Potential of thermal neutrons to correct cosmic-ray neutron soil
moisture content measurements for dynamic biomass effects
Abstract
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.