the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Jan 2026
The WoKaS-Iso Database: Workflow for a Global Compilation of Oxygen-18 and Deuterium Records in Karst Springs and Cave Drip Water for Enhanced Understanding of Karst Systems
Abstract. For analysing karst hydrogeological systems, observations of karst springs and cave drips are considered indispensable. In addition to hydrometric observations, knowing the oxygen and hydrogen stable isotope ratios has improved the understanding of vadose zone and aquifer dynamics, likewise supporting system characterisation and modelling. However, limited accessibility and high costs of the analysis of stable isotopes in karst aquifers have hindered progress in karst research and impeded the accurate understanding of karst processes especially when it comes to comparative or large-scale studies. In this study, we present our workflow to compile the WoKaS-Iso database, the first extensive collection of time series data for Oxygen-18 and Deuterium isotopes in karst springs and cave drip water from diverse sources, encompassing publications, theses, reports, online archives, and collaborative initiatives worldwide. The database incorporates data sourced from 236 springs and 74 caves, comprising in total 997 time series (379 time series for the springs and 618 time series for the cave drip water). These datasets provide coverage across significant karst regions globally, spanning China, the USA, Europe, the Middle East, and Australia. Within datasets, 79% for springs and 68% for cave drip water exhibit resolutions finer than monthly intervals. In addition, by integrating isotopic records with ancillary environmental variables including spring discharge, cave drip rate, precipitation, and rainwater isotopes, the database offers a more comprehensive perspective on hydrological behaviours in karst aquifers, hence advancing hydrogeological characterisation and modelling. The WoKaS-Iso database not only deepens the understanding of the complex systems but also promotes sustainable water resource management as well as the potential to foster collaborative research. The database can be accessed at: https://doi.org/10.25532/OPARA-909.
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Status: final response (author comments only)
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RC1: 'Comment on essd-2025-812', Anonymous Referee #1, 10 Feb 2026
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AC1: 'Reply on RC1', Yining Zang, 02 Apr 2026
Dear Reviewer,
We sincerely thank you for your careful reading of our manuscript and for your positive and encouraging evaluation. We greatly appreciate your recognition of the value of the WoKaS-Iso database and your valuable and constructive suggestions for improving the manuscript.
Please find below our responses to your specific comments.
Comment 1:
Line 301: It could be nice to have one figure with some example of time series, maybe showing some different behavior across North America, Europe and Asia (since these are regions with the more data).Response:
Thank you very much for this helpful suggestion. We agree that including example time series would make the manuscript more informative and would better illustrate the diversity of isotope dynamics represented in the database. In the revised manuscript, we will therefore add a figure showing representative time series from North America, Europe, and Asia. Specifically, we will include three cave drip-water isotope time series and three karst spring isotope time series with overlapping observation periods, in order to facilitate direct visual comparison across continents.Comment 2:
By looking at some time series, I have the impression that Input-FR-S-0016@Lison and Input-FR-S-0017@Loue are the same data. Maybe need to cross checked.Response:
Thank you for this careful observation. We rechecked the dataset and contacted the data contributor to confirm whether there was any mistake.We can confirm that Lison spring and Loue spring share the same precipitation isotope data in our database, whereas their rainfall amount data are different. Our colleague clarified that the isotopic rainfall data for the two springs originate from the same sampling site. The collector for rainfall isotopes was located between two rain gauge stations, one on the Loue spring catchment and the other on the Lison spring catchment. Therefore, the isotope data are identical for both springs.
In contrast, the rainfall amount data are derived from the respective rain gauge stations within each catchment, rather than from the isotope collector itself. To avoid confusion for future users, we will revise the metadata table to describe this setup more explicitly and include the coordinates of the rain gauge stations for reference.
We hope that these revisions address your comments and improve the clarity of the manuscript.
Kind regards,
Yining Zang and Andreas Hartmann on behalf of all authors
Citation: https://doi.org/10.5194/essd-2025-812-AC1
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AC1: 'Reply on RC1', Yining Zang, 02 Apr 2026
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RC2: 'Comment on essd-2025-812', Anonymous Referee #2, 15 May 2026
This paper introduces the world's first systematically integrated database (WoKaS-Iso) containing time-series data on oxygen-18 and deuterium-1 isotopes from karst springs and cave dripwater, providing detailed descriptions of data sources, standardization procedures, quality control measures, global product evaluations, and data structure. The establishment of this database holds significant value for karst hydrogeology, paleoclimate reconstruction in caves, and hydrological model calibration, particularly for large-scale comparative studies and data-scarce regions. The comprehensive workload and robust data integration align with the data paper positioning of the ESSD journal. However, the paper still exhibits notable shortcomings in several aspects that require reconsideration following substantial revisions.
