Published online July 18, 2005

Water and Solute Transport in a Cultivated Silt Loam Soil: 1. Field Observations
Y. Coquet,* C. Coutadeur, C. Labat, P. Vachier, M. Th. van Genuchten, J. Roger-Estrade, and J. Šimůnek

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

ABSTRACT

ated with soil type, vadose zone flow and transport processes are also affected by such external factors as tillage
practices. For example, Petersen et al. (1999, 2001)
showed that surface tillage may strongly affect the dynamics of dye tracers in soils. Trafficking by agriculturalor other machinery has also been shown to modify thehydraulic properties and porosity of soils (Meek et al.,
1989; Ankeny et al., 1990; Mohanty et al., 1994; Gysi etal., 1999). A major challenge is to find useful proxyvariables to assess soil structure heterogeneity (Younget al., 2001).
The structure of tilled soils is known to be strongly
affected by agricultural practices to which the field issubjected. For instance, heavy compaction by the wheelsof farm machinery will produce an abundance of compacted clods within the tilled layer once the soil isplowed (Roger-Estrade et al., 2000), especially for relatively fine-textured soils. On the other hand, surface
tillage may lead to the destruction of compacted clodsthat 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 ofvarious tillage operations. For instance, for cropping
systems that include plowing, the vertical compartments
immediately after sowing may include a seed bed, aplow layer that is unaffected by seed bed preparation,and the underlying undisturbed soil. Compartments weresimilarly separated laterally according to the locationsof wheel tracks of tractors and other farm machinerythat 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 typesof 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 theplow layer below the wheel tracks), or for particularclods within a more heterogeneous compartment (e.g.,
the plow layer between wheel tracks containing compacted clods originating from former wheel tracks, aswell as noncompacted macroporous clods). Several
studies have shown that these different compartments
or clods can have significantly different saturated andunsaturated hydraulic properties (Curmi, 1987; Coutadeur et al., 2002).

Vadose zone flow and transport processes are known to be strongly
affected by both soil structure and soil texture. We conducted a field
experiment to explore the impact of heterogeneity in soil structure
created by agricultural operations such as wheel traffic, plowing, andsurface tillage on water and solute transport. The experiment was
performed on a 4 by 2 m2 field plot perpendicular to the path of a
tractor that had pulled a harrow for seedbed preparation. The plot
was irrigated with a rainfall simulator at a rate of 21 mm h⫺1 for 2 h
and 20 min. An 850 mg L⫺1 bromide solution was subsequently applied
at a rate of 26 mm h⫺1 for 2 h. Soil water contents and pressure heads
during the experiment were monitored with time domain reflectometry (TDR) probes and tensiometers. The soil was sampled for resident
bromide (Br⫺ ) concentrations at the end of the experiment. Waterand bromide fronts were found to be highly heterogeneous. The heterogeneity could be explained by the particular soil structural features
created by the agricultural practices, in particular by the locations
and sizes of compacted soil zones. Very little water and bromide
had penetrated the large compacted zones under the wheel tracks.
Bromide, TDR, and tensiometer measurements all indicated the presence of preferential flow of water and bromide along paths immediately bordering the wheel tracks. The compacted clods in the plowlayer furthermore acted as low-permeability barriers that diverted
water and bromide flow around them. The highly heterogeneous plowlayer between the wheel tracks produced a much higher solute dispersivity as compared to the compacted soil below the wheel tracks.

