Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2006
Publication Date: 7/26/2006
Citation: Evett, S.R., Tolk, J.A., Howell, T.A. 2006. Soil profile water content determination: Sensor accuracy, axial response, calibration, temperature dependence, and precision. Vadose Zone Journal. 5:894-907. Interpretive Summary: Accurate knowledge of soil water content is extremely important for production agriculture and the agricultural and environmental sciences. The neutron moisture meter (NMM) and direct soil sampling are the tested and proven methodologies, but a new class of electromagnetic (EM) sensors has been introduced to the market recently. Comparisons of five EM sensors, the NMM, and direct measurements were made in three soils at water contents ranging from air-dry to saturated and over a range of temperatures, reflecting the variations possible in the field. Factory calibrations were inaccurate for all of the sensors, which required separate soil-specific calibrations. Two of the EM sensors were too sensitive to temperature variations to be useful in the field. Measurement volumes of four of the EM sensors were small, possibly indicating too much sensitivity to small-scale variations in field soil water content to be useful. Of the EM methods, only conventional time domain reflectometry (TDR) was reasonably accurate and temperature insensitive in all soils; but it is not useful for deep measurements in most soils. The NMM remains the only indirect method of soil profile water content determination that can be recommended.
Technical Abstract: Four electromagnetic (EM) soil water content sensors were compared to mass balance water contents, conventional TDR and the neutron moisture meter (NMM) in triply replicated columns of three soils varying in clay content from 17 to 48%. All of the devices were sensitive to temperature except for conventional TDR and the NMM. The Trime T3 and Delta-T PR1/6 devices were so sensitive to temperature (0.015 and 0.009 m**3/m**3 per degree C, respectively, in saturated soil) as to be inappropriate for routine field measurements of soil profile water content. Soil-specific calibrations increased the temperature sensitivity for all except the Trime sensor. Temperature sensitivity was up to 12 times larger at the saturated end compared with values in air-dry soils, corresponding to the much larger bulk electrical conductivities of these soils when saturated. Accuracy of the devices was judged by the root mean squared difference (RMSD) between column mean water contents determined by mass balance and those determined by the devices using factory calibrations. Smaller values of the RMSD metric indicated more accurate factory calibration. The Delta-T system was least accurate, with an RMSD of 1.299 m**3/m**3 at saturation. At the saturated end, the Diviner, EnviroSCAN, NMM and Trime devices all exhibited RMSD values >0.05 m**3/m**3, while conventional TDR exhibited RMSD <0.03 m**3/m**3. All of the devices would require separate calibrations for soil horizons with widely different properties. Measurements indicated small measurement volumes generally for the EM sensors, suggesting that these systems may be susceptible to small-scale variations in soil water content and to soil disturbance close to the access tube caused during installation.