1. Quantitative characterization of the pore network of a macroporous soil using µ X-ray CT Sofie Herman, department of Land Management, K.U. Leuven Sofie.Herman@agr.kuleuven.ac.be

2. Introduction Geometry of pore space: understand water flow Richards’ eq and effective hydraulic properties Macropores (cracks, root channels,…) Preferential flow Pore network models Need to quantify soil structure and pore network of a macroporous soil

3. General research outline Hydraulic characterization K(  ), h(  ) Characterization of porous structure and derivation of macropore network Simulation of flow (and transport) in a pore scale model Comparison between measured and simulated variables sandy loam macroporous soil K(  ), h(  ) Field and laboratory methods: e.g. multistep outflow method, tensio-infiltrometer measurements µCT and image analysis Interaction between different flow domains

4. Microfocus X-ray CT Sample: 5 cm diameter, 5 cm height Scan parameters: 135 kV and 0.1 mA Cu-filter (0.82 mm) to reduce beam- hardening Resolution: 0.1 mm

5. Determination and characterization of the pore network Macropores-matrix separation by binarization Macropore volume: 10 % Pore size distribution and connectivity function by means of mathematical morphology

6. Pore size distribution Opening of the image with spheres of increasing diameter Opening: erosion followed by dilation Original imageErosion of the original image Dilation of the eroded image: Smaller parts removed Struct. Elem.

7. Pore size distribution Result: cumulative PSD, pore size classes depend on pixel size D>0.11mmD>1.02 mm D>3.5 mmD>2.83 mmD>1.92 mm

8. Connectivity function Connectivity: Euler- Poincaré-characteristic: N: number of isolated components C: total number of redundant connections H: number of holes as a function of the pore size class

9. Determination of soil hydraulic properties Generation of a pore network with the same pore size distribution and connectivity function by the Topnet model (Vogel, 1998) Drainage is simulated (initial state: saturation) by applying pressure steps that correspond to a given pore size (Young- Laplace) within the model. Water retention and hydraulic conductivity curves are estimated under drainage

10. Pore network generated by the Topnet model based on the PSD and connectivity data Pores drained at P=-2cm Face-centered cubic grid Cylindrical pores with fixed radius r

11. Distribution of water content  calculated =0.27 cm 3 cm -3  measured =0.32 cm 3 cm -3 - = µ water µ wet µ dry highlow Moisture content

12. Swelling/shrinking Variable aperture of macropores depending on the degree of saturation drywet FWHM dry =0.48mmFWHM wet =0.33mm

13. Conclusions The macropore network was characterized quantitatively in terms of the pore size distribution and connectivity by µCT Effective hydraulic properties were estimated from a static pore network model µCT offers the potential to visualize dynamic phenomena that occur during wetting/drying cycles such as shrinking and swelling of pores

14. Future objectives Describe and measure swelling of pores as a function of moisture content Simulate drainage/imbibition of soil by a dynamic model Incorporate swelling into the model