Soil Science Society of America Journal 63:733-740 (1999)
© 1999 Soil Science Society of America
DIVISION S-1-SOIL PHYSICS
Soft X-ray Radiography of Drainage Patterns of Structured Soils
Y. Moria,
T. Maruyamab and
T. Mitsunoc
a Fac. of Life and Environmental Science, Shimane Univ., Matsue, 690-8504, Japan
b College of Bioresource Sciences, Nihon Univ., Fujisawa, 252, Japan
c Fac. of Agriculture, Kyoto Univ., Kyoto, 606-8502, Japan
yasushim{at}life.shimane-u.ac.jp
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ABSTRACT
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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.
Abbreviations: CT, computed tomography
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INTRODUCTION
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THE CHARACTERIZATION of saturated or unsaturated hydraulic properties of structured soils is one of the major challenges in soil science. Researchers have encountered large deviations in field measurements because of the differences in flow between matrix and macropores. The flow process in structured soils is difficult to determine with the traditional flow theory.
Infiltration measurements from boreholes or simulated macropores have been carried out to characterize structure-induced macropore flow. Philip (1985) conducted approximate analysis of the saturated bulb extending from the borehole. Smettem (1986) found that not only macropore length, but also macropore spacing, could affect water flow. Phillips et al. (1989) demonstrated, using glass tubes, that water under negative pressure could enter simulated macropores after establishing a thin water film on the full length of the macropore walls. Although these studies provide basic information for macropores and macropore flow, information regarding both macropores and the surrounding bulk soil is necessary.
Recently, researchers have stressed the interaction between macropore and matrix. The processes of water redistribution and water drainage were recognized as being initially dominated by macropores, followed by a mixture of macropore and matrix, and finally by the slower process of matrix drainage only. In the two-domain conceptualization, the domain of the soil matrix corresponds to micropores that behave as a homogeneous medium, and the domain of the soil macropores corresponds to the soil porosity component that conducts water faster than the matrix domain (Chen et al., 1993). van Genuchten and Wierenga (1976) designated the two domains of soil water as the mobile and immobile phases, which depend on soil properties and the flow rate, respectively. Hoogmoed and Bouma (1980), White (1985), and Germann and Beven (1985) all examined macropore and matrix flow on the basis of the two-domain approach.
Since macropore flow is considered structure-induced flow, nondestructive measurement of the macropore structure or macropore flow is required to obtain sufficient data on the characteristics of macropore flow. A measurement technique using x-rays would be helpful in this regard. The application of x-rays can provide images of soil structure or soil water distribution without disturbing the soil core. Petrovic et al. (1982), Anderson et al. (1988, 1990), and Grevers et al. (1989) measured the three-dimensional variations in the density or macroporosity within soil samples using computed tomography (CT). Dynamic water movement (Crestana et al., 1985), the breakthrough curve (Peyton et al., 1994), and preferential flow (Anton et al., 1996) were also visualized and analyzed using CT. In addition, gamma rays have been used to measure soil water distribution (Hainsworth and Aylmore, 1983). The image resolution of these systems was reported as 1 mm (Hopmans et al., 1994; Aylmore, 1993); however, some macropore flow has been reported to occur in small macropores having a diameter of <1 mm (Vepraskas et al., 1991; Mori et al., 1999). Moreover, the cost for using x-ray CT and the lack of software modified for soil science research discourage the extensive use of this tool (Aylmore, 1994).
For precise characterization of macropore flow, it is necessary to obtain high-resolution images of the water paths, as well as the dynamic properties of water flow. Therefore, we employed soft x-ray radiography, which is easier to handle and can provide high resolution images, as an alternative approach. Soft x-rays have longer wavelengths of 1 x 10-2 nm to 5 x 10-2 nm and, thus, a lower energy (0–100 kVp) than x-rays used in medical facilities. Low energy x-rays are absorbed selectively by the material and give high-contrast images when contrast media are used. According to a study by Mori et al. (1999), soft x-rays afforded a resolution of 42.3 µm, 20-times higher a resolution than that for natural soils (1 mm), when the contrast medium CH2I2 was used. The highest resolution was obtained at 80 to 85 kVp, a lower energy than that of x-rays used in medical facilities. Mori et al. (1999) visualized macropore flow in saturated structured soils and characterized the geometrical properties of the flow paths and dynamic properties of the flow. Although their images were two-dimensional, profile projection provided sufficient information on the characteristics of the macropore flow paths and flow conditions.