Numerous studies have shown that water and solutetransport in soils can be highly heterogeneous at
the field scale (e.g., Butters et al., 1989; Roth et al.,
1991; Flury et al., 1994, 1995; Hammel et al., 1999). This
fact makes it difficult to define and estimate equivalentor effective parameters for use in relatively simple models describing field behavior (Ellsworth and Jury, 1991;
Mayer et al., 1999). Recently, Vanderborght et al. (1997,
2001) highlighted the importance of soil morphology inexplaining prevailing solute transport properties. The
presence of different horizons or layers within a soil
can be a major factor causing flow heterogeneity (vanWesenbeck and Kachanoski, 1994; Deurer et al., 2001).
In addition to the intrinsic structural properties associ-Y. Coquet, C. Coutadeur, C. Labat, and P. Vachier, 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; J. Roger-Estrade, UMR
INRA/INAPG Agronomy, Institut National de la Recherche Agronomique/Institut National Agronomique Paris-Grignon, B.P. 01,
78850 Thiverval-Grignon, France; M.Th. van Genuchten, USDAARS, George E. Brown, Jr. Salinity Lab., 450 West Big Springs Road,
Riverside, CA 92507; J. Šimůnek, Dep. of Environmental Sciences,
Univ. of California, Riverside, CA 92521. Received 7 Oct. 2004. *Corresponding author (coquet@grignon.inra.fr).
Published in Vadose Zone Journal 4:573–586 (2005).
Original Research Paper
doi:10.2136/vzj2004.0152© Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: HPLC, high performance liquid chromatography; TDR,
time domain reflectometry.

573

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

574

VADOSE ZONE J., VOL. 4, AUGUST 2005

The purpose of this study was to understand how thepresence of different layers, compartments, or clods,
resulting from tillage and trafficking, affect water andsolute transport in an agricultural soil profile. Our hypothesis was that the simultaneous presence of compacted and more macroporous clods and aggregates willlead to considerable heterogeneity in the prevailing flow
and transport processes. We tested this hypothesis ona silt loam soil whose cultivation was representative ofcropping 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 numericalmodel for optimal analysis of the data as described inPart 2 of this two-part series (Coquet et al., 2005).
MATERIALS AND METHODS
Site Characteristics and Tillage Practices
The experiment was conducted in an agricultural field of
the Institut National de la Recherche Agronomique (INRA)
Experimental Station at Grignon (Yvelines, France). The field
was cropped each year since 1962 with maize (Zea mays L.).
The soil is a silt loam Calcic Cambisol (FAO classification)
overlaying dense limestone at a depth of 90 cm. The plowlayer (0–30 cm) consisted on average of 28.2% clay (⬍2 ␮m),
61.2% silt (2–50 ␮m) and 6.5% sand (0.05–2 mm), and contained 2.2% organic matter and 1.3% calcium carbonate. The
particle size distribution was determined using sedimentation
and wet sieving methods after organic matter removal by H2O2
and decarbonation by HCl (Association Française de Normalisation [AFNOR], 1983), whereas the organic matter content
was measured using dry combustion (AFNOR, 1995a), andthe calcium carbonate content using a volumetric method(AFNOR, 1995b).
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 reartires, which were connected to 24-cm wide dual tires, with aspace of 16 cm between the main and dual tires. The innerspacing 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 faceof 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 structureof 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 bycompaction under the wheel tracks of tractors or otherheavy 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., theupper part of the compacted zones below the wheel
tracks, and compacted clods in the upper part of theplow layer).
• ⌫ Structure Formed by the Coalescence of Macroporous
Aggregates or Clods, with Structural Porosity Clearly
Visible to the Eye.
The above three types of structure, as well as regular uncompacted soil (defined here as clods or aggregates ⬍2 cm), arepresent in the plow layer of most cultivated soils. The relative
proportion, size, and location of each of the three structure
types are generally specific of the adopted cropping system(Richard et al., 1999; Roger-Estrade et al., 2000).
The structure of the soil profile was described three times
during the experiment according to above criteria. The firsttime was immediately before installation of the TDR probes
and tensiometers used to monitor soil water status during theexperiment. The probes were inserted in the soil to a distanceof 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 weretaken for concentration measurements of Br⫺ added at thestart of the experiment. The local structure of the soil in termsof ⌬, ⌽, or ⌫ status was determined just before collecting thesoil 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 irrigatedby protecting the spray from wind or local turbulence usinga large framed tent. The plot had its longer dimension perpendicular to the moving direction of the farm machinery, andwas centered along the axis of the path of the tractor pullingthe 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 irrigatedsurface. 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 intensityof 21 mm h⫺1. Electrical conductivity of the tap water was 745
␮S 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 applicationof 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 withthick impermeable plastic to shield the soil from any naturalrain during water redistribution within the soil between 6 Juneand 17 July. No temporal change in soil structure due to575

