Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 18, 2009
Publication Date: November 1, 2009
Citation: Evett, S.R., Schwartz, R.C., Tolk, J.A., Howell, T.A. 2009. Soil profile water content determination: Spatiotemporal variability of electromagnetic and neutron probe sensors in access tubes. Vadose Zone Journal. 8(4):926-941. Interpretive Summary: Studies of root water uptake, crop water use and water use efficiency, irrigation methods and efficiency, and soil hydrology all require accurate determination of volumetric soil water content and of the water content taken as a depth stored in a soil profile. The USDA-ARS, Conservation and Production Research Laboratory, Soil and Water Management Research Unit in Bushland, Texas, studied commercially available electronic water content sensors in comparison with the best scientific methods of water content determination (neutron moisture meter and direct soil sampling) to see if the electronic sensors were capable of determining soil water content with enough accuracy to be useful in irrigation and crop water use studies. The electronic sensors were not accurate enough for these studies. They did not correctly represent the changes in water content over a field or over time in response to irrigation applications. The electronic sensors, while relatively inexpensive and easy to use, are not recommended for scientific or engineering studies where getting the right answer to water resource management problems is necessary. The results of this study are useful in designing new electronic water content sensors that can overcome the problems of those currently available.
Technical Abstract: Since the late 1980s, electromagnetic (EM) sensors for determination on of soil water content from within nonmetallic access tubes have been marketed as replacements for the neutron moisture meter (NMM); however, the accuracy, variability and physical significance of EM sensor field measurements have been questioned. We studied the accuracy and variability of four EM sensors and the NMM, compared with gravimetric measurements, in transects of 10 to 20 access tubes during three field seasons, using soil-specific calibrations. The three capacitance EM sensors produced water content readings for which SD values were up to an order of magnitude larger than those from the NMM. The EM sensor based on travel time (waveguide) principles produced SD values up to six times larger than those of the NMM or gravimetric sampling. The EM sensors would require from two to 72 times as many access tubes to obtain a mean profile water content to a given precision than would the NMM or gravimetric sampling, with more tubes required for drier conditions. The NMM exhibited spatial variation of similar magnitude and pattern as that of gravimetrically sampled profile water contents. The EM methods poorly reproduced the spatial and temporal behavior of NMM and gravimetric sampling and implied spatial variability of profile water content that was not evident in either the NMM or gravimetric data, even though EM sensing volumes were larger than the approximately approximately 75-cm**3 volume of the gravimetric samples. We infer that EM sensors were influenced not only by the mean water content in the sampling volume but by the smaller scale structure of soil electrical properties.