TDR CALIBRATION OF ORGANIC FOREST FLOOR MEDIA

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.