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

www.vadosezonejournal.org

Fig. 1. Soil profiles showing the partitioning of the tilled layer. Compacted ⌬ soil is delineated by solid white lines. Boundaries between theseedbed and the plow layer are indicated by a solid black line. Accumulations of organic matter residues are surrounded by dotted whitelines. (A) Soil profile that was sampled for bromide. (B) Soil profile where time domain reflectometry (TDR) probes and tensiometers were
installed. Note that the bending of the pictures is due to uncorrected parallax effects and not to actual topography.

swelling/shrinkage or dispersion/deposition of clay particles
could be detected during the entire experiment. This was dueto 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 transportof 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.
To accommodate installation of the probes, a trench (70cm deep and 3.1 m wide) was excavated such that its main
face was perpendicular to the moving direction of the farmmachinery. Probes were installed through horizontal holes dug
into one of the main faces (Fig. 3). The lengths of the holes(20 cm for the TDR probes, 29 cm for the tensiometers) weretaken such that the middle parts of the probes (10 cm for the
TDR probes, 1 cm for the tensiometers) would be situated at
positions 30 cm from the main face of the trench. TDR probes
were inserted directly by pushing them through the end of
the holes, whereas holes slightly larger than the ceramic cups
were used for the tensiometers. To improve contact with thesoil, the tensiometer cups were dipped in a soil slurry before
inserting them. The TDR cables and flexible tensiometer tubes
were routed laterally through the holes that were repacked
with soil previously excavated from those locations, and thenup to the soil surface through the refilled trench. Thus, theprobes were placed 30 cm into the undisturbed soil below theirrigated plot (Fig. 3). This setup kept the soil below the irrigatedplot as undisturbed as possible. Also, the refilled installation
trench did not directly receive irrigation water, thus preventing
any water from infiltrating preferentially along the cables or
tubes toward the probes.
All TDR probes were connected to a single acquisition
system (TRASE, Soil Moisture Equipment Corp., Santa Barbara, CA) to obtain volumetric water content (␪) data foreach 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 bysampling the soil destructively 12 h after the rainfall simulation. Soil was collected through a new access trench excavatedin the 4 by 2 m plot that was previously irrigated with water

VADOSE ZONE J., VOL. 4, AUGUST 2005

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

576

Fig. 2. (a) Locations of time domain reflectometry (TDR) probes (large circles) and tensiometers (small squares) within the soil profile. The
enlarged pictures show the position of the TDR probes and tensiometers in relation to the structure of the plow layer. Solid white lines
delineate compacted ⌬ soil as well as the boundary between the seedbed and the plow layer. Accumulations of organic matter residues aredelineated by dotted white lines. (b) Locations of the samples taken from the soil for bromide measurements.

577

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

www.vadosezonejournal.org

Fig. 3. Vertical cross-section showing the installation of the time domain reflectometry (TDR) probes and tensiometers. The cross-section isperpendicular to the main direction of the installation trench and parallel to the moving direction of the farm machinery.

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 verticaldirection 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 framedsquare 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 thelaboratory, weighed, placed in an oven (105⬚C) for 48 h andthen reweighed to obtain their volumetric and gravimetricwater contents. The complete dry samples were subsequently
transferred into 500-mL plastic centrifugation tubes andweighed 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-␮m 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 gradientcontroller (Waters 600E controller system), and a diode array
detector (Waters 996). The HPLC column was of the anionexchange 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-␮mnylon filter (no. FS13N45, A.I.T., Le Mesnil le Roi, France)and placed in the automatic sampler. A volume of 100 ␮L
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 withphosphoric acid. The mobile phase was also filtered througha 0.45-␮m 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 prevailingconditions was 21 min. The chromatography assay lasted about

30 min to allow for the removal of nitrates and chlorides fromthe column before a new injection.

