Bromide and chloride tracer movement in macroporous tile-drained agricultural soil during an annual climatic cycle

Journal of Hydrology 460–461 (2012) 77–89

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Bromide and chloride tracer movement in macroporous tile-drained agricultural soil during an annual climatic cycle Steven K. Frey ⇑, David L. Rudolph, Brewster Conant Jr. University of Waterloo, Department of Earth and Environmental Sciences, 200 University Avenue West, Waterloo, ON, Canada N2L 3G1

a r t i c l e

i n f o

Article history: Received 26 March 2012 Received in revised form 14 June 2012 Accepted 21 June 2012 Available online 29 June 2012 This manuscript was handled by Corrado Corradini, Editor-in-Chief, with the assistance of Juan V. Giraldez, Associate Editor Keywords: Tile drains Macropores Winter melt events Vadose zone flow and transport Seasonally variable hydrology

s u m m a r y This study investigates the dynamics of agricultural tile drain flow and solute capture during normal and wet hydrologic conditions over an annual climatic cycle. In November, bromide (Br) and chloride (Cl) tracers were applied as solutes at 0–2.3 m, and 2.3–4.6 m from a tile drain, respectively, prior to a 9 h simulated precipitation event on a macroporous, silt loam soil. Tracer concentrations were monitored in the tile over the course of the year following tracer application. Results show that the tile drain captured 8% of the applied Br mass within 48 h and 27% within 21 days of tracer application. During a major winter melt event, the tile captured an additional 25% of the total Br mass in less than 10 days. Seven months after application, about 95% of the Br tracer had been captured; however, nearly all the Cl appeared to remain in the soil. Detailed hydrologic monitoring during the melt event indicated that a saturation threshold existed, beyond which, tile effluent tracer concentrations no longer rose in conjunction with discharge but instead dropped as the tile was inundated with dilute event water transmitted through the primarily vertical macropore network within the overlying soil. Although the tile’s rapid capture zone was relatively small, conservative solutes applied post-harvest, and within approximately 2 m of the tile, readily moved to surface water prior to the following growing season. Seasonal melts were identified as the most influential climatic events for solute mobilization. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Tile drains are a significant component of the drainage enhancements required to support productive agriculture on soils with poor natural drainage, but pose environmental risks because of their ability to rapidly transmit nutrients and other potential contaminants to surface water (Skaggs et al., 1994). Agriculture in humid, mid-latitude climatic regions is particularly dependant on tile drains to help regulate the water table position during the growing season and to remove excess soil moisture during the typically cool, wet winters, when large precipitation and melt events occur. As a result, tile drain discharge in these settings is subject to large inter-seasonal, and inter-event, variability. Observations in numerous past studies (e.g. Cambardella et al., 1999; Tomer et al., 2003; Kladivko et al., 2004; Royer et al., 2006; Udawatta et al., 2006; Rozemeijer et al., 2010) have shown that a disproportionately large amount of the annual water and nutrient-mass flux through tile drains occurs in relatively short periods of time. While such evidence is valuable, a more complete understanding of the physical flow and transport processes at work in the soil profile during dif⇑ Corresponding author. Present address: Agriculture and Agri-Food Canada, 960 Carling Avenue, Ottawa, ON, Canada K1A 0C6. Tel.: +1 613 715 5313. E-mail address: [email protected] (S.K. Frey). 0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2012.06.041

ferent climatic conditions is essential for enhancing land management practices on tile drained agricultural land. In addition to seasonal climate differences, flow and transport in tile drained soil is complicated by the presence of macropore features such as earthworm burrows, root holes, and cracks that facilitate preferential water flow and solute transport (Thomas and Phillips, 1979; Beven and Germann, 1982; Jarvis, 2007). Macropores have been shown to transmit nutrients to tile drains (e.g. Laubel et al., 1999; Cook and Baker, 2001; Ball-Coelho et al., 2007), but the extent to which macropore networks are hydraulically active and facilitate preferential flow is heavily dependent on factors such as antecedent moisture content, rainfall intensity, and pore continuity (Beven and Germann, 1982; Jarvis, 2007). In relatively dry soils, light precipitation does not typically induce macropore flow and incident water infiltrates and redistributes within the matrix (Coles and Trudgill, 1985; Köhne and Gerke, 2005); however, in relatively wet soil, macropore flow can be initiated by both heavy (Edwards et al., 1992) and relatively light precipitation (Coles and Trudgill, 1985; Villholth et al., 1998). When macropores are hydraulically active, they can transmit a large proportion of the total flow (Dunn and Phillips, 1991; Mohanty et al., 1996; Lin et al., 1997); however, if the macropores are shorter in length and function more as dead end pores surrounded by a low permeability soil matrix they will transmit relatively little water

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(Bouma et al., 1982; Shipitalo and Gibbs, 2000; Weiler and Naef, 2003; Rosenbom et al., 2008). Because macropores beneath the top soil layer are predominantly vertical features (Steenhuis et al., 1988; Shipitalo and Gibbs, 2000; Frey and Rudolph, 2011), even when macropores are hydraulically active, the tile drain ‘rapid’ capture zone is generally narrow relative to tile spacing (Shipitalo and Gibbs, 2000; Frey et al., 2012b). Past field studies designed to characterize flow and transport processes in tile drained, macroporous soils, have typically utilized surface applied tracers along with controlled irrigation to quantify breakthrough, and dye tracers to identify active flowpaths, (e.g. Villholth et al., 1998; Larsson et al., 1999; Zehe and Flühler, 2001; Stamm et al., 2002) and have often demonstrated rapid arrival of the tracer at the tile drain (e.g. Richard and Steenhuis, 1988; Villholth et al., 1998; Lennartz et al., 1999; Jaynes et al., 2001; Zehe and Flühler, 2001; Stamm et al., 2002; Gish et al., 2004). However, most past research has not considered the topic of seasonal climate influence on tile drain dynamics. Moreover, the few studies that have investigated the physical mechanisms underlying tile drain flow and transport processes in highly characterized soils (e.g. Villholth et al., 1998; Zehe and Flühler, 2001; Stamm et al., 2002) have not extended detailed hydrologic and solute transport monitoring over an entire annual cycle. With these knowledge gaps in mind, this research was designed to improve the understanding of seasonal influences on flow and transport dynamics in macroporous tile drained soils, with a focus on the typically wet periods when tile drain hydraulic activity is the highest. To reflect the high risk scenario where large precipitation events follow the land application of nutrients (in the form of liquid manure) onto wet soil, the year-long tracer test employed in this work was initiated in the fall. It was hypothesized that by implementing a mass balance approach focusing on tracer capture by the tile at a highly characterized site with detailed hydrologic monitoring, it would be possible to assess the role of macroporosity on water and tracer mobility, estimate the tile capture characteristics based on the arrival of the tracers and quantify the influence of seasonal climate variability on the mobility of the tracers over an annual cycle. The results of this study provide new insight into the potential impact that post-harvest (fall) liquid manure applications can have on groundwater and surface water quality in tile drained agricultural settings. 2. Study site characterization 2.1. Site description The work was conducted on active agricultural land near the town of Kintore in southwestern Ontario, Canada (Fig. 1a). The site is characterized by poorly drained, dark gray Gleisolic surface soils underlain by clay, clay loam till, and a complex layering of Quaternary deposits consisting of subglacial diamictons and glacifluvial deposits (Rudolph and Parkin, 1998) that extend to a depth of approximately 45 m. Perforated plastic tile drains, 10 cm in diameter, were installed in 1985. The tiles are systematically spaced 12– 15 m apart and are located at a depth of approximately 0.75 m at the experiment location. Minimum tillage practices have been utilized at the site since 1995. Commercial fertilizers have been regularly applied to the field since 2004. Prior to 2004 the field was regularly fertilized with liquid swine manure, and as a result of the legacy manure applications, chloride (Cl) concentrations in the shallow groundwater at the site ranged from 4 to 10 mg l1 prior to tracer application. However, because the background Cl concentration was very low in relation to the applied tracer concentration, it was not initially deemed a problem for detecting preferential movement of high Cl concentration tracer water to the tile drain. In May 2007, the field was planted in soybean. Fol-

