ARS | Publication request: Field Calibration of Water Content Reflectometers

Submitted to: Soil Science Society Of America Journal
Publication Acceptance Date: March 16, 2004
Publication Date: September 15, 2004
Citation: Chandler, D. Seyfried, M.S., Mcnamara, J.P. 2004. Field Calibration Of Water Content Reflectometers. Soil Science Society Of America Journal. 68(5): 1501-1507.

Interpretive Summary: With the recent advances in electronic technology a number of soil water sensors have been developed that may potentially provide accurate and relatively inexpensive continuous soil water content information for a variety of uses. Unfortunately, the sensors developed to date require some degree of calibration. This can be difficult and expensive in field conditions. We designed an approach to field calibration that allows for accurate, nondestructive field calibration of sensors and tested on field data collected near Boise, Idaho. We found that the approach was accurate and that it is important not only to calibrate for the effects of different soils, but to also consider the variations in individual sensors. The method is useful primarily for other researchers because it requires at least partial access to relatively expensive instruments. Research regarding the effects of soil water on plant yield, soil water and contaminant movement, plant water use and stream flow generation could benefit from this approach.

Technical Abstract: Soil volumetric water content (VWC) is regularly monitored through measurement soil dielectric properties with techniques similar to time domain reflectometry (TDR). The accuracy of these techniques relies on both the sensor uniformity and accuracy of the calibration equation. An in situ technique for calibrating one such instrument, the Campbell Scientific Water Content Reflectometer (WCR), with TDR is presented. The field calibration reduced the error estimate to near ± 1% VWC, from the factory calibration error estimate of ± 3% VWC. The improved accuracy from the in situ calibration resolved some apparent problems in the soil moisture profiles, as represented by the standard calibrations for the WCRs, at soil pits of two different textures.