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There is No Such Thing as a Quick Fix: Travel Times to Subsurface Drains
Eileen Kladivko1,*, Laura Bowling1
Published in Journal of the ASABE 66(6): 1513-1525 (doi: 10.13031/ja.15505). Copyright 2023 American Society of Agricultural and Biological Engineers.
1Department of Agronomy, Purdue University, West Lafayette, Indiana, USA.
*Correspondence: kladivko@purdue.edu
The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License https://creative?commons.org/licenses/by-nc-nd/4.0/
Submitted for review on 20 December 2022 as manuscript number NRES 15505; approved for publication as a Research Article and as part of the “Advances in Drainage: Selected Works from the 11th International Drainage Symposium” Collection by Associate Editor Dr. Mohamed Youssef and Community Editor Dr. Zhiming Qi of the Natural Resources & Environmental Systems Community of ASABE on 25 August 2023.
Highlights
Abstract. Subsurface “tile” drains are an important water management practice for many productive agricultural soils, but drains also deliver nitrate-nitrogen and other soluble chemicals from the bottom of the rootzone to surface ditches and streams. Many studies have focused on determining different ways to reduce the delivery of nitrate and other chemicals to subsurface drains while still providing adequate drainage for crop production. One important consideration in such studies that is often overlooked is the response time of drainage outflow to changes in management at the soil surface. Drains integrate flow from distances immediately adjacent to the drain all the way to the midplane between parallel drains, which means changes in management will not be fully reflected in drainage waters for months to several years, depending on the drainage intensity, soil characteristics, and precipitation. This bromide tracer study was conducted at the Southeast Purdue Agricultural Center (SEPAC) in Indiana, USA. Bromide tracer was applied at the beginning of the drainage season (November) at different distances from the drains on multiple plots of different drain spacings. The bromide concentration in the drainflow was monitored over the next five years to determine breakthrough curves and the amount of time required to flush the bromide from the system. The initial breakthrough of bromide occurred during the first appearance of drainage after application, regardless of the location of the bromide strip, suggesting some preferential flow was occurring. This was further investigated with comparison to curves of theoretical travel time following Kirkham’s analytic solution for flow to a tile drain. The full transport of the bulk of the chemical took 2-3 years, but varied greatly among the different treatments, reflecting the longer travel times for locations further from the drain.
Subsurface “tile” drains are an important water management practice for many productive agricultural soils, but drains also deliver nitrate-nitrogen and other soluble chemicals from the bottom of the rootzone to surface ditches and streams. Due to concerns about nitrate from tile drains being a major contributor to the hypoxic zone in the Gulf of Mexico, many studies have focused on determining different ways to reduce the delivery of nitrate and other chemicals to subsurface drains while still providing adequate drainage for crop production. Some of these practices include cover crops (Kaspar et al., 2012; Hanrahan et al., 2018; Ruffatti et al., 2019), controlled drainage (Christianson et al., 2016; Drury et al., 2014), reduced drainage intensity (Kladivko and Bowling, 2021), and other fertilizer and agronomic management practices (Dinnes et al., 2002; David et al., 2010). One important consideration in such studies that is often overlooked is the response time of drainage outflow to changes in management at the soil surface. Typical field studies only last for 2 to 4 years based on grant funding constraints or before/after treatment experimental design, but such short-duration studies may not be adequate to reflect the full impact of changes in management practices on tile-drained fields. Drains integrate flow from distances immediately adjacent to the drain all the way to the midplane between parallel drains, which means changes in management will not be fully reflected in drainage waters for months to several years, depending on the drainage intensity, soil characteristics, and precipitation.
The question of this lag time between management changes in the field to reductions in load at the watershed outlet has attracted much attention in the last decade (e.g., Ascott et al., 2021; Sprenger et al., 2019; Van Meter et al., 2018). Of relevance to drained agricultural systems, Ascott et al. (2021) argue that storage and transport time of N in the unsaturated zone causes Best Management Practices to not meet water quality targets in expected time frames, due to continued transport of so-called legacy N to the outlet years after the application. Estimates of the travel time for water quality response to management changes vary from a few years to decades (Van Meter et al., 2018, Ballard et al., 2019, Sprenger et al., 2019). Therefore, accounting for storages and travel time, particularly in the unsaturated zone, is important for understanding the influence of water and contaminant flow paths and storages on water quality outcomes (Sprenger et al., 2019). Van Meter et al. (2018) distinguished between two sources of lag time in the transport of contaminants to watershed outlets, the biogeochemical lag, addressing cycling in and out of organic pools, and hydrologic lag, addressing travel time.
