Submitted to: Book Chapter
Publication Type: Book / chapter
Publication Acceptance Date: 2/6/2007
Publication Date: 2/1/2008
Citation: Evett, S.R. 2008. Capacitance sensors for use in access tubes. In: Evett, S.R., Heng, L.K., Moutonnet, P., Nguyen, M.L., editors. Field Estimation of Soil Water Content: A Practical Guide to Methods, Instrumentation, and Sensor Technology. IAEA-TCS-30. International Atomic Energy Agency, Vienna, Austria. ISSN 1018-5518. p. 73-90. 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. It contains detailed information on how to estimate soil profile water content using methods called capacitance methods, which use an oscillating electronic circuit in some ways similar to that of a radio tuner. The chapter provides insight into problems commonly encountered in using these methods. Guidance on use of different instruments in different soils is given, including problem soils that are clayey, stony, gravelly, very hard, or sandy. Instruction on data recording and analysis is also given. Equations for calculation of water contents are given, and common errors discussed. 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. In all cases, the number of access tubes appears to be prohibitive for drier soils in which the variability of readings is larger. The numerous problems with this technology are fully discussed so that users can avoid choosing and using this kind of sensor in soils with high clay content, gravel, salt, or uneven water distribution as often occurs with the newer, more efficient irrigation methods, including drip irrigation.
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. Many indirect methods have been proposed for sensing soil water content with minimal soil disturbance. One of the most popular methods employs a capacitor whose fringing field interacts with the soil outside of an access tube. This chapter discusses equipment and methods for capacitance sensing of soil water content, including the theory of measurement, and practical implications of the sensitivity of this method to soil bulk electrical conductivity, clay content and type, and temperature. Field variability of soil profile water storage estimates is presented for several instruments; as is a discussion of the number of access tubes needed to estimate profile water content to a given precision. In all cases, the number of access tubes appears to be prohibitive for drier soils in which the variability of readings is larger. A discussion of sampling in problem soils, including stony, gravelly or very hard soils, is presented. Quality control, calibration, and data handling are also discussed.