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im
nekd
a UMR INRA/INAPG Environment and Arable Crops, Institut National de la Recherche Agronomique/Institut National Agronomique Paris-Grignon, B.P. 01, 78850 Thiverval-Grignon, France
b UMR INRA/INAPG Agronomy, Institut National de la Recherche Agronomique/Institut National Agronomique Paris-Grignon, B.P. 01, 78850 Thiverval-Grignon, France
c USDA-ARS, George E. Brown, Jr. Salinity Lab., 450 West Big Springs Road, Riverside, CA 92507
d Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521
* Corresponding author (coquet{at}grignon.inra.fr)
Received 7 October 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: HPLC, high performance liquid chromatography • TDR, time domain reflectometry
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The structure of tilled soils is known to be strongly affected by agricultural practices to which the field is subjected. For instance, heavy compaction by the wheels of farm machinery will produce an abundance of compacted clods within the tilled layer once the soil is plowed (Roger-Estrade et al., 2000), especially for relatively fine-textured soils. On the other hand, surface tillage may lead to the destruction of compacted clods that were initially present in the plow layer. This disintegration of clods is generally limited to the surface few centimeters of the plow layer (i.e., the seed bed).
To better describe the effects of agricultural machinery on soil structure and compaction, Manichon (1982) proposed a field method for characterizing soil structure. The method pays particular attention to visible porosity and the compaction status of the soil. The tilled soil for this purpose was divided vertically into several layers or compartments consistent with the depth of various tillage operations. For instance, for cropping systems that include plowing, the vertical compartments immediately after sowing may include a seed bed, a plow layer that is unaffected by seed bed preparation, and the underlying undisturbed soil. Compartments were similarly separated laterally according to the locations of wheel tracks of tractors and other farm machinery that may have compacted the underlying soil. These compacted zones would alternate within the plow layer with noncompacted zones located between the wheel tracks. Manichon (1982) identified three different types of soil structure (
,
, and 
), each having unique internal macroscopic porosities. These types of soil structure may hold for entire compartments if they are homogeneous (e.g., the relatively large compacted zone in the plow layer below the wheel tracks), or for particular clods within a more heterogeneous compartment (e.g., the plow layer between wheel tracks containing compacted clods originating from former wheel tracks, as well as noncompacted macroporous clods). Several studies have shown that these different compartments or clods can have significantly different saturated and unsaturated hydraulic properties (Curmi, 1987; Coutadeur et al., 2002).
The purpose of this study was to understand how the presence of different layers, compartments, or clods, resulting from tillage and trafficking, affect water and solute transport in an agricultural soil profile. Our hypothesis was that the simultaneous presence of compacted and more macroporous clods and aggregates will lead to considerable heterogeneity in the prevailing flow and transport processes. We tested this hypothesis on a silt loam soil whose cultivation was representative of cropping systems that include moldboard plowing. Our study involved a detailed field experiment to be discussed in this paper, as well as the use of a numerical model for optimal analysis of the data as described in Part 2 of this two-part series (Coquet et al., 2005).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Consistent with local cultivation practices, the soil was plowed on 29 Nov. 2000, using a three-furrow moldboard plow. A seedbed (12–14 cm deep) was prepared on 7 May 2001 by means of two runs using a 3.0-m wide rotary harrow following the same tracks. The tractor had 40-cm wide rear tires, which were connected to 24-cm wide dual tires, with a space of 16 cm between the main and dual tires. The inner spacing between the two rear tires was 132 cm. The soil during the entire experiment was kept free from vegetation by manual weeding.
