the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A change in perspective: Downhole cosmic-ray neutron sensing for the estimation of soil moisture
Daniel Rasche
Jannis Weimar
Martin Schrön
Markus Köhli
Markus Morgner
Andreas Güntner
Theresa Blume
Abstract. Above-ground cosmic-ray neutron sensing (CRNS) allows for the non-invasive estimation of the field-scale soil moisture content in the upper decimetres of the soil. However, large parts of the deeper vadose zone remain outside of its observational window. Retrieving soil moisture information from these deeper layers requires extrapolation, modelling, or other methods, all of which come with methodological challenges. Against this background, we investigate CRNS for downhole soil moisture measurements in deeper layers of the vadose zone. To render calibration with in-situ soil moisture measurements unnecessary, we re-scaled neutron intensities observed below the terrain surface with intensities measured above a water body.
An experimental set-up with a CRNS sensor deployed at different depths up to 10 meters below the surface in a groundwater observation well combined with particle transport simulations revealed the response of downhole thermal neutron intensities to changes in soil moisture content at the depth of the downhole neutron detector as well as in the layers above it. The simulation results suggest that the sensitive measurement radius of several decimeters, which depends on soil moisture and soil bulk density, exceeds the one of a standard active neutron probe which is only about 30 cm. We derived transfer functions to estimate downhole neutron signals from soil moisture information and we describe approaches for using these transfer functions in an inverse way to derive soil moisture from the observed neutron signals. The in-situ neutron and soil moisture observations confirm the applicability of these functions and prove the concept of passive downhole soil moisture estimation even at larger depths using cosmic-ray neutron sensing.
Daniel Rasche et al.
Status: final response (author comments only)
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RC1: 'Comment on hess-2022-364', Anonymous Referee #1, 20 Dec 2022
It is intresting about the d-CNRS for deep soil water measurement through groundwater wells, but I only suggest the manuscript should be shorted and the story wil be easy for reading.
Citation: https://doi.org/10.5194/hess-2022-364-RC1 -
AC1: 'Reply on RC1', Daniel Rasche, 20 Jan 2023
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2022-364/hess-2022-364-AC1-supplement.pdf
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AC1: 'Reply on RC1', Daniel Rasche, 20 Jan 2023
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RC2: 'Comment on hess-2022-364', Anonymous Referee #2, 05 Jan 2023
The study is very interesting and the new setup of downhole cosmic-ray neutron sensing for the estimation of soil moisture within vadose zone looks promissing. As it is mentioned by the authors further research is definitely needed to test the proposed approach in different sites.
It would be good to see the estimated soil moistures from above ground CRNS in Figure 10. How different the values measured by above ground CRNS and downhole CRNS at 100 m depth?
Is the precipitation in liquid form during the winter? Does snow affect the downhole neutron intensities?
Citation: https://doi.org/10.5194/hess-2022-364-RC2 -
AC2: 'Reply on RC2', Daniel Rasche, 20 Jan 2023
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2022-364/hess-2022-364-AC2-supplement.pdf
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AC2: 'Reply on RC2', Daniel Rasche, 20 Jan 2023
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RC3: 'Comment on hess-2022-364', Anonymous Referee #3, 20 Mar 2023
The authors introduced a novel way of using the cosmic-ray neutrons sensing technique.
They modeled and measured thermal neutron radiation at below surface dephts, to estimate soil moisture at depths greater than the measurement depth of the more traditional CRNS soil moisture sensor (~40 cm below surface)
To make use of existing monitoring infrastructure, they put CRNS sensors in groundwater level observation wells, at different dephts, up to 10 metres below the surface.
When measurement time was integrated to 25 to 49 hours, sufficiently precise measurements could be obtained up to ~2 metres below the soil surface.
The somewhat eliptical measurement support volume came out larger than that of traditional active below-ground neutron soil moisture sensors, with a radius of ~50 cm at wet conditions.
To avoid calibration against in-situ soil moisture sensors or sampling, they used scaling against above-ground thermal CRNS measurements over a water body.
The authors acknowledged relevant uncertainties on the measurements and their research outcomes and provided a look forward.
Especially regarding the calibration method without soil moisture observations, more research needs to be done, according to the authors.
Moreover, the specific results were deemed valid for the specific sites only, in principle.
To be certain the outcomes can be used at different sites, more research is needed.The authors wrote their manuscript with a clear structure.
The research is novel and relevant to the HESS audience and is written in a way this audience should be able to understand, even with limited knowledge of particle physics.
I recommend a few minor changes only, indicated hereinafter.- In the introduction (Chapter 1), the publication by Kodama et al. (1985, https://journals.lww.com/soilsci/Abstract/1985/10000/APPLICATION_OF_ATMOSPHERIC_NEUTRONS_TO_SOIL.1.aspx) should be mentioned.
This publication is about a passive below-ground neutron sensor and therefore it is needed to briefly mention how the current manuscripts relates to this work from the 1980's.- Chapter 2 Material and Methods, Section 2.4, page 9, lines 196-204: The HESS-reader could be helped by explaining a bit more how the roles protons, muons, and neutrons play, differ and how much different processes dominate in relative terms (moderation and in-soil neutron production)
Please, if explained in the manuscript text, keep this very brief.- Chapter 3 Results, Section 3.1.3, page 7, line 362: Why from wilting point to field capacity (these are rather arbitrary soil moisture contents that, formally, relate to the plant grown) and why not to saturation?
- In chapter 4 Discussion: The possible uncertainties introduced by vertically highly heterogeneous soils should be discussed.
These heterogeneities can affect the outcomes through both vertically variant soil bulk densities and, during certain periods, vertically heterogeneous soil moisture contents.
To what extent does not knowing the exact vertical variation matter to the outcomes and uncertainty?
To what extent would vertically heterogeneous soils affect the applicability of the methods at different sites, given more in-situ sampled data might be needed?- Chapter 4 Discussion, page 23, line 466 "...may lead to different results": such as?
- Chapter 5 Conclusions, page 27, lines 585-590: Please, discuss here and possibly in Chapter 4 Discussion, Section 4.2, that using soil hydraulic models could introduce further uncertainties due to assumptions made during the modelling process. In addition, applying a soil hydraulic model to obtain the results, make these less observations and more model results.
Citation: https://doi.org/10.5194/hess-2022-364-RC3
Daniel Rasche et al.
Daniel Rasche et al.
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