Saturated buffers (SBs) are an effective edge-of-field practice for reducing nitrate loads from agricultural drainage, contributing to improved environmental water quality. However, no software currently exists to design SBs based on site-specific conditions or to quantify their environmental benefits. The objective was to develop and test a DRAINMOD-based tool for predicting drainage discharge and nitrate load removal (NLRSB) under local weather, soil, field drainage, and SB characteristics. We present SBTool, a novel decision-support tool that integrates the DRAINMOD hydrologic model with a nitrate-removal module to simulate SB performance. SBTool was validated using field data from two Iowa sites (2014-2022). Model predictions for discharge (QDD), diverted flow (QDP), and NLRSB showed good agreement with observed data. Prediction errors of QDP and NLRSB were only 5.7 % and 6.1 %, respectively, at the eight-year site, and - 17.5 % and - 13.6 % at the four-year site. Unlike existing design methods, SBTool enables site-specific evaluation and design of SBs, supporting conservation planning and nutrient trading through credible, field-based quantification of nitrate removal.

Phosphorus (P) is essential for crop growth but leaches through subsurface drainage discharge, impacting water quality. This study's objectives are to (1) apply hybrid statistical-machine learning to quantify the contributions of incidental (new) and legacy (old) P in drainage discharge from organic site and inorganic site and (2) evaluate the effect of manure application timing on P loss. We collected data from two on-farm sites in southeast Michigan, USA. A linear regression equation was used to analyze P load based on drainage discharge and fertilizer application timing. The data were split into calibration and validation sets, and machine learning was used for training. The results showed strong model prediction performance. Organic fertilizers contributed approximately twice the observed total phosphorus (TP) loss (7.54 kg ha- 1 vs. 3.73 kg ha- 1) and nearly four times the dissolved reactive phosphorus (DRP) loss (4.90 kg ha- 1 vs. 1.05 kg ha- 1) compared to inorganic P loss, mainly due to the greater P application rate and higher soil test P. When applied during winter months (December-January), organic fertilizer contributed to greater new P loss, whereas early fall applications (October-November) resulted in lower new P loss, showing the importance of application timing. At the organic site, legacy P was the dominant contributor to TP and DRP losses, accounting for 84% and 79% of losses, respectively. At the inorganic site, legacy P was responsible for 97% of TP loss and the entirety (100%) of DRP loss. In conclusion, legacy P was the dominant source of P loss through drainage discharge, and winter organic fertilizer application significantly increased new P loss.

Past studies evaluating the conservation drainage practice of saturated buffers (SB) have used the water flowing over the upstream weir as a control, assuming that upstream weir management does not reduce flow and nitrate load. The objectives are to (1) investigate whether the upstream weir functions as controlled drainage (CD) to provide water-quality benefits beyond those from the buffer flow (BF, defined as water diverted into the buffer), and (2) evaluate the performance of the SB system under high (shallow) and low (deep) weir management levels. From June 2019 to September 2024, we conducted a paired-field study at an on-farm site in Michigan, USA, comparing an SB system to a free drainage control field. Using this method, we separated the two components of the SB system into CD and BF. Results revealed a significant reduction in nitrate-N load (54.5% and 11.1 kg/ha annually, p-value < 0.01) of the SB system compared to free drainage. The CD component emerged as the dominant contributor to nitrate-N load removal (48.0% and 9.6 kg/ha annually), confirming the crucial water-quality role of upstream weir management. Studies that ignored the water-quality effect of the CD component likely underestimated the performance of SB. The BF component contributed to only 6.4% of nitrate-N load reduction. High-level weir setting significantly removed more nitrate-N (0.076 kg/ha daily) than low-level setting (0.036 kg/ha daily). In conclusion, this study shows the significant water-quality contribution of the CD component and underscores the importance of weir management for enhancing nitrate load removal.