Hydrologic lag in the form of theoretical travel time distributions for water and chemicals can be calculated based on the drainage theory of Kirkham (1958) and the further developments of Jury (1975a,b). These calculations provide estimates of required travel times or drainage volumes for chemicals applied at some distance from the drain to arrive at the drain and can be used to estimate the total amount of time or flow required before a chemical applied near the midplane between drains arrives at the drain. Utilizing a flow-corrected time scale is important when precipitation or drainage are not evenly distributed over the year (Buzek et al., 2009). Such analysis provides insight into the duration of the transition of drainflow concentrations from a current management practice to a newly-imposed management practice, or the “lag time” for the full impact of the new practice.
Subsurface drainage studies have also commonly exhibited preferential flow behavior of surface-applied chemicals, including pesticides and tracers (Richard and Steenhuis, 1988; Kladivko et al., 1991, 1999, 2001; Kung et al., 2000; Fortin et al., 2002). Vidon and Cuadra (2010) found that macropore flow accounted for between 11 to 50% of total drainflow over eight drain flow events in May-June on a silty clay loam in Indiana, using hydrograph separation techniques with electrical conductivity (EC) measured in rainwater and base tile flow as the end members. Cation concentration also showed expected responses with “new water” (storm flow) causing concentration increases of surface applied potassium (K) corresponding to hydrograph peaks but corresponding decreases, or dilution, of concentrations of magnesium (Mg) distributed throughout the soil matrix. This pattern of concentration peaks or dilution from new water was clearly evident for large storm events (>6 cm precipitation) but inconsistent for smaller events (<3 cm precipitation), indicating the relative importance of macropore flow is different with different storm event sizes (Vidon and Cuadra, 2010). Williams et al. (2016) found similar results involving both unsaturated and saturated macropore flow using isotopic techniques for hydrograph separation. Early in the storm, event water moves from the surface through the unsaturated macropores and delivers high concentrations of surface-applied chemicals to the drain. Later in the storm, the water delivered to the drains is a mix of event and pre-event water due to mixing along the flowpaths. Williams and McAfee (2021) also observed seasonal differences, with a greater proportion of direct precipitation input in the summer, with greater mixing associated with shallow groundwater recharge in the winter, despite overall low rates of preferential flow.
One of the questions raised in discussions of these preferential flow observations in tile-drained fields was whether the observed preferential flow was simply an artifact of the large soil disturbance caused by the trenching operation to install the tile drains in the field. Messing and Wesstrom (2006) found much higher values of infiltration and saturated hydraulic conductivity in the trench backfill zone than in the soil midway between trenches, even for drains installed up to 45 years earlier. The authors suggested that the potential effects of the spatial difference in flow on chemical transport to the drains should be more thoroughly considered in modeling efforts. However, often it is assumed that the drainage installation trench has returned to average field conditions similar to the undisturbed soil between trenches after a few years (e.g., Williams et al., 2016). The study reported here was designed in part to answer the question of whether preferential flow was occurring only in the close vicinity to the drain itself or whether it occurred more broadly across the field. It was conducted on a drainage field site that had observed preferential flow of pesticides in southeastern Indiana, USA (Kladivko et al., 1991).
The specific objectives of this study were (1) to test whether preferential flow in this field was occurring at distances away from the installation trench, and (2) to compare travel times, at different distances from the drain, with both field measurements and theoretical predictions. The second objective was aimed at answering the question of the hydrologic lag before drainage water reflects newly-applied management practices on the field.
Materials and Methods
Field Site and Methods
The study was conducted on the experimental drainage/water quality field site at the Southeast Purdue Agricultural Center (SEPAC) (39°01’33”N, 85°32’24”W) in southeastern Indiana, USA. The site has been previously described in detail by Kladivko et al. (1991, 1999, 2004) and Kladivko and Bowling (2021). The soil at the site is a Cobbsfork silt loam, formerly called Clermont (fine silty, mixed, superactive, mesic Typic Glossaqualf) and is typical of extensive areas of similar soils across southern Ohio, Indiana, and Illinois. The soil was formed in 50 to 120 cm of loess over glacial till. The surface soil at the study site is a light gray, low organic carbon (0.7%), silt loam containing 66% silt, 22% sand, and 12% clay. The soil is slowly permeable and has a borderline fragipan at about 120 cm depth that severely restricts further downward drainage.
