X-RAY TOMOGRAPHY OF SOIL-WATER DISTRIBUTION IN ONE-STEP OUTFLOW EXPERIMENTS

The application of an optimization procedure to indirectly infer soil hydraulic functions from measured outflow in pressurized soil samples (one-step outflow method), although widely used, has not been fully accepted. Reported problems are associated with the nonuniqueness and instability of the solution, as well as with the flow conditions that exist in the soil as a result of the high pressure difference across the sample. Computed tomography (CT) imaging of two soils with different texture was used to measure soil water distribution during the outflow process. Tomographic results show that unfavorable conditions exist if the soil is initially saturated. Richards' equation, which is employed in the optimization technique, assumes air continuity everywhere in the sample at all times. However, the ceramic plate at the outflow end of a pressurized saturated soil sample prevents the existence of a continuous air phase during initial desorption. We suggest that better results are obtained with the one-step outflow method if the sample is initially unsaturated when pressure is applied.