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 measurements at the field scale. The derivation of soil moisture generally relies on secondary cosmic-ray neutrons in the epithermal-to-fast energy range. Most approaches and processing techniques for observed neutron intensities are based on the assumption of homogeneous site conditions within the measurement footprint of the neutron detector. 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 URANOS neutron Monte-Carlo simulations to investigate measurement footprint dynamics at a study site in north-eastern 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 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 discretization of the measurement footprint in terms of near and far field soil moisture dynamics.

Combining static and portable Cosmic ray neutron sensor data to assess catchment scale heterogeneity in soil water storage and their integrated role in catchment runoff response

“…However, some have identified the need for additionally calibrating the ai parameters to reproduce site-specific SWC dynamics or match local conditions (e.g. Iwema et al, 2015;Rasche et al, 2021;Rivera Villarreyes et al, 2011).…”

COSMOS-Europe: a European network of cosmic-ray neutron soil moisture sensors

“…A4). An example is the CRNS station at the Fuerstensee site in Germany (Rasche et al, 2021). The footprint represented by the CRNS measurement at this site includes a sand lens in the center of the footprint, which is surrounded by peat soils.…”

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).…”