Climate change presents challenges to agricultural water management, necessitating a reevaluation of subsurface drainage design for sustainable crop production. The objectives were to (1) investigate the impacts of climate change on hydrology and corn yield in southeast Michigan, United States, and (2) assess the climate change impact on the optimum drain spacings under two drain depths of 75 cm shallow and 125 cm deep. Using DRAINMOD and climate projections (2030-2059) from nine general circulation models under the SSP2-4.5 pathway, the study predicted hydrological responses and economic returns for various drain spacing designs. The optimum drain spacing was determined as the spacing that maximizes economic return using historical (1994-2023) and future scenarios. Future predictions showed an increased annual temperature (9.6 degrees C for historical vs 12.1 degrees C for future), relatively stable annual precipitation (933 mm for historical vs 928 mm for future), increased evapotranspiration (21%), reduced drainage discharge (21%), and deeper water table (7%) compared to the historical. Drought stress was the primary driver of future yield reductions, averaging 25% for both drain depths. The number of dry days during the growing season is expected to increase in the future due to higher evapotranspiration and a deeper water table. Optimum drain spacings were projected to widen from 7 m to 11 m for shallow drains and from 12 m to 19 m for deep drains to mitigate drought stress. In conclusion, future climate conditions showed a yield decline, suggesting a wider drain spacing may be needed to mitigate the drier growing season.

This supplemental material is linked to the submitted manuscript NRES-15735.

Highlights An empirical equation was embedded in a user-friendly tool to estimate the site-specific design drainage rate. The site-specific design drainage rate was based on the local soil, weather, and economics of the area of interest. The tool uses the site-specific design drainage rate to estimate the optimum drain spacing. The optimum drain spacing maximizes the economic return on investment. Abstract. Properly estimating the subsurface drain spacing is critical to optimizing crop production. The Hooghoudt equation can be used in humid climates to approximate the drain spacing. However, the application of this equation has been limited due to site-specific data requirements and because it is a complicated process that is not usually practical for practitioners. Traditionally, drainage contractors have chosen a drain spacing without using the Hooghoudt equation. The objective of this article is to develop a user-friendly decision-support tool that estimates the site-specific optimum drain spacing for maximum economic return on investment. We developed the Drain Spacing Tool for the Midwest USA based on the Hooghoudt equation and site-specific inputs. The tool automatically acquires the site-specific equivalent saturated hydraulic conductivity of the soil profile and depth to the restrictive layer from the gSSURGO database, and the user manually enters the desired drain depth. The site-specific input of design drainage rate (DDR), that is required in the Hooghoudt equation, is estimated from an empirical equation that was developed from a DRAINMOD modeling study. The site-specific inputs for the empirical equation include site-specific 30-year average growing-season rainfall, drain depth, equivalent saturated hydraulic conductivity, and depth to the restrictive layer, all of which are automatically acquired from gSSURGO, except for the rainfall data, which was acquired from the PRISM Climate Group. The site-specific DDR value from the empirical equation was then used in the Hooghoudt equation to estimate the optimum drain spacing that maximizes economic return on investment. In conclusion, the tool estimates the site-specific optimum drain spacing based on the local soil, weather, and economics of the area of interest. Keywords: Decision-support tool, Design drainage rate, DRAINMOD, Farm profitability, Tile drainage.

Subsurface drainage in humid areas prevents field waterlogging but also transports nutrients to freshwater systems. Controlled drainage (CD) reduces drainage discharge and nutrient transport from fields. Some regions are expected to experience increased precipitation in the future, requiring CD to be evaluated under a changing climate. The objective of this study was to compare the performance of CD under two weir managements for a future period (2030–2059) and historical period (1992–2021). Climate projections were obtained for the shared socioeconomic pathway 245 emission scenario. Aggressive management involved maintaining the weir height at 40 cm during the growing season and 15 cm during the non-growing season, with a longer period of managed flow compared to common management, which maintained the weir height at 50 cm during the growing season and 30 cm during the non-growing season. It was predicted that the 30-year average annual precipitation would not change significantly in the future. The 30-year average mean monthly temperature would increase by 3.0°C in the future compared to the historical period. We performed simulations using the calibrated Root Zone Water Quality Model 2 (RZWQM2). The average drainage discharge in the future indicated a 20% increase in the 30-year average drainage discharge for a field with free drainage. The CD with common and aggressive managements reduced drainage discharge by 59% and 67% for the historical period, whereas the performance of CD was even better for the future period (63% and 72%, respectively). The improved future performance of CD can be attributed to a shift in precipitation patterns, with reduced precipitation during the growing season and increased precipitation during the non-growing season. As a result, the more aggressive weir management during this period created additional opportunities for reducing drainage discharge. In conclusion, aggressive management resulted in a slightly better flow-reducing performance than common management while indicating that both methods would effectively reduce drainage discharge in the likely future scenario.