Structure of the Tilled Soil
The structure of the tilled soil was observed along the face of a large trench perpendicular to the tillage direction. The trench was 70 cm deep and 3.1 m wide to cover one complete unit of the spatially periodic soil structural pattern created by the tillage operation (i.e., seed bed preparation). The structure of the tilled soil was described using the scheme of Manichon (1982), which divides the profile into vertical and horizontal compartments according to the effects of tillage (i.e., soil fragmentation by tillage tools and compaction by wheels) together with a description of the internal structure of each compartment (Fig. 1)
. Manichon (1982) defined three types of internal structure (,, and ) as explained below:
structure with No Structural Porosity and Smoothly Breaking Faces. This type of soil structure is created by compaction under the wheel tracks of tractors or other heavy farm machinery. The structure is also characteristic of clods located in the plow layer between the wheels tracks. Such clods are created by plowing that cuts and fragments the compacted soil formed under former wheel tracks. Structure Stemming from -Structured Soil, but Containing Incipient Cracks Due to Swelling/Shrinkage Processes and Freezing/Thawing Cycles. This type of soil structure appears in the upper part of the tilled soil where the effects of climate are the most significant (i.e., the upper part of the compacted zones below the wheel tracks, and compacted clods in the upper part of the plow layer). Structure Formed by the Coalescence of Macroporous Aggregates or Clods, with Structural Porosity Clearly Visible to the Eye.
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The structure of the soil profile was described three times during the experiment according to above criteria. The first time was immediately before installation of the TDR probes and tensiometers used to monitor soil water status during the experiment. The probes were inserted in the soil to a distance of 30 cm from the face of the trench and distributed over the profile in such a way that most or all compartments and clod types would be sampled by at least one TDR probe and one tensiometer, based on the structure that could be seen along the face of the installation trench. Soil structure type was also determined at the time when gravimetric soil samples were taken for concentration measurements of Br– added at the start of the experiment. The local structure of the soil in terms of , , or status was determined just before collecting the soil samples. Finally, the structure was described a third time when the TDR probes and tensiometers were removed, thus yielding accurate information about the actual , , or structure around each probe.
Experimental Set-up
The experiment started with a rainfall simulation on 6 June 2001, using a large rainfall simulator composed of a Tee Jet 80010 nozzle placed on a swinging arm positioned 3.7 m above the soil surface. A plot of 4 by 2 m2 was uniformly irrigated by protecting the spray from wind or local turbulence using a large framed tent. The plot had its longer dimension perpendicular to the moving direction of the farm machinery, and was centered along the axis of the path of the tractor pulling the harrow for seed bed preparation. The uniformity of water application within the plot had been previously measured several times using 24 tin cans evenly distributed over the irrigated surface. The simulated rainfall had coefficients of variation (CV) ranging from 17 to 30% depending on the feeding pressure of the nozzle. Tap water was used for 260 min at a rainfall intensity of 21 mm h–1. Electrical conductivity of the tap water was 745 µS cm–1; pH was 7.9; total and temporary hardness were 250 and 350 mmol L–1, respectively; and chloride, nitrate and sulfate concentrations were 39, 30, and 64 mg L–1, respectively. The Br– concentration of the tap water was <0.1 mg L–1.
The initial rainfall simulation was followed by application of a KBr solution at 850 mg L–1 Br– for 2 h at a rainfall intensity of 26 mm h–1. The CV of the Br– application was 21% and that of the water application before Br– was 25%. After Br– application, the irrigated surface was covered with thick impermeable plastic to shield the soil from any natural rain during water redistribution within the soil between 6 June and 17 July. No temporal change in soil structure due to swelling/shrinkage or dispersion/deposition of clay particles could be detected during the entire experiment. This was due to the fact that the clay fraction of the soil contained a limited amount of swelling clay minerals. No anion exclusion was expected in the soil as previous experiments with the same soil showed that bromide transport was similar to the transport of water isotopic tracers. Additional details about the experimental setup are given by Desbourdes-Coutadeur (2002).
Soil Water Content and Pressure Head Measurements
Thirty 20-cm-long TDR probes and 30 mini-tensiometers were installed in the soil profile (Fig. 2a)
. The mini-tensiometers had 2-cm-long, 6-mm-diam. ceramic ends mounted on 80-cm-long soft translucent PVC tubes connected to mercury manometers.
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) data for each probe every 11 min (the minimum time required to scan all 30 probes), while the mercury manometers of the tensiometers were read manually. The TDR and tensiometer measurements were recorded essentially continuously for 380 min during the infiltration part of the experiment. The measurements during redistribution were performed at increasingly larger time intervals from 4 h to 14 d (6 June–17 July 2001).
