Continuous Macropores Research Articles

Preferential Transport of Solute through Soil Columns Containing Constructed Macropores

AbstractImplication of macropore flow in enhancing solute transport has prompted much research. The objective of this study was to characterize solute transport in constructed macropores in three soils of different water conductivities. The macropores were constructed by inserting metal rods into uniformly packed soil columns of 0.2‐m i.d. and 0.3 m long. A set of nine soil columns was prepared and a total of 52 displacement experiments was conducted using NO3 as a tracer. Significant preferential flow occurred in the silt loam containing seven continuous macropores of 3‐mm diam. even at fluxes <0.5K, where K is saturated hydraulic conductivity of a column matrix. At fluxes far smaller than those causing saturation, no preferential flow took place and the breakthrough curves (BTCs) displayed single peaks, indicating that only the matrix contributed to solute transport. As fluxes increased, BTCs exhibited double peaks, and the relative magnitudes and positions of the two peaks were flux dependent. These two‐curve or double‐peak BTCs may represent solute transport through the macropores and the matrix, respectively. Due to the difficulty in keeping macropores open in silica sand, especially at fluxes ≥K, little evidence of preferential flow was observed in this system. This was also ascribed to the porous and conductive nature of the medium. Preferential flow also occurred in the loamy sand, but a greater flux was needed to get the same degree of preferential flow as in the silt loam. In general, in the coarser soil, less preferential flow occurred even at similar normalized flow rates. Macropore continuity has been shown to be increasingly important in macropore flow when flux increases.

Preferential flow in sandy loam soils as affected by irrigation intensity

Dye-tracer studies in the field using Brilliant Blue FCF as tracer were performed to investigate the effect of irrigation intensity and soil heterogeneity on preferential flow. In two fields, both level and newly tilled in terms of seed bed preparation, to plots of 1.6 × 1.6 m were applied 50 mm of dye solution at rates of 10 and 50 mm h −1. In the second year level, plots of grass of similar size were applied with 25 mm dye solution at a rate of 3.1, 6.2, 12.5, and 25 mm h −1. For all plots the stained patterns were examined one or two days after application of dye solution by the excavation of 11 vertical cross sections of 100 × 100 cm and 10 cm apart from each other. Flow patterns were digitized and depth functions for the degree of dye coverage and the number of activated flow channels were calculated. Furthermore, the structural features of each cross section were examined visually. The results show that deep penetration of water into the soil profile took place as preferential flow through macropores, mainly earthworm channels, with much of the water thus bypassing the soil matrix. In the top 0–25 cm layer, the degree of dye coverage tended to be larger for the lower irrigation intensities indicating that water flow in the top soil took place through a relatively great proportion of the pores in the soil matrix. In the 35–100 cm subsoil layer the number of stained macropores tended to be larger for the higher irrigation intensities. Thus, at higher irrigation intensity a positive pressure potential apparently developed more extensively in the topsoil initiating preferential flow through a greater number of macropores in the subsoil. In the newly tilled soil, water flow took place through a relatively great part of the topsoil matrix. Deeply penetrating stained earthworm channels originated, predominantly, in the well defined transition zone between topsoil and subsoil. In the soil left untilled and grass covered for about one year the continuity of macropores was more pronounced, and stained channels could frequently be traced from the subsoil all the way to the soil surface, in particular at low irrigation intensity.

Catchment infiltration II: Contributing areas

This study considers two issues of interest to the hydrologic and geographical information systems community. One deals with identifying the spatial distribution of infiltration and runoff contributing areas. The other addresses process modelling within a GIS framework. The study operates on the premise that partitioning of precipitation into runoff or infiltration depends on rainfall intensity and on soil properties. The problem is that neither local rainfall intensity, nor soil properties such as infiltration capacity and macroporosity are known well enough for all points of a catchment and need to be estimated. We infer local intensity from the interpolated distribution of cumulated rain depths over the catchment and record duration at the official met site. Measured values of sorptivity and hydraulic conductivity define infiltration. Negative head infiltration describes macroporosity. To scale‐up measured point values to larger areas and to model infiltration and macropore continuity at a catchment scale we use geostatistical kriging and conditional simulation together with standard GIS techniques of overlay manipulation. Results delineate areas contributing to runoff and infiltration and relate them to macroporosity. By intersecting overlays of precipitation with those of infiltration we create alternate GIS masks targeting specific portions of the watershed as either runoff or infiltration contributing zones. Choice of cell size and time interval define the scales of averaging for the application. Kriged surfaces illustrate the distribution of catchment infiltration, while conditional simulation provides a mechanism to define model uncertainty.