In this study, drainage patterns of structured soils were visualized using soft x-ray radiography. A contrast medium was used to observe the drainage process with high-resolution images. Domain concept was examined by image analysis, where macropore or matrix contribution to drainage was investigated. Furthermore, outflow experiments were employed to obtain the hydraulic properties along with the geometrical properties of the drainage process. Outflow experiments are flexible in initial and boundary conditions, and yield fast results across a wide range of water content. Since we focused on macropores, the unsaturated hydraulic conductivity for high water content was calculated. The objectives of this study were to investigate the drainage process using soft x-ray projection images of drainage patterns within an undisturbed soil and to characterize the soil properties containing macropores, based on both hydraulic and geometrical data.
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Materials and method
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Soils
Undisturbed soil cores were collected from three areas under different management: paddy field (Gray Lowland soils), upland field (Para-Kuroboku soils), and forest (Brown Forest soils). These are typical soils in Japan. Soil samples were taken from the plow layer and subsoil because these are reported to have different soil structures, which have been affected by tillage (Mori et al., 1992). The physical properties of these soils are shown in Table 1
. Undisturbed soil samples were taken using duralumin square sample holders of 5 x 5 x 5 cm with a 2-mm wall thickness. Duralumin has lower x-ray attenuation than steel.
X-ray Radiography with Outflow Experiments
Soft x-ray radiography was performed according to the procedure of Mori et al. (1999) using a side-beam soft x-ray apparatus (AW-100 extra, SOFTEX, Tokyo). A pressure cell was placed in the apparatus as presented in Fig. 1
, and outflow experiments were conducted. The hydraulic properties of the porous plate of the pressure cell are presented in Table 2
.
The soil sample was saturated for 2 d with 0.05 M CaSO4 solution to avoid clay particle dispersion. Outflow experiments were performed with a suction of 31.6 kPa below the porous plate for paddy soils, and 10 kPa for upland field and forest soils. One step was performed for the 10-kPa suction experiments, whereas a multistep of 10, 20, and 31.6 kPa was applied for the 31.6-kPa suction experiments. Detailed discussions on outflow experiments are presented by Kool et al. (1985), Parker et al. (1985), Hopmans et al. (1992) and Eching and Hopmans (1993b). The program MLSTPM (Eching and Hopmans, 1993a) was used to simulate outflow for parameter optimization of the hydraulic model. The model used in the program is the combination of van Genuchten's 
(h) model (1980),
 | (1) |
 | (2) |
with the pore-size distribution model of Mualem (1976) to yield (van Genuchten, 1980):
 | (3) |
where Se is the effective saturation (0 
Se 1), r (m3 m-3), and s (m3 m-3) are the residual and saturated water contents, respectively; Ks (cm h-1) is the saturated hydraulic conductivity;
, and l (assumed to be 0.5) are empirical parameters.
The initial and boundary conditions for the experiments are:
 | (4) |
where h(0,z) represents the initial soil matric head distribution, L is the total height of the soil sample with the porous plate, ha(t) is the applied pressure, and z = 0 corresponds to the top of the soil sample.
The objective function O(b) to be minimized is
 | (5) |
where b is a vector containing the optimized parameters r, s, 
, and n. Eching and Hopmans (1993b) recommended obtaining soil retention data to avoid the nonuniqueness problem. However, in this study soil disturbance by the tensiometer might create preferential flow. As an alternative, according to van Dam et al. (1992), the soil retention curve was measured beforehand and parameters for the retention curves were calculated by RETC (van Genuchten et al., 1991). Subsequently, the parameters were applied as initial values for optimization of outflow experiments to yield best-fit parameters for the unsaturated hydraulic conductivity curves.
The physical properties of the contrast medium, CH2I2, are presented in Table 3
. About 1 cm3 of contrast medium was applied dropwise to the soil sample. As water drained, the contrasting medium was drawn into the soil sample so that pore geometry could be observed from radiographs. Because CH2I2 has a lower surface tension than water, the contrast medium easily followed the water movement. Therefore, the resulting tracing pattern was considered to represent the movement of the drainage front. An experiment on the effects of contrast media that used glass capillary tubes was reported by Mori et al. (1999). They found that the effect of CH2I2 was negligible when the volume fraction of CH2I2 to water was <0.1. Another preliminary experiment was carried out to examine whether the contrast medium would pass the water and intrude into the saturated soil by its own weight. The contrast medium stayed on the soil surface for >24 h and intruded into the soil only when suction was applied.
