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Comparison of Newly Proposed and Existing Design Approach for Saturated Buffers
Yousef Abdalaal1, Ehsan Ghane1,*
Published in Journal of the ASABE 66(2): 431-440 (doi: 10.13031/ja.15246). Copyright 2023 American Society of Agricultural and Biological Engineers.
1Biosystems and Agricultural Engineering, Michigan State University, East Lansing, Michigan, USA.
* Correspondence: ghane@msu.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 23 June 2022 as manuscript number NRES 15246; 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. Celso Castro Bolinaga and Community Editor Dr. Kati Migliaccio of the Natural Resources & Environmental Systems Community of ASABE on 17 January 2023.
Highlights
Abstract. A saturated buffer (SB) is a conservation drainage practice that removes nitrate from subsurface drainage discharge. The reported wide range of nitrate load removal necessitates improvements in design approaches for more consistent performance. There are two SB design approaches: Illinois Natural Resources Conservation Service (Design 1) and McEachran et al. (2020) (Design 2). We proposed a new Design 3 that builds on the previous two designs. In Design 3, the nitrate load removal was simulated for buffer widths ranging from 3 to 30 m with a 0.3-m interval, and the buffer width that maximized the annual nitrate load reduction over the long term was chosen as the SB design. The objective of this study was to identify the best design approaches for maximizing nitrate load removal based on field data. Daily drainage discharge data from two field sites in Michigan were used to design a hypothetical SB length and width for each approach. The designs were compared by applying an identical method to estimate the nitrate load removal for each hypothetical SB system. The method extends Designs 1 and 2 by incorporating a hydrological and nitrate removal component. The results showed that using the minimum recommended buffer width of 9.1 m and the minimum 5% SB design capacity of Design 1 resulted in 25% to 35% of diverted flow to the buffer and 14% to 16% nitrate load removal at the two field sites. However, Design 1 resulted in the lowest nitrate removal compared to Designs 2 and 3 (i.e., 0.3% to 3.4% lower). Designs 2 and 3 consistently provided maximum nitrate load removal regardless of the site conditions, whereas the performance of Design 1 was inconsistent. In conclusion, Designs 2 and 3 were equally good and resulted in higher nitrate load removal compared to Design 1.
Although subsurface drainage is essential to increase crop production, it also transports nitrate to surface water (Ghane et al., 2016). Elevated nitrate levels in surface water cause societal, ecological, and economic concerns (USEPA, 2013). The hypoxic zone in the northern Gulf of Mexico is caused by excess nitrate, primarily originating from subsurface-drained farms in the Upper Mississippi River basin (USEPA, 2018). Therefore, there is a critical need to reduce nitrate loss from subsurface-drained farms using conservation practices.
Saturated buffers (SB) are a conservation drainage practice that is designed to reduce nitrate loss from subsurface-drained farms (NRCS standard 604). In this system, a portion of the drainage discharge is redirected into perforated distribution pipes that run underground along the length of the buffer (Jaynes and Isenhart, 2014). As water moves out of the perforated distribution pipes, it seeps through the soil toward the ditch, and nitrate is removed via denitrification (Groh et al., 2019). Streeter and Schilling (2021) have also found that the nitrate concentration of the field drainage discharge decreased at a rate of 0.11 mg/L per meter width of the buffer in an SB in Iowa. The SB system removed an average of 45% of the total nitrate load from drainage discharge in Iowa (Jaynes and Isenhart, 2019). Other studies showed the potential of using SB systems as a strategy to alleviate N loss in the Midwest and help meet N reduction targets (Chandrasoma et al., 2022, 2019; Tomer et al., 2020).
The USDA Natural Resources Conservation Service (NRCS) uses the Illinois NRCS design spreadsheet to design SB. That design approach is based on choosing a distribution pipe length to handle 5% of the drainage capacity from the drainage area providing water to that buffer (Design 1), which is a one-size-fits-all minimum 5% design approach. Wide ranges in nitrate load removal were reported in an assessment of existing SB systems across multiple sites in Iowa, despite the soil similarity between some of the investigated sites (Jaynes and Isenhart, 2019). Macrae et al. (2021) reported that a one-size-fits-all approach may not result in a considerable nutrient reduction. Thus, it is even harder to predict the performance of the same design in regions that have very different climates, soils, topography, and land uses, which necessitates implementing a site-specific approach as stated in Macrae et al. (2021). Annual percentages of nitrate load removal ranged from 8% to 84% (Jaynes and Isenhart, 2018). The wide range of nitrate removal necessitates improvements in design approaches, so all saturated buffers are designed to have satisfactory performance.