Solute Concentration Measurements
A map of Br– resident concentrations was constructed by sampling the soil destructively 12 h after the rainfall simulation. Soil was collected through a new access trench excavated in the 4 by 2 m plot that was previously irrigated with water and Br– (Fig. 3). The trench had the same orientation as the trench used for installing the TDR probes and tensiometers. The main face oriented toward the center of the irrigated plot (i.e., toward the instrumented face of the probe-installation trench; Fig. 3) was intensively sampled using small cylinders (4-cm diam., 2-cm height) every 8 cm in the horizontal direction along a distance of 300 cm, and every 4 cm in the vertical direction starting at a depth of 4 cm down to 48 cm, and also at the 56- and 64-cm depths (Fig. 2b). A few vertical sampling profiles were not completed below the dual tire tracks where little Br– transport was expected. A total of 597 small cores were taken from precisely measured locations using a framed square grid. The trench was not refilled, but its face was covered with plastic. The Br–-sampling trench was thought to be far enough (1.2 m) from the TDR probes and tensiometers to limit its impact on the flow and transport measurements.
After sampling, the cores were brought immediately to the laboratory, weighed, placed in an oven (105°C) for 48 h and then reweighed to obtain their volumetric and gravimetric water contents. The complete dry samples were subsequently transferred into 500-mL plastic centrifugation tubes and weighed again. After adding about 40 mL of deionized water to the dry samples (i.e., between once and twice their dry weight), the tubes were agitated for 20 h and centrifuged for 20 min at 8000 rpm. The supernatant was filtered through a 1.2-µm glass microfiber filter (Whatman GF/C cat 1822 090, Whatman International Ltd, Maidstone, UK) and stored <30 d in a 30-mL plastic flask at 4°C in a cold chamber before analysis for Br–.
Bromide concentrations of the extracted liquid were measured using a high performance liquid chromatography (HPLC) chain composed of an automatic sampler (Waters 717 autosampler, Waters Corp., Milford, MA), a pump and a gradient controller (Waters 600E controller system), and a diode array detector (Waters 996). The HPLC column was of the anion-exchange type (Waters IC Pack HC) composed of a polymethylacrylate resin grafted with quaternary ammoniums, and preceded by a pre-column of the same type. A 1-mL aliquot of the extracted liquid phase was filtered again on a 0.45-µm nylon filter (no. FS13N45, A.I.T., Le Mesnil le Roi, France) and placed in the automatic sampler. A volume of 100 µL was injected into the column with a mobile phase of potassium dihydrogen phosphate at 30 mmol L–1 containing 20% acetonitrile. The mobile phase was adjusted to pH of 2.8 with phosphoric acid. The mobile phase was also filtered through a 0.45-µm membrane filter (Durapor cat. HVLPO4700, Millipore Corp., Billerica, MA) before entering the column at a rate of 2 mL min–1. The wavelength of detection was set at 205 nm, while the retention time of Br– under the prevailing conditions was 21 min. The chromatography assay lasted about 30 min to allow for the removal of nitrates and chlorides from the column before a new injection.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The seed bed consisted of mostly very loose soil with only a few small and clods (<10 cm) that were left unfragmented by the harrow. The plow layer below the wheel tracks appeared in the form of large homogeneous zones. These compacted zones extended slightly in lateral directions from the wheel tracks (Fig. 1) because of mechanical stress exerted by the wheels. The plow layer between the wheel tracks was the most heterogeneous, containing both and clods. The clods originated from old compacted zones below former wheel tracks (e.g., during harvesting of the preceding maize crop) but displaced and fragmented by the last plowing. clods were almost absent during the year of the experiment, which was consistent with the fact that the freezing depth during the relatively mild winter of 2000/2001 was only about 10 cm. Finally, the untilled soil appeared very homogeneous at the macroscopic scale; the only visible structural features were earthworm (Lumbricus terrestris) holes and decayed plant root channels.
