Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 2/6/2007
Publication Date: 2/1/2008
Citation: Evett, S.R. 2008. Gravimetric and volumetric direct measurement of soil water content. 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. 23-37. 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 worldwide audience of agriculturalists, including scientists and practitioners. It contains detailed information on how to accurately measure soil water content using direct methods. It provides insight into problems commonly encountered in using these methods, which are the standard by which all indirect methods (soil water sensors) are judged. Guidance on effective use of different samplers in different soils is given, including problem soils that are 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 sample size, sample number, and how well samples represent the field is discussed, and guidance is given for deciding on sample number and size. Also discussed is the design of samplers that minimize sample disturbance in order to obtain correct soil water contents. Sampling using soil coring tubes to obtain samples from depths throughout the crop root zone is also presented.
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. However, direct measurement methods are still the standard by which indirect methods are judged. This chapter discusses equipment and methods for direct measurement of soil water content, resulting in both mass basis and volumetric basis water content values. Sampler design that minimizes compression of samples is thoroughly discussed, with standards for the ratio of wall thickness to sample cross-sectional area given. Equipment that is commonly used, but does not meet standards is identified. Also discussed are sampler designs with relieved internal diameters behind the cutting edge to minimize friction between the sample and sampler wall. Correct sampler insertion is identified as a key element in avoiding sample compression or dilation. Designs and methods for sampler insertion that minimize sampler skewing off the axis of insertion are presented. Equations for water content calculation, and calculation of standard deviation and number of samples needed are given. A discussion of sample volume vs. the representative elemental volume (REV) for soil water content is given in the context of how the REV changes with time since wetting, crop growth, depth of sampling, and soil type. Methods for sampling in problem soils, including stony, gravelly or very hard soils, are presented. Also discussed is sampling for calibration of indirect methods. Quality control and data handling are also discussed.