Highlights Novel three-bed, cascading-inlet bioreactor treated agricultural drainage from a 249-ha catchment. Nitrate removal rates and load reduction efficiencies were similar to those of traditional single-field bioreactors. Sedimentation problems reduced bed life; a sediment sensing and exclusion system solved them. This scale provides opportunities for centralized management and nutrient reduction verification. Abstract. Denitrifying bioreactors, a structural practice deployed at the field scale to meet water quality goals, have been underutilized and require additional evaluation at the small catchment scale. The objective of this study was to quantify the performance of a large, multi-bed denitrifying bioreactor system sized to treat agricultural drainage runoff (combined drainage discharge and surface runoff) from a 249-ha catchment. Three woodchip bioreactor beds, 7.6 m wide by 41 m long by 1.5 m deep, with cascading inlets, were constructed in 2016 in southern Minnesota, U.S. The beds received runoff for one water year from a catchment area that is 91% tile-drained row crops, primarily maize and soybeans. Initial woodchip quality differed among the three beds, affecting flow and nitrate removal rates. Bioreactor flow was unimpeded by sediment for twelve events from September 2016 to July 2017, during which time 55% of the discharge from the catchment was treated in the bioreactor beds. Average daily nitrate removal rates ranged from 2.5 to 6.5 g-N m-3 d-1 for the three bioreactor beds, with nitrate-N load removal of flow through the beds between 19% and 27%. When accounting for untreated by-pass flow, the overall nitrate-N removal of the multi-bed system was 12.5% (713 kg N). During high-flow events, incoming sediment clogged the reactor beds, decreasing their performance. There was 4,520 kg of sediment trapped in one bed, and evidence suggests the other two trapped a similar load. To solve this problem and prolong the bioreactor’s lifespan, we installed a shutoff gate that activated when inflow turbidity exceeded a threshold value. Finally, the findings indicate that catchment-scale denitrifying bioreactors can successfully remove nitrate load from agricultural runoff, but sediment-prevention measures may be required to extend the bioreactor's lifespan. Keywords: Bioreactor, Denitrification, Nitrate removal, Sedimentation, Subsurface drainage.

To minimize the environmental impact of phosphorus (P) loss from subsurface-drained fields to freshwa-ter water bodies like the Great Lakes, a detailed understanding of P transport dynamics is vital. The main objective of this study was to investigate P transport dynamics using high-frequency monitoring. We used the HydroCycle-PO4 instrument to measure total reactive P (TRP) concentration at a high resolution from a subsurface-drained farm with continuous no-till and Blount loam soil. We used a dataset contain-ing hourly TRP concentration and hourly drainage discharge measurements in the analysis. Results showed that there was a good relationship between TRP concentration and drainage discharge (R -squared = 0.60) such that TRP had a transport-limited chemodynamic pattern, that is TRP concentration tended to increase with increase in flow during events. A 1% increase in drainage discharge resulted in a 1.36% increase in TRP load, indicating a significant increase in P concentration during high flows. We found that flow events substantially contributed to P loss (89%) because of capturing the rapid increase in P concentration during high flows. The rate of increase in P concentration during the rising limb ranged from 0.02 to 0.66 mg/L per hour. The highest 7.7% of drainage flow transported 75% of the TRP load during the monitoring period. The hysteresis pattern tended to be positive (clockwise) during the study period, indicating that preferential flow was a pathway for TRP loss. Most flow events (30 out of 36) displayed a flushing effect in which P concentration increased with rise in drainage discharge. In conclusion, high -frequency P sampling showed that management and conservation practices should target flow events to reduce P loss. & COPY; 2023 The Author(s). Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

The first two figures of the supplementary materials provide images of water ponding in our field site for two different occasions. The last figure shows the 1:1 scatterplots of observed and predicted values of subsurface drainage discharge, dissolved reactive phosphorus, and total phosphorus.

