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United States Department of Agriculture

Agricultural Research Service


item Evett, Steven - Steve

Submitted to: International Atomic Energy Agency
Publication Type: Book / chapter
Publication Acceptance Date: 4/13/1999
Publication Date: N/A

Interpretive Summary: The need for reliable soil water content measurement systems is increasing in irrigation, resource conservation, civil engineering, environmental protection; and in research activities of many kinds relating to these activities. For more than 40 years, the neutron probe has been the most widely applicable, well-tested method for water content measurement. But, this probe contains a low-level radioactive source (of fast neutrons) that causes it to come under regulation and licensing, and that render it inapplicable to problems requiring unattended data collection. Research into the time domain reflectometry (TDR) and capacitance probe (CP) electron technologies has resulted in several instruments that are offered as replacements for the neutron probe. This paper reviews the problems associated with effective use of the three methods and concludes that the neutron probe is still the most widely applicable, best understood method for soil profile water content measurement. But, the TDR and CP methods have the advantage of unattended, automatic data collection that lowers the cost of data collected. Both the TDR and CP methods may have problems in soils high in certain types of clay or in salty soils. The CP method is particularly susceptible to problems due to nonuniform soil. For work that requires measurement of small volumes of soil, such as in pots or for studies of plant root water relations, both the TDR and CP methods have advantages.

Technical Abstract: Soil water measurement methods encounter particular problems related to the physics of each method which are reviewed here. For time domain reflectometry (TDR), these related to wave form shape changes caused by soil, soil water, and TDR probe properties. Methods of wave form interpretation that overcome these problems are discussed. The TDR method is more difficult, or even impossible, to use in saline soils or in soils high in swelling clays. But, soils high in kaolinitic clays act more like sands and behave well for the TDR method. Probe length may be limited to as short as 10 cm in some problem soils, but in sands, probes may be as long as 1.5 m. Small measurement volume is both a strength and a weakness of TDR. But, the capacity to custom tailor measurement volume by changing rod length and spacing in probes is a major advantage of TDR. Neutron scattering (NS) is well understood, but calibration methods remain critical lto accuracy and precision and are discussed with recommendations for field calibration and use. The NS method is the only one discussed that may not be used for unattended monitoring. Capacitance probes tend to exhibit very small radii of influence and thus are sensitive to small scale changes in soil properties and are difficult or impossible to field calibrate. Capacitance methods are sensitive to soil content of some clays, to salinity, and to temperature. Field comparisons of neutron and capacitance probes are presented. The NS method is still the best method overall for soil profile water content measurement. Both TDR and capacitance methods may be used for unattended, automatic data logging; and have small measurement volumes; features that are advantageous for certain measurement problems.

Last Modified: 8/24/2016
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