From Space to Soil: Revealing Environmental Water with Cosmic-Ray Neutron Sensing

The Cosmic-ray neutron sensing (CRNS) technique is a modern technological solution that facilitates continuous measurement of the average water content in the environment, including soil, snow, and vegetation. The sensor monitors an area of 10 to 15 hectares and soil depths up to 50 centimeters. This configuration enables the non-invasive and field-scale measurement of the soil water content or snow water equivalent that is insensitive to small-scale heterogeneities. The method has the potential to serve as an alternative to conventional in-situ sensors or costly soil or snow samples, while also demonstrating the capability to bridge the gap between point measurements and remote sensing data in both horizontal and vertical directions. The passive CRNS technology measures the natural cosmogenic background radiation and detects changes in the moderation of cosmic-ray induced fast neutrons by hydrogen atoms. The neutron radiation measured with CRNS originates primarily from high-energy cosmic rays that interact with nuclei in the Earth's atmosphere, producing secondary particles such as neutrons. These secondary neutrons penetrate the atmosphere and interact with the soil, where their moderation is influenced by the hydrogen content, including soil moisture. CRNS is flexible and robust because it operates non-invasively and requires no direct soil contact, making it suitable for diverse terrains and conditions. Additionally, it is largely unaffected by small-scale soil heterogeneity and provides continuous, real-time data with minimal maintenance. The CRNS can also be employed on mobile platforms for the purpose of on-demand soil moisture mapping at the field, regional, or even national scale. Presently, a network of CRNS stations being established on global scale, with several hundred sensors already in operation. In this presentation, we will examine recent experiments and prospective future applications of CRNS, with the objective of extending the boundaries of this technology.