Development of a site-specific standardised Cosmic Ray Neutron Sensor (CRNS) calibration protocol

Abstract

Soil moisture is a key soil characteristic that must be taken into account when planning agricultural management activity e.g. trafficking of land or timing of nutrient application. Traditionally, soil moisture is measured using point-scale sensors or directly through field sampling followed by drying-weighing of the soil sample in the laboratory.  However, due to the spatial heterogeneity of soils, soil moisture varies significantly within a few meters within a single paddock, which limits the usefulness of the traditional point-scale methods for management decisions. In contrast, satellite measurements of soil moisture are too coarse in their resolution to give accurate descriptions of soil moisture at sub-field scale.   The Cosmic-Ray Neutron Sensor (CRNS) is a neutron sensing instrument that can be installed at field sites to monitor soil moisture conditions over intermediate scales. This approach uses the principle of elementary particle attenuation with depth to assess concentration of water molecules in soil within a footprint of 75-300 m radius, with a varying accuracy for the chosen footprint radius. Further exploration of both the CRNS footprint and accuracy is needed, necessitating the development of a standardized site-specific calibration process (the focus of the present work) to enable enhanced data integration across sites and improve the accuracy of neutron counts (NC) to volumetric soil moisture content (VMC) conversion within the defined footprint. For that purpose, the newly established Irish Soil Observation Network (ISMON) has installed ten CRS2000/B neutron sensors presenting an opportunity to create such a standardised protocol. This work presents the first results of the ISMON CRNS stations calibration and introduces a new vision towards development of standardised CRNS calibration protocol for grassland sites.