Field-scale solute transport in a heavy clay soil : J. J. B. Bronswijk, W. Hamminga & K. Oostindie, Water Resources Research, 31(3) 1995, pp 517–526

The transport of a bromide tracer was studied in a heavy clay soil. After applying the tracer on day 0, water and bromide content profiles in the unsaturated zone were determined on days 6, 46, 209, 335, and 572. Furthermore, water samples were taken frequently from the groundwater and drain discharge and analyzed for bromide content. In the clay soil studied, solute transport occurred in three domains: large continuous macropores, smaller more tortuous mesopores, and the pores inside the soil aggregates. Bromide transport through the first domain, i.e., preferential flow through large macropores, amounted only to a few percent. This resulted, however, in maximum bromide concentrations in groundwater and drain discharge. Quantitatively, the most important solute transport occurred through the mesopores. This transport was characterized by high spatial variability, significant lateral transport, and low mobile water volumes (6%). At the field scale this resulted in the rapid leaching of solute in comparison with earlier experiments on sand and loam soils. After only 6 days and 34 mm net precipitation, the average bromide concentration peak had reached a depth of 55 cm. In spite of the heterogeneous transport and the presence of preferential flow, field-average solute concentration profiles exhibited regular and smooth shapes. Solute transport inside the soil aggregates played only an indirect role, i.e., via solute retardation as a result of convection and diffusion into the aggregates.

FLOW MECHANISMS THROUGH CONTINUOUS AND BURIED MACROPORES

Tillage destroys stable continuous macropores and at the same time creates unstable tillage pores within the tilled layer. The buried macropores below the tilled layer are not harmed. Flow through continuous pores is usually considered to be laminar. The objectives of this study were to determine the conditions under which water would flow into a buried macropore in a soil column, and to determine the flow regime inside a macropore. Packed columns (457 mm length, 76 mm diameter) were constructed with a sloping base to measure separately outflow from the artificial macropore (6 mm diameter) and the soil matrix. The columns were then tilled artificially by removing the top 12 cm and repacking. In addition, a study was conducted in a sloping box of soil with four macropores beneath a tilled layer. Dye was added to two 76-mm-diameter rings within the box, either immediately or 1 day before ponded infiltration into the rings. Ponded outflow rates from continuous 6-mm macropores ranged from 5 to 11 ml/s, which is equivalent to Reynolds numbers as high as 2370. Outflow also occurred from buried macropores, although the rate was reduced (0.01 to 0.3 ml/s) compared with continuous macropores. If dye was added immediately before infiltration, dye appeared in macropore outflow, but if dye was added 1 day before infiltration, no dye appeared in macropore outflow but only in matrix outflow. This indicated either preferential flow carrying the dye (if no time for binding to soil) or preferential flow of water around the dye. Even buried macropores can function as preferential flow pathways, and turbulent flow might occur through large, continuous macropores.

Field‐Scale Solute Transport in a Heavy Clay Soil

The transport of a bromide tracer was studied in a heavy clay soil. After applying the tracer on day 0, water and bromide content profiles in the unsaturated zone were determined on days 6, 46, 209, 335, and 572. Furthermore, water samples were taken frequently from the groundwater and drain discharge and analyzed for bromide content. In the clay soil studied, solute transport occurred in three domains: large continuous macropores, smaller more tortuous mesopores, and the pores inside the soil aggregates. Bromide transport through the first domain, i.e., preferential flow through large macropores, amounted only to a few percent. This resulted, however, in maximum bromide concentrations in groundwater and drain discharge. Quantitatively, the most important solute transport occurred through the mesopores. This transport was characterized by high spatial variability, significant lateral transport, and low mobile water volumes (6%). At the field scale this resulted in the rapid leaching of solute in comparison with earlier experiments on sand and loam soils. After only 6 days and 34 mm net precipitation, the average bromide concentration peak had reached a depth of 55 cm. In spite of the heterogeneous transport and the presence of preferential flow, field‐average solute concentration profiles exhibited regular and smooth shapes. Solute transport inside the soil aggregates played only an indirect role, i.e., via solute retardation as a result of convection and diffusion into the aggregates.