The second design approach was developed by McEachran et al. (2020), which proposed using a theoretically developed equation for estimating the SB width (Design 2). The authors’ equation provides an optimum buffer width without the need for using the drainage capacity as an input. McEachran et al. (2020) did an indirect comparison between Designs 1 and 2 for six existing SB sites in Iowa in terms of the buffer widths and the effectiveness ratios at the optimal width versus effectiveness at the current widths. The previously mentioned two design approaches (NRCS and McEachran) have not been directly compared to each other in terms of saturated buffer performance (i.e., average annual nitrate load removals and diverted flows) using variable field flow data. Therefore, there is a need to conduct a comparison between Designs 1 and 2 using field data to determine which design approach maximizes nitrate load removal.
Here, we propose a new design approach for SB design. In our proposed design, daily simulations were performed for all possible buffer widths from 10 to 100 ft (3 to 30 m). The long-term diverted flow and nitrate load removal were estimated for each buffer width. Finally, the buffer width with the maximum nitrate load removal was chosen as the optimum buffer width (Design 3). We also included the proposed Design 3 in the comparison to determine which design approach maximizes nitrate load removal. The objective of this study was to identify the best design approaches for maximizing nitrate load removal based on field data. The value of this study is that it will inform decision-makers about which method maximizes nitrate load removal of the saturated buffer.
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| Figure 1. Top view and cross-section B-B showing design parameters of saturated buffer and parameters of Dupuit and head loss formulas (figure not to scale). |
Materials and Methods
Governing Equations of a Saturated Buffer
The governing equations of an SB system can be classified into hydrological and biogeochemical components. The hydrological component is responsible for water movement from the pipes to and through the buffer soil and to the open ditch. The biogeochemical component is responsible for the process for removing nitrate. In this section, we describe the governing equations.
Flow equation: Based on McEachran et al. (2020), the flow through buffer soil can be represented by the Dupuit equation, which describes water movement through porous media in an unconfined aquifer as expressed in equation 1. In this setting, steady-state flow conditions are assumed to be an applicable representation of water movement through the buffer soil:
(1)
where
QDP = diverted volumetric flow into the buffer soil (cm3/day)
K = saturated hydraulic conductivity of the buffer soil (cm/day)
h1 = hydraulic head at the distribution pipe (cm)
h2 = hydraulic head at the open ditch (cm)
W = width of the buffer (cm)
LDP = length of the distribution pipe (cm) (fig. 1).
Head loss equation: in subsurface drainage, entrance head loss occurs as flowlines converge toward the pipe perforation (Skaggs, 1991). In an SB system, the distribution pipe functions as a subirrigation pipe, where exit head loss occurs as water moves out of the pipe and enters the surrounding soil (Skaggs, 1991). The diverted flow to the buffer and the exit head loss can be represented by the effective radius of the distribution pipe (Skaggs, 1991) in equation 2:
(2)
where
h0 = hydraulic head inside the control structure (cm)
d = distance from the distribution pipe center to the impermeable layer (cm)
Ref = effective radius of the distribution pipe (cm).
Nitrate removal equation: McEachran et al. (2020) assumed that denitrification is the primary mechanism that removes nitrate from the diverted water to buffer soil and used a first-order kinetics equation (eq. 3) to estimate the nitrate concentration of the diverted flow as it moves through the buffer soil:
(3)
where
CDD = initial nitrate concentration of diverted flow (mg/l)
? = nitrate removal coefficient (1/day)
C = final nitrate concentration of diverted flow at the maximum width of the buffer (mg/l)
Tu = time of travel for water to move from the distribution pipe through the buffer width (day).
The time of travel (Tu) can be calculated using equation 4 (McEachran et al., 2020):
(4)
where
ne = effective porosity (cm3/cm3).
Design Approaches for Saturated Buffers
There are two SB design approaches: the Illinois Natural Resources Conservation Service (NRCS) design spreadsheet (Design 1) and McEachran et al. (2020) (Design 2). We proposed a new approach (Design 3) that builds on the previous two designs. In this section, we describe these design approaches.
