Direct measurement of the soil water retention curve using X-ray absorption

X-ray absorption measurements have been explored as a fast experimental approach to determine soil hydraulic properties and to study rapid dynamic processes. As examples, the pressure-saturation relation theta(psi) for a uniform sand column has been considered as has capillary rise in an initially dry sintered glass column. The theta(psi)-relation is in reasonable agreement with that obtained by inverting a traditional multi-step outflow experiment. Monitoring the initial phase of capillary rise reveals behaviour that deviates qualitatively from the single-phase. local-equilibrium regime described by Richards' equation.