 typical observation wells were selected…”.

Sentence has been rewritten

12- Title of Table 6: Suggest this title: “Validation results for typical observation wells in the study area.”

Suggestion has been taken in consideration in the manuscript.

13- Table 6, third column: should be Validation value (m), second column title Observed value (m).

Changes have been made in the table.

14- Referring to comment 13, my concern is that the calibrated model was recalibrated on the validation period. If this was the case, this is no longer a validation!

This section has been rewritten as it may cause confusion. The calibrated model was not recalibrated during the validation period.

15- Figure 3: Should add labels (a), (b), and (c) inside or on top of the figures. The legend box for (b) and (c) are hardly legible. Please enlarge.

Labels (a), (b), (c) have been added and the legend boxes enlarged