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 now characteristics in soils of different management: paddy, upl and field, and forest. A constant-head saturated hydraulic conductivity exp eriment was conducted using the soft X-ray apparatus. A contrast medium, CH 2I2, was applied as a tracer to obtain contrast images of macropore now. Th e 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 o f mass attenuation coefficients. CH2I2 has a larger attenuation coefficient than bulk soil and has a kinematic viscosity similar to water, It was intr oduced dropwise to allow movement with the water now. Macropore now was cap tured using a soft X-ray TV camera, whereas now 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 obtain ed was 42.3 mu m This corresponded well to the calculated value of 30 mu m at an energy level of 60 to 70 keV, The resultant images showed that only a bout 30% of potentially available macropores conducted water now. Macropore now paths were affected by land management: straight isolated cylindrical paths in paddy field soils; a network of tortuous paths in upland field soi ls; and round cloudy interaggregate paths in forest soils. Macropore now ve locity was estimated from CH2I2 movement. Because the now area was restrict ed to a small number of macropores, the Reynolds number of now ranged from 51.6 to 88.6. The now was in a transition region from laminar to turbulent now. Under actual conditions of macropore now, the assumption of laminar no w was not realized, Both structural and dynamic analyses of macropore now e xplained the now characteristics successfully.