The use of a simple field air permeameter as a rapid indicator of functional soil pore space

Currently there is a lack of quantitative data on the functional classification of soil structure, particularly in routine soil surveys, because field physical measurements of flow characteristics are slow and require expertise. A simple air permeameter was developed which allows rapid, in situ measurements to be made of the intrinsic permeability of the soil to air ( k a). Measurements of k a are highly dependent on the structure of the soil, in particular the size and continuity of macropores, and therefore can be used as an estimate of functional pore space (pore space contributing to the transmission of air and water) in the soil. Values of k a ranged from 5.6 μm 2 for a compacted alluvial soil under pasture to 276.8 μm 2 for a red-brown earth under native grass, cultivated one year before the time of measurement. Development of a functional pore space scoring table for use in the field has shown that k a 0.5 can be estimated from morphological descriptions of the soil. However, field descriptions are qualitative, subjective and difficult to apply consistently. The air permeameter can discriminate between the structural differences in soil under different management practices. The air permeameter should appeal to pedologists who wish to have a quantitative aid in the interpretation of soil structure.

Earthworm Macropores and Preferential Transport in a Long‐Term Manure Applied Typic Hapludalf

AbstractDeep burrowing earthworm species have been found to be present in soils with a history of manure application. This study was designed to quantify the effects of long‐term application of liquid dairy manure and inorganic fertilizer on the distribution of earthworm macropores and in turn on the preferential transport of water and tracer through a typical soil of the karst area of the upper mid‐western USA. Large (≈ 30 cm diam. by 90 cm long) undisturbed soil columns were taken from plots where liquid dairy manure or inorganic fertilizers had been applied continuously for 8 yr. The number and size distribution of macropores in soil columns were nearly the same for both inorganic and manure treatments, however, visible surface macropores were continuous to much deeper depth in soil columns taken from manure than from the inorganic fertilizer plot. Identification of the earthworms a year later showed the presence of Apporectodea tuberculata, A. trapezoides, and Lumbricus rubellus, subsurface burrowers, as well as, L. terristris, a deeper burrowing species in the manure applied plot. Apporectodea tuberculata was the only species present in the inorganic fertilizer plot. Number of macropores and macroporosity varied with soil depth. The maximum macroporosity was <2.5% and it occurred at 2‐cm depth. The predominant macropore sizes were between 1‐ and 2‐mm radii for both treatments. During breakthrough experiments, Cl− appeared earlier in soil columns taken from the manure plot thereby indicating a greater continuity of macropores in the manure compared with the inorganic fertilizer treatment. Thearly appearance of Cl− in the manure treatment, however, was much slower than one would expect based on the number of macropores and their continuity estimated from the serial sectioning. This suggests that intrusive serial sectioning and image analysis techniques probably overestimate the continuity of macropores possibly due to vacuuming of the earthworm casts and other debris that plugs the macropore channels. Based on macropore size distribution with depth and related breakthrough curves, it is likely that most existing models of water and contaminant transport that simulate macropore flow, will not accurately predict the transport of water and contaminant because of their assumption that surface visible macropores are continuous to deeper soil depths. Data from this study showed that macropore size distribution could be described by a normal or log‐normal distribution function. These functions in combination with information on continuity and tortuosity of macropores may be sufficient, when used in some current macropore models, to adequately describe the conducting efficiency of macropores in soils.