RESULTS AND DISCUSSION
Structure of the Tilled Soil
Three layers could be identified in the soil profile(Fig. 1): a seed bed (0–12 cm) which had been createdby harrowing the plow layer, a plow layer (12–30 cm)
unaffected by surface tillage, and soil below the plowlayer (⬎30 cm; not shown in Fig. 1) which had not been
disturbed by tillage. No plow pan could be identified
since the plowing depth had been increased to 30 cm
by the farmer the year the experiment took place, instead of 28 cm in previous years. The second layer (theplow layer that had not been harrowed) could be further
divided laterally into compacted zones affected by thewheels of the tractor pulling the harrow, and zones between the wheel tracks that were not affected by compaction.
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 thewheel 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 precedingmaize crop) but displaced and fragmented by the lastplowing. ⌽ clods were almost absent during the year of
the experiment, which was consistent with the fact thatthe 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 plantroot channels.
To evaluate soil structural variability in the longitudinal direction (i.e., in the direction the tractor moved),
we compared the different profile descriptions madealong the same tractor path. The compacted ⌬ zones
below the wheel tracks (surrounded by white lines inReproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

578

VADOSE ZONE J., VOL. 4, AUGUST 2005

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 thewheel tracks have similar shapes and extents in bothprofiles. 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 thesame row and buried by plowing. Also notice the large⌬ clods (split into two pieces in Profile A) 12 cm to theright of the left wheel track (the grid mesh is 4 cm inthe 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, withquite similar locations within the profiles.
The similarities between the soil profiles within thesame 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 interms of a two-dimensional vertical analysis (i.e., alonga cross-sectional area perpendicular to the path of thetillage 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 (oneTDR probe, No. 6, was found to be unreliable becauseof bending of the rods during installation). Time zero
was at the start of the rainfall simulation. The initialwater 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 ofthe seed bed between 2 and 6 cm depth all indicatedsimilar water contents vs. time, with a rapid increase in␪ shortly after starting the rainfall simulation. The watercontents then reached a plateau at values (苲0.20–0.30
m3 m⫺3) which were very similar for Probes 2, 3, 4, and5, 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 wasincreased from 21 to 26 mm h⫺1 during Br⫺ application.
Water contents of the plow layer are plotted separately in Fig. 4b for probes in the compacted ⌬ zones,
and in Fig. 4c for probes in the macroporous ⌫ zones.
The initial water content was generally higher and lessvariable in ⌬ zones than in ⌫ structures. Probes locatedwithin the ⌬ zones were further separated into twogroups. 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 probesof 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 theseed bed. Also, a small crack was visible in the top partof this ⌬ clod directly above the right waveguide of theprobe. That probe had its measurement volume partly
in a ⌫ zone and partly in a ⌬ clod, thus recording watercontents 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 areawith 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 wereclose to the saturated water content of the ⌫ soil (estimated to be 0.39 m3m⫺3 from laboratory measurementson large cylinders as done for the seedbed). The initialwater content measured with each probe reflected thedepth 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 moreheterogeneous 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 thecompacted ⌬ 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 flowof 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 locatedat 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 variationsin 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Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

www.vadosezonejournal.org

579

Fig. 4. Time domain reflectometry (TDR) probe responses to infiltration.

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) inthe 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 thediscrepancy between the sampling volumes of the relatively small tensiometers (0.6 cm diam., 2 cm long) andthe 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. Thislag in the response of tensiometers relative to the TDR
probes may have been due in part also to the fact thattensiometers generally have a response time of several
minutes when outfitted with manometers (Klute andGardner, 1962; Vachaud, 1969).
Only one tensiometer (15) gave useful results for the⌫ 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) showeda 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. Tensiome-

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

580

VADOSE ZONE J., VOL. 4, AUGUST 2005

Fig. 5. Tensiometer responses to infiltration.

