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Potential of thermal neutrons to correct cosmic-ray neutron soil moisture content measurements for dynamic biomass effects
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  • Jannis Christoph Jakobi,
  • Johan A. Huisman,
  • Hannah Fuchs,
  • Harry Vereecken,
  • Heye Bogena
Jannis Christoph Jakobi
Forschungszentrum Jülich GmbH, Forschungszentrum Jülich GmbH
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Johan A. Huisman
Forschungszentrum Jülich, Forschungszentrum Jülich
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Hannah Fuchs
Wasserverband Eifel-Rur (WVER), Wasserverband Eifel-Rur (WVER)
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Harry Vereecken
Forschungszentrum Jülich GmbH, Forschungszentrum Jülich GmbH
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Heye Bogena
Forschungszentrum Juelich, GmbH, Forschungszentrum Juelich, GmbH

Corresponding Author:h.bogena@fz-juelich.de

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Cosmic ray neutron sensors (CRNS) allow to determine field-scale soil moisture content non-invasively due to the dependence of aboveground measured epithermal neutrons on the amount of hydrogen. Because other pools besides soil contain hydrogen (e.g. biomass), it is necessary to consider these for accurate soil moisture content measurements, especially when they are changing dynamically (e.g., arable crops, de- and reforestation). In this study, we compare four approaches for the correction of biomass effects on soil moisture content measurements with CRNS using experiments with three crops (sugar beet, winter wheat and maize) on similar soils: I) site-specific functions based on in-situ measured biomass, II) a generic approach, III) the thermal-to-epithermal neutron ratio (Nr) and IV) the thermal neutron intensity. Calibration of the CRNS during bare soil conditions resulted in root mean square errors (RMSE) of 0.097, 0.041 and 0.019 m3/m3 between estimated and reference soil moisture content of the cropped soils, respectively. Considering in-situ measured biomass for correction reduced the RMSE to 0.015, 0.018 and 0.009 m3/m3. When thermal neutron intensity was considered for correction, similarly accurate results were obtained. Corrections based on Nr and the generic approach were less accurate. We also explored the use of CRNS for biomass estimation. The use of Nr only provided accurate biomass estimates for sugar beet. However, significant site-specific relationships between biomass and thermal neutron intensity were obtained for all three crops. It was concluded that thermal neutron intensity can be used to correct soil moisture content estimates from CRNS and to estimate biomass.