the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Aug 2024
Synchronization frequency analysis and stochastic simulation of multisite flood flows based on the complicated vine-copula structure
Abstract. Accurately modeling and predicting flood flows across multiple sites within a watershed presents significant challenges due to potential issues of insufficient accuracy and excessive computational demands in existing methodologies. In response to these challenges, this study introduces a novel approach centered around the use of vine copula models, termed RDV-Copula (Reduced-dimension vine copula construction approach). The core of this methodology lies in its ability to integrate and extract complex data information before constructing the copula function, thus preserving the intricate spatial-temporal connections among multiple sites while substantially reducing the vine copula's complexity. This study performs a synchronization frequency analysis using the devised copula models, offering valuable insights into flood encounter probabilities. Additionally, the innovative approach undergoes validation by comparison with three benchmark models, which vary in dimensions and nature of variable interactions. Furthermore, the study conducts stochastic simulations, exploring both unconditional and conditional scenarios across different vine copula models. Applied in the Shifeng Creek watershed, China, the findings reveal that vine copula models are superior in capturing complex variable relationships, demonstrating significant spatial interconnectivity crucial for flood risk prediction in heavy rainfall events. Interestingly, the study observes that expanding the model's dimensions does not inherently enhance simulation precision. The RDV-Copula method not only captures comprehensive information effectively but also simplifies the vine copula model by reducing its dimensionality and complexity. This study contributes to the field of hydrology by offering a refined method for analyzing and simulating multisite flood flows.
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Status: open (until 07 Oct 2024)
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RC1: 'Comment on egusphere-2024-2266', Anonymous Referee #1, 30 Aug 2024
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This study introduces a novel RDV-Copula (Reduced-dimension vine copula) approach to improve the modeling and prediction of flood flows across multiple sites within a watershed. The method integrates complex spatial-temporal data while reducing the computational complexity of vine copula models. By identifying key variables and constructing RDV-Copula functions, the approach effectively captures spatial-temporal relationships between sites, ensuring accurate flood risk assessment. Applied to the Shifeng Creek watershed, the method revealed strong spatial connectivity, highlighting the increased risk of downstream flooding during heavy rainfall events. Validation against benchmark models showed that increasing model dimensions does not always enhance simulation accuracy, and in some cases, can complicate the model. The RDV-Copula method strikes an optimal balance between information accuracy and simplicity. This approach proves particularly useful for flood risk analysis and management, providing a refined methodology for multisite runoff simulations, and supporting decision-making for flood control and event scheduling.
Overall, the methods seem more likely to be reliable and the originality of the research is undoubted. The analyses in this study are well organized and the results are reasonable. In addition, the presentation of this article is generally clear. It is a valuable study and within the scope of this journal. Therefore, I recommend minor revisions prior to final acceptance.
General comments for the authors’ reference:
- Subsection 2.3.1: In this subsection, the description of the text and the presentation of Figure 3 focus on illustrating the process of how to choose the key variables. But after selecting the key variables, how to construct the RDV-Copula model needs further elaboration. Please supplement this section and modify the picture if necessary.
- Subsection 3.2.2.2: Why are these three different sets of structures chosen as benchmark models? What is the significance of the comparison of each set of benchmarks? Please explain how is it possible to validate the effectiveness of the proposed RDV-Copula method by comparing it with the three sets of benchmarks?
- Line 418-421: What does the symbol of the “*” in Figure 7(a) indicate? There is no explanation in the text or in the picture.
- Line 198-201: The methodology provides a brief introduction to the differences and characteristics of C-vine and D-vine copula. However, there is still a possibility that it may confuse readers who do not have the knowledge of this area. I recommend that some schematic diagrams may be added to the introduction to assist comprehension.
- In the process of constructing the joint distribution function, why is only the relationship between yesterday's runoff and today's runoff considered when identifying the relationship in the time dimension? Why not consider the effect of the runoff from two days ago on today's runoff?
- Line 378-379: “This chosen structure is then further compared with other copula functions to validate its efficacy.” Based on my understanding, the phrase “This chosen structure” refers to the structure selected after comparing the RDV-Cvine and RDV-Dvine. However, the sentence is somewhat ambiguous, so I am uncertain if my interpretation is correct. Could you please clarify and rewrite the sentence?
- I think there is a repetition of the information presented in the table and the picture in Supplement D. Please select one of them (table or picture) for the information presentation.
- Line 98-100: “This complexity can complicate the copula's structure determination, inflate computational demands during parameter fitting, and potentially diminish the accuracy of stochastic simulations.” In this sentence, the phrase “copula's structure determination” should be revised to “copula structure's determination”.
- Line 365-367: “The subsequent step involves identifying the site with the most significant correlation to its preceding day's inflow, which is then used as a as a variable to represent the temporal relationship on that day.” There is an error in this sentence. “as a” is repeated, please delete the redundant one.
