the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Jan 2025
Technical note: Towards a paradigm change in observing soil water content via cosmic-ray neutron sensing
Abstract. Observing soil moisture via cosmic-ray neutron sensing has seen a rapid methodological development and expansion during the last decade. However, to foster its application some change in perspective may be useful. We reformulate the most common calibration equation used when working with CRNS data, the Desilets equation, and provide a simple and insightful form. This leads us also to a new option for calibration of CRNS time series without any local sampling of soil moisture, and also without knowledge on CRNS detector sensitivity. At the same time it delivers the basis for a quantitative expression on how heterogeneities in the footprint contribute to the CRNS-derived soil moisture as well as why statistical errors in practice may be larger than assumed usually. Finally, we suggest to also define the area (and volume) represented by the CNRS observation in a more pragmatic way and complement it by indicating needs for better standardization.
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RC1: 'Comment on egusphere-2024-4141', Anonymous Referee #1, 04 Mar 2025
This interesting manuscript describes a revised formulation and physical interpretation of the Desilets calibration equation, which has been widely used to estimate soil water content based on cosmic-ray neutron sensing. The results show that the equation can be considered as fraction where the numerator represents the difference between the maximum neutron count rate at a site and the current neutron count rate, and the denominator represents the difference between the current neutron count rate and the minimum neutron count rate for the site. This is a small but meaningful and insightful extension of the revised Desilets equation provided by Kohli et al. (2021). This seems like the core contribution of the manuscript.
The manuscript also shows that there should be a fixed ratio between maximum and minimum neutron count rates for a site; thus it is only necessary to determine one of these parameters via calibration. This formulation suggests a calibration approach where either the maximum or minimum neutron count rate is identified from a suitable time series of neutron counts at the site.
I think it would be good, and perhaps essential, to strengthen this contribution by providing some actual quantitative examples of this calibration approach and how it compares with the standard calibration approach based on soil sampling. Only some qualitative examples are shown currently, with no assessment of accuracy.
I did not find much value in the other sections of the manuscript, i.e. sections 2.4 and following. Perhaps other readers will find some benefit there. Section 3 seems mostly speculative.
I have provided 36 specific questions, comments, and suggested edits in an attached pdf version of the manuscript.
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AC1: 'Reply on RC1', Sascha Oswald, 03 Apr 2025
Thank you to reviewer #1 in general and also in particular for seeing the main development of the manuscript as a '... meaningful and insightful extension of the revised Desilets equation ...'.
As a general remark we would like to stress that the manuscript aims for providing pragmatic approaches that could complement existing, rather complex ones; doing so based on algebraic reformulations and calculations that should be simple but insightful, and which could be useful for some applications, especially studies without in-depth scientific tools such as Monte Carlo simulations or extensive measurement campaigns. To reflect that, we will also slightly change the title of the manuscript towards its pragmatic direction in a revised version.To strengthen the contribution, a need identified by the reviewer, we will
(i) include a quantitative derivation of a local maximum neutron count rate that accounts for additional hydrogen pools and using real values for lattice water content soil organic matter water equivalent; plus similarily at the lower end define a local minimum neutron count rate accounting for the site bulk density and including that quantitatively.
(ii) add a diagramm demonstrating how the suggested calibration parameters (Nbg, Nmax or (Nmax - Nbg)-window), including the alternative ones as in (i), relate to a real long term time series of corrected neutron counts, based on an already calibrated N0 value.
(iii) add an example how the suggested simple direct calibration could improve the results when a CRNS device is transfered to another, different location without adjusting the N0 value calibrated at the former site only (no calibration at the new site).
(iv) add text describing that these parameters could be used for checking of CRNS times series in respect to outliers, unusual deviations, and drifts or jumps in the CRNS device used. This is an broader application that could be useful for more cases than the direct calibration.The reviewer has not criticised the mathematical development, with one exception of an ancillary equation, but suggested to streamline it in some places. This will be done in a revised manuscript.
To enhance section 3.1 we will add a new suggestion how to define a representative footprint that is substantially different to how the horizontal footprint so far is defined and handled, especially for heterogenous soil moisture distribution at the monitoring site. Other parts of section 3, that the reviewer sees as 'mostly speculative' and the second reviewer as perspective will be shortened and moved to the discussion.
The reviewer has provided 36 specific questions, comments, and suggested edits in a pdf, thanks for that. A revised manuscript will account for the specific comments and suggestions. Most of them will be done as suggested. The others are discussed or questions answered in the supplementary pdf attached here.
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AC1: 'Reply on RC1', Sascha Oswald, 03 Apr 2025
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RC2: 'Comment on egusphere-2024-4141', Anonymous Referee #2, 05 Mar 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-4141/egusphere-2024-4141-RC2-supplement.pdf
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AC2: 'Reply on RC2', Sascha Oswald, 03 Apr 2025
Thank you to reviewer #2 for the specific and technical comments.
We would like to stress that the manuscript aims for providing pragmatic approaches that could complement existing, rather complex ones; doing so based on algebraic reformulations and calculations that should be simple but insightful, and which could be useful for some applications, especially studies without in-depth scientific tools such as Monte Carlo simulations or extensive measurement campaigns. To reflect that, we will also slightly change the title of the manuscript towards its pragmatic direction in a revised version.To further convince the reviewer about the usefulness of the manuscript in a revised version we will
(i) include a quantitative derivation of a local maximum neutron count rate that accounts for additional hydrogen pools and using real values for lattice water content soil organic matter water equivalent; plus similarily at the lower end define a local minimum neutron count rate accounting for the site bulk density and including that quantitatively.
(ii) add a diagramm demonstrating how the suggested calibration parameters (Nbg, Nmax or (Nmax - Nbg)-window), including the alternative ones as in (i), relate to a real long term time series of corrected neutron counts, based on an already calibrated N0 value.
(iii) add an example how the suggested simple direct calibration could improve the results when a CRNS device is transfered to another, different location without adjusting the N0 value calibrated at the former site only (no calibration at the new site).
(iv) add text describing that these parameters could be used for checking of CRNS times series in respect to outliers, unusual deviations, and drifts or jumps in the CRNS device used. This is an broader application that could be useful for more cases than the direct calibration.The reviewer has provided a number of specific and technical comments that are answered in the supplementary pdf attached here. Besides those the reviewer has not questioned the mathematical development provided within the manuscript.
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AC2: 'Reply on RC2', Sascha Oswald, 03 Apr 2025
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