Towards disentangling heterogeneous soil moisture patterns in cosmic-ray neutron sensor footprints

Abstract

Abstract. Cosmic-ray neutron sensing (CRNS) allows for non-invasive soil moisture estimations at the field scale. The derivation of soil moisture generally relies on secondary cosmic-ray neutrons in the epithermal to fast energy ranges. Most approaches and processing techniques for observed neutron intensities are based on the assumption of homogeneous site conditions or of soil moisture patterns with correlation lengths shorter than the measurement footprint of the neutron detector. However, in view of the non-linear relationship between neutron intensities and soil moisture, it is questionable whether these assumptions are applicable. In this study, we investigated how a non-uniform soil moisture distribution within the footprint impacts the CRNS soil moisture estimation and how the combined use of epithermal and thermal neutrons can be advantageous in this case. Thermal neutrons have lower energies and a substantially smaller measurement footprint around the sensor than epithermal neutrons. Analyses using the URANOS (Ultra RApid Neutron-Only Simulation) Monte Carlo simulations to investigate the measurement footprint dynamics at a study site in northeastern Germany revealed that the thermal footprint mainly covers mineral soils in the near-field to the sensor while the epithermal footprint also covers large areas with organic soils. We found that either combining the observed thermal and epithermal neutron intensities by a rescaling method developed in this study or adjusting all parameters of the transfer function leads to an improved calibration against the reference soil moisture measurements in the near-field compared to the standard approach and using epithermal neutrons alone. We also found that the relationship between thermal and epithermal neutrons provided an indicator for footprint heterogeneity. We, therefore, suggest that the combined use of thermal and epithermal neutrons offers the potential of a spatial disaggregation of the measurement footprint in terms of near- and far-field soil moisture dynamics.

A change in perspective: Downhole cosmic-ray neutron sensing for the estimation of soil moisture

“…This is done for different reasons. Firstly, thermal neutrons respond to changes in environmental hydrogen content and thus, soil moisture (e.g., Hubert et al, 2016;Weimar et al, 2020;Rasche et al, 2021). Secondly, we expect the neutron intensity (i.e.…”

“…A bare counter tube is then more effective as the HDPE shielding of a moderated tube would not only slow down but also reflect a certain percentage of potentially countable neutrons away from the instrument and would thus reduce the observed intensity. Furthermore, it has been shown that thermal neutrons can be potentially used to obtain soil moisture information from larger depths compared to epithermal neutrons (Rasche et al, 2021).…”

“…Several different Monte Carlo-based particle transport simulation toolkits have previously been used for the investigation of secondary cosmic-ray neutrons at the soil-atmosphere interface in the context of CRNS including GEANT4 (Hubert et al, 2016;Brall et al, 2021), MCNP (Zreda et al, 2008;Franz et al, 2012;Andreasen et al, 2016Andreasen et al, , 2017Weimar et al, 2020;Köhli et al, 2021) and URANOS (Köhli et al, 2015(Köhli et al, , 2021Li et al, 2019;Rasche et al, 2021) which only simulates neutrons of different energies. Although simulating only neutrons and not e.g.…”

“…1). The site is of one of three permanently operating CRNS stations (Heidbüchel et al, 2016;Rasche et al, 2021) in the Terrestrial Environmental Observatory TERENO-NE (Zacharias et al, 2011;Heinrich et al, 2018). The observatory is located in the cfb climatic zone following the Köppen-Geiger classification (Bogena et al, 2022) The study site is located on a glacial terminal moraine formed during the Pommeranian phase of the Weichselian glaciation in the Pleistocene (Börner, 2015).…”

Potential of Thermal Neutrons to Correct Cosmic‐Ray Neutron Soil Moisture Content Measurements for Dynamic Biomass Effects

“…Such investigations are particularly important given that the intensity of thermal neutrons also depends on soil moisture content and soil chemistry, since thermal neutrons are particularly strongly absorbed by certain elements in the soil (Andreasen et al, 2016;Zreda et al, 2008). In addition, recent studies have shown that the sensing volume of thermal neutrons is much smaller than in the case of epithermal neutrons (Bogena et al, 2020;Rasche et al, 2021). Using neutron transport simulations, it was found that thermal neutrons have a radial footprint of approximately 45 m, which increases only slightly with soil moisture content, and a sensing depth that increases from 10 to 65 cm with decreasing soil moisture content from 0.50 to 0.01 m 3 /m 3 (Jakobi et al, 2021).…”