Design 1: The USDA NRCS design spreadsheet determines the length of an SB system that would divert a minimum of 5% of the drainage system capacity into the buffer soil based on a minimum buffer width of 9.1 m (Standard Code 604). This design does not have an optimization function to maximize nitrate removal. The NRCS design approach is based on one set of inputs at one point in time (i.e., peak flow). The set of inputs includes the hydraulic head at the control structure and at the open ditch, as well as the drainage system capacity.
Design 2: The second SB design approach was theoretically developed by McEachran et al. (2020) to determine the optimum width for maximum nitrate removal as follows:
(5)
where
W* = optimum width of the buffer (cm)
hw = weir level inside the control structure referenced to the restrictive layer (cm).
This design approach is based on one set of inputs at one point in time. The set of inputs include the hydraulic head inside the control structure (approximated as the weir level) and at the open ditch, and the removal coefficient ?, which can be calculated as suggested by McEachran et al. (2020):
(6)
where
Kd = first-order denitrification coefficient (1/day)
fT = reduction coefficient affected by temperature (unitless) that can be calculated as reviewed by Heinen (2006):
(7)
where
T = the soil temperature (°C)
Tr = reference temperature (°C)
Q10 = factor with a typical value ranging between 2 and 3.
Design 3: This design builds on the previous two design approaches that used the Dupuit equation and first-order nitrate removal kinetics. Similar to the existing designs, Design 3 assumes that steady state can be used to represent water movement through the buffer system. This design includes a new mechanism that considers exit head loss as water leaves the distribution pipe through perforations and flows into the surrounding soil. Design 3 also considers the behavior of the SB system daily over several years by accounting for variations in the drainage discharge from the field (QDD), and consequently accounting for variations in the hydraulic head at the distribution pipe h1, as well as the variations in the nitrate removal coefficient (?) as affected by variations in soil temperature. This contrasts with Designs 1 and 2, which only use single values for h1, QDD, and/or ? at one point in time.
In Design 3, daily nitrate load removal was simulated using the main routine presented in the following section. The daily nitrate load removal was estimated for all buffer widths ranging from 3 to 30 m in increments of 0.3 m. The buffer width that maximized long-term nitrate load reduction was chosen as the SB design.
Main Routine for Estimating the Daily Nitrate Load Removal
The main routine builds on Designs 1 and 2, and it is composed of a hydrological and nitrate removal component. For the main routine, we assumed that the drainage discharge comes from a free drainage system into the two-chamber control structure, and the distribution pipe comes out of the upstream chamber. The main routine is described as follows:
Hydrology component: The hydrology component of the main routine includes a water balance model (eqs. 1, 2, 8, 9, and 10), in which two water balance equations are defined. The first water balance equation is a conservation of mass inside the control structure:
(8)
where
QDD = the field drainage discharge (cm3/day)
QDP = the diverted flow (cm3/day)
QBY = bypass flow (cm3/day).
The bypass flow was calculated using the calibrated weir equation developed by Chun and Cooke (2008). The authors’ equations can be used for any structure with a rectangular weir. For the purpose of this article, we used the 250-mm control structure, written as:
(9)
(10)
where
H = head over the rectangular weir (cm)
h0 = hydraulic head inside the control structure (cm)
Lweir = length of the weir in the control structure (cm)
Hweir = height of the weir from the bottom of the control structure (cm)
QBY = bypass flow (cm3/day)
Unit conversion factor = 1.8144 × 106.
The second water balance is applied at the interface between the buffer soil and the distribution pipe and states that the flow exiting the distribution pipe through pipe perforations (eq. 2) should be equal to the flow moving through the buffer soil toward the open ditch (eq. 1).
Nitrate removal component: The nitrate load removal component of the main routine involved three steps. Based on McEachran et al. (2020), the first step estimated the final nitrate concentration of the diverted water that reached the ditch after passing through the buffer soil using first-order kinetics for nitrate removal (eq. 3). The second step calculated the nitrate load as the product of the volumetric flow times its nitrate concentration (eqs. 11, 12, and 13). The third step calculated the nitrate load removal of the system as the difference between the nitrate load in the drainage discharge coming from the subsurface-drained field (the case if there was a free drainage system without an SB system) and the nitrate load reaching the ditch (sum of the loads from the diverted and bypass flows) (eqs. 14 and 15).
