Quantifying the City-Scale Impacts of Impervious Surfaces on Groundwater Recharge Potential: An Urban Application of WRF–Hydro

Round 1

Reviewer 1 Report

This manuscript uses the WRF-Hydro to establish the quantification of the city-scale impacts of impervious surfaces on groundwater recharge potential. The manuscript provides an interesting topic and has interesting results. The mathematical model theory of the manuscript is quite complete. The analysis results of the case study presented are valuable. The research contents, achievements and discussions of the manuscript are also complete.

However, the content of the manuscript lacks conclusions and suggestions. Please put forward conclusions of this study and suggestions for further promotion, application and research in the future.

 

 

 

Author Response

Dear Reviewer,

I would like to thank you on behalf on my co-authors for taking the time to read and review our work.

We are grateful for your comments, which we have used to improve our manuscript.

We have reworked and reorganized the Discussion section and added a Conclusion section to address your comments. Please see the following for additional details:

“However, the content of the manuscript lacks conclusions and suggestions. Please put forward conclusions of this study and suggestions for further promotion, application and research in the future.”

A separate Conclusion section was added to expand on concluding remarks that were previously included in the Discussion section, developing on suggestions for further promotion and how to build on this study’s proposed approach and findings:

“  5. Conclusion

 

The proposed approach shows how an integrated modeling system like WRF–Hydro can be adapted to investigate land surface processes in an urban context and at city-scale while retaining the necessary level of detail to depict the physics of surface and subsurface flow. The approach was used to quantify the potential for groundwater recharge in LA by comparing pre- and post-urban development scenarios, showing the spatially distributed impact of urban impervious surfaces on the city’s water budget. While there are many ways in which urbanization alters the water cycle in cities other than the spread of impervious surfaces, and further research should look to include critical features such as underground sewer systems, our findings help assess the multi-scale land surface processes that can limit the potential recharge of groundwater resources in water-scarce regions.

Like LA, many cities around the world are increasingly faced with the threats of flooding and droughts. The introduction of urban imperviousness was built on WRF–Hydro’s physics-based modeling capability allowing for further applications in other cities with limited need for parameterization. When modified to fit urban environments, WRF-Hydro’s multi-scale capabilities could further assist urban adaptation efforts to identify areas to prioritize for measures such as green infrastructure. Moreover, WRF–Hydro’s modular structure offers further opportunities for hydrometeorological research with the two-way coupling of the WRF atmospheric model [34]. Previous research has for example found a statistical link between urbanization and more pronounced rainfall events in the US [47], which could be physically tested using the modified WRF–Hydro system coupled to WRF. The presented approach can provide a valuable tool to inform on urban land use management, the development of adaptive urban solutions, helping to increase the resilience of cities.”

We look forward to hearing back from you.

Kind regards,

Ulysse Pasquier

Author Response File: Author Response.docx

Reviewer 2 Report

This study has assessed the impact of urbanization on the ground water recharge in the Los Angeles city using modified WRF–Hydro, a physics-based hydrological modeling system. The study has has addressed the impact of  urban imperviousness on the urban water budget by comparing the pre-and post-urban development hydrological response to an intense precipitation event.

General Comments: Thank you very much for providing me with this opportunity to review this manuscript. Overall, I would like to congratulate the authors for this rigorous work. In the manuscript, all the statements, requiring citation, have been well referenced. The research rationale/hypothesis has been clearly communicated. The study objectives have been defined clearly. The introduction gives sufficient background information about the study. The study approach is consistent with the research objective. The methodology is appropriate and rigorous. The results appear complete and accurate and the findings are well‐supported by the data. The findings have been appropriately discussed in the context of previous literature.

However, there are some suggestions that the authors must incorporate in the revised version.

1. The study limitations have to be clearly mentioned.

2. The conclusions at present are a bit exaggerated and must be tailored to information about the limitations and the future scope of this work.

3. The discussion needs to be organized as main findings, comparison with previous studies, key strengths, key limitations, and a concluding paragraph of the overall work.

 

4. The conclusion must discuss the wider implications of the study findings and future research avenues.

5. Moreover, I would suggest using a model for comparison with the results of WRF-Hydro.

Best of luck, and keep up the good work.

Author Response

Dear Reviewer,

I would like to thank you on behalf on my co-authors for taking the time to read and review our work.

We are grateful for your comments, which we have used to improve our manuscript.

We have reworked and reorganized the Discussion section and added a Conclusion section to address your comments. Please see the following for additional details:

1. The study limitations have to be clearly mentioned.

The paragraphs starting on lines 457, 464 and 473 were reorganized and rewritten to put more emphasis on the main key limitations of this study discussed in the context of previous research in the field of urban hydrology.

 

“          While this method can help assess the impact of urbanization on the water cycle, other studies in urban hydrology have argued that identifying the fraction of TIA that is directly physically connected to storm networks or streams can improve the representation of the hydrological response of urban watersheds [40]. Uncertainties of scale moreover remain and while a 900 m LSM resolution can be considered fine resolution compared to most WRF-Hydro applications, lowering this value can improve the accuracy of simulations of river discharge and the water budget [23].

