X-ray imaging of water flow through soil

In this study a laboratory experiment on preferential flow was conducted with the aim to quantify the importance of water repellency, soil structure and irrigation rate on water infiltration into soil. To do so, the study was conducted as follows: (a) comparison between the wetting fronts of the irrigations done on the same sample; (b) the tortuosity of the wetting fronts, this was analysed calculating a tortuosity index; (c) the average infiltration velocity and (d) a visual comparison between the 3D images. The aim of the latter was to see whether there were differences in the structure before and after the irrigations, and to understand how the structure influenced the water infiltration through the soil. Furthermore, we tested whether the X-ray computed tomography is a suitable technique for this kind of studies. In order to use this technique, an irrigation device and fifteen quasi-2D boxes were specially designed to fit in the X-ray scanner. The experiments were conducted on three different kinds of soil samples: a bare mixture of soil and peat (50/50 by volume), a planted mixture of soil and peat (50/50 by volume) and garden soil samples that once collected and prepared where buried underground for four weeks. The images obtained with the x-ray machine were analysed using the computer program ImageJ. The main outcomes of this project were found to be: (a) the infiltration patterns through bare and garden soil samples remained identical throughout different irrigations runs while they changed in the sown soil samples; (b) high irrigation rates reduced the wetting front tortuosity; (c) the tortuosity of the wetting front increased with time; (d) macroporosity and initial water content strongly influenced the water infiltration velocity through the soil, the bigger they were the higher the infiltration velocity; (e) plants roots and long consolidation times exerted a stabilizing effect on the soil structure. The X-ray computed tomography, despite the setup can be improved, demonstrated to be a useful technique to investigate water infiltration through the soil as it was able to very well detect water in the soil samples. If used together with an image analysis software it has a huge potential as it can be used even for 3D infiltration experiments. During this project just a small part of the X-ray computed tomography potential was used, to estimate its potential many more similar projects are needed