(11)
(12)
(13)
(14)
(15)
where
NLDD = drainage discharge nitrate load coming from the subsurface-drained field (kg/day)
NLRDB = nitrate load reaching ditch after moving through the buffer soil (kg/day)
NLBY = nitrate load reaching the ditch via bypass flow (kg/day)
NLLD = nitrate load reaching the ditch through both buffer and bypass (kg/day)
RNLSB = reduction in nitrate load due to the SB system (kg/day)
Unit conversion factor = 10–9.
Drainage Discharge Measurement and Water Sampling from the On-farm Site
We collected daily drainage discharge from two field sites (CL and BL) in Michigan over three years, from January 2019 to the end of December 2021. The data were used to design a hypothetical SB length and width for each approach at each site. The sites provided real-world field variations in flow and hydraulic head to have a better comparison of the design approaches. The dominant soils were Blount Loam at CL and Ziegenfuss Clay Loam at BL. The average saturated hydraulic conductivities were 3.09 cm/h at CL and 4.19 cm/h at BL. Details for each site are in Supplementary Materials S1 and S2.
To measure hourly drainage discharge, the control structure at each on-farm site was instrumented with a V-notch weir with a water-level logger setup for measuring low to moderate unsubmerged flows (Shokrana and Ghane, 2021) and an area-velocity sensor for measuring flow conditions that did not satisfy the required conditions of the V-notch setup. Details of these measurements are presented in Dialameh and Ghane (2022) and in Supplementary Materials S2. An automated sampler was used to collect daily composite water samples, which were analyzed for nitrate concentration using the colorimetric nitrate reductase analysis method at the Water Quality Lab of Michigan State University. The average nitrate-N concentrations in drainage discharge were 10.3 mg/L at the CL site and 16.8 mg/L at the BL site.
Comparison of the Designs and Inputs for the Three Design Approaches
The three design approaches were compared by applying an identical main routine that was presented earlier in the methods section to estimate the nitrate load removal for each hypothetical SB system. To compare the nitrate load removal of the three design approaches, first we had to determine the design parameters (length and width). Due to the differences in the three design approaches (table 1), a specific approach was followed to calculate the SB design parameters for each of the three design approaches. This approach defined the design parameters for each design. The sequence of calculations is explained as follows: The considered design parameter in this study was the buffer width, which was based on the data presented in table 1. The sequence of calculations for the three design approaches started with the use of field data to define the inputs for Design 1. Then, Design 1 was used to determine the length of the distribution pipe. After that, the same input values from Design 1 were used to calculate the optimum buffer width for Design 2. Finally, the calculated length of the distribution pipe from Design 1 and the daily field values of drainage discharge and soil temperatures were used to determine the optimum width of the buffer in Design 3.
| Table 1. Main differences between the three design approaches of a saturated buffer system. | |||
| Category of differnces | Design 1 | Design 2 | Design 3 |
| Considered time span | One point in time. (Variations over time are not considered). | One point in time. (Variations over time are not considered). | Any Period. (Variations over time are considered). |
| Buffer width | Input: Minimum of 9.1 m. | Output | Output |
| Distribution pipe length | Output | Does not require a length input. | Input |
| Optimization function | None | Maximize effectiveness of nitrate removal. | Maximize effectiveness of nitrate removal. |
| Processes considered | Flow through buffer soil. | Flow through buffer soil. First-order kinetics for nitrate removal. | Flow through buffer soil. First-order kinetics for nitrate removal. Exit head loss of water moving out of the distribution pipe. |
In Design 1, the drainage system capacity input was set as the maximum drainage discharge that occurred over the three-year period. The maximum values of the measured daily flows were 2596 m3/day at the CL site and 1267 m3/day at the BL site. The input value for the water control weir elevation was set so that it would be 30.5 cm from the soil surface (NRCS code 604). The depth to the restrictive layer, the top of the clay pan, at each site was determined using a combination of field investigation and the reported data in the Gridded Soil Survey Geographic Database (gSSURGO; United States Department of Agriculture Natural Resources Conservation Service, n.d.). These values were 305 cm at CL and 170 cm at BL. This resulted in h1 values of 274 cm at CL and 149 cm at BL. The input value for baseflow water elevation in the ditch was taken as the geometrical mean value of the ditch water level across the three-year period at the CL site, which resulted in the h2 value of 38.4 cm, assuming that the bed of the ditch was 30 cm above the impervious layer. The h2 value at the BL site was assumed to be the same because the water elevation was not monitored at BL. The inputs discussed in this paragraph were used in the NRCS Illinois spreadsheet to determine the length of the distribution pipe.