The field experimental site had drains (10-cm diameter) installed in 1983 at spacings of 5-, 10-, and 20-m at an average depth of 75 cm and a slope of 0.4% (fig. 1). Three drain lines (225-m length) were installed at each spacing, with the outside drain lines on each spacing acting as border drains between treatments. Each spacing was replicated in two blocks separated by a 40-m distance. The center drains of the 5-, 10-, and 20-m plots discharged into observation culverts at the lower end of the field before being routed to the main drain. Subsurface drainflow volumes were monitored continuously with tipping-bucket flow gauges connected to a datalogger. Flow-proportional drainflow samples were continuously collected with automatic water samplers (Isco, Lincoln, NE) during all time periods in which there was flow, with the samples being retrieved every weekday. Water samples were frozen until subsequent laboratory analyses by commercial labs or on campus. Bromide concentrations were measured using Latchat instrumentation. Chemical mass losses (loads) were calculated as the product of concentration and water flow volume for each day and were expressed on a per hectare basis, assuming that each drainline collects water from midplane to midplane.
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| Figure 1. Plot diagram of SEPAC experimental drainage field. Inset shows locations of bromide application strips. |
The bromide (Br) tracer experiment was conducted starting on November 10, 1989, and continued for approximately 10 yrs, until the bromide concentrations dropped to near pre-experiment values. The application timing was after the corn harvest and slightly before the winter flow season normally began. Potassium bromide was applied in solution at the rate of 400 kg Br ha-1 in the area of the application strips. This high application rate was chosen based on previous experience and because two of the plots would only receive Br on 15% of the plot area, causing dilution of the Br signal in the drainflow. Different widths and placements of application strips were applied to the six plots to test the preferential flow and travel time objectives (fig. 1). The 5 m east (drain 2) and west (drain 5) plots received Br over the entire width of the plot, from midplane to midplane centered over the tile, giving a final application rate of 400 kg Br ha-1 to the plot. This application over the entire plot was to test the chemical outflow as it occurs from the typical chemical inputs across a uniformly managed field. The 10 m (drain 1) and 20 m (drain 3) east plots received a 3-m wide application strip of bromide, centered over the tile. This resulted in final application rates of 120 and 60 kg Br ha-1 equivalent over the entire plot width for the 10 m and 20 m plots, respectively. These centered strips tested transport primarily in the old trench and short distances away from the drain. The 10 m (drain 4) and 20 m (drain 6) west plots also received a 3-m wide application strip of bromide, but it was offset 1.5 m from the tile drain, for final application rates of 120 and 60 kg Br ha-1. This placement was done to avoid any potential lingering effects of soil disturbance in the drain installation trench and to evaluate the potential preferential flow from distances away from the drain. It was also done to evaluate the travel times for different drain spacings as depicted in Jury (1975a,b).
Soil cores were collected in November 1990, one year after the start of the experiment. Two transects of 10 cores each were taken in a line parallel to the drain, near the center of the application strip for the offset plots and ~0.75 m away from the drain for the other four plots. Additional cores were taken outside of the application strip for the offset plots to assess the transport of the Br towards the drain. Soil cores were divided into 15-cm depth increments from 0 to 120 cm. Individual samples were air-dried and analyzed for total Br concentration and soil bulk density to obtain the total mass of Br. Representative corn stover and grain samples were collected from a few locations in the field to obtain an estimate of Br uptake by the crop. Soil, stover, and grain were extracted with water, and the Br was analyzed on Latchat instrumentation. The mass recovery at the end of Year 1 was calculated as a percent of total Br applied in each plot by summing the mass remaining in the soil, taken up by the crop, and lost in drainflow over the year.
Data Processing and Analysis
Since there were trace concentrations of bromide occasionally detected in drainflow before the current experiment, a threshold concentration was determined by finding the highest observed bromide concentration during the period of observation from 1 January 1988 to 10 November 1989. These threshold concentrations were then used to establish the latest time period of observation after the application of November 1989. The threshold concentrations for all six drains were between 1.5 and 1.6 mg Br liter-1. Most drains fell below their threshold concentration by 1995, but the 5 m west plot continued to show Br loss into 1999. Loads were accumulated for each drain from the application day until the last date, when a concentration above the threshold was observed.
