Soil water sensing by neutron scattering

Authors: Evett, Steven - Steve

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 3/9/2022
Publication Date: 4/26/2022
Citation: Evett, S.R. 2022. Soil water sensing by neutron scattering. In: Mahendran, N., Bentahar, N., editors. Reference Module in Earth Systems and Environmental Sciences. Encyclopedia of Soils in the Environment. 2nd edition. Online:Elsevier. https://doi.org/10.1016/B978-0-12-822974-3.00046-X.
DOI: https://doi.org/10.1016/B978-0-12-822974-3.00046-X

Interpretive Summary: Neutron scattering methods have several important uses in both agriculture and civil engineering. Several USDA agencies use neutron scattering methods, including the neutron moisture meter (NMM), to determine soil water content, and for safety monitoring of dams and levees. The NMM is used by USDA stakeholders for the same purposes, including using it for irrigation scheduling of high value crops, ensuring high crop quality, and efficient use of water. Although such techniques have been in existence for decades, there is not a recent review that summarizes this information in one place. A USDA ARS scientist at the Conservation and Production Research Laboratory, Bushland, Texas, wrote this chapter to explain the theory of the neutron scattering method, the equipment used and how to calibrate and use it. The chapter also explains the use of a new neutron scattering method that relies on cosmic ray collisions with atoms in the air to produce neutrons that are then scattered by interaction with water, allowing water content to be estimated. Better understanding and use of these methods will lead to more productive, efficient, and sustainable use of water in agriculture as well as safer dams and levees. This book chapter will be a useful resource for both new users and people who are experienced users.

Technical Abstract: Soil water content can be sensed by counting thermal (energy at room temperature) neutrons that are scattered from collisions of more energetic (fast) neutrons with the nuclei of atoms. Repeated collisions reduce the energy of fast neutrons until they are at thermal energy, a process called moderation. Hydrogen atoms, mainly found in soil water, are more effective moderators than are most elements found in soils, so the count of thermal neutrons is a good indicator of the amount of water in soil. Fast neutron sources can be man-made or natural (cosmic ray collisions with atoms). The neutron scattering methods are widely used for sensing of soil water content in agricultural and environmental research, monitoring, and management. It is the most accurate method of determining soil profile water content when correctly calibrated and used. This chapter describes neutron moisture meter (NMM) equipment, its theory, how it is used in access tubes in the soil to sense water content at multiple depths, the theory and statistics of sensing, best practices, and interferences with the method. The chapter also describes the cosmic ray neutron scattering (CRNS) method, which counts neutrons above the soil surface and which relies on fast neutrons produced naturally by cosmic ray impacts on the nuclei of atoms in the atmosphere near the soil surface. The CRNS equipment and use are described along with elementary theory, and common interferences from vegetation and humidity in the atmosphere.