Large Area Soil Moisture Measurement Using Cosmic Rays Neutrons: The Australian CosmOz Network

Field measurement of soil moisture is undertaken traditionally using point based measurement techniques such as neutron probes or time domain reflectrometry (TDR). Recently, a new technique has been developed that can be used to derive soil moisture at larger spatial scales by measuring neutrons that are generated by cosmic rays within the air and soil, and emitted back into the atmosphere. A study by Hendrick and Edge (1966) in the mid 1960s showed that the intensity of the fast neutrons above the ground varied with soil moisture content. The intensity of the neutron is mainly moderated by hydrogen ions located in the water and soil, and the density is inversely correlated with soil moisture. To soil scientists and hydrologists, this has opened up the possibility of measuring surface soil moisture automatically over an area of ~40 ha to a depth of ~0.5 m. The technique has the potential to fill the gap between point scale measurements (neutron probe or TDR) and soil moisture estimated using earth observation techniques (remote sensing). In Australia, 11 probes have been deployed across a range of agro-ecological zones to demonstrate the potential for larger scale soil moisture monitoring.