MEASURING WATER CONTENT OF SOIL COLUMNS IN THE LABORATORY: A COMPARISON OF GAMMA RAY ATTENUATION AND TDR TECHNIQUES

Volumetric water content was measured by time domain reflectometry (TDR) and by a single-beam γ-ray attenuation at eight positions on each of three soil columns. Both methods measured equally well at single locations and both showed some position dependence, which was greater for γ-ray attenuation than for TDR. An error analysis showed the major contributor to variance in the γ-ray measurement was the unknown variability of bulk density which gave rise to uncertainty of ± 0.05 cm3∙cm−3 for three soils. For the TDR technique all sources of variance resulted in a total uncertainty of ± 0.02 cm3∙cm−3. The greater sampling volume of the TDR as used in this experiment was advantageous with the naturally structured heterogeneous soils. Key words: Soil structure, error analysis, γ-ray attenuation, time domain reflectometry.