ter 4 started to react at 1.7 h, approximately the sametime 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 bytensiometer 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 theneighboring ⌫ 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 othertensiometers 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 gavevery similar pressure head values. Tensiometers 5 and6 below the right wheel track (Fig. 2b) barely responded
to water addition. Tensiometer 5 started to respondafter 3 h, while Tensiometer 6 responded only minimallyafter 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 functionof the log of time (t) (Fig. 6). Such graphs are typicallyused for field measurements of the hydraulic conductivity using the internal drainage method (Libardi et al.,
1980; Chong et al., 1981), when ␪ is approximately alinear 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 withlog(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 endof the experiment, 42 d after the infiltration, ␪ wasslightly higher than the initial values as measured before
the infiltration started.
The tensiometers generally responded very similarlyReproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

www.vadosezonejournal.org

581

Fig. 6. Water redistribution within the soil profile as recorded with time domain reflectometry (TDR) probes. Vertical bars mark end of infiltration.

as the TDR probes (Fig. 7). For example, the pressure
head in the seed bed also decreased with log(t) (Fig.
7a), and at increasing rates near the end of the experiment, again most probably because of some evaporation. The pressure heads may have been more variable
also toward the end of the experiment because of temperature effects on the readings during the summer(Haise and Kelley, 1950). Several tensiometers in thecompacted soil below the wheel tracks (e.g., tensiometer
3, Fig. 7b) and in the soil undisturbed by tillage (tensiometers 17, 23, 29, Fig. 7d) still recorded increasing pressure
heads with time after irrigation ceased (i.e., after 6 h 20min). This indicates that water was still percolating into
these depths of the profile after irrigation had ceased.

Shortly afterward, however, the pressure heads in theseparts of the profile started to decrease at rates thatwere quite similar to other locations with the same soil
structure (i.e., seed bed, ⌬, ⌫, or untilled soil).

Solute Concentrations after Infiltration
The soil was sampled for Br⫺ concentrations 12 h after rainfall simulation. Each square in Fig. 8a represents
a sampling location. A kriged contour plot was constructed using the Surfer software (Golden Software,Inc., Golden, CO) for ease of visualization (Fig. 8b).
The maps show considerable heterogeneity in the Br⫺
concentration of both the plow layer and the undis-

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

582

VADOSE ZONE J., VOL. 4, AUGUST 2005

Fig. 7. Water redistribution within the soil profile as recorded with tensiometers. Vertical bars mark end of infiltration.

turbed layer between the wheel tracks. Also, concentrations in the undisturbed deeper layer between the wheel
tracks were relatively high next to the wheel tracks. This
reflects preferential flow of water and Br⫺ in the plowlayer along the edge of the compacted ⌬ zones below
the wheel tracks. Some Br⫺ actually reached the bottomof 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.
The ⌬ clods within the plow layer between the wheel
tracks had the same effect as the wheel tracks on theinfiltration 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 bywhite 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 thelateral position within the soil) as well as below theseclods (e.g., clods between x ⫽ 142 and 154 cm). These
results show that the infiltration front moved aroundthese 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 theunderlying soil, much like an umbrella protects a personfrom 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 thewheel tracks. Although the average mass balance forthe entire plot was 72%, a minimum of only 45% wasrecovered for a vertical profile through the wheel trackat 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 wereobserved 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 lateraltransport by creating umbrella effects similar to thoseobserved 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 thesoil 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⫺, alongwith 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

583

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

www.vadosezonejournal.org

Fig. 8. Observed bromide concentrations within the soil profile in terms of (a) point values, and (b) an interpolated map of concentrations.
Enlargements are shown at the bottom of the figure.

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

584

VADOSE ZONE J., VOL. 4, AUGUST 2005

Fig. 9. Average bromide concentration profiles (a) under the left wheel track, (b) between the wheel tracks, and (c) under the right wheel track.
Fitted analytical solutions of the one dimensional advection–dispersion equation are shown as solid black lines. Fitted values of soil dispersivity(␭) are also indicated.

(Fig. 8b). This location is between the main and thedual 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 thebase 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 thestandard one-dimensional advection–dispersion equation (assuming steady-state water flow) for step inputs
using the STANMOD parameter inversion program (Šimůnek et al., 1999). The analytical solution did not account for the effects of preferential flow. The purposeof 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 and1.44 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 theright wheel track had a dispersivity twice that of the profile
below the left wheel track, thus showing the effect ofheterogeneity created by the presence of two types oftires (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 thesoil. 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.