Theoretical curves for bromide accumulation vs. drainflow were calculated using the concepts of Kirkham (1958) built upon by Jury (1975a,b), in which dimensionless travel time from an injection point at the surface of the water table is solved as a function of the dimensionless distance (X) of the point of injection from the tile line (see fig. 2). The travel time in years for steady state conditions can then be calculated for SEPAC using the dimensionless travel times presented in Jury (1975a). The travel time calculation implies that the annual average drainflow is evenly distributed throughout the year. Since drainflow typically occurs for about nine months of the year at SEPAC, it is preferred to solve for the drainflow (Qsat) required for travel through the saturated zone from distance X as:
(1)
where
T = dimensionless travel time associated with distance X, from Jury (1975a)
? = porosity (assumed equal to 0.4)
S = half-spacing distance of the plot.
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| Figure 2. Streamlines followed by water flowing in steady state to the tile line. Shaded area shows path followed by water injected between X=0.85 and X=0.90. Adapted from the conceptual diagram of travel time estimates from Jury 1975a. (The red box illustrates the offset application zone for the 20 m west plot, and the blue box shows the offset application zone for the 10 m west plot). |
The values of TX for each 5th percentile distance from the drain to the drainage divide were approximated from figure 3 in Jury (1975a). For the 5 m plots, where Br was applied to the full plot width, each Qsat,X represents the accumulated drainflow required to move the Br from 5% of the applied area (or 5% of the applied Br) from the water table to the drain. For the 10 m and 20 m east plots, Br was only applied over X = [0,0.3] and X = [0,0.15], respectively, so only those values are used. For the 10 m and 20 m west plots, Br was applied from X = [0.3,0.9] and X = [0.15,0.45].
Travel time in the unsaturated zone was calculated for drains 4 (10 m W) and 6 (20 m W) in two different ways. The first approach was a simple piston flow estimate of the amount of water required to move the bromide from the surface to the water table depth of 0.65 m and 0.63 m for drains 4 and 6, respectively. Water table depth was estimated from the average observed mid-plane water table position on days with drainflow and water table above the drain depth from 1987-1990. The field-average water table position was estimated by averaging the mid-plane depth and the drain depth. Assuming an average volumetric water content of 0.299 (estimated from neutron probe data from 10/23/89), the piston flow calculation yields that 19.4 and 19.0 cm of net infiltration was needed for bromide to reach the water table. A similar approach was used by Richard and Steenhuis (1988) as they were assessing their bromide drainage results for evidence of preferential flow. The second approach to calculating unsaturated travel time used the travel-time density function derived through the solution of the one-dimensional Convective Dispersion Equation (CDE), solved in terms of water infiltration rate, I, rather than time (Jury and Horton, 2004). The two parameters of the distribution are the area-averaged vertical pore-water velocity (V) and a field-scale dispersion coefficient (D). To adjust for the fact that some mass was already gone from the soil profile, having come out in the drain, the parameters were fitted to the distribution of Br mass in the soil cores collected one year from the start of the experiment, normalized by the total mass in the soil plus the mass lost in tile drainage in the first year, by minimizing the sum of square errors between predicted and observed soil Br concentration in the upper 60 cm of the soil column.
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| Figure 3. Cumulative bromide load as a function of cumulative drainflow for all six drains. |
The fitted parameters were used to solve for the distribution of travel time (expressed as the depth of applied water) to the mean water table depth, Z, described above. Using this calculation meant that 25 cm and 21 cm of net infiltration was needed for the bromide peak to reach the water table for drains 4 and 6, respectively. Net infiltration was estimated as the depth of drainflow, plus the 6 cm of precipitation that fell between the application of bromide in November 1989 and the first drainflow event in December. The calculation was repeated for each 10th percentile, i.e., the drainflow required to move the first 10% of bromide to the water table. In order to create a cumulative plot of Br loss, as a percentage of total Br load, versus drainflow, the time required for each 10% to reach the water table was added to the drainflow required for saturated flow for each spatial position X to which Br was applied.
Results and Discussion
Preferential Flow
Bromide arrived at the drain as soon as drainflow started after bromide application on all plots (fig. 3). There was no water flow into drains for the first ~30-40 days after application, but as soon as drainage began, bromide was present in the drainflow. Approximately 6 cm of rain fell between application on November 10 and the first flow in late December, with 0.3, 4, and 0.3 cm coming on November 14, 15, and 16, respectively. This early 4 cm rainfall may have moved some of the Br down into the profile through bypass flow (Vidon and Cuadra, 2010; Williams et al., 2016), delivering it to deeper depths where it could more quickly arrive at the water table when the late December rains occurred and drainage began. All drains had 1% of their total load within 70 days of application (table 1). The first 10% of the load exited all drains within 5 months [range from 3-5 months] during the first flow season (table 1). For this table, the percent of load was calculated based on the total mass that eventually arrived at the drain outflow and not on the total mass applied, which is discussed later.