Ponded infiltration as a cause of the instability of continuous macropores

A model is presented which yields the range of the hydraulic parameters that are critical for the collapse of continuous macropores in a given soil under certain hydrological circumstances. These continuous macropores are important, as they control the hydraulic conductivity at saturation and the infiltration capacity of most soils. The conceptual model is based on data from earlier infiltration experiments on soil columns in situ; it consists basically of a soil matrix with discontinuous macropores which is traversed by a vertical macropore channel. The experiments suggested that under the transient flow conditions that occur in the soil when ponded infiltration ends, the wall of a macropore channel can become unstable. Collapse will occur if the shear stress caused by the seepage to the channel exceeds the shear strength of the soil particles in the wall of the macropore channel. The model ascertains the magnitude of this shear stress from the transient gradient of the hydraulic head G over a length L which is the distance between the wall of the macropore channel and air encapsulated by the water in the soil matrix. For given parameters L, k (hydraulic conductivity) and μ (specific storativity) the behaviour of G as a function of time is derived from the equation for non-stationary, horizontal flow of soil water. The solution consists of a series of exponential functions; one term is characterized by the emptying time of the macropore channel and the others by the relaxation time т = 4L 2μ π 2rmk . It is shown how, for a certain soil, a domain of k and μ values conducive to collapse can be found. Each point within this domain defines a length scale for L. The moment of collapse is found to be proportional to √т. The domains of k, μ and L are determined for three different soil textures (silt loam, sandy loam and loamy sand). The application of the model shows that ponded infiltration can create conditions appropriate for the collapse of macropores and, therefore, for the decrease of the saturated conductivity.

A Laboratory Analysis of the Effect of Macropores on Solute Transport

AbstractMacropores play an important role in many soils in relation to ground‐water contamination by providing preferential pathways from the root zone to the water table. Surface‐applied agrochemicals may hereby be carried quickly to the shallow ground water. The reduced retention combined with a small contact area between the flowing water and the soil implies that little removal from physical, chemical, and microbiological processes may take place, thus increasing the risk of ground‐water contamination.A laboratory procedure for evaluating the effect of macropores in a given soil is proposed and tested. The procedure involves the following sequence of experiments on two undisturbed soil monoliths: (1) measurements of the distribution of outflow, (2) measurements of breakthrough curves, (3) dye application for visual observation of macropores; and (4) horizontal slicing of the monoliths to measure macropore distribution and continuity of macropores. The experimental procedure is based on well‐documented techniques that combined will provide evidence on the significance of macropores in relation to ground‐water contamination and guidance for selecting the appropriate model for simulating flow and transport at the field in question.The laboratory procedure was tested on two large, undisturbed soil monoliths (30 cm in diameter) removed from the unsaturated zone at a clayey moraine agricultural field in Denmark. This soil was known to have many macropores. The investigations documented the strong influence of these structures on flow and transport suggesting that a traditional flow and transport model would be inadequate for simulating the processes occurring in the field.

MACROPORE CHARACTERIZATION BY INDIRECT METHODS

Macropores are important in preferential flow; therefore, function and continuity of macropores are often more important than visual observation. The objective of this research was to compare several indirect methods of determining macropore volume. Ponded and tension infiltration measurements were made in situ at the soil surface and 0.25 m deep in a Waukegan silt loam (fine-silty over sandy, mixed, mesic Typic Hapludoll), and air-filled porosity at each pressure head was calculated from conductivity determinations. Undisturbed, unconfined samples were taken from the 0.25− to 0.35-m depth. Air-filled porosity as a function of pressure head was determined on these samples using wetting and draining equilibrium curves in rotated cores (to even out the gravitational gradient), measured/calculated after steady-state lab conductivity, and calculated from ped size distribution (assuming a ped shape factor and ped size/crack size relation, with known bulk density and ped density). Water adsorption/desorption in rotated cores in only four out of 20 samples displayed an “air-entry” pressure head, and no samples displayed a “water-entry” pressure head (that is the head at which no more water enters pores during equilibrium soil water adsorption). All methods tested estimated air-filled porosity in the same range. Air-filled porosity determined from ped-size distribution and from ponded and tension infiltration measurements was much less tedious than from rotated core measurements.

The nature, distribution and management of sodic soils in New-South-Wales

Accurate data on the distribution of the various types of sodic soils in New South Wales are not available. However, general observations suggest that large areas are affected by structural instability as a result of sodicity, particularly on grey clays and red-brown earths of the Murray-Darling Basin. There is a strong need for new sodicity surveys because the production of crops and pasture often is well below potential on these lands. Exchangeable sodium data on their own do not adequately describe sodic soil behaviour, so information is also required about related factors such as electrical conductivity, exchangeable magnesium, clay mineralogy, pH, calcium carbonate content, degree of remoulding, and the frequency of continuous stable macropores. Critical limits for sodicity that are used by soil management advisory services need to be redefined. Considerable research into the reclamation and management of sodic soils has occurred in the irrigation areas and rainfed cropping districts of the Murray-Darling Basin in New South Wales. Mined and by-product gypsum, and to a lesser extent lime, have been shown to greatly improve the physical condition and profitability of production from soils with a dispersive surface. However, the responses to these treatments are less likely to be economical when sodicity is confined to the subsoil. Adequate supplies of gypsum and lime are available in New South Wales, but further research is required to determine economically optimal and environmentally acceptable rates and frequencies of application, particle sizes and chemical compositions for different farming systems that utilize the various types of sodic soil.