SUMMARY AND CONCLUSIONS
Several previous studies have shown that tillage practices can create different types of soil structure within
agricultural soils, with each structure having its own
hydraulic properties. In this study, a field experimentdemonstrated that differences in soil structure createdby tillage have a strong impact on the spatial and temporal dynamics of water content and solute concentration.
A 4 by 2 m2 plot was subjected to a step input infiltrationof Br⫺. After applying 52 mm of the tracer solution, theBr⫺ front was found to be distributed very heterogeneously in the profile. Little Br⫺ penetrated the compacted soil below the wheel tracks (moving to a depthof only 17 cm), while the Br⫺ front moved to a depthof more than 64 cm in areas between the wheel tracks.
The heterogeneous leaching pattern was controlled
mostly by the presence of compacted soil zones andclods. These compacted zones were not only locatedbelow recent wheel tracks, but were also present in theform of large clods between the tracks as a result of thedisplacement (by plowing) of compacted soil formed
under earlier wheel tracks. The compacted clods provided umbrella (or shadow) effects to the infiltratingwater, thus causing substantial heterogeneity in the moisture and Br⫺ fronts.
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 waterand solute transport is used in attempts to reproduce
the field data presented in this paper.

www.vadosezonejournal.org

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

ACKNOWLEDGMENTS
We wish to thank all people involved in this large field
experiment, Bernard Renaux from INRA-Orléans, Nathalie
Bernet-Lempereur and Valérie Pot from INRA EGC-Grignon, Jacques Troizier and all of the staff of the Experimental
Unit of INRA-Grignon. This work was supported by the Research and Education Department of the French Ministry ofAgriculture, Alimentation, Fisheries and Rural Affairs. Support from OECD (a fellowship under the OECD Co-operative
Research Program: Biological Resource Management for Sustainable Agriculture Systems) and INRA for J. Šimůnek’s
summer stay at the UMR INRA/INAPG EGC-Grignon isgreatly acknowledged.

REFERENCES
Ankeny, M.D., T.C. Kaspar, and R. Horton. 1990. Characterizationof tillage and traffic effects on unconfined infiltration measurements. Soil Sci. Soc. Am. J. 54:837–840.
Association Française de Normalisation. 1983. Soil quality. Particle
size determination by sedimentation. Pipette method. AFNOR
Standard X 31–107. (In French.) AFNOR, Paris.
Association Française de Normalisation. 1995a. Soil quality. Determination of organic and total carbon after dry combustion (elementary analysis). AFNOR Standard NF ISO 10694. (In French.)
AFNOR, Paris.
Association Française de Normalisation. 1995b. Soil quality. Determination of carbonate content. Volumetric method. AFNOR Standard NF ISO 10693. (In French.) AFNOR, Paris.
Butters, G.L., W.A. Jury, and F. Ernst. 1989. Field scale transport ofbromide in an unsaturated soil. 1. Experimental methodology andresults. Water Resour. Res. 25:1575–1581.
Caron, J., S. Ben Jemia, J. Gallichand, and L. Trépanier. 1999. Field
bromide transport under transient-state: Monitoring with time domain reflectrometry and porous cup. Soil Sci. Soc. Am. J. 63:
1544–1553.
Chong, S.K., R.E. Green, and L.R. Ahuja. 1981. Simple in situ determination of hydraulic conductivity by power function descriptions ofdrainage. Water Resour. Res. 17:1109–1114.
Coquet, Y., J. Šimůnek, C. Coutadeur, M. Th. van Genuchten, V.
Pot, and J. Roger-Estrade. 2005. Water and solute transport ina cultivated silt loam soil: II. Numerical analysis. Available atwww.vadosezonejournal.org Vadose Zone J. 4:587–601 (this issue).
Coutadeur, C., Y. Coquet, and J. Roger-Estrade. 2002. Variation of
hydraulic conductivity in a tilled soil. Eur. J. Soil Sci. 53:619–628.
Curmi, P. 1987. Intrinsic physical behavior of clods with different
macroporosities. (In French.) p. 53–58. In G. Monnier and M.J.
Goss (ed.) Soil compaction and regeneration. Proc. of the Workshop on Soil Compaction: Consequences and Structural Regeneration Processes, Avignon, France. 17–18 Sept. 1985. A.A Balkema,
Rotterdam, The Netherlands.
Desbourdes-Coutadeur, C. 2002. Study of water transport in a plowed
soil. Bidimensional modeling of infiltration and redistribution insoil with an heterogeneous structure. (In French, with English
abstract.) Thèse de doctorat de l’Institut National AgronomiqueParis-Grignon, INA-PG, Paris.
Deurer, M., W.H.M. Duijnisveld, J. Böttcher, and G. Klump. 2001.
Heterogeneous solute flow in a sandy soil under a pine forest:
Evaluation of a modeling concept. J. Plant Nutr. Soil Sci. 164:
601–610.
Ellsworth, T.R., and W.A. Jury. 1991. A three-dimensional field studyof solute transport through unsaturated, layered, porous media. 2.
Characterization of vertical dispersion. Water Resour. Res. 27:
967–981.
Ferré, P.A., J.H. Knight, D.L. Rudolph, and R.G. Kachanoski. 1998.
The sample areas of conventional and alternative time domainreflectometry probes. Water Resour. Res. 34:2971–2979.
Ferré, T.P.A., H.K. Nissen, and J. Šimůnek. 2002. The effect of thespatial 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.
Flury, M., H. Flühler, W.A. Jury, and J. Leuenberger. 1994. Suscepti-

