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

Authors: Yakubova, Galina; Kavetskiy, Aleksandr; Prior, Stephen - Steve; Torbert, Henry - Allen; GAUTAM, SIDHARTH - Auburn University

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/6/2024
Publication Date: 9/23/2024
Citation: Yakubova, G.N., Kavetskiy, A.G., Prior, S.A., Torbert III, H.A., Gautam, S. 2024. Measuring and mapping soil moisture in agricultural fields by neutron-gamma analysis [abstract]. International Soil and Tillage Research Organization (ISTRO), 23-27 September 2024, Virginia Beach, VA.

Interpretive Summary:

Technical Abstract: Knowledge of subsurface soil water content distribution over agricultural fields is important for optimizing modern agricultural practices and enhancing soil science knowledge. It is important in selecting appropriate tillage, land use, and irrigation management practices. Collecting and processing representative field soil cores for traditional laboratory analysis or measuring soil field moisture by portable devices is labor intensive, time consuming and represent “point” soil moisture content. There are other methods that measure soil characteristics using remote sensors (from aircraft and satellites), but their estimates cover large spatial areas with relatively large associated errors. Pulsed fast thermal neutron analysis (PFTNA) technology for measuring and mapping soil moisture is a good alternative to the previous mentioned methods. PFTNA is based on detecting gamma lines that appear due to neutron nuclei interactions. Each neutron-nucleus interaction give a gamma line of particular energy for that interaction. Soil water content is correlated with the 2.22 MeV gamma signal from hydrogen in soil. State-of-the-art nuclear physics methodologies and instrumentation, combined with commercial availability of portable pulse neutron generators, high-efficiency gamma detectors, reliable electronics, and measurement and data processing software, have currently made the application of neutron-gamma analysis possible for routine measurements in various fields of study. Our mobile PFTNA system (paired with GPS) was used for moisture measurements by determining hydrogen peak areas in the thermal neutron capture gamma spectra acquired when scanning agricultural fields. A calibration dependency was used for converting hydrogen peak area to soil moisture content by comparing PFTNA hydrogen peak area to other soil moisture determination methods (gravimetric, time domain reflectometry, and nuclear moisture/density gauge). Long-range (100-300 m) trends of changing water content are of interest when mapping soil water distribution for agricultural purposes. This distribution can be represented as isolines overlaying a geographical map of a studied field. Data suitable for mapping are prepared as datasets of geographical coordinates and corresponding hydrogen content. Isolines across the field can be created from such dataset by different methods. Examples of the water content maps of real agricultural fields suitable for use in agricultural practices will be presented. While data acquired by gravimetric or instrument methods can be labor intensive and time consuming, PFTNA scanning of a 10-ha field can acquire needed mapping data in ~1 h. Thus, PFTNA scanning can be recommended as a more efficient method for measuring and mapping soil moisture in agricultural fields.