We carried out a time domain reflectometry (TDR) calibration for 25 fo
rest 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 predic
t the calibration Line parameters from theoretical values with a refra
ctive mixing model. However, when the apparent dielectric constant of
the water was considered, it was possible to predict the offset parame
ter because the low forest floor bulk density caused the calibration l
ine slope and offset to be almost reversely proportional. Calibration
parameters for different forest floor materials are presented. Allowin
g a somewhat higher margin of error we could also establish general ca
libration curves which were in close agreement with a published calibr
ation curve (Malicki et al., 1994). An error analysis showed that deco
mposition of organic matter, residual water and temperature effects ha
ve negligible effects on the calibration parameters. Shrinkage of the
organic material significantly influenced both the volumetric water co
ntent and the TDR reflection times. If not corrected, both effects yie
lded systematic errors of approximately 0.02 cm(3) cm(-3) for a strong
ly shrinking H horizon.