ARS | Publication request: Water content reflectometer calibration, field versus laboratory

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 24, 2008
Publication Date: January 21, 2009
Citation: Logsdon, S.D. 2009. Water content reflectometer calibration, field versus laboratory. Soil Science Society of America Journal. 73(1):1-6.

Interpretive Summary: The newer soil water sensors are easier to use, but often require site-specific calibration. This study showed that the laboratory calibration was linearly-related to soil water content and temperature. The laboratory calibration resulted in calculated soil water contents that were too high for data collected in the field, even though the same field soils were used in the laboratory. The field calibration was not linear; the mid-range soil water contents produced sensor values that were much higher than the laboratory values while the wet and dry field values were closer to the laboratory values. This information is important for scientists who use newer sensors to monitor soil water content. Increasingly non-scientists (i.e., regulators and irrigation specialists) are using these sensors because of low-cost and ease of use, and they need to be aware of the need for field calibration of the sensors to ensure accurate results.

Technical Abstract: For soils with large amounts of high-charge clays, site-specific calibrations for the newer permittivity probes that operate at lower frequencies, often have higher permittivity values than factory calibrations. The purpose of this study was to determine site-specific calibration of water content reflectometers (CS616) for soils with "superactive" mineralogy. Field calibration CS616 values (converted to square root of apparent permittivity, or SRAP) were determined as a function of water content from neutron probe. Uncertainties of neutron probe and CS616 measurements were included in the analysis. Laboratory (lab) calibrations were determined from undisturbed columns taken horizontally from the same depths where the CS616 probes had been in the field. A third study compared data for two packed columns, one with wet and dry zones and the other with homogeneous water content. The laboratory calibration was linear with SRAP. The field calibration was nonlinear, and the SRAP values were even higher than the laboratory values, emphasized more in the intermediate water content range. The column with wet and dry soil had higher SRAP than the homogeneous column. Uneven soil water content could have contributed to the higher SRAP and nonlinear SRAP (water content) for field data than for laboratory calibration.