Fig. 1. (a) General location of the research site in southwestern Ontario, Canada; and (b) experiment plot location relative to the field topography and boundaries.

lowing soybean harvest in September 2007 and just prior to the initiation of this study, the field was planted in winter wheat. The wheat was subsequently harvested in July 2008 and the field remained fallow until the end of this study in November 2008. The same field has been the subject of past research on nitrate movement, and riparian zone denitrification (Rudolph and Parkin, 1998; Cey et al., 1998, 1999). 2.2. Soil physical and hydraulic properties The tracer experiment was conducted in the southern part of the agricultural field where the A and B horizon soils are classified as sandy loam and silt loam, respectively, and the surface slopes gently (1.6% grade) toward a perennial steam that runs along the southern edge (Fig. 1b). Physical and hydraulic characteristics of the soil at the site have been previously reported in detail (Frey and Rudolph, 2011; Frey et al., 2012a) and are summarized in Table 1. The A horizon soil extends to a depth of approximately 20 cm and is underlain by a compacted plowpan surface. Because the A horizon hydraulic properties have been found to exhibit significant temporal variability (Frey et al., 2012a), additional surface soil hydraulic property characterization was conducted in November 2008 in relatively wet soil located 5 m down-gradient from the tracer application area in order to reflect conditions during tracer application. The hydraulic property characterization consisted of six tension and double-ring infiltrometer tests that were conducted along two rows (three test positions in each row) on an evenly spaced 2 m by 3 m grid, with the long edge positioned

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Table 1 Arithmetic mean values of selected physical properties of the soil at the experimental plot, and geometric mean B horizon saturated hydraulic conductivity (Ksat) (from Frey and Rudolph (2011)). Soil horizona b

A Bc

Sand (%)

Silt (%)

Clay (%)

Gravel (%)

Organic matter (%)

Bulk density (Mg m3)

Ksatd (cm d-1)

53.4 (1.5) 31.4 (3.9)

32.5 (1.5) 52.7 (3.1)

14.1 (0.7) 15.9 (1.2)

7.0 (1.7) 9.1 (3.2)

6.3 (0.3) –

1.2 (0.0) 1.9 (0.1)

– 0.25

Values in parentheses denote standard deviation. a The A–B horizon boundary exists at approximately 20 cm depth. b n = 6. c n = 4. d Measured with a falling head permeameter according to Reynolds (2008a).

above the tile drain, following the methodology described in Frey et al. (2012a) to obtain near saturated (K(w)), and field saturated (Kfs), hydraulic conductivity. 2.3. Macropore characteristics A graph of mean macroporosity versus depth on forty-eight 0.25 m2 horizontal surfaces between 0.02 m and 0.76 m depth, within a 6 m by 4 m test area positioned approximately 5 m upgradient from the tracer application area, is shown in Fig. 2; and is derived from Frey and Rudolph (2011), where macropores at the site were physically counted. The highest proportion of macroporosity was observed near surface where it amounts to approximately 0.35% of the soil area. Total macroporosity gradually declines with increasing depth, and accounts for approximately 0.06% of the soil area at tile drain elevation. Cylindrical pores less than 5 mm in diameter compose most of the macroporosity between the surface and tile depth, and macropores greater than 5 mm in diameter are most abundant near the top of the B horizon where they compose approximately 35% of the total macroporosity. Analysis of dye infiltration patterns in Frey and Rudolph (2011) showed that the soil matrix in the A horizon was extensively stained. However, the staining in the B horizon soil matrix was only observed in areas surrounding predominantly vertical macropores, and within cylindrical macropores and cracks that were located within and along the edges of relic, tile drain installation scars (i.e. the soil above the tile drain that is conglomerated as a result of a tile plow ripping in the trench for the drain to lay) that extended from the top of the B horizon to tile depth. The depth of dye

Fig. 2. Arithmetic mean macroporosity at the experiment plot from physical counts of circular macropores of different sizes, and total macroporosity including fractures, from the surface to tile drain depth (modified from Frey and Rudolph (2011)).

penetration into the B horizon was dependent on the penetration depth of those macropores that were able to move dye through the heavily compacted plowpan layer at the base of the tillage zone. As previously reported by others (Haria et al., 1994; Kohler et al., 2003; Shipitalo et al., 2004), the shallow low permeability layer appeared to have promoted localized ponding and lateral flow immediately above it. 2.4. Site instrumentation Instrumentation installed and monitored during the experiment included: saturated and unsaturated zone equipment; a meteorological station; tile discharge monitoring equipment; and a rainfall simulator, herein, referred to as the irrigation system (Fig. 3). Seven piezometers constructed with 25.4 mm inside diameter (ID) PVC pipe and having 0.6 m long screens were installed. Five of the piezometers were shallow with screens centered at a 1.2 m depth (W1–W5), and two piezometers were deep (W6, W7) with screens centered at depths of 2.5 m, and 4.5 m, respectively.

Fig. 3. Experimental plot configuration showing the bromide and chloride tracer application areas relative to the: irrigation area, shallow (W1, W2, W3, W4, W5) and deep (W6, W7) piezometers, multilevel TDR probe, and tile drain monitoring station.

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Model 3001 LT LeveloggerÒ pressure transducers (Solinst Canada Ltd., Georgetown, ON) were placed in each of the piezometers to record water levels at 15 min intervals. Soil moisture content was measured at depths of 7.5, 23, 53, 83 and 113 cm, on 1 h intervals, with a model MP017 multilevel TDR probe (ESI Environmental Sensors Inc., Sidney, BC) positioned 1.5 m from the tile drain, midway along the irrigated section. The simulated precipitation was applied with a sprinkler system that had an MP3000 spray nozzle (Hunter Industries Inc., San Marcos, CA) positioned at each corner of the 9 m by 9 m irrigated plot area, and was capable of supplying 13 mm h1 of water (Fig. 3). Tile discharge was monitored in 15 min intervals using a collection reservoir drained by a Rule 20R submersible pump (ITT Flow Control, Gloucester, MA) that was coupled to an electronic flow-meter (Omega Technologies Co., Laval, QC) with an approximate 800 l h1 peak capacity. Tile drain water samples were obtained with an ISCO model 6712 automatic water sampler (Teledyne ISCO Inc., Lincoln, NE) installed in an insulated enclosure. A meteorological monitoring station was installed between July 2007 and October 2009 approximately 25 m from the plot, that recorded hourly data using a Model CR23X micrologger (Campbell Scientific Inc., Edmonton, AB) from a model HMP45 air temperature and humidity sensor (Vaisala Inc.) and a Model TE525W tipping bucket rain gauge (Texas Electronics Inc., Dallas, TX) that was outfitted with a Model CS705 snowfall conversion adapter (Campbell Scientific Inc., Edmonton, AB) to measure precipitation in the winter time. 3. Experiment description 3.1. Hydraulic response and tracer tests Two preliminary field tests were conducted to assess the hydraulic response of the tile drain at the experimental plot to simulated precipitation events. The first test was conducted on 22 October 2007 when antecedent soil conditions were relatively dry (0.32 m3 m3 soil moisture content at 7.5 cm depth) and the second test was conducted 2 days later, after 15 mm of natural rain had fallen at the site and the soil was wetter (0.37 m3 m3 soil moisture content at 7.5 cm depth). Both of the preliminary tests involved 3 h of irrigation using water application rates of 8.1 and 8.3 mm h1, for the first and second tests, respectively. Local municipal water was used for both of the preliminary tests because of low flow in the adjacent stream and lack of other water sources at the site, however, stream flow had increased by early November, and therefore stream water was used to feed the irrigation system during the tracer experiment. The tracer experiment began on 7 November 2007, with the application of 250 g m2 of bromide (Br) to a 2.3 m by 6.1 m patch of soil immediately adjacent to the tile drain, and the application of 250 g m2 of chloride (Cl) to a 2.3 m by 6.1 m patch of soil located further way from the tile (2.3–4.6 m) and immediately adjacent to the Br application zone (Fig. 3). Both tracer ions were uniformly applied in dissolved form (125 g l1) from NaBr and NaCl source material with watering cans over a 25 min period, which is the equivalent of 2 mm of incident water. The irrigation system was activated immediately after tracer application and 5 mm h1 of water was applied to the plot for 9 h, using water pumped from the adjacent stream. While Br concentrations in the stream water were negligible, Cl concentrations were approximately 6 mg l1 and were therefore comparable to the background concentrations in the tile effluent and not deemed to interfere with the ability to detect rapid movement of high Cl concentration tracer water to the tile drain. The application rate is an approximate representation of a 3 year return period, 9 h rain event at the site. Minimal surface runoff was observed.