Preferential flow in drained systems is also sometimes recognized by the coincidence of peak chemical concentrations with peak drainflow (Kladivko et al., 1991,1999; Pleur et al., 2020; Williams et al., 2016; Vidon and Cuadra, 2010). Figure 4 shows the first year of drainflow and Br concentrations for drain 5 (5 m West, whole plot application) and drain 6 (20 m West, offset application). During the first month of flow, there were two or three hydrograph peaks that coincided with Br peaks, suggesting preferential flow. Most of the remaining flow season did not have coincident peaks, indicative of Br being distributed mostly in the matrix, as is nitrate (Kladivko et al., 2004). There were a few coincident peaks in spring 1990, especially in drain 5, where the application included distances close to the drain, which might be expected to have a greater preferential flow occurrence. Even without the obvious coincidence of concentration and flow peaks, macropore flow was still an important contribution to tile drainflow in the study of Vidon and Cuadra (2010).
The fast initial arrival and the early coincidence of flow and concentration peaks, even for those plots where the Br application strip was offset from the tile, is evidence of preferential flow, potentially in both unsaturated and saturated zones. This is consistent with other work at this site with pesticides and tracers (Kladivko et al., 1991,1999; Kung et al., 2000). So preferential flow is not just a lingering effect of the drain trench itself.
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| Figure 4. Time series of drainflow (cm) and bromide concentration (mg/L) for year 1 for drains 5 and 6. The dots indicate the date of the local maxima in drain flow and concentration, identified as a data sample that is larger than its two neighboring samples. If a peak is flat, the first day of the peak is identified. The gray bars indicate the days when drain flow peaks and concentration peaks both occurred. |
Hydrology
Drainflow per unit area from the different drain spacings and replicate blocks has shown consistent behavior over the 31 years of the study (Kladivko and Bowling, 2021; Kladivko et al., 2004). Flow data during the years of the bromide study are shown in figure 5. Note the strong seasonality of drainflow at this site, as shown by flat portions of the cumulative drainflow graph during most summer seasons. In general, the drainflow per area is greatest for the 5 m spacing, intermediate for the 10 m spacing, and lowest for the 20 m spacing, as would be expected. The west block (drains 4-6) has consistently had greater drainflow than the east block (drains 1-3) throughout the study. Specific reasons for the greater flow on the west block than the east block have not been determined, as the site has a uniform slope and soil characteristics, as well as surface drains surrounding the area that minimize surface runoff onto the plots. The restricting layer is also at a relatively uniform depth (1.2 m) across the field.
Bromide Recovery
Not all the Br applied to the plots was recovered when sampled a year later, which puts limits on the interpretation of mass load delivery to the drain. The prolonged time period for Br to flush from the system is still a significant finding, but the discussion of total mass delivered to the drain must keep in mind the overall low recovery.
Table 2 shows recoveries in soil, crop, and cumulative drainflow, as well as estimated losses to seepage and overland flow at the end of Year 1. It also presents the cumulative Br lost in drainage when the Br concentration dropped below the threshold 5 to 10 years later. At the end of Year 1, total Br recovery in soil, crop, and drainflow was approximately 47 to 55% in the whole plots (drains 2, 5) and centered strip plots (drains 1, 3) and was approximately 64 to 87% in the offset plots (drains 4, 6). Potential losses of Br from the system include surface runoff from the 4 cm of rainfall that occurred five days after application and seepage (vertical and lateral). Previous studies at both field scale and experimental boxes have found that bromide losses in overland flow are typically less than 1% of the amount applied to dry soils (e.g., Burgoa et al., 1993; Walton et al., 2000). However, Walton et al. (2000) found Br loss rates up to 15% for soils with large aggregate formation or preferential flow pathways, which they attributed to solute being held in the soil matrix near the surface being available for transport by overland flow. Referring to figure 2, the concentration of Br in vertical seepage will reflect Br applied at a relative distance of 90%-100% from the drain, which is essentially zero for the offset applications, and relatively low in year 1 for the full width application, because Br applied at a high relative distance would not yet have traveled across the width of the plot. Seepage rates were estimated based on DRAINMOD model simulations (Wang et al., 2006), multiplied by the observed drainflow concentrations from drain 4 (since it is the lowest, and seepage concentrations are expected to be low). This resulted in estimated seepage losses for Year 1 of <1% for all drains except drain 6. Therefore, although it is possible that some of the missing bromide was lost to overland flow and seepage, these mechanisms are not expected to account for the large discrepancy in recovery during one year. Total recovery in drainage by the end of the experiment ranged from 26 to 39% for all plots except drain 6, which had a much higher total recovery of 61% (fig. 6, table 2). Most of the unaccounted-for Br had likely flowed laterally below the soil profile or vertically through the slowly permeable restrictive layer over the years. Although not all Br was accounted for, the lengthy travel times are an important finding of this study.
