Calibration and correction procedures for cosmic‐ray neutron soil moisture probes located across Australia

Abstract

The cosmic-ray probe (CRP) provides continuous estimates of soil moisture over an area of $30 ha by counting fast neutrons produced from cosmic rays which are predominantly moderated by water molecules in the soil. This paper describes the setup, measurement correction procedures, and field calibration of CRPs at nine locations across Australia with contrasting soil type, climate, and land cover. These probes form the inaugural Australian CRP network, which is known as CosmOz. CRP measurements require neutron count rates to be corrected for effects of atmospheric pressure, water vapor pressure changes, and variations in incoming neutron intensity. We assess the magnitude and importance of these corrections and present standardized approaches for network-wide analysis. In particular, we present a new approach to correct for incoming neutron intensity variations and test its performance against existing procedures used in other studies. Our field calibration results indicate that a generalized calibration function for relating neutron counts to soil moisture is suitable for all soil types, with the possible exception of very sandy soils with low water content. Using multiple calibration data sets, we demonstrate that the generalized calibration function only applies after accounting for persistent sources of hydrogen in the soil profile. Finally, we demonstrate that by following standardized correction procedures and scaling neutron counting rates of all CRPs to a single reference location, differences in calibrations between sites are related to site biomass. This observation provides a means for estimating biomass at a given location or for deriving coefficients for the calibration function in the absence of field calibration data.

JASMIN: A prototype high resolution soil moisture analysis system for Australia

“…CosmOz is a network of cosmic ray soil moisture probes installed at a number of locations around Australia (Hawdon et al 2014). The effective sensing depth depends strongly on soil moisture itself, decreasing non-linearly with increasing soil moisture and ranges from about 70 cm to 10 cm.…”

Monitoring Environmental Water with Ground Albedo Neutrons from Cosmic Rays

“…Since 2008 more than one hundred sensors have been installed in the USA, Europe, Asia, Africa and Australia (e.g. [5], [6]). Most of the data is freely available at http://cosmos.hwr.arizona.edu.…”

“…where g = 1 − 0.075(R − R NM ), R is the cutoff rigidity at the CRNS site, and R NM is the cutoff rigidity at the neutron monitor station used ( [5], [6]). …”

Examining the relationship between intermediate-scale soil moisture and terrestrial evaporation within a semi-arid grassland

“…The site is classified as a semi-arid region with latitude −32.87 • , longitude 148.52 • , and an elevation of 438 m above mean sea level. The Baldry experimental catchment formed part of the Australian National Cosmic Ray Soil Moisture Monitoring Facility (CosmOz) network, established by the Commonwealth Scientific and Industrial Research Organization (CSIRO) (Hawdon et al, 2014). A CRNP was put in place at the site in March 2011, complimenting existing meteorological and soil moisture sensors, as well as an eddy-covariance flux tower.…”

“…Although coupling between evaporation and soil moisture is expected to be high in arid and semi-arid regions, the dynamics of surfaceatmosphere feedbacks are not well understood in such environments (Wang et al, 2012). Several studies have attempted to describe these links, with the aim of predicting one variable through knowledge of the other (Mintz and Walker, 1993;Wetzel and Chang, 1987) or to use developed relationships to inform upon linked hydrological responses such as evaporation (McCabe et al, 2005;Stisen et al, 2011), soil moisture (Liu et al, 2012), drought (Entekhabi et al, 1992;Fischer et al, 2007;Oglesby and Erickson, 1989), precipitation (Findell et al, 2011;Held et al, 2005), and even vegetation response to soil moisture stress (Liu et al, 2011). A common feature of such studies is the use of model estimates of terrestrial evaporation across a wide range of study areas and land cover and biome types.…”