Tillage effects on measured soil hydraulic properties

The effect of no-tillage (NT), ridge tillage imposed on long-term NT (NT/RT), moldboard plow (MBD), and chisel tillage (CHT) treatments on soil water retention and movement on three soils was studied using field and laboratory measurements. Field measurements of saturated hydraulic conductivity ( K fs) were made with a Guelph permeameter; laboratory measurements of saturated hydraulic conductivity ( K sat) were made using the constant head method on 30 and 7.5 cm diameter cores. The geometric mean of K sat from 30 cm cores was greater in NT than in MBD for both the Ap and B horizons of Nicollet clay loam (fine-loam, mixed, mesic Aquic Hapludoll) and Rozetta silt loam (fine-silty, mixed, mesic Typic Hapludalf); however, K sat from 7.5 cm cores was not significantly different for the two tillage treatments. The K sat for the Ap horizon of Seaton silt loam (fine-silty, mixed mesic, Typic Hapludalf) in NT/RT was lower than in CHT, possibly because of surface layer compaction produced during manure injection. A greater number and total area of stained macropores were found in NT than in MBD. Comparisons between methods of saturated hydraulic conductivity measurements showed that K sat values from 30 cm cores were the greatest for all treatments and depths, which may be attributed to the role of continuous macropores in large core samples. The K fs values obtained from the Guelph permeameter ( α = 12 m −1) were smaller than K sat values from 30 and 7.5 cm cores. Unsaturated hydraulic conductivity was measured with a disk permeameter both in the field and laboratory. For water potential values from −30 to −150 mm, the unsaturated hydraulic conductivity ( K suc) values were similar for the NT and the MBD treatments and were much lower than the K sat values. To include the effects of macropores, larger core samples are preferred.

Tillage‐ and Traffic‐Induced Changes in Macroporosity and Macropore Continuity: Air Permeability Assessment

AbstractQuantitative analysis of macropore geometry in undisturbed soils has been hindered by difficulties in creating conditions where macropore measurements can be separated from matrix pore measurements. Such analysis is important, however, for evaluating tillage management effects on both root growth and solute movement. Inferences regarding management effects on macroporosity were made possible by combining the use of improved air‐permeability techniques with two recently developed methods to analyze air‐permeability (Ka) data, using undisturbed soil samples from a tillage study. Log10 Ka values exhibited a near‐normal distribution. In wheel‐traffic interrows (Wh), macropore air permeability was significantly less under conventional‐tillage corn (CT) than no‐tillage corn (NT) and no‐tillage alfalfa (ALF). Tillage differences were usually not significant in non‐wheel‐traffic interrows (NWh). Wheel traffic significantly decreased air permeability for CT, but not for NT or ALF. The two methods of analyzing air‐permeability data yielded similar inferences regarding macropore geometry. Conventional tillage decreased bulk density (increasing total porosity) for NWh only, but seemed to decrease the stability, number, and continuity of macropores relative to NT and ALF. Macroporosity and macropore geometric factors were affected little by traffic for NT and ALF, but traffic significantly affected CT macropores in this study where samples were taken approximately 1 mo after corn planting.