| Table 2. Mass recovery of bromide in soil, drainflow, and crop at end of Yr. 1, and mass recovery in drainflow by end of experiment, as % of bromide applied to each plot. | |||||||||||||||
| Whole Plot | Offset, 3 m Strip | Centered, 3 m Strip | |||||||||||||
| Drain | Drain 2 | Drain 5 | Drain 4 | Drain 6 | Drain 1 | Drain 3 | |||||||||
| Spacing/Block | 5 mE | 5 mW | 10 mW | 20 mW | 10 mE | 20 mE | |||||||||
| % Mass recovery of applied Bromide at end of Year 1 | Soil | In-Strip | 35.0 | 35.2 | 47.0 | 53.6 | 26.3 | 27.6 | |||||||
| En-Route[a] | X | X | 1.1 | 6.5 | X | X | |||||||||
| Crop[b] | 7.1% | 7.1% | 7.1% | 7.1% | 7.1% | 7.1% | |||||||||
| Drainflow | 12.4% | 12.7% | 9.2% | 19.7% | 13.3% | 18.2% | |||||||||
| Seepage[c] | <1% | <1% | <1% | 1.2% | < 1% | <1% | |||||||||
| Overland Flow[d] | 1-15% | 1-15% | 1-15% | 1-15% | 1-15% | 1-15% | |||||||||
| Approx. Total | 56-70% | 56-70% | 65-79% | 88-102% | 48-62% | 54-68% | |||||||||
| % Mass recovery (Br) in drain- flow by end of experiment | Drainflow | 31.0% | 38.9% | 35.3% | 60.8% | 26.5% | 37.3% | ||||||||
| [a] En-Route is estimated from soil cores taken between the edge of the application strip and the drain, for the offset plots. [b] Estimated from average crop yield in 1990 and the water-extractable Br from the crop. [c] Seepage loss is estimated based on simulated seepage rates from Wang et al. (2006), multiplied by observed average concentration from drain 4. [d] Potential overland flow losses are estimated based on a review of relevant literature, e.g., Burgoa et al. (1993) and Walton et al. (2000). | |||||||||||||||
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| Figure 5. Cumulative drainflow vs. time for all six drains. |
Bromide Loss from 5 m East and West Pair
The 5 m east and west pair had bromide applied to the entire plot, and they have the same spacing, and thus this was the pair of drains most indicative of leaching behavior that would occur with uniform field chemical applications on an intensively drained field. The two plots had the same bromide load vs. time behavior during the first two flow seasons, and then they diverged (fig. 7). Again, the end of each flow season is visible by the flat portions of the cumulative curve occurring each summer season. These results suggest several things. First, the bromide arriving at the drain during the first two flow seasons is expected to have originated at the same distances from the tile in both plots, yielding the same mass load to the tiles. Although the bromide loads were the same, the flow was greater in the west plot (drain 5) than in the east plot (drain 2) (fig. 5), suggesting some “clean water” may have entered the upper end of the west part of the field. This would be consistent with the long-term observation of overall greater flow in the west block than in the east. Second, the divergence of bromide load delivered to the tile after the second season suggests that some bromide was lost to seepage in the east block, from the outer 20 to 35% of the distances from the drain. At the end of the second flow season, when the plots had delivered equal bromide loads, the west plot had delivered 65% of its eventual total load, whereas the east had delivered 80% of its eventual total load (fig. 8). So, bromide in the outer 35% of the distances was apparently not completely captured in the east plot.
