TDR calibration of organic forest floor media

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 refractive mixing model. However, when the apparent dielectric constant of the water was considered, it was possible to predict the offset parameter because the low forest floor bulk density caused the calibration line slope and offset to be almost reversely proportional. Calibration parameters for different forest floor materials are presented. Allowing a somewhat higher margin of error we could also establish general calibration curves which were in close agreement with a published calibration curve (Malicki et al., 1994). An error analysis showed that decomposition of organic matter, residual water and temperature effects have negligible effects on the calibration parameters. Shrinkage of the organic material significantly influenced both the volumetric water content and the TDR reflection times. If not corrected, both effects yielded systematic errors of approximately 0.02 cm³ cm^-3 for a strongly shrinking H horizon.