Tile flow rates were continuously measured from 10 October to 28 November, 2007; at which time the monitoring system pump was damaged by the accumulation of tile discharge sediment in the pump reservoir. The pump was subsequently replaced; however, the flow record between 29 November, 2007 and 9 January, 2008 was lost as a result of the pump failure. In early January 2008, a major winter thaw and rainfall event occurred. During this event, tile flow rates occasionally exceeded the system’s 800 l h1 capacity and tile flow rates were then measured manually using a bucket and stopwatch method to supplement the dataset from the automatic monitoring system. The pump had to be repaired because of excessive sediment buildup on three additional occasions during January 2008 and as a result, the measured tile flow data for the month of January is somewhat sporadic. The automated tile discharge record ends at the end of January 2008 due to equipment failure. Model estimates (described below) of tile discharge at the site were made to facilitate tracer mass flux estimates for the duration of the study. 3.2. Water quality monitoring Chemical tracer monitoring of the tile water began immediately after the tracers were applied and continued for 1 year. The sampling frequency was initially once every 15 min at the tile drain; however, the sampling frequencies were progressively decreased as follows: day 3 (after tracer application), one every hour; day 8, one every 4 h; day 21, one every 6 h; day 69, one every 12 h; and day 169, one every 24 h. The reduction in sampling frequency coincided with the gradual stabilization of tracer concentrations in the tile water that was observed over time. Tile water samples were extracted directly from the tile drain pipe, therefore, tracer monitoring was unaffected by the problems that adversely affected the tile flow monitoring system. Br and Cl concentrations in the water samples were measured using a Model ICS-3000 ion-chromatograph (Dionex Corp., Sunnyvale, CA). Background concentrations of Br and Cl in the tile drain discharge were measured prior to tracer application. 3.3. Quantifying residual tracer mass in soil To determine the amount of residual tracer retained in the soil near the end of the year long tracer test, sixteen, 0.05 m diameter by 2 m long, soil cores were collected in September 2008 from an evenly spaced grid within a 6 m by 6 m area where the tracers were originally applied. In total, 134 subsamples were removed from the cores in 0.1 m increments and gravimetric soil moisture content was measured. Soil pore water from each subsample was extracted by centrifugation and analyzed for Br and Cl concentrations. Where the length of soil recovered in a core was less than the 2 m theoretical length, the position of each subsample was estimated based on its relative depth within the core barrel. The core analysis results were assembled into a three-dimensional dataset that was spatially interpolated with ordinary kriging using a spherical variogram in order to generate three dimensional Br and Cl concentration and mass distributions. The variogram model was deemed appropriate pffiffiffiffiffiffiffiffiffiffiffiby ffi verifying that the Q1 statistic (Kitanidis, 1997) was 62= n  1, where n is number of samples. 3.4. Consideration of tile baseflow effects Because the land surface area contributing to tile flow at the monitoring station is large in relation to the size of the irrigated test plot, and there was tile baseflow at the time of the 7 November 2007 experiment, measured tracer concentrations are diluted in relation to the actual concentrations transmitted to the tile from the tracer application area. Such dilution masks the temporal

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dynamics of the tracer transport to the tile drain. The irrigation induced tile discharge was obtained by linearly interpolating tile base flow between the beginning and end points of the clearly visible tile hydrograph discharge peak and subtracting it from the total hydrograph flow. Estimates of undiluted tracer concentration (M L3) were then made by dividing mass flux (M T1) by irrigation induced tile discharge (L3 T1). A tracer breakthrough curve could then be estimated that more accurately reflects short term flow and transport processes beneath the tracer application area.

exception of K(w0cm), surface soil hydraulic conductivity tended to be higher in the infiltration test areas located immediately above the tile drain, which suggests that tracers applied closer to the tile may infiltrate more readily than those applied further away.

3.5. Water level – tile drain discharge correlation

The transient nature of the tile drain flow rate in response to natural and simulated precipitation events provides valuable insight into the hydraulic behavior of the tile under various hydrologic conditions. For example, although the magnitude and duration of irrigation events were similar during both the 22 October, and 24 October, 2007 hydraulic response tests, there was a greater increase in tile discharge during the later of the two tests (Fig. 4a). Between these two tests, on October 23, a precipitation event occurred that resulted in a noticeable increase in tile flow and also increased soil water content in the vicinity of the tests. A similar situation arose prior to the commencement of the 7 November irrigation event (Fig. 4b). It is suggested that an increase in soil moisture content, as a result of the precipitation events, enhanced the transmission of infiltrating water to the tile drain by increasing preferential flow. During both the 24 October and 7 November irrigations, tile flow began to increase within 1 h after irrigation commenced, and declined rapidly upon irrigation cessation. It is interesting to note that although the tile flow rates increased relatively quickly after the start of the natural precipitation events, the decline rate was much slower than for the irrigation events (Fig. 4). This is likely because of the much more regional scale of the precipitation event and larger overall volume of water infiltrating along the upstream section of the tile. Hydrograph separation conducted for the 24 October and 7 November irrigation events shows that the amount of additional tile discharge induced by irrigation is small in relation to the amount of water applied, with approximately 17% and 14%, respectively, of the applied water volume being recovered within 24 h. Additional insight into the hydraulic response of the soil profile and tile drain system can be drawn from the transient soil moisture data (Fig. 5). While the near surface soil moisture content at 7.5 cm depth increased substantially during both October tests (Fig. 5a and b) there was minimal increase at a depth of 23 cm (top of plowpan) on 22 October, whereas on 24 October, water content at a depth of 23 cm approached saturation. Because of the similarity in initial soil moisture content profile prior to the start of the 24 October and 7 November tests, the magnitude and rate of soil moisture increase in response to irrigation were similar (Fig. 5b and c). In addition, although the magnitude and duration of irrigation was different in each case, rapid near surface soil drainage (i.e. decreased soil moisture content) occurred soon after irrigation ended in both cases, illustrating the dynamic nature of the shallow flow system in the vicinity of the tile. Results from the irrigation tests are consistent with previous work at this site that showed the importance of the plowpan (Frey and Rudolph, 2011), and suggest that soil moisture content at the base of the A horizon is a good indicator of tile discharge response to water application on the field surface. When the duration and/or intensity of precipitation is sufficient to cause localized saturation at the top of the low K layer, there is increased likelihood that water will laterally disperse until vertically continuous macropores or a plowpan discontinuity are encountered that facilitate preferential flow into the B horizon, and ultimately into the tile drain (e.g. Kamra et al., 1999; Kohler et al., 2003; Shipitalo et al., 2004; Frey and Rudolph, 2011). As