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| Figure 6. Cumulative bromide load as percent of total applied bromide in each plot vs. time. Graphs of each drain are ended when the concentrations fell below the threshold determined at start of experiment (see text), except drain 5 continued slow accumulation until April 1999. |
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| Figure 7. Cumulative bromide load vs. time for all six drains. |
The observed mean travel times, based on when 50% of the cumulative total Br delivery occurred, were essentially identical at ~410 days (Dec 23 and 31, 1990, see table 1), but drain 5 had a much longer tail on its distribution than drain 2 (fig. 8). This mean travel time is similar to the nitrate transit time of 400 days found by Buzek et al. (2009) using isotopic techniques and a 2-component model of drainflow, and slightly longer than the 6 to 12 months found by Dimitrova-Petrova (2020), Williams et al. (2016), and Williams and McAfee (2021). All these studies discuss that transit times to drains vary with rainfall patterns, soil type, and underlying geologic and groundwater flow conditions.
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| Figure 8. Cumulative bromide load as a percentage of total load delivered from each drain vs. time. |
Bromide Loss from Offset Pair, 10 m and 20 m West Block
The bromide application strip was offset from the tile trench by 1.5 m on the 10 m and 20 m spacing plots in the West block. As discussed earlier, bromide arrived at the tile drain as soon as drainflow began, even though the closest bromide was 1.5 m away from (and 0.75 m above) the tile. Thus, preferential flow was occurring in the main part of the field, undisturbed by the trenching installation. Cumulative bromide delivery to the tile was slower for these two plots than for the other four plots with centered application strips (fig. 3), reflecting the absence of bromide at short distances from the tile with short streamtubes.
Bromide loads from both the 10 m and 20 m plots were similar in the first flow season after application. In Years 2 and 3, the bromide loads were greater in the 20 m spacing. In year 4, with approximately 60cm of flow, the 10 m plot started accumulating greater loads than the 20 m plot (figs. 3 and 7). This crossover in accumulation rates between the two plots reflects the difference in relative distances from the drain and, thus, the difference in streamtubes and flowpaths to the drain. Although the absolute distance of the 3 m wide application strip is the same for both plots (1.5 to 4.5 m from the drain), the relative distances are different (0.15 to 0.45 for the 20 m and 0.3 to 0.9 for the 10 m). Thus, the 10 m plot has longer and deeper streamtubes to travel to the drain (fig. 2), requiring a longer time to completely flush the chemical from the system.
Predictions from the Jury (1975a,b) approach are shown in figures 9 and 10. Using the simplified assumptions of 19.4 and 19.0 cm of water for drains 4 and 6, respectively, for the unsaturated zone piston flow to the water table and then the calculated streamtubes, the theoretical lines in figure 9
clearly show the divergence in behavior for the 10 m and 20 m plots. The greater spread of the measured breakthrough curves compared to the theoretical are due to two factors. The immediate arrival of bromide at the drain, in spite of being applied 1.5 m away from the drain, is evidence of preferential flow, as discussed earlier. Beyond this early arrival from preferential flow, there would also be hydrodynamic dispersion, causing spreading around the piston flow estimate for the unsaturated zone. This is illustrated with our estimation of the dispersion coefficient and travel time in the unsaturated zone shown in figure 10. Due to the dispersion of the bromide in the unsaturated zone, some bromide is estimated to reach the drain earlier than a piston flow estimate, but it still lags behind the observations, further supporting the idea of preferential flow.
Although the theoretical lines show both drains starting to deliver bromide to the drain with approximately the same amount of drainflow (figs. 9 and 10), the 10 m plot takes much more drainflow to deliver most of its load than does the 20 m, due to the longer streamtubes. Both theoretical and measured curves show a larger amount of flow required for the 10 m plot than the 20 m plot.
Bromide Loss from Centered Strip Pair, 10 m and 20 m East Block
A 3 m wide strip was centered over the tile drain for the 10 m and 20 m plots in the east block. This treatment differed from the 5 m plots in that the centered strip was only 3 m wide and therefore did not include the full plot widths. The two plots showed nearly identical accumulation of bromide mass vs. drainflow initially (fig. 11), due to being directly over the tile and at short distances from it. The 10 m plot required more water to clean out the plot than did the 20 m, probably due to the greater relative distances as discussed for the offset plots above.