In Design 2, the head difference (?h) was calculated as the difference between the values of h1 and h2 that were used in Design 1. The coefficient of nitrate removal was calculated using equations 6 and 7, and the literature-reported values in McEachran et al. (2020) for Q10 (2.5), T (7°C), Tr (20°C), and Kd (0.55 day-1). The coefficient of nitrate removal (?) was calculated to be 0.165 day-1. The input values were used in equation 5 to calculate the optimum width of the buffer for Design 2, which does not require a distribution pipe length as an input.
In Design 3, multiple simulations were conducted using multiple values of the buffer width that were coupled with other fixed inputs. The fixed inputs were the h2 value and length of distribution pipe from Design 1, the distribution pipe properties and depth, the daily values of the drainage discharge, the long-term average daily values of the soil temperature, the whole-period average of the nitrate concentration of the drainage discharge, and the weir levels. The distribution pipe properties were identified for an 8-row, 100-mm diameter perforated pipe, so the effective radius was 1.6 cm (Ghane, 2022; Ghane et al., 2022). The distribution pipe depth in the buffer soil was taken as the commonly used value of 80 cm. The weir level was set to be 100 cm from the soil surface during the periods of 1 April to 15 May and 1 October to 1 November, 45 cm from the soil surface during the growing season (16 May to the end of September), and 30 cm from the soil surface during the rest of the year. The long-term average daily values of soil temperature were calculated using historical measured daily values at Kellogg Station, Michigan, at a depth of 100 cm (this depth was chosen because it was the closest to the distribution pipe depth).
The buffer width value from each design approach was coupled with the constant value of the length of the distribution pipe from Design 1 to represent a hypothetical SB system. This resulted in a total of six hypothetical SB systems (three designs and two sites).
To conduct a fair comparison between the three design approaches, we used the same method to estimate the nitrate load reduction for each of the hypothetical SB systems representing the three design approaches. At each field site, the main routine (presented in the methods section) was applied to the daily measured field data to calculate the amount of nitrate load removal for each of the hypothetical SB systems. Finally, the design approach that resulted in the greatest nitrate load removal was chosen as the best design approach.
Results and Discussion
Distribution Pipe Length and Buffer Width of Design 1
Using the minimum buffer width of 9.1 m, the distribution pipe length was calculated so that it would divert 5% of the maximum drainage discharge to the buffer at each field site (NRCS Standard Code 604). The calculated distribution pipe lengths were 433 m at the CL site and 643 m at the BL site. The required distribution pipe length decreased as the maximum drainage discharge decreased and as the hydraulic gradient across the buffer width increased.
Optimum Buffer Width of Design 2
For Design 2, the calculated optimum buffer widths were 16.3 m at the CL site and 11.1 m at the BL site. These optimum widths were greater than the 9.1-m minimum width of NRCS Standard Code 604. McEachran et al. (2020) estimated the optimum widths for six sites in Iowa, and they found four optimum widths greater than the minimum width and two smaller than the minimum width recommended by NRCS. This shows that having a one-size-fits-all 9.1-m minimum width may not result in optimum performance.
The calculated optimum buffer width increased as the hydraulic head difference between the control structure and the open ditch increased. The faster flow velocity through the buffer soil at the CL site, as calculated by equation 4, required a wider buffer as compared to the BL site to effectively reduce the nitrate concentration of the diverted flow before it exited the buffer soil into the open ditch. This was the case as the nitrate removal in Design 2 was represented through a function that used a fixed daily rate for the removal coefficient, and the width was the only parameter that affected the final nitrate concentration for the same site.
Optimum Buffer Width of Design 3
For Design 3, the calculated optimum buffer widths were 17.1 m at the CL site and 11.0 m at the BL site. These optimum widths were greater than the 9.1-m minimum width of the NRCS Standard Code 604. For a fixed distribution pipe length, the maximum value of the diverted flow was at the minimum buffer width, and it decreased as the width increased (fig. 2). Consequently, the bypass flow increased as the buffer width increased because of the conservation of mass at the control structure.
