Discriminating the influence of soil texture and management-induced changes in macropore flow using soft X-rays

Soft X-ray radiography, a nondestructive technique, was employed to examine macropore flow characteristics in soils of different management : paddy, upland field, and forest. A constant-head saturated hydraulic conductivity experiment was conducted using the soft X-ray apparatus. A contrast medium, CH 2 I 2 , was applied as a tracer to obtain contrast images of macropore flow. The visualization efficiency has been increased by the low-energy system that enables high-contrast images to be obtained as a result of a wider range of mass attenuation coefficients. CH 2 I 2 has a larger attenuation coefficient than bulk soil and has a kinematic viscosity similar to water. It was introduced dropwise to allow movement with the water flow. Macropore flow was captured using a soft X-ray TV camera, whereas flow paths were photographed on X-ray films. Light duralumin, was used for the sample holder instead of the conventionally used stainless steel in order not to cancel the efficiency induced by the contrast medium. We found that the highest resolution obtained was 42.3 μm. This corresponded well to the calculated value of 30 μm at an energy level of 60 to 70 keV. The resultant images showed that only about 30% of potentially available macropores conducted water flow. Macropore flow paths were affected by land management: straight isolated cylindrical paths in paddy field soils; a network of tortuous paths in upland field soils; and round cloudy interaggregate paths in forest soils. Macropore flow velocity was estimated from CH 2 I 2 movement. Because the flow area was restricted to a small number of macropores, the Reynolds number of flow ranged from 51.6 to 88.6. The flow was in a transition region from laminar to turbulent flow. Under actual conditions of macropore flow, the assumption of laminar flow was not realized. Both structural and dynamic analyses of macropore flow explained the flow characteristics successfully.