Laboratory calibrations of water content reflectometers and their in-situ verification
Roland Stenger A D , Greg Barkle A B and Craig Burgess A CA Lincoln Environmental Research, Private Bag 3062, Hamilton, New Zealand.
B Present address: Aqualinc Research Limited, PO Box 14041, Hamilton, New Zealand.
C Present address: Department of Earth Sciences, University of Waikato, Hamilton, New Zealand.
D Corresponding author. Email: stenger@lvlham.lincoln.ac.nz
Australian Journal of Soil Research 43(5) 607-615 https://doi.org/10.1071/SR04177
Submitted: 14 December 2004 Accepted: 3 May 2005 Published: 8 August 2005
Abstract
Like time domain reflectometers, cheaper CS615 water content reflectometers (WCRs) also measure dielectric properties of the soil to determine its volumetric water content (VWC), but are more affected by environmental factors. Quadratic equations described the laboratory data from 12 horizons of 4 soils of vastly differing properties slightly better than linear equations. Root mean squared errors (RMSE) averaged over the 3 horizons increased slightly in the order Gley (av. 1.0%), Pumice (av. 1.3%), Recent (av. 1.6%), and Allophanic soil (av. 1.9%). Using the manufacturer’s standard calibration resulted in significantly higher RMSE (av. 6.2–25.3%) and mean errors (av. –5.5% to +21.5%), with the VWC of the 2 soils of volcanic origin being underestimated. An atypical dielectric response of water stored in volcanic soils has been attributed to their low bulk density, high porosity, and large specific surface area. The in-situ verification was hampered by the variability observed between the data from duplicate WCRs. Measuring the inter-sensor variability in air and water indicated that this could account for a significant part of this variability, while small-scale variation of VWC in-situ was also observed. Nevertheless, the laboratory calibrations were usually better, or at least similarly suited, to describe the in-situ data than the manufacturer’s calibrations.
Additional keywords: soil moisture, dielectric properties, volcanic soils.
Acknowledgments
We thank the Foundation for Research, Science and Technology for funding this project (LVLX0006 and LVLX0302) and Andrew Western (University of Melbourne) for making unpublished parameter values for his temperature correction equation available to us. Appreciation is also given to Landcare Research Ltd for access to the lysimeter facility. Thanks also to Ian Woodhead (Lincoln Technology) and 2 anonymous reviewers who improved this paper by providing constructive criticism.
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