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| Figure 2. Flow dynamics in saturated buffer with varying buffer widths based on long-term daily simulations at BL site. |
Design 3 showed that increasing the buffer width resulted in a reduction in the nitrate load of the diverted flow and an increase in the nitrate load of the bypass flow (fig. 3). The reduction in the nitrate load of the diverted flow was not linear; the intensity of the reduction (slope of the curve) was steep at low buffer widths and reached an asymptote at large buffer widths. Large buffer widths essentially eliminated nitrate load in the diverted flow, and these widths were approximately 19 m at the BL site and 24.4 m at the CL site. At small widths, the travel time was too small for considerable nitrate removal to occur. At large widths, the bypass was too large, thereby decreasing the diverted flow, which resulted in small nitrate removal.
The chosen optimum buffer widths for Design 3 that resulted in the largest total nitrate load removal were at smaller buffer widths (11.0 m at BL and 17.1 m at CL) than the widths where the nitrate load in diverted flow was essentially eliminated. These chosen widths had higher total nitrate load removal since they also considered the increase in nitrate load from bypass flow that resulted from the increase in buffer width.
Comparison of Design Approaches
The comparisons among the three design approaches across the two field sites showed that Designs 2 and 3 yielded the best designs (table 2). Design 1 resulted in the largest percentage of diverted flow to the buffer and the least nitrate removal at both field sites. Design 1 had the largest diverted flow because it had the smallest width, and the Dupuit equation indicates that smaller buffer widths will have larger diverted flows. The smaller buffer widths of Design 1 did not provide enough time for the buffer soil to remove larger amounts of nitrate from the diverted water as it passed through the buffer soil when compared to Design 2 or Design 3. Design 1 removed the least nitrate load because it did not include optimization for nitrate removal and used the minimum allowable buffer width (9.1 m). Therefore, choosing a smaller buffer width that diverts more flow into the buffer may not always result in larger nitrate load removal.
As shown in table 2, Designs 2 and 3 consistently provided maximum nitrate load removal at two distinct sites. These two sites had different soil properties and drainage designs (drain depth and spacing). However, Design 1 removed 20% ([352.9-282.1]/352.9) and 2% ([247.8-243.4]/247.8) of the nitrate load compared to Design 3 at the CL and BL sites, respectively. As evident, Design 1 performed well at the BL site but poorly at the CL site. This shows that Design 1 did not have consistency in nitrate load removal from one site to another. Therefore, Designs 2 and 3 consistently provided maximum nitrate load removal regardless of the site conditions, whereas the performance of Design 1 was inconsistent.
 |
| Figure 3. Nitrate load dynamics of saturated buffer with varying buffer widths based on long-term daily simulations at BL site. |
Designs 2 and 3 resulted in similar nitrate load removal, mainly because both used first-order removal kinetics. The first-order removal kinetics in Designs 2 and 3 assumed that denitrification was the primary mechanism affecting the nitrate balance within the buffer soil and did not account for other mechanisms (such as plant uptake, mineralization, and immobilization), which can be affected by the hydrology changes that may arise from using different buffer widths. The water-table position and soil temperature within the buffer soil can limit denitrification or affect other processes mediating the nitrogen balance, such as organic carbon decomposition or nitrogen mineralization/immobilization (Youssef et al., 2005). The water-table position in the buffer soil defines the available soil-water pore space, the available soil organic carbon content, and the soil organic carbon decomposition (i.e., shallower layers have more labile organic carbon with a higher content) (Youssef et al., 2005). Also, the first-order kinetics method does not have a limit for the amount of nitrate substrate that can undergo denitrification, unlike other methods (e.g., the Michaelis-Menten model). Therefore, we recommend an improvement of the method used to represent nitrate removal in Designs 2 and 3.