Using fractal dimensions of stained flow patterns in a clay soil to predict bypass flow

Methylene blue staining patterns of five undisturbed unsaturated soil cores, 200 mm long and 200 mm in diameter, taken from a well-structured clay soil classified as a Hydric Fluvaquent, were characterized by using the fractal dimension of structure to predict measured bypass flow. Cumulative outflow curves in all cores were well described by a spherical model. Outflow in each core started after a significant time lag from the start ofirrigation. Outflow rates during irrigation in all cores were almost equal to irrigation rates; nearly all the water applied, after outflow had started, contributed to bypass flow. Total outflow ( O m, mm) was regressed on the time lag ( T 1, min) as: O m = −0.181 T 1 + 9.85. This time lag was caused by the effect of internal catchment of discontinuous macropores and surface storage. The three-dimensional fractal dimension of structure ( D s3) was calculated for the upper and lower halves of the core, by using values of D s2 and stained area in cross-sections. A statistically significant empirical equation, relating the total amount of outflow ( O m) to both upper and lower values of D s3 and to the volume fraction of stained parts ( V s) is: O m = −230.6( V s D s3−1 ) upper + 232.4( V s D s3−1 ) lower + 12.6 Thus, a greater D s3 value in the upper half of the core and a lower D s3 value in the lower half of the core induce larger amounts of outflow: hence vertically continuous macropores, such as fragments of cracks or tubes, play a significant role in the process of bypass flow.

Herbicide and Tracer Movement in Soil Columns Containing an Artificial Macropore

AbstractPreferential flow has been shown to be an important mechanism affecting water and solute movement in some soils. The movement of agricultural chemicals to groundwater is of special concern. Soil columns containing an artificial macropore were used to study alachlor [2‐chloro‐N‐(2,6‐diethylphenyl)‐N‐(methoxymethyl)acetamide], cyanazine [2‐{[4‐chloro‐6‐(ethylamino)‐1,3,5‐triazin‐2‐yl]amino}‐2‐methyl‐propanenitrile], pendimethalin [N‐(1‐ethylpropyl)‐3,4 dimethyl‐2,6 dinitrobenzenamine], and chloride movement. Packed columns were modified by removing a 6‐mm diam. core from the center. Herbicides and chloride were applied to the soil surface and columns were irrigated with 63 mm of a 0.0075 M calcium sulfate solution. Initial chloride breakthrough occurred much sooner in columns with a continuous macropore than in columns with either a partial or no macropore. Total chloride loss in column drainage, however, was less in columns containing a continuous macropore than in those without a continuous macropore. In contrast, alachlor, cyanazine, and pendimethalin were only detected in drainage from columns with a continuous macropore. Because herbicides were not detected in column drainage unless a continuous macropore was present, leaching studies using packed soil columns may significantly underestimate the extent of herbicide movement through a structured soil. Pesticide leaching experiments using soil columns with an artificial macropore may provide estimates that are more representative of field behavior and could be used to supplement current pesticide mobility studies required by the USEPA to support product registration.

Macropore Characterization for Two Tillage Systems Using Resin‐Impregnation Technique

AbstractThis study used resin impregnation and image analysis to characterize macroporosity of a Nicollet loam soil (fine‐loamy, mixed, mesic Aquic Hapludoll) under no‐tillage and conventional tillage. Soil samples (7.5 by 5 by 5 cm) were taken at each 5‐cm depth interval to determine macroporosity after conducting solute‐leaching experiments on undisturbed soil columns. These soil samples were impregnated with either polyester or epoxy resin. Impregnated soil blocks were sectioned in the middle and ground to a smooth finish. Photographic slides were made of the horizontal faces and an automatic image analyzer was used to calculate the percentage of area occupied by macropores, and total perimeter, number, and size‐frequency distribution of macropores. Percentage of area, perimeter, and number of macropores were not statistically different for the two treatments. Macroporosity data obtained from the samples did not support the observations made in the solute‐leaching studies on saturated soil columns (i.e., a greater degree of preferential flow in no‐tillage columns) because the air‐dried samples and the saturated columns had different porosity characteristics and because small, two‐dimensional images were unable to sample any less frequently occurring larger pores. Another source of discrepancy between the results of the two studies may be macropore continuity. Two‐dimensional analysis of porosity images does not provide a measure of pore continuity, which can be a decisive factor in solute transport through soil columns under different tillage systems. Results from the resin‐impregnation study were not consistent with the results of a related field study. Inconsistency between the macroporosity data of these two studies were attributed to: the difference in minimum cut‐off diameter of macropores in the two studies, the difference in sample size, and the difference in moisture status of the field and lab samples.