X-ray radiographs were taken at various times, while cumulative outflow was measured as a function of time using a measuring cylinder. The experiments were conducted continuously for 24 to 72 h, until equilibrium was attained (Kool et al., 1985). X-ray radiographs were taken by stereography. Soil samples were moved to the left and right from the center of the beam during the exposure time. Since x-rays radiate, the resulting photograph becomes a stereograph. Soil pores were observed by stereoradiography to avoid erroneous recognition of the pore type: cylindrical, interaggregate, or matrix.
Image Analysis
After the time-sequential imaging of drainage patterns, image analysis was carried out using ImagePC (Scion Corp., Frederick, MD). Gray scale images of 512 by 512 pixels were imported to a computer. Changes in the drainage patterns were visualized by subtracting the reference image from the image of interest. The resultant images showed the time sequence of drainage patterns.
Macropores appeared as clearly delineated high-contrast regions, whereas the soil matrix appeared faint in soft x-ray images. Since these two regions were clearly distinguishable, the areal fraction of macropores to the total sample area was calculated by pixel counting.
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Results and discussion
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Geometrical Properties of Drainage from Undisturbed Soil Core
Subtracted images for pressurized desorption after sample saturation are shown in Fig. 2
. The numerical values in the figures show the average volumetric water content calculated using cumulative outflow volume data. The darker areas correspond to regions where water has drained. Macropore contributions to drainage were calculated from the subtracted images and are presented in Fig. 3
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Fig. 2 Time sequential drainage patterns of (a) paddy soil, (b) upland field soil, and (c) forest soil
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Fig. 3 Macropore contribution to drainage for (a) Gray Lowland paddy soil, (b) Para-Kuroboku field soil, and (c) Brown Forest soil
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As many researchers have noted, two-domain flow was observed in most of the soils. For most of the paddy field soils, cylindrical macropores drained first, followed by the soil matrix. This was especially notable for the core sample taken from a depth of 50 cm. This drainage pattern is in agreement with that described by Germann and Beven (1981). These macropores were caused by rice roots (Mori et al., 1994), which have large, straight vertical and small, horizontal roots. Hence, projection images showed that drainage from paddy field soils is strongly affected by root channels. Macropore contribution by the cylindrical macropores decreased after drainage of 0.06 pore volume (Fig. 3a). Similarly, for the Para-Kuroboku upland field soils, drainage originated first from the cylindrical macropores, followed by matrix flow. Macropore contribution decreased after 
0.1 pore volume (Fig. 3b). These transition pore volumes of 0.06 (3.3% of total volume) for paddy field and 0.10 (7.5% in total volume) for upland field soils corresponded well with the measured macroporosity of 2.1% and 9.7% in Table 1, respectively. Although the contrast medium was not real water, the contrasted areas showed good agreement with measured macroporosity. This means that contrast medium can trace the soil water drainage successfully.
In the third image of field soils, scattered black points were observed. This means that drainage continued through the whole soil volume for the whole duration of the outflow experiment and agreed with the results of Hopmans et al. (1992). There was no clear drainage pattern for forest soil, and images showed random patterns, indicating that flow occurred only in the interaggregate macropores or matrix.
Unsaturated Hydraulic Properties of Structured Soils
Figure 4
shows the unsaturated hydraulic conductivity of the three examined soils. The parameters of hydraulic conductivity were estimated from the outflow data. Some soils showed nonuniqueness problems when saturated hydraulic conductivity was applied as fixed values. Since the image analysis showed that the paddy field and upland field soils exhibit two-domain flow, we anticipated a discontinuity in the hydraulic conductivity curve near saturation. Therefore, the saturated hydraulic conductivity was assumed to be variable in the parameter estimation procedure. In Fig. 4, the unsaturated hydraulic conductivity curve was drawn using the fitted saturated hydraulic conductivity, whereas the actual measured saturated hydraulic conductivity is shown as a single data point. Although the Mualem model of the van Genuchten equation included in the MLSTPM code does not account for two-domain flow, discontinuity near saturation implied that two-domain flow has also been observed in numerical analysis. The deeper soils exhibited two-domain flow, whereas shallower soils did not. This could be explained by differences in bulk density. When soils are tightly packed, the effect of macropores becomes larger, resulting in two-domain flow. Differences between saturated and unsaturated conductivity increased in the order forest soils, upland field soils and paddy field soils. Paddy field soils showed differences of two orders of magnitude, with differences for upland field and forest soils being about one order of magnitude or less. This also agreed with the differences in bulk density; paddy field soils were tightly packed, whereas upland field and forest soils were not.
