Soil moisture (SM) can highly vary in space and time, and this can have a significant impact on cotton crop yield and fibre quality. Irrigated cotton accounts for over a quarter of all irrigated agriculture in Australia. With water scarcity increasing due to increased demand across industries and climate change, water use efficiency must continue to improve. The Australian cotton industry is already a world leader in water use efficiency, improving whole farm water use efficiency from 57% to an estimated 70% in the past three decades. The remaining 30% of water is lost across the farm, due to field seepage and evaporation (Roth et al., 2014).

There are limitations with how moisture is currently measured and estimated on a field scale. Technologies such as capacitance probes are only capable of measuring SM in a small area directly around the probe, whereas large scale remote sensing technology such as satellites cannot measure beyond the soil surface. Especially with the rise of new irrigation systems such as bankless irrigation, accurate paddock scale SM measurements will allow growers to better determine their irrigation schedules.

The CosmOz Rover, developed by CSIRO, contains a large-scale cosmic ray probe. Primary cosmic rays from outer space, usually in the form of protons, interact with the atmosphere to form secondary cosmic rays; high energy neutrons. These collide with hydrogen atoms, losing energy, to become fast neutrons, and eventually reach a state of thermal equilibrium. Cosmic ray probes measure the flux of fast neutrons, which is inversely proportional to the amount of hydrogen atoms whether it be in air, water, or organic compounds. As water molecules are the dominant source of hydrogen atoms in soil, then close to the Earth’s surface, SM content can be inferred from neutron fluxes (Desilets et al., 2010).

The CosmOz Rover has a 300 m radius horizontal footprint, and can measure to a depth between 0.1-0.7 m depending on SM content, with saturated profiles only measurable to shallower depths (Hawdon et al., 2014). Cosmic ray measurements are passive, non-invasive and mostly insensitive to variations in soil characteristics such as texture, surface roughness, bulk density and the state of water (Desilets et al., 2010; Zreda et al., 2008). Measurements can be taken at a fixed point for temporal data, or additionally moved around a field/farm for spatial data. Key factors that affect measurements include soil organic carbon.

This technology has been studied with success in natural vegetation and dryland agricultural systems, but has not been trialled with irrigated systems. This novel technology has the potential to provide more representative paddock-scale SM measurements compared to other technologies, as a tool to guide decision making for irrigation schedules and improve water-use efficiency.