Physical and Morphological Characterization of Bypass Flow in a Well‐Structured Clay Soil

AbstractBypass flow governs solute movement in well‐structured clay soils. Combined use of physical and morphological methods was made in this study to better characterize the process. Flow processes in five 200‐mm‐long undisturbed cores of 200‐mm diameter were monitored at 11‐min intervals by using 102 small transducer tensiometers, divided among five samples and installed at three depths. Flow patterns along macropores were stained with methylene blue. Twenty‐five tensiometers in or within a distance of a few millimeters of stained macropores reacted very quickly when 10 mm of simulated rain was applied at an intensity of 13 mm h−1, and showed a drying pattern after the end of the simulated rainfall. Forty‐three tensiometers inside peds reacted more slowly and showed continued wetting after the end of the simulated rainfall indicating internal catchment of water in the bottom of discontinuous macropores followed by redistribution of water. Internal catchment increased with depth in the samples, as indicated by both physical and morphological data. Using tensiometer measurements as a point count, it is estimated that 33% of the soil volume was in close contact with continuous macropores, while 42% was influenced by the effects of internal catchment. An average of 5.2 ± 0.96 mm of the applied 10 mm of water left the cores through bypass flow, while an estimated average of 3.3 ± 0.96 mm contributed to internal catchment. The observed patterns of water movement illustrate the inadequacy of the concept of mobile/immobile water.

Cropping system effects of maize on infiltration, runoff and erosion on loess soils in South-Limbourg (The Netherlands): A comparison of two rainfall events

Two natural rainfall events are compared to evaluate the effects of three cropping systems of silage maize on soil moisture content, infiltration, runoff and erosion. Both rainfall events took place in early summer. One was a low intensity event with 27.6 mm of rain in 9 hours, and the other a high intensity event with 33.4 mm in 42 minutes. Cropping systems were: 1. (I) a spring tilled system (conventional), 2. (II) an autumn and spring tilled system with summer barley as spring cover crop, and 3. (III) an autumn tilled system with winter rye as winter cover crop and direct drilling of silage maize. During the low intensity event, soil moisture content of the top 5 cm rose to field capacity on all three cropping systems. No runoff was generated. During the high intensity event, soil moisture content rose to field capacity on the two spring tilled cropping systems but was only slightly raised in the direct drill system, in spite of 17.7 mm of infiltrated rain. Runoff coefficients of the high intensity event were 41.7% (conventional system), 14.9% (autumn and spring tilled system) and 47.0% (direct drill system). The direct drill system showed a severely slaked soil surface in early summer, caused by winter rain. The response to rainfall of soil moisture content is ascribed to: 1. (I) a predominance of matrix infiltration on all cropping systems during the low intensity event and on the spring tilled systems during the high intensity event, and 2. (II) a predominance of infiltration via continuous macropores, open to the surface (of biologic origin), by-passing the soil matrix, on the direct drill system during the high intensity event. The presence of continuous, vertical macropores on the direct drill system explains its surprisingly high infiltration capacity, considering its strongly slaked appearance. The smooth soil surface of the direct drill cropping system may have delayed infiltration during the flooded stage of the high intensity event by not providing vent points for the escape of soil air. Soil loss from the direct drill system during the high intensity event was only 15.6% of that from the conventional system. This is ascribed to low detachment rates of soil material by drop impact and/or overland flow, due to the presence of winter rye remains and, especially, the relatively high soil surface shear strength of the direct drill cropping system in early summer.

The influence of soil deformations on the permeability to air

SUMMARYSamples were reconstituted in the laboratory from a cracking grey clay soil. Some samples were created with a granular structure in which there was no preferred macropore orientation, others were created with a strongly preferred macropore orientation. The samples were compressed and their air permeability measured. They were then sheared and their air permeability measured again.In the samples with a granular structure the change in permeability was dominated by the change in the volume of the pore space. Volume increase led to permeability increase and volume decrease led to permeability decrease. At low shear strains and low volume increases there was some loss of pore continuity leading to small permeability decreases. In contrast, permeability change in the samples with a preferred macropore orientation was dominated by macropore continuity. Permeability decreased both when volume decreased and also over a wide range of volume increases. These results are interpreted in terms of critical‐state soil mechanics and used to construct ‘critical state for permeability’ lines analogous to the usual critical‐state lines.It is suggested that these forms of behaviour encompass the likely bounds of behaviour of soils in many circumstances.