To evaluate soil structural variability in the longitudinal direction (i.e., in the direction the tractor moved), we compared the different profile descriptions made along the same tractor path. The compacted zones below the wheel tracks (surrounded by white lines in Fig. 1) are clearly visible in the two profiles, and intruded into the area between the wheel tracks. The plow layer between the wheel tracks is shown enlarged in Fig. 1. The continuous white lines in the enlarged pictures again delineate compacted zones, whereas the discontinuous white lines highlight organic matter residues. Notice that the compacted zones on the sides below the wheel tracks have similar shapes and extents in both profiles. In particular, the compacted zone on the left side shows the same inclusion of organic matter derived from maize stems and root residues located along the same row and buried by plowing. Also notice the large clods (split into two pieces in Profile A) 12 cm to the right of the left wheel track (the grid mesh is 4 cm in the pictures) in both profiles. These large clods resulted from a compacted band under a former wheel track, which was cut and displaced by the last plowing. Other clods of smaller sizes are visible in both profiles, with quite similar locations within the profiles.
The similarities between the soil profiles within the same tractor passage suggest that longitudinal variability in soil structure created by tillage is much smaller than lateral variability. This finding motivated us to experimentally and numerically study the transport problem in terms of a two-dimensional vertical analysis (i.e., along a cross-sectional area perpendicular to the path of the tillage machinery).
Soil Water Dynamics in the Tilled Soil
Observed Water Contents During Infiltration
Volumetric water contents () vs. time were measured at 29 of 30 TDR locations within the tilled soil (one TDR probe, No. 6, was found to be unreliable because of bending of the rods during installation). Time zero was at the start of the rainfall simulation. The initial water content of the seed bed (Fig. 4a) was homogeneous and very low (close to 0.05 m3 m–3). Similar low values in the very top of seed beds were previously measured by Sillon (1999) for a comparable soil during evaporation. The five TDR probes in the upper part of the seed bed between 2 and 6 cm depth all indicated similar water contents vs. time, with a rapid increase in shortly after starting the rainfall simulation. The water contents then reached a plateau at values (
0.20–0.30 m3 m–3) which were very similar for Probes 2, 3, 4, and 5, and slightly higher for Probe 1. These values are much lower than the saturated water content of the seed bed which had been estimated to be 0.54 m3 m–3 based on slow resaturation of large soil cylinders (15-cm diam., 7-cm height) in the laboratory. The plateau values increased slightly after 4.5 h when the rainfall intensity was increased from 21 to 26 mm h–1 during Br– application.
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zones, and in Fig. 4c for probes in the macroporous zones. The initial water content was generally higher and less variable in zones than in structures. Probes located within the zones were further separated into two groups. The first group, which included Probes 11, 15, and 18, had high initial water contents (about 0.38 m3 m–3) and showed little or no response to the infiltration. The second group, consisting of Probes 9, 19, and 20, showed a noticeable increase in water content following infiltration. The response of each of the TDR probes of this second group can be explained by considering their particular location within the tilled soil (Fig. 2). For example, Probe 9 was located within a zone, but at the very top of a large clod immediately below the seed bed. Also, a small crack was visible in the top part of this clod directly above the right waveguide of the probe. That probe had its measurement volume partly in a zone and partly in a clod, thus recording water contents that were intermediate between the zones (i.e., the probes of Fig. 4c) and zones (i.e., Probes 11, 15, and 18 in Fig. 4b). Probe 20 next to the left wheel track was actually in a zone, while Probe 19 was located immediately below a high organic matter area with high porosity as judged from visual examination.
The TDR probes in the zones showed substantial increases in water content after 1 h of infiltration (Fig. 4c). Similarly, as for the seed bed, plateau values slightly increased when the rainfall intensity was changed. The plateau values of the water content of the zones were close to the saturated water content of the soil (estimated to be 0.39 m3m–3 from laboratory measurements on large cylinders as done for the seedbed). The initial water content measured with each probe reflected the depth of that probe. For instance, Probes 12 and 14 were located at 24 cm depth and Probe 8 at 16 cm depth.
