Use of neutron measurements for soil-moisture determination

Neutron measurement of soil water content is based on the slowing down of fast neutrons emitted from a localized source and then on the diffusion of slow neutrons. Counting rate of slow neutrons depends on soil water content and chemical characteristics of the medium: hydroxide or organic matter content and neutron absorption or diffusion cross sections. Neutron measurements require a prior calibration, either by gravimetry or theoretical from the determination of FaFd on a soil sample. Both calibration methods are described and deviations are determined from absolute values of water content depending on soil nature. Spatial distribution of the parameters FaFd and its influence on the calibration curve of the meters are examined. Some practical rules are given concerning the water balance estimation in agriculture