Laboratory calibrations of water content reflectometers and their in-situ verification

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.