Tile drain discharge (Q) during the periods when the flow monitoring system was inoperable was estimated using an empirical model that utilized the high frequency groundwater level data from the piezometers. As part of the model development, crosscorrelation between groundwater levels in each of the seven piezometers, and Q was determined using the Matlab R2011b (The Mathworks Inc., Natick, MA) ‘corrcoef’ function, to examine water level – Q relationships. In the model, QT (tile discharge normalized with a power transform function) was hypothesized to be a function of both water table elevation and hydraulic head in the deeper piezometers and was estimated for each 15 min time step (i) according to

Q Ti ¼ A½Z s;i þ Z d;i ðkÞ þ B where A and B are two empirical fitting coefficients, and water levels in one shallow piezometer (Zs), and one deep piezometer (Zd) that were smoothed with a centered moving average over k, 15 min, time intervals. The optimal shallow and deep piezometer water-level combination, as well as values for A (2.374), B (1.767), and k (1550), were determined by coupling the model with the PEST parameter estimation software package (Doherty, 2004). The Nash–Sutcliffe (Nash and Sutcliffe, 1970) efficiency coefficient was used to assess the performance of the model. 3.6. Tile drain bromide mass capture estimates Estimates of bromide mass (Mbr) discharged through the tile drain were determined according to

M br ¼

n X Q i Ci i¼1

where Qi, and Ci are tile flow, and Br concentration respectively, for each 15 min time step (i). Measured and modelled values of Qi were used to estimate Br mass flux over the respective 3 weeks (n = 2016), and 1 year (n = 35136) periods that followed tracer application. When the time interval between water samples was more than 15 min, linear interpolation between measured data points was used to estimate tile effluent Br concentrations. Because the reduction in sampling frequency coincided with the progressive stabilization of Br concentrations in the tile effluent, the interpolation is not anticipated to have resulted in significant estimation error. 4. Results and discussion 4.1. Soil hydraulic properties Based on the results from the November 2008 infiltrometer tests, the A horizon soil has a geometric mean Kfs of 27.5 cm d1 (based on the row 1 and row 2 Kfs values given in Table 2) in conditions similar to those encountered on 7 November 2007. Both the Kfs and K(w5 to 0cm) of the surface soil (Table 2) are considerably higher than the B horizon Ksat of 0.25 cm d1 (Table 1). With the

4.2. Tile drain hydraulic response to natural and simulated precipitation

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Table 2 Arithmetic mean initial (hi) and final (hf) moisture contents; and near-saturated (K(w)) and field saturated (Kfs) hydraulic conductivity for the surface soil at the experimental plot during the November 2008 infiltration tests. Note that row 1 and row 2 are located above and 2 m away from the tile drain, respectively.

a b

Row

Position

hi

hf

K(5 cm)a (cm d-1)

K(2 cm)a (cm d-1)

K(1 cm)a (cm d-1)

K(0 cm)a (cm d-1)

Kfsb (cm d-1)

1

i ii iii

0.36 0.32 0.34

0.42 0.43 0.42

4.0 6.4 4.5

10.3 43.0 13.3

18.8 90.9 37.8

86.3 284.8 225.1

84.1 273.3 47.6

2

i ii iii

0.36 0.37 0.33

0.42 0.43 0.43

2.6 3.9 1.9

8.1 13.2 8

20.6 27.4 17.4

717.8 569.3 119.8

11.6 2.1 16.5

Measured with a tension infiltrometer according to Reynolds (2008b). Measured with a double-ring infiltrometer according to Reynolds et al. (2002).

Fig. 4. Tile discharge relative to precipitation and irrigation water input for (a) October 22–26, and (b) November 6–10.

will be shown, such a flow and transport mechanism has important implications for solute movement to tile drains during major infiltration/snow melt events. 4.3. Tile discharge – water level correlation For all six of the piezometers assessed, water levels are strongly correlated with transformed tile discharge QT (Fig. 6), with crosscorrelation coefficients ranging from 0.74 to 0.88. Lag time between water level fluctuation and tile discharge is minimal (from 3 to +1 h) for the four shallow piezometers, W1, W2, W4, and W5; however, water level fluctuation in deep piezometers W6 (screened at 2.5 m), and W7 (screened at 4.5 m), lags tile discharge by 12, and 15 h, respectively. The hydraulic response of W3 to precipitation and irrigation was muted because of its close proximity (approximately 0.15 m) to the tile drain (Fig. 3). Accordingly,

Fig. 5. Soil moisture content fluctuation in response to the 3 h irrigation events on: (a) October 22; (b) October 24; and (c) the 9 h irrigation event on November 7.

there was very little water level fluctuation in W3 during all but the most intense hydrologic events. The strongest correlation between modelled and measured tile discharge was achieved by coupling the raw water level in W1 with water level from W7 that had been smoothed using a 16.2 day (k = 1550) centered moving average. The combination of high frequency water-table fluctuation, with the low frequency signal contained within the deep water level fluctuation, yielded an empirical model (in the form described in Section 3.5) that was able capture both the high frequency variability in tile discharge associated with individual precipitation events and the low frequency variability in tile base flow (Fig. 7). The cross-correlation coefficient for the modelled versus measured transformed tile discharge relationship is 0.93, with less than 1 h of lag, and the Nash–Sutcliffe model efficiency coefficient as determined by comparing modelled versus observed tile flow for the October

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Fig. 6. Cross-correlation between tile discharge and water levels in piezometers W1, W2, W4, W5, W6, and W7.

10th to November 27th time period is 0.59. While it should be noted that for individual rainfall events, model estimates of tile discharge can differ from the observed discharge, the general trends displayed in the modelled data are consistent with the observed data (Fig. 7c). The correlation between modelled and measured discharge suggests that predicting tile discharge as a function of water levels can be an alternative to long term continuous tile monitoring. 4.4. Short term tracer movement to the tile drain The presence of Br in the tile effluent within 1 h of application (Fig. 8) indicates that preferential flow influenced tracer movement during the early phase of the experiment and is consistent with results from previous work that show the highest tracer concentrations in tile effluent are observed soon after application (e.g. Villholth et al., 1998; Jaynes et al., 2001; Stamm et al., 2002; Köhne et al., 2006). As indicated in the observed breakthrough curve

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(Fig. 8b), Br concentrations in the tile effluent continuously increase throughout the 9 h irrigation interval and peak approximately 1 h after irrigation stops. Following the initial peak, Br concentration in the tile effluent declines to less than 25% of the maximum value within 24 h, at which point it continues to undergo a more gradual decline during the next 20 days. While numerous small precipitation events do not have significant influence on tile effluent Br concentration or tile flow rates during the 3 weeks following tracer application, relatively high intensity events on 21 and 27 November both induce temporary increases in Br concentration and tile flow and suggest a threshold for the initiation of preferential solute leaching (McGrath et al., 2010). The hydrologic response to the series of precipitation events during the latter part of November shows that in order for precipitation to notably influence tile effluent Br concentration, rainfall intensity needs to exceed approximately 2 mm h1, which is very close to K(w5cm) for the soil at the site (Table 2). Because macropore hydraulic activity increases substantially when pressure heads reach 10 to 6 cm (Jarvis, 2007), it can be surmised that precipitation in excess of 2 mm h1, along with the antecedent soil moisture conditions that existed during the 3 weeks following tracer application, initiated preferential flow that facilitated increased Br leaching from the near surface soil matrix. Similar precipitation rates have been reported to induce preferential flow by past researchers. For example, Coles and Trudgill (1985) found that in wet but not saturated silt and clay loams in the UK, 2.5 mm h1 of precipitation was a threshold value, while Villholth et al. (1998), who conducted their work on silt and sand loam in Denmark, found that 2–3 mm h1 was adequate. In contrast, Köhne and Gerke (2005) found that 2 mm h1 of irrigation was insufficient to induce preferential flow in silt loam to sandy silt soil in Germany. Such differences highlight that in addition to site specific conditions, antecedent soil moisture is an important factor, as was observed in the tile drain hydraulic response to the two October irrigation tests, as well as in other previous work (e.g. Kung et al., 2000a; Zehe and Flühler, 2001; Vidon and Cuadra, 2010). Although Br concentrations in the tile effluent rise quickly after tracer application, and the wet soil conditions reflect ‘high risk’ conditions for tile drainage contamination from surface applied solutes, the amount of Br mass captured by the tile within 48 h of tracer application is only about 8% of the applied mass

Fig. 7. (a) Water level in relation to tile drain elevation in the seven piezometers, (b) the two water level signals that compose the empirical tile discharge model, and (c) modelled versus measured transformed tile discharge.

