Point Soil Water Retention Curves Determined Using Neutron Radiography.

The soil water retention curve is needed for simulating flow and transport processes in a variety of vadose zone applications, such as irrigation of agricultural crops, groundwater recharge, and the fate of spilled or leaking chemicals. Traditional techniques for measuring this curve, such as the hanging water column method, yield height-averaged curves which have to be modeled using inverse procedures to extract relevant point parameters. We have developed a technique for directly determining point soil water retention curves using neutron radiography. 2-D images of quasi-equilibrium volumetric water contents at nine imposed basal suctions were obtained during monotonic drying of Flint sand at Oak Ridge National Laboratory's High Flux Isotope Reactor CG1-D beamline. All of the images were normalized with respect to the oven dry image. Volumetric water contents were calculated on a pixel-by-pixel basis using Beer-Lambert’s law after taking into account beam hardening and geometric corrections.  A grid of 120 points was superimposed on the images, and the volumetric water content values at these locations were combined with information on the known distribution of suction within the sample to give 120 point soil water retention curves.  These curves were well described by the Brooks and Corey equation.  Median values of the resulting parameter estimates closely matched those for the point curve obtained by inverse modeling of height-averaged volumetric water content versus suction data measured independently by the hanging water column method. The advantage of using neutron radiography for these measurements is that multiple point curves are obtained for a single sample, thereby permitting detailed investigation of the spatial variation in this hydraulic property at the mesoscopic scale.  The data can also be up-scaled to predict the water retention curve for any height increment of interest.