Importance of Accounting for Exit Head Loss
Exit head loss occurs as water moves out of the distribution pipe perforations and enters the surrounding soil (Skaggs, 1991). To demonstrate the importance of exit head loss in SB design, the percentage of diverted flow and total nitrate load removal were compared for the two cases of considering or not considering exit head loss for Design 1. The results at the CL site showed that the designed SB system (9.1-m width and 433-m length) diverted 5% of flow when exit head loss was not considered, whereas the same design diverted 4.4% of flow when exit loss was considered (Supplementary tableS3). Therefore, not accounting for exit head loss in Design 1 resulted in an undersizing of the buffer length for a given width. Simulations using the main routine and neglecting exit head loss showed that the total diverted volume at the CL site over three years increased by about 4400 m3 (2.2% difference), and total nitrate-N removal increased by 7.7 kg (0.3% difference) compared to the scenario of considering exit head loss. Similar results for Designs 2 and 3 are presented in Supplementary Materials S3. Regardless of the buffer dimensions, the amount of diverted flow to the buffer and the amount of nitrate load removed were overestimated when exit head loss was neglected. The impact of the exit losses was also more evident at the field site (CL site) which was characterized by higher flow rates.
| Table 2. Comparison between the saturated buffer parameters, and the hydrology and nitrate removal performances of three design approaches from 2019 January to 2021 December. | ||||||||
| CL Site | BL Site | |||||||
| Design Approach | Design 1 | Design 2 | Design 3 | Design 1 | Design 2 | Design 3 | ||
| Buffer length LDP (m) | 433 | 643 | ||||||
| Buffer width W (m) | 9.1 | 16.3 | 17.1 | 9.1 | 11.1 | 11.0 | ||
| Total drainage discharge over three years (m3) | 197573 | 89196 | ||||||
| Total nitrate-N load from field over three years (kg) | 2035 | 1498 | ||||||
| Total diverted flow over three years (m3) | 68776 | 46686 | 45125 | 22625 | 19750 | 19963 | ||
| Total diverted flow over three years percentage (%)[a] | 34.8% | 23.6% | 22.8% | 25.4% | 22.1% | 22.4% | ||
| Nitrate-N load removed over three years (kg) | 282.1 | 352.5 | 352.9 | 243.4 | 247.8 | 247.8 | ||
| Nitrate-N load removed percentage (%)[b] | 13.9% | 17.3% | 17.3% | 16.2% | 16.5% | 16.5% | ||
| Average annual field drainage (m3 / yr) | 65858 | 29732 | ||||||
| Average annual diverted flow (m3 / yr) | 22926 | 15562 | 15042 | 7542 | 6583 | 6654 | ||
| Average annual nitrate-N removed (kg / yr) | 94.0 | 117.5 | 117.6 | 81.1 | 82.6 | 82.6 | ||
[a]Percentages are out of the total drainage discharge. [b]Percentages are out of the total nitrate-N load from field. | ||||||||
The saturated buffer is not the only system that is affected by exit head loss. Simulations showed that exit head loss over the drainage pipes can reach more than 13 cm after a whole day of steady subirrigation (Skaggs, 1991). Like the saturated buffer, woodchip bioreactors will be undersized if exit head loss of the water forced out of the distribution pipe and entrance head loss of the water entering the collection pipe are not considered. This is because the hydraulic head in the inlet control structure will be higher than the hydraulic head in the upstream end of the bioreactor due to exit head loss, and the hydraulic head in the outlet control structure will be lower than the hydraulic head in the downstream end of the bioreactor due to entrance head loss. The same concept applies to phosphorus removal structures where water is forced out of a perforated pipe into a porous media. Therefore, exit and entrance head losses should be incorporated in the designs of conservation drainage practices that involve water movement into and out of a perforated pipe surrounded by a porous media.
Practical Application of Design 3
Climate, soil, and drainage design vary from one location to the other, thereby creating a distinct hydrologic response from the subsurface-drained farms. As shown in table 2, the optimum buffer widths for Design 3 were 17.1 and 11.0 m at the CL and BL sites, respectively. This difference between the optimum widths at the two sites was because of the differences in the hydrologic response of the two farms. Therefore, SB design requires site-specific determination of length and width. Design 3 accounts for site-specific conditions to estimate the annual nitrate load removal of the SB system. Designs 1 and 2 use only one point in time, so they cannot calculate the annual nitrate load removal. Consequently, Design 3 is more suitable for incorporation into decision-support tools because it shows the value of this practice by quantifying the nitrate load removal compared to Designs 1 and 2. Decision-support tools can accelerate the adoption of saturated buffers by increasing knowledge of the value of this practice. This approach informs state and federal efforts to support nutrient-trading programs. In a nutrient-trading program, the farmer receives a payment for the nitrate load removed based on their site-specific conditions. Overall, Design 3 has the potential for application in decision-support tools to increase the adoption of SB.