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Fig. 8. (a) November 7–28 tile discharge relative to precipitation and irrigation water inputs, and (b) tile effluent bromide and chloride concentrations and cumulative bromide and chloride mass as a percent of the total mass applied.

(Fig. 8b). A similar amount of Br (9%) was captured by the tile within 48 h of the large precipitation event that began on 21 November, where 23 mm of rainfall was received over 24 h. After 3 weeks of relatively wet soil/hydrologic conditions where 48 mm of rainfall was received, the Br mass captured by the tile drain accounts for 27% the total amount applied. While Fig. 8b clearly shows that Br reaches the tile drain quickly, the short term temporal dynamics of tracer transport at the site become clearer after the effects of baseflow dilution have been removed. The modified Br breakthrough curve (Fig. 9) shows that tile flow from outside the irrigation area has a significant impact on the concentration of Br in the tile drain. Instead of peak concentrations arriving approximately 1 h after irrigation stopped as shown in Fig. 8b, the modified breakthrough curve shows that Br concentration in the water draining from the test plot actually peaked 1 h after irrigation began, with a maximum value reaching approximately 1.2% of the initial tracer concentration, and is consistent with previous work that has shown tracer induced concen-

tration spikes can precede maximum tile discharge (Laubel et al., 1999; Kung et al., 2000b). The Br concentrations are relatively low while irrigation induced tile discharge remains elevated, and it is hypothesized that ‘tracer free’ irrigation water is being preferentially transmitted to the tile drain rather than Br laden water. As discharge rates decline upon the cessation of irrigation, Br concentrations again increase as solute laden matrix pores in the A horizon sequentially drain. In contrast to the rapid arrival of the Br tracer, there is no indication of increasing levels of Cl during the early stages of the infiltration experiment (Fig. 8b). Over the 3 week period following tracer application, a mass of chloride equivalent to 10% of that which was applied was captured by the tile. However, the captured Cl is expected to have mostly been background Cl that existed prior to tracer application, because a simple multiplication of cumulative tile discharge by the pre-tracer application Cl concentration yields a mass capture estimate equal to 9% of the applied mass. The lack of rapid breakthrough of Cl to the tile drain provides further evidence that the preferential flow features influencing the transport of the applied tracers in this macroporous soil are primarily vertical in nature, as previously suggested in the detailed site characterization work (Frey and Rudolph, 2011). A similar observation was made by Villholth et al. (1998) who noted that at one of their three test plots, Cl applied to a soil strip 1–1.5 m from the tile drain was not rapidly transmitted to the drain. In comparison, in a study where steady state preferential flow conditions were established prior to tracer application on a loamy soil in Switzerland, Stamm et al. (2002) observed rapid lateral movement of Cl that was applied 2.5 m from the drain. Such contrasting observations again highlight the site specific nature of tile drain tracer experiments, and emphasize the need for additional work that characterizes near surface preferential flow paths in different soil, land use, and climatic settings. 4.5. Long term tracer movement to the tile drain

Fig. 9. Irrigation water input and tile discharge, and bromide (Br) concentration after the dilutive influence from tile discharge sourced from outside the test plot was removed using hydrograph separation.

With the exception of short duration winter melts and/or precipitation events, Br concentrations in the tile effluent tended to decline logarithmically over time (Fig. 10a). Because tile discharge

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rates increased in the late fall (Fig. 10e) the rate of Br mass capture by the tile drain (Fig. 10f) remained relatively stable over the first two and a half months, even though concentrations tended to decline. It is only after a major winter melt event in mid January 2008 (when rates briefly increase) that the Br mass capture rate declined appreciably, at which point over 75% of the Br mass (Fig. 10f) had been accounted for. The next most significant decline in Br mass capture rate occurred following the spring melt at the end of March (when the rates again briefly increased), after which both tile flow rates and Br concentrations experienced concurrent declines, and at which time approximately 95% of the Br mass had already been captured. While significant precipitation events in early May and early June both induce temporary increases in the Br concentration, pre spring melt levels were not reached, which in part reflects Br depletion in the soil. At the beginning of August, Br concentrations dropped below 0.1 mg l1 and remained undetectable until tile monitoring ceased, with the exception of one period of heavy precipitation in mid-September that caused a brief increase to approximately 1 mg l1. Direct comparison of results from different tile drain tracer tests is complicated by the wide variety of influential factors (e.g. season, climate, soil type, scale, land management practices, topography); however, even a cursory analysis of existing work shows that large percentages of surface applied tracers are typically captured

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by tile drains during the late fall to early spring time frame in areas with similar seasonal climatic conditions. For example, Lennartz et al. (1999) showed that between 45% and 70% of Br applied to loam soil in a 0.5 ha field in Northern Germany was captured after 384–462 mm of rainfall during the winter months of their 3 year study. Also in Northern Germany, Köhne et al. (2006) found that tile drains captured approximately 24% of Br applied to 0.16– 0.24 ha plots with silty clay soil, between late November and late April, during which time 477 mm of rain was received. On 900 m2 loamy sand plot in Sweden, Larsson et al. (1999) found that 46% of Br was captured by tiles over a winter period with approximately 360 mm of precipitation. As would be expected, results from these field scale tracer tests consistently show lower percentages of captured Br than what was observed in the current plot scale work, where the tracer was applied directly adjacent to the tile. In contrast to the comparatively high Br capture rates observed for tracer tests initiated in the fall, Fortin et al. (2002) found that only 3.2–9.6% of the Br applied during the spring on loam and silt loam soil under different management regimes, in Quebec Canada, was captured by tiles prior to the following winter, and after 739 mm of rainfall. The graph of cumulative Br mass captured (Fig. 10f) shows the relative importance of late fall precipitation, winter precipitation

Fig. 10. Hydrologic and solute transport characteristics between October 2007 and November 2008. (a) Tile effluent bromide and chloride concentration, (b) air temperature, (c) precipitation rate, (d) cumulative precipitation since tracer application, (e) tile discharge, and (f) bromide captured by the tile drain (as% of total mass applied).