585

bility of soils to preferential flow of water: A field study. Water
Resour. Res. 30:1945–1954.
Flury, M., J. Leuenberger, B. Studer, and H. Flühler. 1995. Transportof anions and herbicides in a loamy and a sandy field soil. Water
Resour. Res. 31:823–835.
Gysi, M., A. Ott, and H. Flühler. 1999. Influence of single passes withhigh wheel load on a structured, unploughed sandy loam soil. Soil
Tillage Res. 52:141–151.
Haise, H.R., and O.J. Kelley. 1950. Causes of diurnal fluctuations oftensiometers. Soil Sci. 70:301–313.
Hammel, K., J. Gross, G. Wessolek, and K. Roth. 1999. Two-dimensional simulation of bromide transport in a heterogeneous fieldsoil with transient unsaturated flow. Eur. J. Soil Sci. 50:633–647.
Kamau, P.A., T.R. Ellsworth, C.W. Boast, and F.W. Simmons. 1996.
Tillage and cropping effects on preferential flow and solute transport. Soil Sci. 161:549–561.
Klute, A., and W.R. Gardner. 1962. Tensiometer response time. Soil
Sci. 93:204–207.
Kulli, B., M. Gysi, and H. Flühler. 2003. Visualizing soil compaction
based on flow pattern analysis. Soil Tillage Res. 70:29–40.
Libardi, P.L., K. Reichardt, D.R. Nielsen, and J.W. Biggar. 1980.
Simple field methods for estimating soil hydraulic conductivity.
Soil Sci. Soc. Am. J. 44:3–7.
Manichon, H. 1982. Influence of cropping systems on the soil tillage
profile: Development of a diagnostic method based on morphological observation. (In French.) Thèse de doctorat de l’Institut National Agronomique Paris-Grignon, INA-PG, Paris.
Mayer, S., T.R. Ellsworth, D.L. Corwin, and K. Loague. 1999. Identifying effective parameters for solute transport models in heterogeneous environments. p. 119–133. In D.L. Corwin et al. (ed.) Assessment of non-point source pollution in the vadose zone. Geophysical
Monogr. 108. Am. Geophysical Union, Washington, DC.
Meek, B.D., E.A. Rechel, L.M. Carter, and W.R. DeTar. 1989.
Changes in filtration under alfalfa as influenced by time and wheel
traffic. Soil Sci. Soc. Am. J. 53:238–241.
Mohanty, B.P., M.D. Ankeny, R. Horton, and R.S. Kanwar. 1994.
Spatial analysis of hydraulic conductivity measured using disc infiltrometers. Water Resour. Res. 30:2489–2498.
Petersen, C.T., S. Hansen, and H.E. Jensen. 1999. Depth distributionof preferential flow patterns in a sandy loam soil as affected bytillage. Hydrol. Earth Syst. Sci. 4:769–776.
Petersen, C.T., H.E. Jensen, S. Hansen, and C. Bender Koch. 2001.
Susceptibility of a sandy loam soil to preferential flow as affectedby tillage. Soil Tillage Res. 58:81–89.
Richard, G., H. Boizard, J. Roger-Estrade, J. Boiffin, and J. Guérif.
1999. Field study of soil compaction due to traffic in northern
