Water segmentation and reconstruction of the four-phased volume

Once sand particles and root system elements are identified, water and air phases can be segmented. The remaining voxels not assigned to root or sand are separated by a threshold value. Similarly to the sand segmentation, the threshold is chosen such that the volume of water identified from the image at Day 0 matches the water volume initially introduced in the sample.

In the case of the fine sand, pore sizes and more particularly the size of zones filled with water when the sand is partially saturated (i.e., pores are only partially filled by water) may be close to the voxel size. Therefore, most of the voxels attributed to soil pores can include both air and water phases and cannot be attributed rigorously to one of the two phases only. Consequently, water phase identification is only qualitative at the pore scale in the fine sand. In the coarse sand, pores are larger and water phase identification is more representative of the local water distribution. Figure 4 shows examples of reconstructed four-phase volumes.

Phase segmentation of two root-soil specimens with the looser sand state – top row: fine Hostun sand (HN31), bottom row: coarse Hostun sand (HN1.5-2). (a, e) Greyscale volumes; (b, f) four-phased segmented volumes; (c, g) vertical section; (d, h) horizontal section.