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and melts, and spring melt events on the progressive leaching of Br to the tile drain. The single most significant event with respect to Br movement was a 10 day winter melt event in early January 2008 (Fig. 11), during which time the average temperature was 3 °C, 62 mm of precipitation was received at the site, and approximately 25% of the initial Br mass (or 50% of the remaining mass) was discharged through the tile drain (Fig. 10f). In comparison, approximately 50% of the initial Br mass was discharged over the initial 2 month time frame preceding the January melt. Chloride concentrations in the tile effluent remained quite stable throughout the monitoring period and generally remained within the bounds of background levels (4–11 mg l1) measured prior to tracer application (Fig. 10a). Following tracer application, Cl concentrations only exceeded 11 mg l1 on three very wet occasions (early December, mid January, and early May), when maximum concentrations of 15 mg l1 were reached. Chloride concentrations also experienced significant (albeit temporary) declines to as low as 1 mg l1 during extremely wet conditions on numerous occasions, such as the melt events in mid January, mid February, and the end of March when Br concentrations were also observed to decline. The lack of a noticeable increase in tile Cl concentrations during the study suggests that 1 year was insufficient time for the majority of the Cl to move laterally from beneath the Cl application area to the tile, which also infers that the lateral Cl movement was primarily facilitated by relatively slow soil matrix flow and transport, and not by macropore flow and transport. 4.6. Flow and transport to the tile drain during melt events Although high solute concentrations in tile discharge can be toxic to aquatic life and should not be disregarded, very little of the applied tracer mass reached the tile as a result of the initial preferential flow induced concentration spike. In actuality, the

Fig. 11. (a) Bromide and chloride concentrations in the tile effluent, (b) air temperature, (c) precipitation, (d) water content of soil at 23 cm depth, and (e) horizontal hydraulic gradient between W5 and W3, during the January 2008 precipitation/melt event.

majority of the tracer mass was transmitted to the tile in response to large precipitation and/or melt events that occurred throughout the late fall and early winter. During these significant but infrequent hydrologic events, tile drain baseflow was much higher, and there was a major influx of clean event water to the tile; as a result, there existed greater opportunity for dilution. Results from this work show that total solute mass loading to surface water during the major winter melt far exceeds the immediate post-application period, even when the post-application period reflects ‘worst case’ conditions for solute land application (i.e., on wet soil with tiles flowing and followed by heavy precipitation). Although this work was conducted at a plot scale, the event driven solute transport dynamics compare well to larger scale studies. In example, as part of a long term study of nutrient exports from three Illinois watersheds, Royer et al. (2006) found that >50% of NO3–N was transmitted when stream flow rates were in the top 10 percentile. During the major melt event, Br concentrations in the tile effluent initially rose on January 6th (Fig. 11a) in response to increasing temperatures (Fig. 11b), and light precipitation (Fig. 11c) which suggests increased drainage from the solute laden soil matrix in the vicinity of the tile. Had the intensity of the melt event not continued accelerating, the observed increase in solute concentrations along with increased tile flow would likely have corresponded with results from past larger scale work where positive correlation between flow rates and solute concentration was observed (Tiemeyer et al., 2006). However, as melting accelerated, the water content at 23 cm depth continued to increase (Fig. 11d) until saturation was reached, at which point there is a concurrent decrease in tile effluent tracer concentration even though tile flow rate continued to increase. It is postulated here that this change in the characteristic leaching pattern represents a process level threshold (Zehe and Sivapalan, 2009), at the extreme wet end of the plot scale hydrologic response spectrum, that reflects when tracer laden water leaching from the soil matrix is inundated by event water transmitted to the tile via preferential flow. It is during this most intense period of the melt event that the solute concentration – discharge relationship behaves similar to the observations of Vidon et al. (2009) and Vidon and Cuadra (2010) from work conducted in Central Indiana, who noted that Cl concentrations in stream discharge, and Mg2+ concentrations in tile flow, respectively, tended to decline during large storms. An assessment of water levels and hydraulic gradients in the vicinity of the tile provides additional support for the concept of a hydrologic threshold in extremely wet tile drained fields. As precipitation intensity increased on 9 January (Fig. 11c) the water level in the piezometer closest to the tile drain (W3) increased rapidly to the point where the water table in the vicinity of the tile was relatively flat and lateral hydraulic gradients were effectively absent. During these brief periods, vertical flow through the soil above the tile drain is hypothesized to be the dominant source of tile discharge; and as a result, soil in the immediate vicinity of the tile drain will be subject to increased solute leaching as compared to more distant soil. Under such extremely wet conditions when the entire field is saturated, it would be expected that the solute concentrations would decline appreciably after the majority of solutes have been leached from the soil above the tile drain, which is evidenced by the temporary decline in Br concentrations to 1 mg l1 on 9 January (Fig. 11a) that tends to coincide with the precipitation peak (Fig. 11c) and the point when lateral gradients are lowest (Fig. 11e). 4.7. Long term tracer mass balance The final Br mass balance overestimated total mass by 15%, (i.e. 3.5 kg was applied and 4.01 kg was accounted for as tile discharge

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Fig. 12. Residual bromide and chloride distribution within the 6 m  6 m  2 m soil volume that was extensively cored in mid-September 2008, relative to the initial tracer application areas identified on top (ground) surface.

capture (3.94 kg) and as soil residual (0.07 kg)). Although it appears more mass was ultimately recovered than actually applied, it is still a relatively good mass balance given the duration of the study, rapid variations in flows and concentrations, and the difficulties associated with trying to accurately measure tile drain discharge. Because of the presence of Cl in the soil profile prior to tracer application, and the persistently low Cl concentrations in the tile discharge throughout the monitoring period that were generally within ±5 mg l1 of the pre-tracer application concentrations, a cumulative Cl mass balance is not presented. 4.8. Residual tracer distribution For the 16 soil cores that were extracted in September 2008, tracer concentrations in the soil pore water ranged from 0 to 113 mg l1 for Br, and 5 to 194 mg l1 for Cl (Fig. 12). Kriging results indicate that less than 2% of the original Br is retained in the soil, of which the majority is localized in the topographically down-gradient end of the plot within the A horizon. Because of past liquid swine manure applications at the site, the amount of residual Cl tracer in the soil was difficult to determine; however, if the 50 mg l1 isosurface is used to approximate the position of the Cl tracer, it is apparent that the majority of the mass is retained between the ground surface and tile drain depth and the centroid of the residual mass has moved down the topographic gradient and towards the tile drain. Even though Cl concentrations in the tile effluent did not appear to rise significantly over the 10 month period between tracer application and core extraction, results from the core analysis suggest that the Cl tracer had reached the tile drain at the down gradient end of the plot by mid-September 2008 (Fig. 12). When interpreting the residual tracer profiles at this site, it is important to note that based on the hydraulic heads in W6 and W7 (Fig. 3), the vertical component of groundwater movement at the site is generally directed upwards; as a result, the hydraulic gradient will restrict the downward migration of surface applied solutes in the groundwater. 5. Conclusions and implications This study provides valuable insight into the dynamic nature of solute movement to tile drains over the course of an annual climatic cycle and during a major winter melt event. Unique aspects of the work include the combination of detailed soil characterization, high resolution hydrologic monitoring, and longer term (1 year) cumulative Br mass balance; as well as the simple use of groundwater levels to predict tile discharge. Based on the results of the mass balance, it was demonstrated that mobile solutes

(i.e., Br) applied on the land surface within 2.3 m of the tile (post-harvest) can potentially be leached from the soil profile via tile drainage prior to the next growing season, whereas elevated pore water concentrations of solute applied beyond 2.3 m from the tile (i.e., Cl) were observed to have just reached the tile 10 months after application. When it is considered that tile drains are often spaced less than 20 m apart and may have an ‘over-winter’ capture zone 4 m wide, it can be expected that tile drains will capture upwards of 20% of the soluble nutrients (i.e. nitrate) applied or derived from liquid manure application on post-harvest tile drained fields prior to the following growing season. At this site, early solute breakthrough (i.e. within 48 h of tracer application) to the tile accounted for a relatively small proportion (8%) of the applied Br mass even though hydrologic conditions were ideal for macropore flow. The relatively low proportion of Br rapidly transmitted to the tile, as well as the long delay in the arrival of Cl, can be attributed to the primarily vertical orientation of the macropore network that tends to direct preferential flow in a downward direction. For the majority of solute mass transmitted into the B horizon by macropores that do not terminate near the tile, the permeability and hydraulic conditions of the soil matrix, and the associated groundwater flow, will ultimately govern lateral solute movement. For the solute applied nearest the tile, the temporal pattern of solute breakthrough was closely related to magnitude of the precipitation and/or melt events, and the proportion of solute mass remaining in the soil profile. For example, approximately 25% of the initial Br mass (or 50% of the mass still present in the soil) was captured by the tile drain during a single winter melt event that spanned a 10 day interval in mid January. In the late fall and early winter, smaller precipitation events also caused notable increases in the amount of Br mass captured by the tile; however, rainfall intensity was important and a minimum value of approximately 2 mm h1 was required before concentrations in the tile effluent increased noticeably. By the following spring, 7 months after the application of the surface tracers, approximately 95% of the Br tracer had been captured by the tile drain while during the same time interval, Cl concentrations in the tile discharge had not increased above background levels. A detailed assessment of the soil hydraulic conditions, along with high resolution tile effluent tracer concentration data, identified a sequence of leaching conditions that exist as tile drained soil wets up to near flooded conditions during a major winter melt. Early in the wetting process, hydraulic gradients in the vicinity of the tile drain increase, which causes increased leaching from the soil matrix in the vicinity of the tile and an associated increase in tile effluent tracer concentrations. As the wetting progresses, a saturation threshold is reached, upon which time the lateral hydraulic gradients are diminished and the tile is inundated with event

