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United States Department of Agriculture

Agricultural Research Service

Title: FIELD ESTIMATION OF SOIL WATER CONTENT

Authors
item Hignett, Cliff - SELF-EMPLOYED
item Evett, Steven

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: February 6, 2007
Publication Date: N/A

Interpretive Summary: As a chapter in a book to be published by the International Atomic Energy Agency/FAO Joint Division, this publication will reach a world wide audience of agriculturalists, including scientists and practitioners. This introductory chapter contains an overview of field methods of soil water measurement and estimation, concentrating on those methods that can be used for water content determination in the entire soil profile to well below the root zone. Such methods are essential for determination of crop water use, water use efficiency, irrigation efficiency, and the effects of fertilization on these. The chapter provides insight into problems commonly encountered in using these methods, which include direct sampling (soil cores), methods based on nuclear physics, and those based on the electromagnetic properties of soils. The relationship between scatter in the data, number of samples needed in the field (and thus cost), and how well samples represent the field is discussed, and guidance is given for deciding on sample number and size. Calibration of indirect methods of water content estimation is discussed, as are methods of quickly field checking a calibration.

Technical Abstract: Accurate soil water content measurements to considerable depth are required for investigations of crop water use, water use efficiency, irrigation efficiency, and the hydraulic properties of soils. As a chapter in a book to be published by the International Atomic Energy Agency/FAO Joint Division, this publication will reach a world wide audience of agriculturalists, including scientists and practitioners. This introductory chapter contains an overview of field methods of soil water measurement and estimation, concentrating on those methods that can be used for water content determination in the entire soil profile to well below the root zone. Such methods are essential for determination of crop water use, water use efficiency, irrigation efficiency, and the effects of fertilization on these. The chapter provides insight into problems commonly encountered in using these methods. Methods discussed include direct sampling, neutron thermalization, and the electrometric methods (time domain reflectometry, frequency domain, and related methods). Common interferences with these methods are discussed, including soil bulk electrical conductivity (salinity and high content of charged clays), and temperature. The relationship between scatter in the data, number of samples needed in the field (and thus cost), and how well samples represent the field is discussed in the context of the representative elemental volume, and guidance is given for deciding on sample number and size. Calibration of indirect methods of water content estimation is discussed, as are methods of quickly field checking a calibration.

Last Modified: 9/1/2014
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