- Line 442-443: “The obvious dark colored blocks in the graph indicate the high probabilities of being the high-water or the low-water concurrently.” This sentence seems a bit confused. Please rewrite it to avoid ambiguity.
- Line 445-447: “While the LSM site's synchronization probabilities with the other sites are comparatively lower, they still exceed 50%, recorded at 58.29% with the LX site, 61.25% with the QS site, and 57.15% with the SD site.” The sentence is not clear enough, please revise it and replace the word “recorded”.
- Line 653-655: “Depending on the number of hydrometric stations, Wang and Shen (2023b) established the 7-dimensional regular vine (R-vine) copula models to depict the complex and diverse dependence.” Please delete the “the” before “7-dimensional regular vine”. Please replace the “dependence” by “dependencies”.
- Line 700: “The conditional simulation is a double-edged sword.” Please remove the “the” before the conditional simulation.
Citation: https://doi.org/10.5194/egusphere-2024-2266-RC1 -
RC2: 'Comment on egusphere-2024-2266', Anonymous Referee #2, 16 Sep 2024
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The manuscript is concerned with synchronization frequency analysis and stochastic simulation of multisite flood flows based on the complicated vine-copula structure, which is interesting. It is relevant and within the scope of the journal.
However, the manuscript, in its present form, needs some further improvements. Once the adequate revisions to the following points are implemented, the paper may be acceptable for publication. There are some specific comments that might help the authors further enhance the manuscript's quality.
* Introduction:
It is generally well-written, with good structure and clarity, but there are a few areas where improvements can be made for better precision and readability.
* Line 31-33: “As is reported by Centre for Research on the Epidemiology of Disasters (CRED)”. The use of "is" here is unnecessary and makes the sentence sound awkward. Remove the word “is” from the sentence.
* Line 37: “Large floods often result from the amalgamation of floods from multiple sub-watersheds” . Please replace "amalgamation" with "merging" for more concise language.
* Line 64-65: “Copula function is widely applied in hydrological fields...” . "Copula function" should be plural here.
* Method:
* Many equations are presented in the paper, and most look OK. However, please check carefully whether all equations are necessary and whether the quantities involved are properly explained. Line 232-237: For equations (6), (7), and (8), the variables are not clearly explained. Please elaborate on the meaning of each variable, such as u_ph^1,u_ph^2,u_ph^3,u_ph^4 and u_pl^1,u_pl^2,u_pl^3,u_pl^4 . This ensures the reader understands the variables used in the equations.
* Figure2. The colors used to represent elements such as reservoirs and cross sections are a bit confusing. Use the same color for the same elements throughout the figure. For instance, if reservoirs are represented by a specific color, maintain that color consistently. Please revise it.
* Figure4. How is it possible to distinguish between conditional simulation and unconditional simulation through Figure 4? There is no clear explanation or visual distinction between conditional and unconditional simulation in the figure. Please provide a clear legend or add explanations for (a) and (b) in Figure 4, highlighting the difference between conditional and unconditional simulations.
* Case study:
* Line 316-317: "This study focuses on four major sites within the Shifeng Creek catchment" The reason for selecting these sites is unclear. Clarify that these sites are chosen due to their strategic importance. Can you explain briefly why these particular sites were selected for the study?
* Line 344-346: The combinations of [X-H, Y-H, Z-H, W-H], [X-M, Y-M, Z-M, W-M], and [X-L, Y-L, Z-L, W-L] mean what synchronization respectively? The abbreviations are used without prior explanation. Before utilizing the abbreviations please correspond the abbreviated letters to the original meaning. If not, it will be harder for readers to understand.
* Line 320-321: “To achieve this, daily runoff data of August, covering a span from 2000 to 2020, have been compiled.” This sentence would be more properly expressed in the past tense. Change "have been compiled" to "were compiled" for better tense consistency.
* Results
* This section is well written. The only area of weakness is the display of pictures. The font size of figure 10 is a little small. It could be considered to keep only a lower number of sub-figures and the rest could be placed in supplementary part of the article.* Discussion & Conclusion
* Line 666-668: “Although the eight-dimensional vine copula model takes more variables into account, including both temporal and spatial correlation, the model is too complicated due to many variables, which makes the simulation less efficient on the contrary.” This sentence is too long and complex. Pease simplify it.
* Minor comments
The grammar in the article is generally correct. However, there are some words that are not used appropriately. A few examples are given below:
* Line 254-255: “This strategy aims to distill essential spatial-temporal information, thereby reducing the vine copula function's dimensionality to simplify the model structure.” Here, the meaning of the word “distill” is not suitable for this application. It would be more appropriate to replace it with “extract”.* Line 271-273: “The core distinction between these two simulation methods hinges on whether certain data points are pre-determined at the time of simulation.” There is something wrong with the logic of this statement.
* Line 347: “The calculation equations can be referenced in Appendix B.” I think “provided” may be better than “referenced”.
* The supplemental content section is a bit too redundant. Think about keeping the important parts.
Citation: https://doi.org/10.5194/egusphere-2024-2266-RC2
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