SOIL TECHNOLOGY Soil Technology I1 (1996) 205-217 TDR calibration of organic forest floor media M.G. Schaap *, L. de Lange, T.J. Heimovaara Landscape and Environmental Research Group, UniversiQ of Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ Amsterdam The Netherlands Accepted 19 September 1996 Abstract We carried out a time domain reflectometry (TDR) calibration for 25 forest floor samples of five different forest stands. Linear regression was used to estimate the volumetric water content from the refraction index. Due to the presence of bound water, it was impossible to predict the calibration line parameters from theoretical values with a refractivemixing model. However, when the apparent dielectricconstant of the water wasconsidered, it was possible to predict the offset parameter because the low forestfloor bulk density caused the calibration line slope andoffset to be almost reverselyproportional. Calibration parameters for different forest floor materials are presented. Allowing a somewhat higher margin of error we couldalso establish general calibration curveswhich werein closeagreement with a published calibration curve (Mali&i et al., 1994). An error analysis showed that decomposition of organicmatter,residual water and temperature effects have negligible effects on the calibration parameters. Shrinkage of the organicmaterial significantly influenced both the volumetric water contentand the TDR reflection times.If not corrected. both effectsyielded systematic errorsof approximately 0.02 cm3 cmP3 for a strongly shrinking H horizon. Keywords: Time domain reflectometry; Soil water; Forest floor; Calibration 1. Introduction Time domain reflectometry (TDR) has become a widely accepted method to deter- mine soil water contents in a quick, accurate, and non-destructive way. Such character- istics make TDR also very suitable for application in the forest floor since water contents in this layer are highly dynamic, both in time and space. A large amount of TDR related work concerns TDR travel time-water content calibrations (Topp et al., * Corresponding author. Present address: U.S. Salinity Laboratory, 450 Big Springs Road, Riverside, CA 92507, USA. Tel.: + 31-20-5257450; fax: + 31-20-5257431; e-mail: mschaap@ussl.ars.usda.gov. 00933-3630/96/$17.0 Copyright 0 1996 Elsevier Science B.V. AU rights reserved. PII SO933-3630(96)00128-6