Design 3 can be further coupled with economic analysis or other user-specified objective functions that target specific annual nitrate load reduction goals. For example, the economic analysis could identify a buffer width that maximizes profit for landowners without reducing the effectiveness of the system to remove nitrate loads. As shown in figure 4, the range around the optimum width value with minimal variations in the annual nitrate load removal (the near-plateau section of the curve) showed that a smaller buffer width can be used instead of the optimum width. This reduction in buffer width would increase the cultivated area of the field and increase the profits of landowners without sacrificing nitrate load removal. A demonstration of this analysis is presented in Supplementary Materials S5. Overall, other objective functions can be coupled with Design 3 to achieve the desired goals.
The distribution pipe length could be limited by the field dimensions or the slope of the buffer in the direction parallel to the ditch. The NRCS Code 604 recommends that the distribution pipe be level or slope downward away from the control structure. The buffer slope in the direction parallel to the ditch should have the minimum soil cover over the distribution pipe, and the pipe depth should not exceed the maximum possible operation depth of the installation machine. Our results showed that when the buffer length was smaller than the calculated design length from Design 1, the average reduction in the percentage of total diverted flow of the three designs was 14.3%, and the average reduction in the percentage of total nitrate removal was 9.1% (Supplementary Materials S4). We believe that the best SB design approaches should first determine the maximum allowable buffer length as an input, then estimate the buffer width in a way that would utilize an optimization function to meet the target nitrate load removal and financial demands of the user. This is because the length of the buffer considerably impacts the total amount of nitrate load removal (Supplementary Materials S6). The longer the length of the buffer, the larger the diverted flow and nitrate load removal.
 |
| Figure 4. Diverted flow and nitrate load removal percentages of design approaches of saturated buffers at CL site using maximum available buffer length suitable for distribution pipe installation. |
Conclusions
We conducted a comparison between three saturated buffer design approaches to determine the best design. Our study resulted in the following key conclusions:
- Used Design 1, with the minimum width stated in NRCS Code 604 and the minimum recommended treatment capacity of 5% of the drainage system capacity, which resulted in a system with the maximum diverted flow to the buffer and the lowest nitrate removal. The main reason for the low nitrate removal was that the current version of Design 1 did not optimize for nitrate removal and only targeted the amount of diverted flow to the buffer.
- Choosing a smaller buffer width that diverts more flow into the buffer may not always result in larger nitrate removal.
- For a fixed buffer length, increasing the buffer width reduced the diverted flow to the buffer and increased the bypass flow. Increasing the buffer width reduced the nitrate load of the diverted flow and increased the nitrate load of the bypass flow.
- Assuming that the nitrate removal within the buffer soil followed first-order removal kinetics, Designs 2 and 3 resulted in saturated buffer widths with comparable nitrate load removal that were better than those obtained with Design 1.
- Designs 2 and 3 consistently provided maximum nitrate load removal, regardless of the site conditions. Design 1 did not provide consistent nitrate load removal from one site to another.
- The first step in SB design should be the determination of the maximum allowable buffer length, followed by an estimation of the buffer width that would maximize nitrate removal. The longer the length of the buffer, the larger the diverted flow and nitrate load removal.
- Neglecting exit head loss (when water exited the distribution pipe) resulted in an overestimation of the diverted flow into the distribution pipe and an overestimation of nitrate load removal by the saturated buffer.
In conclusion, the results indicated that Designs 2 and 3 were both good, and they were both better than Design 1 in terms of maximizing nitrate load removal. Design 3 follows a process-based approach to estimate the annual nitrate removal for site-specific conditions. The advantage of Design 3 is that it can be used in decision-support tools to increase knowledge of the value of this conservation drainage practice, thereby accelerating the adoption of saturated buffers. This approach informs state and federal efforts to support nutrient-trading programs. To further improve Design 3, we recommend including more sophisticated nitrate removal and hydrology components that can capture other processes that affect nitrogen dynamics in soil and its interaction with hydrology.
Supplemental Materials
Supplementary materials associated with this article can be found at: https://doi.org/10.6084/m9.figshare.20132174.v4
Acknowledgments
This work was funded in part by the Michigan Department of Agricultural and Rural Development (791N7700580), the USDA NRCS Classic Conservation Innovation Grant (USDA-NRCS-NHQ-CIG-20-GEN0010808), and the Michigan Alliance for Animal Agriculture (AA-19-006).
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