France: Pore space and morphological analysis of the compacted
zones. Soil Tillage Res. 51:151–160.
Roger-Estrade, J., G. Richard, H. Boizard, J. Boiffin, J. Caneill, and
H. Manichon. 2000. Modelling structural changes in tilled topsoil
over time as a function of cropping systems. Eur. J. Soil Sci. 51:
455–474.
Roth, C.H., K.L. Bristow, J. Brunotte, M. Facklam-Moniak, and G.
Wessolek. 1999. Impact of tillage and field traffic induced variabilityof hydraulic properties on unsaturated conductivity and waterbalance calculations. p. 489–496. In M.Th. Van Genuchten et al.
(ed.) Characterization and measurement of the hydraulic propertiesof unsaturated porous media. Part 1. Univ. of California, Riverside.
Roth, K., W.A. Jury, H. Flühler, and W. Attinger. 1991. Transportof chloride through an unsaturated field soil. Water Resour. Res.
27:2533–2541.
Schulin, R., M.Th. van Genuchten, H. Flühler, and P. Ferlin. 1987.
An experimental study of solute transport in a stony field soil.
Water Resour. Res. 23:1785–1794.
Sillon, J.F. 1999. Experimental and modeling study of the effects ofthe structure of the plowed layer on the moisture regime of a baresoil during desiccation: Application to the prevision of the available
days for cropping operations. (In French, with English abstract.)Thèse de Doctorat de l’Institut National Agronomique Paris-Grignon, INRA-Unité d’Agronomie, Laon.
Šimůnek, J., M.Th. van Genuchten, M. Šejna, N. Toride, and F.J. Leij.
1999. The STANMOD Computer Software for Evaluating Solute
Transport in Porous Media Using Analytical Solutions of the Con-

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

586

VADOSE ZONE J., VOL. 4, AUGUST 2005

vection-Dispersion Equation. Version 2.0. U.S. Salinity Lab., ARS,
USDA, Riverside, CA.
Vachaud, G. 1969. Measurement of capillary pressures in unsaturated
soils. (In French) Bull. du B.R.G.M. (deuxième série). Sect. III
4:15–20.
Vanderborght, J., C. Gonzalez, M. Vanclooster, D. Mallants, and J.
Feyen. 1997. Effects of soil type and water flux on solute transport.
Soil Sci. Soc. Am. J. 61:372–389.
Vanderborght, J., M. Vanclooster, A. Timmerman, P. Seuntjens, D.
Mallants, D.J. Kim, D. Jacques, L. Hubrechts, C. Gonzalez, J.

Feyen, J. Diels, and J. Deckers. 2001. Overview of inert tracer
experiments in key Belgian soil types: Relation between transportand soil morphological and hydraulic properties. Water Resour.
Res. 37:2873–2888.
van Wesenbeck, I., and G. Kachanoski. 1994. Effect of variable horizon thickness on solute transport. Soil Sci. Soc. Am. J. 58:1307–
1316.
Young, I.M., J.W. Crawford, and C. Rappoldt. 2001. New methods
and models for characterising structural heterogeneity of soil. Soil
Tillage Res. 61:33–45.