For the deeper soil layer that remained undisturbed by tillage, we separated between probes located below the wheel tracks (Fig. 4d) and those between the wheel tracks (Fig. 4e). The initial water content was more heterogeneous between the wheel tracks than below. This was most likely due to heterogeneity in water fluxes within the plow layer between the wheel tracks before the experiment. The water content of probes between the wheel tracks increased more rapidly than those below the wheel tracks, except for Probe 26, which was located very close to the right wheel track (Fig. 2a). Assuming that little or no water infiltrated through the compacted zones below the wheel tracks, the slow increase in water content as shown by the TDR probes in the untilled soil below the wheel tracks (Fig. 4d, Probes 25, 30) must have been due to some lateral flow of water that infiltrated between the wheel tracks.
Observed Pressure Heads During Infiltration
Initial pressure heads in the seed bed (Fig. 5a)
varied between –100 and –600 cm without much correlation with depth. Tensiometers 1, 7, 13, and 25 were all located at a depth of 3 cm, but had initial pressure heads of –180, –420, –100, and –520 cm, respectively. We attribute these relatively large differences to local variations in the redistribution of water that either was added initially around the tensiometer cups during installation, or had leaked from the cups during flushing of the tensiometers to remove air bubbles before the experiment. Similar to the TDR probes, the tensiometers recorded sharp increases in pressure head as the water front reached the ceramic cups. The first tensiometers to record the arrival of the infiltration front (Probes 1, 7, 13, and 25) were located at the shallowest depth (3 cm) in the seed bed. The other tensiometers responded later, but not in a consistent manner in terms of their depths. Except for 13, all tensiometers reacted later than the TDR probes. One possible explanation for this is the discrepancy between the sampling volumes of the relatively small tensiometers (0.6 cm diam., 2 cm long) and the TDR probes (8 cm wide, 20 cm long). Because of their larger sampling volume (Ferré et al., 1998, 2002; Caron et al., 1999), the TDR probes may have detected the infiltration front earlier that the tensiometers. This lag in the response of tensiometers relative to the TDR probes may have been due in part also to the fact that tensiometers generally have a response time of several minutes when outfitted with manometers (Klute and Gardner, 1962; Vachaud, 1969).
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zones of the plow layer (Fig. 5c), indicating that the moisture front passed this tensiometer after 1.8 h. All tensiometers in the zones and clods (Fig. 5b) showed a relatively gradual increase in the pressure head, except tensiometer 4. Although placed at the base of a wheel track (Fig. 2a), this tensiometer was located in a slightly less compacted zone below the 16-cm wide spacing between the main and dual tires of the tractor. Tensiometer 4 started to react at 1.7 h, approximately the same time as tensiometer 15 at a depth of 22 cm in a zone with a considerable amount of organic matter residues (just below a clod). The pressure head recorded by tensiometer 27, located at the edge of a clod, responded early at 1.2 h, but changed only very slowly in time because of slow movement of water from the neighboring zone toward the clod. Tensiometers 16 and 21 responded similar to tensiometer 27, whereas tensiometer 3, located in the low-permeability compacted zone below the right main tire track, responded the slowest (after 2 h). Initial pressure heads in the deeper untilled soil layer (Fig. 5d) measured with seven tensiometers (three other tensiometers in this zone did not work satisfactorily) varied little, ranging between –200 and –300 cm. Only tensiometer 12 (Fig. 2a) showed a sharp infiltration front at 2.2 h. This is consistent with TDR Probe 26, which recorded the infiltration front after 1.9 h (Fig. 4d). By contrast, tensiometers 17, 23, and 29, located at depths between 34 and 39 cm between the wheel tracks, recorded water arrival between 2.1 and 2.9 h and gave very similar pressure head values. Tensiometers 5 and 6 below the right wheel track (Fig. 2b) barely responded to water addition. Tensiometer 5 started to respond after 3 h, while Tensiometer 6 responded only minimally after 7 h.
