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

Abstract

Cosmic-ray neutron sensors (CRNS) enable noninvasive determination of field-scale soil moisture content by exploiting the dependence of the intensity of aboveground epithermal neutrons on the hydrogen contained in soil moisture. However, there are other hydrogen pools besides soil moisture (e.g., biomass). Therefore, these hydrogen pools should be considered for accurate soil moisture content measurements, especially when they are changing dynamically (e.g., arable crops, deforestation, and reforestation). In this study, we test 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) based on high-quality reference soil moisture: (a) site-specific functions based on in-situ measured biomass, (b) a generic approach, (c) the thermal-to-epithermal neutron ratio (Nr), and (d) the thermal neutron intensity. Bare soil calibration of the CRNS resulted in high root mean square errors (RMSEs) of 0.097, 0.041, and 0.019 m³/m³ between estimated and reference soil moisture content for sugar beet, winter wheat, and maize, respectively. Considering in-situ measured biomass for correction reduced the RMSE to 0.015, 0.018, and 0.009 m³/m³. The consideration of thermal neutron intensity for correction was similarly accurate. We also explored the use of CRNS for biomass estimation and found that Nr only provided accurate biomass estimates for sugar beet. In contrast, we found significant site-specific relationships between biomass and thermal neutron intensity for all three crops, suggesting that thermal neutron intensity can be used both to improve CRNS-based soil moisture content measurements and to quantify crop biomass.