## Abstract

The soil water retention function is needed for modeling multiphase flow in porous media. Traditional techniques for measuring the soil water retention function, such as the hanging water column or pressure cell methods, yield average water retention data which have to be modeled using inverse procedures to extract relevant point parameters. In this study, we have developed a technique for directly measuring multiple point (pixel-scale) water retention curves for a repacked sand material using 2-D neutron radiography. Neutron radiographic images were obtained under quasi-equilibrium conditions at nine imposed basal matric potentials during monotonic drying of Flint sand at the High Flux Isotope Reactor (HFIR) Cold Guide (CG) 1D beamline at Oak Ridge National Laboratory. All of the images were normalized with respect to an image of the oven dry sand column. Volumetric water contents were computed on a pixel by pixel basis using an empirical calibration equation after taking into account beam hardening and geometric corrections. Corresponding matric potentials were calculated from the imposed basal matric potential and pixel elevations. Volumetric water content and matric potential data pairs corresponding to 120 selected pixels were used to construct 120 point water retention curves. Each curve was fitted to the Brooks and Corey equation using segmented non-linear regression in SAS. A 98.5% convergence rate was achieved resulting in 115 estimates of the four Brooks and Corey parameters. A single Brooks and Corey point water retention function was constructed for Flint sand using the median values of these parameter estimates. This curve corresponded closely with the point Brooks and Corey function inversely extracted from the average water retention data using TrueCell. Forward numerical simulations performed using HYDRUS 1-D showed that the cumulative outflows predicted using the point Brooks and Corey functions from both the direct (neutron radiography) and inverse (TrueCell) methods were in good agreement with independent measurements of cumulative outflow determined with a transducer. Our results indicate that neutron radiography can be used to quantify the point water retention curve of homogeneous mineral particles. Further research will be needed to extend this approach to more heterogeneous porous media.

Original language | English |
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Pages (from-to) | 1-8 |

Number of pages | 8 |

Journal | Advances in Water Resources |

Volume | 65 |

DOIs | |

State | Published - Mar 2014 |

### Funding

This Research was supported by the Laboratory Directed Research and Development (LDRD) Program of Oak Ridge National Laboratory and the Joint Directed Research and Development (JDRD) Program of the University of Tennessee UT-ORNL Science Alliance. The Authors thank Sophie Voisin, Computational Sciences and Engineering Division, ORNL for her contributions to the development of the MATLAB image analysis code. The Authors also acknowledge the assistance of various HFIR support groups and individuals, including the Machine Shop, the Instrument Development Group, Lakeisha Walker, Jaimie Werner, and Brent Taylor. This Research at Oak Ridge National Laboratory’s High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy, which is managed by UT-Battelle, LLC.

## Keywords

- Neutron radiography
- Point water retention curves
- Quantification