Preprints
https://doi.org/10.5194/hess-2021-202
https://doi.org/10.5194/hess-2021-202

  19 Apr 2021

19 Apr 2021

Review status: this preprint is currently under review for the journal HESS.

Towards disentangling heterogeneous soil moisture patterns in Cosmic-Ray Neutron Sensor footprints

Daniel Rasche1, Markus Köhli3,4, Martin Schrön5, Theresa Blume1, and Andreas Güntner1,2 Daniel Rasche et al.
  • 1GFZ German Research Centre for Geosciences, Section Hydrology, 14473, Potsdam, Germany
  • 2University of Potsdam, Institute of Environmental Sciences and Geography, 14476, Potsdam, Germany
  • 3Physikalisches Institut, Heidelberg University, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany
  • 4Physikalisches Institut, University of Bonn, Nussallee 12, 53115 Bonn, Germany
  • 5UFZ – Helmholtz Centre for Environmental Research GmbH, Dep. Monitoring and Exploration Technologies, Permoserstr. 15, 04318, Leipzig, Germany

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.

Daniel Rasche et al.

Status: open (until 14 Jun 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'reviewer comments', Anonymous Referee #1, 30 Apr 2021 reply
  • RC2: 'Reviewer comments', Anonymous Referee #2, 07 May 2021 reply

Daniel Rasche et al.

Daniel Rasche et al.

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Short summary
Cosmic-Ray Neutron Sensing provides areal average soil moisture measurements. We investigated how distinct differences in spatial soil moisture patterns influence the soil moisture estimates and present two approaches to improve the estimate of soil moisture close to the instrument by reducing the influence of soil moisture further afield. Additionally, we show that the heterogeneity of soil moisture can be assessed based on the relationship of different neutron energies.