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Fig. 4 Optimized unsaturated hydraulic conductivity. Parameters, which were obtained from measured soil water retention curves, were used as initial values. Paddy 30 cm, Paddy 50 cm, Field 50 cm, and Forest 70 cm showed discontinuity near saturation. Saturated hydraulic conductivities of these soils were drawn as single markers (see arrows)
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Measured soil water retention curves used for the same parameter estimation procedure are shown in Fig. 5
, where symbols denote independently measured data, and curves are the result of parameter optimization. Small deviations from measured data were observed because the parameters were used as initial values and were not fixed. However, we concluded that the optimizations were successful.
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Fig. 5 Measured and optimized soil water retention curves. "m" and "o" ahead of the sample names represent measured and optimized, respectively. GP is Gray Lowland paddy soils, KU is Kuroboku upland field soils, and BF is Brown Forest soils
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For the soils that exhibited two-domain flow, water contents at the transition points were estimated from the x-ray photographs, and the corresponding matric potentials at these points were calculated using the measured soil water retention curves. The transition water contents were 0.53, 0.66, and 0.60, and the corresponding matric potentials were -42.5, -14.1, and -10.6 cm for paddy field soil (50-cm depth), upland field soil (50-cm depth), and forest soil (70-cm depth), respectively. These values are slightly larger than those reported by Mohanty et al. (1997), where the respective transition matric potentials were -1.5, -3.0, and -30 cm. These deviations probably arise from the large gradient in the soil water retention curve near saturation in our data. Tensiometers were not used in this research to avoid soil disturbance; however, direct measurement of matric potential would have been desirable.
In addition to bulk density being the controlling factor with regard to the presence of two-domain flow, we also concluded that paddy field soils from deeper layers can have significant macropore flow. The deeper soils have lower density than shallower soils, yet differences in unsaturated conductivity at saturation and near saturation were much larger than in shallower soil. Thin sections of the paddy field soils are presented in Fig. 6 . Alluvial materials accumulated around a subsoil macropore in Fig. 6a, which produced a black belt around the macropore in the gray scale figure. They show a color of 7.5YR 8/8 (yellow orange) and polarized under crossed Nicol light; therefore, they are oriented clay particles. On the other hand, the alluvial materials were not found in the plow layer in Fig. 6b. Clay particles are believed to flow through the soil with the irrigation water. Argillan and flood coatings have often been observed in soil macropores and indicate the existence of downward movement of colloidal suspensions (Mitsuchi, 1992). Subsoil is unaffected by tillage, so macropores would be conserved, allowing accumulation of clay particles. The coated inner wall of cylindrical macropores would be physically strong and able to allow macropore flow, even in low bulk density soils. Wang et al. (1994) described the hydrological properties of ants' burrows that are caused by wax and exudates that coated the macropore wall. A similar observation was made in this study, in which decayed root macropores were coated with clay particles. An influence of fracture coatings on fracture–matrix flow (Thoma et al., 1992) was also observed in agricultural field soils. In light of these findings, when we develop water flow models of soil containing macropores, we should take into consideration not only the two-domain flow, but also the resistance caused by these alluvial materials.
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Fig. 6 Thin section of root channel in Gray Lowland paddy soils: (a) 50 cm in depth; and (b) 20 cm in depth. Each photograph has a side length of 0.48 mm
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Conclusions
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We visualized drainage patterns using soft x-rays and characterized the geometrical and hydraulic properties of structured soils. As a result, the following conclusions were obtained.- Drainage patterns showed that drainage proceeded in sequence according to soil structure. For paddy and upland field soils, 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. Macropore contributions to drainage decreased 0.06 and 0.10 pore volume for paddy and upland field soils, respectively.
- Unsaturated hydraulic conductivity data showed a discontinuity near saturation, providing evidence of two-domain flow. This is explained by differences in bulk density. When soils are tightly packed, the effect of macropores becomes larger, resulting in two-domain flow. The differences increased in the order: forest, upland field, and paddy field soils.
- The thin section photographs showed that the macropore wall of paddy soils had been coated with alluvial materials. In this case, the coated inner wall of cylindrical macropores allows macropore flow, even in relatively low bulk density soils.
- Soft x-ray radiography provided fine images of drainage patterns. Furthermore, measurements of hydraulic properties along with the geometrical properties were useful for examining water flow in structured soils.
Soil Science Society of America 1997
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ACKNOWLEDGMENTS
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The authors are grateful to Dr. Nobuo Toride, Saga University, for his valuable comments during the development of this study. We are also thankful to Dr. Mitsuhiro Inoue, Tottori University, for his suggestions on using a multistep outflow experiment. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan.
Received for publication June 22, 1998.
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TOP
ABSTRACT
INTRODUCTION