Redistribution
Water redistribution started at 6.3 h (i.e., immediately after the end of the rainfall simulation). Water contents during redistribution are often best plotted as a function of the log of time (t) (Fig. 6)
. Such graphs are typically used for field measurements of the hydraulic conductivity using the internal drainage method (Libardi et al., 1980; Chong et al., 1981), when is approximately a linear function of log(t). The end of the infiltration process is reflected by the quick nonlinear drops in at the beginning of the plots for most probes, except for those that reacted only minimally to infiltration (Probes 11, 15, and 18 in Fig. 4b and 6b; Probe 16 in Fig. 4d and 6d). After 7.8 h, decreased linearly with log(t) for all probes, except for those in the seedbed where the rate of decrease of was not steady but increased with log(t). This was likely due to some evaporation during the relatively warm summer of 2001. While the soil surface was covered with plastic, only a few large stones kept the plastic in place. The plastic likely was also somewhat permeable to water vapor. In all, the behavior of the TDR probes during redistribution was consistent with that during infiltration. At the end of the experiment, 42 d after the infiltration, was slightly higher than the initial values as measured before the infiltration started.
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, , or untilled soil).
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zones below the wheel tracks. Some Br– actually reached the bottom of the sampled domain (64 cm), although only 52 mm of Br– solution had been applied. These results are consistent with the TDR and tensiometer data (Fig. 4 and 5), especially TDR 26 and tensiometer 12 which reacted quickly after irrigation began.
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clods within the plow layer between the wheel tracks had the same effect as the wheel tracks on the infiltration pattern. In Fig. 8, this region of the soil profile is enlarged and compared with the structural map of the soil where the blocks are delineated by white lines. One can see that the advance of the Br– front was slowed by the clods. Br– concentrations were lower in these clods (e.g., the clod at x = 120 cm in Fig. 8, where x is the abscissa corresponding to the lateral position within the soil) as well as below these clods (e.g., clods between x = 142 and 154 cm). These results show that the infiltration front moved around these clods. The difference in hydraulic conductivity between the clods and the surrounding soil forces the flow streamlines to diverge around the clods, while impeding flow into the clods themselves as well as the underlying soil, much like an umbrella protects a person from rainfall. This "umbrella effect" of the low-permeability clods is most clearly visible for the clod at x = 120 cm in Fig. 8. This same effect, but at a larger scale, also explains the preferential flow of Br– around the wheel tracks. Although the average mass balance for the entire plot was 72%, a minimum of only 45% was recovered for a vertical profile through the wheel track at x = 50 cm, while a maximum of 120% was recovered along a vertical concentration profile close to the wheel track at x = 194 cm. These results indicate significant lateral flow and redistribution of infiltrating water toward more permeable nearby zones between the wheel tracks. Similar variabilities in mass recovery rates were observed by Kamau et al. (1996), and also by Schulin et al. (1987) for a stony soil. Schulin et al. (1987) attributed the erratic recovery rates to lateral advective transport caused by water uptake by roots and/or anisotropy in the hydraulic conductivity. We believe that the stones present in their soil may have contributed to lateral transport by creating umbrella effects similar to those observed in our experiment for the zones and clods. Kulli et al. (2003) showed that compaction under wheel tracks can promote preferential flow into earthworm burrows, provided that water ponding occurs at the soil surface. Our study suggests that this same mechanism can be extended to any compacted () zone within the soil that impedes water and solute fluxes and redirects water laterally toward more permeable () zones.
Bromide concentrations were found to be the highest in the seed bed. This is consistent with the step-input top boundary condition used for applying the Br–, along with subsequent advective dispersive transport. Bromide concentrations were lowest in the compacted soil underneath the wheel tracks, although some heterogeneities in the infiltration patterns occurred here also, particularly within the right wheel track at x = 262 cm (Fig. 8b). This location is between the main and the dual tire (between 252 and 268 cm) where the soil had been slightly less compacted. Again, this is consistent with the relatively fast response of Tensiometer 4 at the base of the wheel track at x = 260 cm (Fig. 2a and 5b).
