Status and Perspectives on the Cosmic-Ray Neutron Method for Soil Moisture Estimation and Other Environmental Science Applications

Since the introduction of the cosmic-ray neutron method for soil moisture estimation, numerous studies have been conducted to test and advance the accuracy of the method. Almost 200 stationary neutron detector systems have been installed worldwide, and roving systems have also started to gain ground. The intensity of low-energy neutrons produced by cosmic rays, measured above the ground surface, is sensitive to soil moisture in the upper decimeters of the ground within a radius of hectometers. The method has been proven suitable for estimating soil moisture for a wide range of land covers and soil types and has been used for hydrological modeling, data assimilation, and calibration and validation of satellite products. The method is challenged by the effect on neutron intensity of other hydrogen pools such as vegetation, canopy interception, and snow. Identifying the signal of the different pools can be used to improve the cosmic-ray neutron soil moisture method as well as extend the application to, e.g., biomass and canopy interception surveying. More fundamental research is required for advancement of the method to include more energy ranges and consider multiple height levels.