Using TDR to estimate hydraulic conductivity and air entry in growing media and sand

Gas relative diffusivity measurements are key indicators of the quality of growing media. Previous studies have shown that this property can be estimated indirectly from measurements of the point of air entry (ψ_a), air-filled porosity (θ_a), and saturated hydraulic conductivity (K_s). Different tools are required to measure these parameters and this paper investigates how a single tool, time domain reflectrometry (TDR), already used to determine θ_a from measurements of volumetric water content (θ), could be utilized to measure ψ_a and K_s in growing media. Cylinders were filled with 13 different substrates and coarse sand. A transient-state technique (vertical infiltration at -1 hPa of water potential) was used to calculate K_s from θ measurements in time. In growing media, calculated K_s values from transient-state experiment were statistically equal to estimates obtained from steady-state measurements at a potential of -1hPa. However, both methods underestimated the K_s values obtained under steady-state conditions after a pulse of water had been applied or after prolonged wetting. For sand, TDR-based measurements and steady-state infiltration at -1 hPa, provided estimates of K_s equal to those obtained after prolonged saturation. To estimate ψ_a, TDR probes in a horizontal and a vertical position were tested in addition to a pressure transducer technique. For growing media, the horizontal positioning of the probes provided more consistent estimates of ψ_a than the other two techniques. Estimates of ψ_a with TDR in sand, both in vertical and horizontal position, were similar.