Average concentration profiles below the wheel tracks and between the wheel tracks were obtained by laterally averaging concentrations of vertical Br– distributions either under the left wheel track, between the wheel tracks, or under the right wheel track (Fig. 9)
. These profiles were analyzed with an analytical solution of the standard one-dimensional advection–dispersion equation (assuming steady-state water flow) for step inputs using the STANMOD parameter inversion program (imnek et al., 1999). The analytical solution did not account for the effects of preferential flow. The purpose of this analysis was to see if one could obtain an equivalent one-dimensional solute dispersivity by considering a simplified description of the structure of the profile in terms of only compacted soil below the wheel tracks and soil between the wheel tracks. This simplification would allow the use of one-dimensional models to simulate water and solute movement in tilled soils as proposed by Roth et al. (1999). The input concentration was fixed at 850 mg L–1, and the water flux density at 26 mm h–1. The Br– distribution between the wheel tracks (Fig. 9b) showed a much higher dispersivity (
= 4.4 cm) than the data below the wheel tracks (0.66 and 1.44 cm). This was expected because of the high heterogeneity of the plow layer between the wheel tracks, which consisted of mixture of compacted and noncompacted clods and soil (Fig. 1). The profile below the right wheel track had a dispersivity twice that of the profile below the left wheel track, thus showing the effect of heterogeneity created by the presence of two types of tires (main and dual) within the same wheel track. Also, notice that the average Br– concentration profile between the wheel tracks (Fig. 9b) could not be described well with the standard one-dimensional advection–dispersion equation. A significant proportion of the Br– mass moved to depths beyond 30 cm in this part of the soil. This behavior can be described only using either a one-dimensional dual-porosity transport model or, as shown in the companion study (Coquet et al., 2005), a more elaborate two-dimensional process-based flow/transport model that accounts explicitly for the different heterogeneities in the profile.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The abundance, size, and location of compacted clods depend on agricultural practices. Modeling the impact of such clods on water and solute transport may be useful for understanding the potential environmental effects of alternative cropping and tillage systems. In a subsequent paper, a two-dimensional model of water and solute transport is used in attempts to reproduce the field data presented in this paper.
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ACKNOWLEDGMENTS |
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imnek's summer stay at the UMR INRA/INAPG EGC-Grignon is greatly acknowledged. |
REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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imnek, C. Coutadeur, M. Th. van Genuchten, V. Pot, and J. Roger-Estrade. 2005. Water and solute transport in a cultivated silt loam soil: II. Numerical analysis. Available at www.vadosezonejournal.org. Vadose Zone J. 4:587–601 (this issue).imnek. 2002. The effect of the spatial sensitivity of TDR on inferring soil hydraulic properties from water content measurements made during the advance of a wetting front. Vadose Zone J. 1:281–288.imnek, J., M.Th. van Genuchten, M. ejna, N. Toride, and F.J. Leij. 1999. The STANMOD Computer Software for Evaluating Solute Transport in Porous Media Using Analytical Solutions of the Convection-Dispersion Equation. Version 2.0. U.S. Salinity Lab., ARS, USDA, Riverside, CA.This article has been cited by other articles:
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  J. Vanderborght and H. Vereecken Review of Dispersivities for Transport Modeling in Soils Vadose Zone J., January 24, 2007; 6(1): 29 - 52. [Abstract] [Full Text] [PDF]
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J. Vanderborght and H. Vereecken One-Dimensional Modeling of Transport in Soils with Depth-Dependent Dispersion, Sorption and Decay Vadose Zone J., January 24, 2007; 6(1): 140 - 148. [Abstract] [Full Text] [PDF]
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D. L. Corwin, J. Hopmans, and G. H. de Rooij From Field- to Landscape-Scale Vadose Zone Processes: Scale Issues, Modeling, and Monitoring Vadose Zone J., March 8, 2006; 5(1): 129 - 139. [Abstract] [Full Text] [PDF]
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Y. Coquet, J. Simunek, C. Coutadeur, M. Th. van Genuchten, V. Pot, and J. Roger-Estrade Water and Solute Transport in a Cultivated Silt Loam Soil: 2. Numerical Analysis Vadose Zone J., July 18, 2005; 4(3): 587 - 601. [Abstract] [Full Text] [PDF]
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