Measuring and mapping moisture content in agricultural fields by neutron-gamma analysis

All currently applied methods for soil moisture measurement and mapping in agricultural fields are labor-intensive and time-consuming. The Pulsed Fast Thermal Neutron Analysis (PFTNA) method, described in this article, can provide in situ soil moisture distribution data across agricultural fields by field scanning with a mobile PFTNA system in a reasonably short time. This method is based on acquiring soil gamma ray responses when irradiated by fast neutrons. The response gamma spectra [thermal neutron capture (TNC) gamma spectra] contain the gamma ray peak related to hydrogen present in soil. Since the majority of hydrogen is contained in soil water, soil moisture can be determined from the value of the hydrogen peak area in TNC spectra. A power dependency with a non-zero constant term was used to convert the hydrogen peak area to soil moisture content. To create this dependency, the hydrogen peak area in the TNC spectra was plotted against moisture data obtained using other methods (gravimetric, time domain reflectometry, nuclear radiation transmission) in the same agricultural fields. Developed methods for PFTNA field scanning and data processing provided data for moisture mapping; this scanning method involved moving at 5 km h−1, simultaneously recording GPS coordinates and TNC gamma spectra every 30 s, and scanning paths that uniformly covered surveyed fields. Comparison of these maps with those created using data from traditional soil moisture measurement methods (gravimetric, time domain reflectometry, nuclear radiation transmission) demonstrated good agreement. Note that PFTNA scanning of a 20-hectare field can acquire the data needed for mapping in approximately one hour. Thus, PFTNA scanning can be recommended as a more efficient method for measuring and mapping soil moisture in agricultural fields.