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water transmitted through the hydraulically continuous preferential flow pathways above the tile that is concomitant with a marked drop in tile effluent tracer concentration. The residual tracer mass distribution observed beneath the tracer application area almost 1 year after tracer application identified the importance of the regional shallow-groundwater flow system on water and solute flux in tile drained agricultural settings. At locations similar to the site investigated here in southwestern Ontario, where the vertical component of the shallowgroundwater flow direction is upward (typical of many tile drained regions), the hydraulic gradients act as a protective barrier that inhibits surface applied solutes from migrating deep into the soil profile. In these settings, tile drains, and ultimately surface water bodies, are the primary receptors for water and solutes that leach from the soil profile. Acknowledgements The authors would like to thank: Ontario Pork, Ontario Ministry of the Environment, and the Natural Sciences and Engineering Research Council of Canada for financial support; Jeff Melchin, Joana Niemi, Julia Charlton, Sean Sinclair, Paul Johnson, and Bob Ingleton for assistance with field and lab activities; and especially Frank Aarts and his family for providing access to the site. References Ball-Coelho, B.R., Roy, R.C., Topp, E., Lapen, D.R., 2007. Tile water quality following liquid swine manure application into standing corn. J. Environ. Qual. 36 (2), 580–587. http://dx.doi.org/10.2134/jeq2006.0306. Beven, K., Germann, P., 1982. Macropores and water flow in soils. Water Resour. Res. 18 (5), 1311–1325. Bouma, J., Belmans, C.F.M., Dekker, L.W., 1982. Water infiltration and redistribution in a silt loam subsoil with vertical worm channels. Soil Sci. Soc. Am. J. 46 (5), 917–921. Cambardella, C.A., Moorman, T.B., Jaynes, D.B., Hatfield, J.L., Parkin, T.B., Simpkins, W.W., Karlen, D.L., 1999. Water quality in Walnut Creek watershed: nitrate– nitrogen in soils, subsurface drainage water, and shallow groundwater. J. Environ. Qual. 28 (1), 25–34. Cey, E.E., Rudolph, D.L., Parkin, G.W., Aravena, R., 1998. Quantifying groundwater discharge to a small perennial stream in southern Ontario, Canada. J. Hydrol. 210 (1–4), 21–37. Cey, E.E., Rudolph, D.L., Aravena, R., Parkin, G.W., 1999. Role of the riparian zone in controlling the distribution and fate of agricultural nitrogen near a small stream in southern Ontario. J. Contam. Hydrol. 37 (1–2), 45–67. Coles, N., Trudgill, S., 1985. The movement of nitrate fertilizer from the soil surface to drainage waters by preferential flow in weakly structured soils, Slapton, S. Devon. Agric. Ecosyst. Environ. 13 (3–4), 241–259. Cook, M.J., Baker, J.L., 2001. Bacteria and nutrient transport to tile lines shortly after application of large volumes of liquid swine manure. Trans. ASAE 44 (3), 495– 503. Doherty, J., 2004. PEST Model-Independent Parameter Estimation User Manual, fifth ed. Watermark Numerical Computing. Dunn, G.H., Phillips, R.E., 1991. Macroporosity of a well-drained soil under no-till and conventional tillage. Soil Sci. Soc. Am. J. 55 (3), 817–823. Edwards, W.M., Shipitalo, M.J., Dick, W.A., Owens, L.B., 1992. Rainfall intensity affects transport of water and chemicals through macropores in no-till soil. Soil Sci. Soc. Am. J. 56 (1), 52–58. Fortin, J., Gagnon-Bertrand, E., Vézina, L., Rompré, M., 2002. Preferential bromide and pesticide movement to tile drains under different cropping practices. J. Environ. Qual. 31 (6), 1940–1952. Frey, S.K., Rudolph, D.L., 2011. Multi-scale characterization of vadose zone macroporosity in relation to hydraulic conductivity and subsurface drainage. Soil Sci. Soc. Am. J. 75 (4), 1253–1264. http://dx.doi.org/10.2136/ sssaj2010.0403. Frey, S.K., Rudolph, D.L., Parkin, G.W., 2012a. Spatial and temporal influences on hydraulic properties in tile-drained soil. Soil Sci. Soc. Am. J. 76 (2) 350–360. http://dx.doi.org/10.2136/sssaj2011.0194. Frey, S.K., Rudolph, D.L., Lapen, D.R., Ball-Coelho, B.R., 2012b. Viscosity dependant dual-permeability modelling of liquid manure movement in layered, macroporous, tile-drained soil. Water Resour. Res. 48, W00L11. http:// dx.doi.org/10.1029/2011WR010809. Gish, T.J., Kung, K.J.S., Perry, D.C., Posner, J., Bubenzer, G., Helling, C.S., Kladivko, E.J., Steenhuis, T.S., 2004. Impact of preferential flow at varying irrigation rates by quantifying mass fluxes. J. Environ. Qual. 33 (3), 1033–1040. Haria, A.H., Johnson, A.C., Bell, J.P., Batchelor, C.H., 1994. Water movement and isoproturon behavior in a drained heavy clay soil: 1. Preferential flow processes. J. Hydrol. 163 (3–4), 203–216.

