Multiple pixel-scale soil water retention curves quantified by neutron radiography |
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| Institution: | 1. Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, USA;2. Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA;3. Department of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, TN, USA;4. Department of Geosciences, Texas Tech University, Lubbock, TX, USA;5. Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA;1. Maier-Leibnitz Zentrum (FRM II), Technische Universität München, Lichtenbergstraße 1, D-85748 Garching, Germany;2. Laboratory of Radiochemistry and Environmental Chemistry, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland;3. Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT),, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany;1. Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, No. 1, Xikang Road, Nanjing 210098, China;2. College of Civil Engineering & Architecture, Shandong University of Science and Technology, No. 579, Qianwangang Rd., Huangdao, Qingdao, 266590, China;1. LEN Technologies, Oak Hill, VA, USA;2. Department of Civil Engineering, The University of Texas at Arlington, Arlington, TX 76019-0308, USA;1. Department of Civil and Building Engineering, and Architecture (ICEA), Università Politecnica delle Marche, Italy;2. Department of Civil, Environmental and Architectural Engineering (ICEA), University of Padova, Italy |
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| 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. |
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| Keywords: | Point water retention curves Neutron radiography Quantification |
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