Suitability of the cosmic-ray fast neutron soil moisture monitoring method in a low-latitude plateau

The cosmic-ray fast neutron soil moisture monitoring method has a large monitoring range, is unaffected by soil salinity, allows real-time and non-destructive in situ measurement, and is of significance for drought and flood warning, water-saving irrigation, ecological environment protection, and soil productivity improvement. To clarify the suitability of the cosmic-ray fast neutron soil moisture monitoring method in the low-latitude plateau, this study conducted a large-scale soil moisture investigation using oven-drying method in the southern Chinese city of Dali (25°42ʹ14″N, 100°10ʹ34″E) from May to September 2020 to verify the accuracy of the cosmic-ray fast neutron method. At the experimental site, a cosmic-ray fast neutron soil moisture station, frequency domain reflectometer (FDR) soil moisture station, and rainfall monitoring station were installed. FDR was used for continuous monitoring and comparison, and the correlation of soil moisture in the 0–50 cm soil layer was analyzed. Combined with the observation data of rainfall stations, the response ability of cosmic-ray fast neutrons and FDR to continuous changes in soil moisture was studied. The experimental results showed that the determination coefficient of the linear equation (R²), root-mean-square error (RMSE), and absolute error between the soil moisture content measured by the cosmic-ray fast neutron method and by the multi-point mean value of the oven-drying method were 0.898, 0.013 cm³∙cm^–3, and 0.027 cm³∙cm^–3, respectively. This shows that the cosmic-ray fast neutron method can accurately measure the regional soil water content in low-latitude plateaus. In the long time series, the cosmic-ray fast neutron method and FDR have a consistent trend of soil moisture changes in the 10 cm soil layer, and the determination coefficient of the linear equation (R²) was 0.839. This shows that the cosmic-ray fast neutron method can respond to soil moisture changes over time, similar to FDR. In terms of sensitivity to precipitation, if the precipitation amount was more than 2 mm for the precipitation process, and both methods had obvious responses. For the precipitation process with precipitation below 2 mm, the cosmic-ray fast neutron method was slightly better than FDR. However, more obvious sensitivity was not observed in this experiment, especially for the precipitation process with precipitation below 1 mm, which may be related to the loose texture of the surface soil in the experimental area and the rapid shift of sunny and rainy weather. In this experiment, the cosmic-ray fast neutron soil moisture monitoring method was suitable in the low-latitude plateau region, was not sensitive to the spatial variability of soil moisture, and was accurate and reliable for measuring the average soil moisture content in the range of 100 m. It meets the current soil moisture observation requirements, can provide real-time soil moisture information, effectively improving the efficiency and accuracy of mesoscale soil moisture monitoring, and provides a reference for related soil moisture monitoring research and applications.