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| Figure 9. Cumulative bromide load as percentage of total load, vs. net applied water, for offset bromide application strips. Solid lines are observed, and dashed lines show theory with piston flow for unsaturated zone and streamtubes for saturated zone (Approach #1 in text). Net applied water is estimated as the cumulative drainflow, plus the 6 cm of precipitation that fell between application of the bromide and the beginning of drainage in December 1989. |
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| Figure 10. Cumulative bromide load as percentage of total load vs. net applied water, for offset bromide application strips. Solid lines are observed, and dashed lines show theory with dispersion for unsaturated zone and streamtubes for saturated zone (Approach #2 in text). Net applied water is estimated as the cumulative drainflow, plus the 6 cm of precipitation that fell between application of the bromide and the beginning of drainage in December 1989. |
Comparison of Bromide Results to Other Field Observations
The bromide results illustrate the amount of time for a chemical tracer application to be “flushed out” of the system. Although the drains did respond almost immediately to the bromide application, even when not directly over the drain, it took years to flush the system. Table 1 shows it took between 10 and 18 months for 50% of the eventual load to reach the drain, and between 18 and 64 months for 90% to reach the drains. Although mean travel times of drainage water have been estimated at 6 to 12 months (Williams et al., 2016; Dimitrova-Petrova et al., 2020), 245 days (Williams and McAfee, 2021), or 400 days (Buzek et al., 2009), which are similar to our 10 to 18 months, the tail of the distribution is much longer. Other studies have begun to examine this complete travel time distribution and have found that long tails in water age distribution can result from multi-scale heterogeneities of hydraulic conductivity, as well as variable flow path connectivity and mixing intensity (Sprenger et al. 2019).
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| Figure 11. Cumulative bromide load as percentage of total load vs. cumulative drainflow, for centered application strips. |
This study has quantified the hydrologic lag time for bromide applied at the soil surface for a subsurface drained agricultural system and found that for a range of drain spacings common in the Eastern Corn Belt, non-reactive chemicals applied near the mid-point between two tile drains may take 5-10 years to reach a new equilibrium at the tile outlet, following a shift in management practices, although 50% of the load differential reached the drainage outlet within 2 years. This is consistent with other studies that have shown that drained agricultural watersheds can exhibit water quality improvements within a few years of management changes (Smith et al., 2013; McIsaac et al., 2016; Jaynes et al., 2004), while overall legacy effects may last from a few years to multiple decades (Meals et al., 2009; McIsaac et al., 2016). Many typical field studies are conducted for 2 to 3 years, which means that the tested management practice changes will not be completely reflected in the outflow during the time of the study. In addition, this hydrologic lag is just part of the overall lag time to be expected from a non-conservative tracer such as reactive nitrogen because of potential transformations (Van Meter et al., 2018), and estimates of these legacy time scales remain highly uncertain (Van Meter et al., 2018; Ballard et al., 2019).
Another illustration of this cleanout time for drained fields occurred on these plots in 2005-2006, as described in Kladivko and Bowling (2021). Two plots had an herbicide leak on large portions of the plots, killing the corn crop and resulting in much less N uptake on those plots. Figure 12 illustrates one of the damaged plots (10 m east) vs. its undamaged replicate plot (10 m west). Nitrate-N concentrations in the drainflow increased sharply after the herbicide leak, due to excess nitrate-N with no crop to take it up. The concentrations stayed higher than the undamaged plot for over a year, gradually being flushed from the plot by drainage and returning to a similar concentration as its replicate. There were no additional fertilizer N applications made in 2006 due to soybeans being grown, and thus the response curve represents the one-time application of N fertilizer in 2005.
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| Figure 12. Nitrate-N concentration vs. time for two replicate drains at SEPAC, one with high nitrate-N concentration due to accidentally killed corn crop. It took about a year to “clean out” the excess nitrate-N from the plot (see text). From Kladivko and Bowling, 2021. |
Summary and Conclusions
Preferential flow to tile drains occurs even when the chemical is applied offset from the drain trench. This suggests that preferential flow is not just a result of the original trenching operation. It also suggests there is some preferential flow occurring both in the unsaturated and saturated zones.
Applications of chemicals at the same physical distance from the tile will require different amounts of time to reach the tile based on the overall tile spacing. Chemicals applied closer to the midplane between drains require more time than those further away from the midplane, even if the absolute distances are similar. This affects the “clean-out” time for plots.
Drainflow is a time-averaged indicator of soil management. When changing field management practices, such as fertilizer rate, the drainflow will reflect some of the change immediately, and some will appear in later months or years in the drainage, arriving from further distances away. Care must be taken not to attribute drainage responses, or lack thereof, to a management practice that occurred just a short time previously.
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