- The text repeatedly mentions that the database is built upon WoKaS and SISAL_mon_v1, but fails to systematically elaborate on its incremental contributions compared to these two foundational databases (e.g., the number of newly added sites, time series length, data types, etc.). It is recommended to include a "Comparison with Previous Work" section in the introduction or methods section to clearly outline the incremental features.
- The evaluation of the three precipitation isotope modelsIsoGSM, Isoscape, and Sine Curve (Section 2.2.2) was based solely on data from 55 stations, encompassing a mix of cave, spring, and weather station sites, which may introduce bias in the results. The criteria for spatiotemporal matching should be specified more rigorously (e.g., whether matching is performed monthly or whether precipitation amounts are weighted); the assessment findings for δ²H should also be presented in the main text rather than merely mentioned in the appendix.
- In the quality scoring system (Table 6), "Data Collection" receives 3 points, "Digital Data" receives 2 points, and "Global Products" receives 1 point. This scoring criterion does not account for variations in the accuracy of digital data or differences in the reliability of global products across specific regions. It is recommended to introduce more detailed quality metrics (such as data missing rate, temporal continuity, and deviations from adjacent sites) or at least conduct a sensitivity analysis of this scoring system.
- Figure 3 simultaneously displays data combination types (colors) and sampling resolution (symbol sizes), making them visually difficult to distinguish. It is recommended to split the data or adopt a more intuitive legend design.
- Section 2.1.3 contains significant overlap with the content of Figures 4 and 5; it is recommended to consolidate and streamline the content.
- Although detailed CSV structures and MATLAB scripts are provided, typical use cases (e.g., how to extract time series from a specific site or perform simple analyses using global product data) are lacking. It is recommended to include a concise example or flowchart in Section 3.4, "Usage Notes."
- L80: "The database incorporates data sourced from 236 springs and 74 caves, totaling 997 time series..." It does not specify how many of these time series contain both δ¹⁸O and δ²H, or how many contain only one of them.
- The statement L91 ("serving 10%–25% of the global population (Ford and Williams, 2007; Stevanovic, 2019)" cites literature from nearly two decades ago, making its data somewhat outdated; more recent global assessments could be incorporated.
- L145 “they largely omit δ¹⁸O and δ²H data from karst springs and cave drip waters”GNIP/GNIR. The design itself was not intended for karst water; the criticism seems somewhat excessive.
- Table 1 contains only figure captions; the main text does not explain the steps illustrated in Figure 1.
- In Figure 3, colors represent data combinations (flow rate + δ¹⁸O + δ²H, etc.), while symbol sizes indicate resolution; however, the legend does not explain the meaning of these sizes.
- “only GNIP stations within 25 km were retained”The reason for choosing 25 km was not explained, nor were altitude differences considered.
- The stated IsoGSM spatial resolution of "200 km" in Table 2 is inaccurate. IsoGSM typically has a T42 spectral resolution (approximately 2.8 × 2.8, or about 300 km); please verify this.
- Section 3.3 mentions that extraction scripts for three products are provided, but IsoGSM and Isoscape are not included. Explain the reason or provide a simple NetCDF reading example.
Citation: https://doi.org/10.5194/essd-2025-812-RC2
Data sets
The WoKaS-Iso Database: Workflow for a Global Compilation of Oxygen-18 and Deuterium Records in Karst Springs and Cave Drip Water for Enhanced Understanding of Karst Systems Yining Zang https://doi.org/10.25532/OPARA-909
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Overall opinions
The manuscript aims to showcase the WoKaS-Iso database. It consists of very useful isotope measurement dataset for various applications in karst hydro(geo)logy and make the data easily accessible to the whole scientific community. The isotope measurements come with precipitation MSWEP, temperature ERA5 and evapotranspiration GLEAM, giving a ready to use dataset with independent estimations of P, T and ET.
The manuscript is well written and pleasant to read. The figures are well designed and clear. The data are well described and the author give clear recommendation on the use of the data by affecting various flags depending on the data quality. This is really useful for the reader and future user of the dataset.
I would recommend this manuscript for publication in ESSD after addressing some minor concerns given as specific comments below. This initiative is of clear interest for the scientific community and will benefit from additional contributions in the future.
Specific comments
Line 301: It could be nice to have one figure with some example of time series, maybe showing some different behavior across north America, Europe and Asia (since these are regions with the more data).
By looking at some time series, I have the impression that Input-FR-S-0016@Lison and Input-FR-S-0017@Loue are the same data. Maybe need to cross checked