Theory and application of measuring mesoscale soil moisture by cosmic-ray fast neutron probe

In recent years, measurement of surface soil moisture by the cosmic-ray fast neutron probe has gradually attracted more attention The intensity of the fast neutrons above the ground is sensitive to water content changes, largely insensitive to soil chemistry and inversely correlated with hydrogen content of the soil. Measurement of this intensity with a portable neutron detector placed above the ground permits the possibility of monitoring soil moisture. By this passive, non-invasive and intermediate scale measurement, soil moisture at a horizontal scale of around 660 m and depths of 12 to 76 cm can be inferred. And the large footprint makes this method suitable for weather and short-term climate forecast initialization and for validation of soil moisture inversed from satellite sensors. In this paper, the results of the cosmic-ray instruments well reflected the variation trend of soil moisture at the field scale. We found that irrigation and infiltration were two most important factors that affected the variation of soil moisture during a growing season of maize in Zhangye and the soil moisture of different period in the growing season of maize was very different. At last, Wireless Sensor Network data were used to validate this result and we found their correlation was very well with a RMSE of 0.0275 m³m⁻³ in the non-irrigation time.