One of this study’s key limitations is the lack of representation of underground piped drainage systems. These systems not only play a central role the urban water cycle, studies have shown that their management can influence groundwater recharge. Previous research has for example found that leakages in the sewerage or water distribution systems can compensate for the increase in runoff and even lead to an increase in groundwater recharge [41,42]. While the addition of such infrastructure represents an important future improvement to the modified WRF-Hydro model, this study was interested in depicting the land surface processes that control the potential for increased groundwater recharge in a water-scarce city like LA that currently manages a water resource deficit.

Other studies have also underlined the importance of urban irrigation and its contribution to the urban water budget and microclimate in cities like LA [43–45]. In this precipitation event-based approach, urban irrigation was not considered but would have to be included in a longer-term simulation of LA’s hydrology. A longer-term simulation would moreover allow for the assessment of actual groundwater recharge. In the current approach potential groundwater recharge is determined based on infiltration and deep drainage below the LSM soil column. Actual recharge to aquifers could be determined with the use of dedicated three-dimensional groundwater flow models such as ParFlow [46]. Still, the focus on land surface processes at metropolitan scale can help water scarce cities maximize infiltration and fully harness water resources during rare rainfall events.”

 

2. The conclusions at present are a bit exaggerated and must be tailored to information about the limitations and the future scope of this work.

 

A separate Conclusion section was added to expand on concluding remarks that were previously included in the Discussion section. The concluding remarks were reframed within the context of the study’s key limitations and particularly the lack of representation of underground sewer systems.

 

“ 5. Conclusion

 

The proposed approach shows how an integrated modeling system like WRF–Hydro can be adapted to investigate land surface processes in an urban context and at city-scale while retaining the necessary level of detail to depict the physics of surface and subsurface flow. The approach was used to quantify the potential for groundwater recharge in LA by comparing pre- and post-urban development scenarios, showing the spatially distributed impact of urban impervious surfaces on the city’s water budget. While there are many ways in which urbanization alters the water cycle in cities other than the spread of impervious surfaces, and further research should look to include critical features such as underground sewer systems, our findings help assess the multi-scale land surface processes that can limit the potential recharge of groundwater resources in water-scarce regions.

Like LA, many cities around the world are increasingly faced with the threats of flooding and droughts. The introduction of urban imperviousness was built on WRF–Hydro’s physics-based modeling capability allowing for further applications in other cities with limited need for parameterization. When modified to fit urban environments, WRF-Hydro’s multi-scale capabilities could further assist urban adaptation efforts to identify areas to prioritize for measures such as green infrastructure. Moreover, WRF–Hydro’s modular structure offers further opportunities for hydrometeorological research with the two-way coupling of the WRF atmospheric model [34]. Previous research has for example found a statistical link between urbanization and more pronounced rainfall events in the US [47], which could be physically tested using the modified WRF–Hydro system coupled to WRF. The presented approach can provide a valuable tool to inform on urban land use management, the development of adaptive urban solutions, helping to increase the resilience of cities.”

 

3. The discussion needs to be organized as main findings, comparison with previous studies, key strengths, key limitations, and a concluding paragraph of the overall work.

 

 

The discussion section was entirely reorganized to match the following subsections:

• Main findings (line 421)
• Comparison with previous studies (line 440)
• Key strengths (line 448)
• Key limitations (lines 457, 464, 474)

 

Concluding paragraphs of the overall work were moved to a separate Conclusion section and further developed.

 

4. The conclusion must discuss the wider implications of the study findings and future research avenues.

 

As shown in response to comment 2, the conclusion was added in a separate section and expanded upon. We discuss the transferability of the proposed approach to other cities as well as its use in adaptation-focused studies. Future search avenues are identified such as moving from a standalone version of WRF-Hydro to its online configure, coupled with the WRF atmospheric model.

 

5. Moreover, I would suggest using a model for comparison with the results of WRF-Hydro.

 

Few other studies have used WRF-Hydro in an urban context. We discuss some of our results in comparison with other modeling efforts, including MIKE-SHE and a precipitation-runoff model, the latter also being applied to Los Angeles. A benchmarking of WRF-Hydro compared to a set of other similar models would be valuable but fell outside the scope of the current study.

 

“           The findings are consistent with existing research showing the hydrological impacts of urbanization and land use change, where peak streamflow and surface runoff increase while infiltration decreases due to the development of urban impervious surfaces [36,37]. For example, using the MIKE-SHE model for an urban watershed, [38] associated a 10 % increase in urbanization with a 24.8 % increase in overland flow. Using a daily precipitation-runoff model in the Los Angeles Basin, [39] found comparable values for LA’s current water budget, with a minimum evapotranspiration rate of 0 mm.yr^-1 over impervious areas, 26 % of water inflow derived as surface runoff and only 8 % contributing to groundwater recharge. “

We look forward to hearing from you.

Kind regards,

Ulysse Pasquier

Author Response File: Author Response.docx