Soft x-ray radiography of drainage patterns of structured soils

Characterization of unsaturated hydraulic and geometrical properties of undisturbed soils is essential for understanding water flow and solute transport in soils. We visualized the drainage pattern by soft x-ray radiography using a contrast medium. Outflow experiments were conducted simultaneously and unsaturated hydraulic conductivity was calculated by parameter estimation. Time sequential drainage patterns were obtained by the subtraction technique, which was done by subtracting a reference image from the image of interest. Drainage patterns of paddy and upland field soils showed that drainage occurs through macropores first, followed by interaggregate macropores, and then finally through the soil matrix. The drainage pattern resembles two-domain flow of macropore and matrix. On the other hand, drainage occurred through the entire sample in forest soils. Unsaturated hydraulic conductivity data showed a discontinuity near saturation, providing evidence of two-domain flow. This was explained by differences in bulk density. Tightly packed soils allowed macropore flow. The differences increased in the order: forest, upland field, and paddy field soils. An exception was observed in paddy field soils, where roughly packed deeper soils showed significant macropore flow. The thin section photograph of paddy field soils from deeper layers revealed that the macropore wall was coated with alluvial materials that prevented smooth water flow across the macropore wall. In this regard, we should also consider the resistance caused by these materials. Soft x-ray radiography of the drainage pattern was helpful in characterizing the unsaturated hydraulic properties of structured soils.