Jarvis, N.J., 2007. A review of non-equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality. Eur. J. Soil Sci. 58 (3), 523–546. http://dx.doi.org/10.1111/j.13652389.2007.00915.x. Jaynes, D.B., Ahmed, S.I., Kung, K.J.S., Kanwar, R.S., 2001. Temporal dynamics of preferential flow to a subsurface drain. Soil Sci. Soc. Am. J. 65 (5), 1368–1376. Kamra, S.K., Michaelsen, J., Wichtmann, W., Widmoser, P., 1999. Preferential solute movement along the interface of soil horizons. Water Sci. Technol. 40 (2), 61– 68. Kitanidis, P.K., 1997. Introduction to Geostatistics: Applications to Hydrogeology. Cambridge University Press, Cambridge, New York, 249 p. Kladivko, E.J., Frankenberger, J.R., Jaynes, D.B., Meek, D.W., Jenkinson, B.J., Fausey, N.R., 2004. Nitrate leaching to subsurface drains as affected by drain spacing and changes in crop production system. J. Environ. Qual. 33 (5), 1803–1813. Kohler, A., Abbaspour, K.C., Fritsch, M., Schulin, R., 2003. Using simple bucket models to analyze solute export to subsurface drains by preferential flow. Vadose Zone J. 2 (1), 68–75. Köhne, J.M., Gerke, H.H., 2005. Spatial and temporal dynamics of preferential bromide movement towards a tile drain. Vadose Zone J. 4 (1), 79–88. Köhne, S., Lennartz, B., Köhne, J.M., Šimu˚nek, J., 2006. Bromide transport at a tiledrained field site: experiment, and one- and two-dimensional equilibrium and non-equilibrium numerical modeling. J. Hydrol. 321 (1–4), 390–408. Kung, K.J.S., Kladivko, E.J., Gish, T.J., Steenhuis, T.S., Bubenzer, G., Helling, C.S., 2000a. Quantifying preferential flow by breakthrough of sequentially applied tracers: silt loam soil. Soil Sci. Soc. Am. J. 64 (4), 1296–1304. Kung, K.J.S., Steenhuis, T.S., Kladivko, E.J., Gish, T.J., Bubenzer, G., Helling, C.S., 2000b. Impact of preferential flow on the transport of adsorbing and non-adsorbing tracers. Soil Sci. Soc. Am. J. 64 (4), 1290–1296. Larsson, M.H., Jarvis, N.J., Torstensson, G., Kasteel, R., 1999. Quantifying the impact of preferential flow on solute transport to tile drains in a sandy field soil. J. Hydrol. 215 (1–4), 116–134. Laubel, A., Jacobsen, O.H., Kronvang, B., Grant, R., Andersen, H.E., 1999. Subsurface drainage loss of particles and phosphorus from field plot experiments and a tiledrained catchment. J. Environ. Qual. 28 (2), 576–584. Lennartz, B., Michaelsen, J., Wichtmann, W., Widmoser, P., 1999. Time variance analysis of preferential solute movement at a tile-drained field site. Soil Sci. Soc. Am. J. 63 (1), 39–47. Lin, H.S., McInnes, K.J., Wilding, L.P., Hallmark, C.T., 1997. Low tension water flow in structured soils. Can. J. Soil Sci. 77 (4), 649–654. McGrath, G.S., Hinz, C., Sivapalan, M., Dressel, J., Pütz, T., Vereecken, H., 2010. Identifying a rainfall event threshold triggering herbicide leaching by preferential flow. Water Resour. Res. 46, W02513. http://dx.doi.org/10.1029/ 2008WR007506. Mohanty, B.P., Horton, R., Ankeny, M.D., 1996. Infiltration and macroporosity under a row crop agricultural field in a glacial till soil. Soil Sci. 161 (4), 205–213. Nash, J.E., Sutcliffe, J.V., 1970. River flow forecasting through conceptual models, Part I – a discussion of principles. J. Hydrol. 10, 282–290. Reynolds, W.D., 2008a. Saturated hydraulic properties: laboratory methods. In: Carter, M.R., Gregorich, E.G. (Eds.), Soil Sampling and Methods of Analysis, second ed. CRC Press, Boca Raton, pp. 1013–1024. Reynolds, W.D., 2008b. Unsaturated hydraulic properties: field tension infiltrometer. In: Carter, M.R., Gregorich, E.G. (Eds.), Soil Sampling and Methods of Analysis, second ed. CRC Press, Boca Raton, pp. 1107–1128. Reynolds, W.D., Elrick, D.E., Youngs, E.G., 2002. Single-ring and double- or concentric-ring infiltrometers. In: Dane, J.H., Topp, G.C. (Eds.), Methods Soil Anal. Part 4 – Phys., Methods. Soil Sci. Soc. Am. Madison, pp. 821–826. Richard, T.L., Steenhuis, T.S., 1988. Tile drain sampling of preferential flow on a field scale. J. Contam. Hydrol. 3 (2–4), 307–325. Rosenbom, A.E., Ernstsen, V., Flühler, H., Jensen, K.H., Refsgaard, J.C., Wydler, H., 2008. Fluorescence imaging applied to tracer distributions in variably saturated fractured clayey till. J. Environ. Qual. 37 (2), 448–458. Royer, T.V., David, M.B., Gentry, L.E., 2006. Timing of riverine export of nitrate and phosphorus from agricultural watersheds in Illinois: implications for reducing nutrient loading to the Mississippi River. Environ. Sci. Technol. 40 (13), 4126– 4131. http://dx.doi.org/10.1021/es052573n. Rozemeijer, J.C., van der Velde, Y., van Geer, F.C., Bierkens, M.F.P., Broers, H.P., 2010. Direct measurements of the tile drain and groundwater flow route contributions to surface water contamination: from field-scale concentration patterns in groundwater to catchment-scale surface water quality. Environ. Pollut. 158 (12), 3571–3579. Rudolph, D., Parkin, G., 1998. Partitioning of Solutes from Agricultural Fields within the Hydrologic System at Two Sites in Southern Ontario and the Subsequent Impact on Adjacent Aquatic Ecosystems, Edited, COESA Report: RES/MON-010/ 97. Shipitalo, M.J., Gibbs, F., 2000. Potential of earthworm burrows to transmit injected animal wastes to tile drains. Soil Sci. Soc. Am. J. 64 (6), 2103–2109. Shipitalo, M.J., Nuutinen, V., Butt, K.R., 2004. Interaction of earthworm burrows and cracks in a clayey, subsurface-drained, soil. Appl. Soil Ecol. 26 (3), 209–217. Skaggs, R.W., Brevé, M.A., Gilliam, J.W., 1994. Hydrologic and water quality impacts of agricultural drainage. Crit. Rev. Environ. Sci. Technol. 24 (1), 1–32. Stamm, C., Sermet, R., Leuenberger, J., Wunderli, H., Wydler, H., Flühler, H., Gehre, M., 2002. Multiple tracing of fast solute transport in a drained grassland soil. Geoderma 109 (3–4), 245–268. Steenhuis, T.S., Richard, T.L., Parlange, M.B., Aburime, S.O., Geohring, L.D., Parlange, J.Y., 1988. Preferential flow influences on drainage of shallow sloping soils. Agric. Water Manage. 14 (1–4), 137–151.

S.K. Frey et al. / Journal of Hydrology 460–461 (2012) 77–89 Thomas, G.W., Phillips, R.E., 1979. Consequences of water movement in macropores. J. Environ. Qual. 8 (2), 149–152. Tiemeyer, B., Kahle, P., Lennartz, B., 2006. Nutrient losses from artificially drained catchments in North-Eastern Germany at different scales. Agric. Water Manage. 85, 47–57. Tomer, M.D., Meek, D.W., Jaynes, D.B., Hatfield, J.L., 2003. Evaluation of nitrate nitrogen fluxes from a tile-drained watershed in central Iowa. J. Environ. Qual. 32 (2), 642–653. Udawatta, R.P., Motavalli, P.P., Garrett, H.E., Krstansky, J.J., 2006. Nitrogen losses in runoff from three adjacent agricultural watersheds with claypan soils. Agric. Ecosyst. Environ. 117 (1), 39–48. http://dx.doi.org/10.1016/j.agee.2006.03.002. Vidon, P., Cuadra, P.E., 2010. Impact of precipitation characteristics on soil hydrology in tile-drained landscapes. Hydrol. Process. 24 (13), 1821– 1833.

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Vidon, P., Hubbard, L.E., Soyeux, E., 2009. Seasonal solute dynamics across land uses during storms in glaciated landscape of the US Midwest. J. Hydrol. 376 (1–2), 34–47. Villholth, K.G., Jensen, K.H., Fredericia, J., 1998. Flow and transport processes in a macroporous subsurface-drained glacial till soil I: field investigations. J. Hydrol. 207 (1–2), 98–120. Weiler, M., Naef, F., 2003. An experimental tracer study of the role of macropores in infiltration in grassland soils. Hydrol. Process. 17, 477–493. http://dx.doi.org/ 10.1002/hyp. 1136. Zehe, E., Flühler, H., 2001. Preferential transport of isoproturon at a plot scale and a field scale tile-drained site. J. Hydrol. 247 (1–2), 100–115. Zehe, E., Sivapalan, M., 2009. Threshold behaviour in hydrological systems as (human) geo-ecosystems: manifestations, controls, implications. Hydrol. Earth Syst. Sci. 13 (7), 1273–1297.