Controlled drainage (CD) is a valuable management practice for reducing drainage volume and nutrient loss, but its impact on corn (Zea mays L.) production is not completely understood. The objectives of this study were to investigate the regional effect of CD on corn grain yield compared to free drainage (FD), investigate the factors influencing corn yield response to CD, provide management recommendations for optimizing corn yield under CD, and identify future research needs for corn production on poorly drained soils with subsurface drainage systems. This synthesis included data collected from 13 field sites where corn was planted under both FD and CD in six U.S. Midwestern states and North Carolina totaling 55 site-years of data from 2006 to 2017. On average, there was no statistically significant difference in corn grain yield between CD (10.62 Mg/ha) and FD (10.53 Mg ha−1). However, 42% of the dataset indicated that CD either increased or decreased corn yield by 4% or more compared to FD. Further analysis was conducted on this subset of data in order to evaluate underlying factors (i.e., weather conditions during the season, soil type, and drainage system design and management) influencing corn yield response to CD. Results of this analysis showed that CD was effective in alleviating plant stress caused by mild to moderate drought conditions and subsequently increased corn grain yield by 4–14% in 12 site-years. In contrast, CD reduced corn grain yield by 4–10% during wet growing seasons (6 site-years). Variability in growing season precipitation has been identified as a key factor influencing corn grain yield under CD, and more active management or CD system automation is recommended. General recommendations are provided for managing manually operated CD systems in the U.S. Midwest to improve growing season water management and corn yield. Additional research to develop technologically advanced water management systems for crop production on poorly drained soils is needed in order to adapt to changing weather patterns.

Highlights More diverted flow to the buffer does not necessarily mean more nitrate load removal. A design approach should incorporate a nitrate removal component to maximize nitrate load removal. The newly proposed design follows a process-based approach to estimate the annual site-specific nitrate removal. The newly proposed design provided more consistent nitrate load removal regardless of the site conditions. Neglecting exit head loss in the design process leads to an overestimation of diverted flow and nitrate load removal. 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. Keywords: Conservation practice, Exit head loss, Nitrate, Subsurface drainage, Tile drainage, Water quality.

HighlightsFor 3- or 4-row regular-perforated pipes, the effective radius ranged from 0.3 to 0.9 cm (average 0.6 cm).For 8-row regular-perforated pipes, the effective radius was 1.9 cm.The effective radius of sock-wrapped pipes ranged from 5.7 to 6.0 cm (average 5.9 cm).The 8-row sand-slot pipes had a lower initial system cost than the sock-wrapped pipes.The 8-row regular-perforated pipes had a lower initial system cost than the 4-row regular-perforated pipes.Abstract. Knitted-sock envelopes are applied in agricultural subsurface drainage to prevent sediment clogging of the drain pipes. In the United States and Canada, sand-slot pipes are sometimes used as a cheaper alternative to sock-wrapped pipes. However, their initial system cost has not been compared. The main objective of this study was to evaluate the effect of pipe material on drain spacing and initial system cost. First, the theoretical effective radius of each pipe material was estimated. Then, the drain spacing was calculated for each pipe material in a drainage design, such that each pipe would provide the same design drainage rate (i.e., same water removal rate). The results showed that the effective radius of sock-wrapped pipes (average 5.9 cm) was much greater than that of 4-row (average 0.4 cm) and 8-row perforated sand-slot pipes (average 1.6 cm). Rows refer to number of longitudinal rows of perforations. The sock-wrapped pipes considerably increased the effective radius of the pipe by reducing the entrance head loss. Furthermore, the sock-wrapped pipes allowed for a wider drain spacing (ranging from 0.8 to 5.4 m wider) in soil with risk of drain sedimentation, thereby reducing the total length of the lateral drain pipe needed for drainage design compared to both 4- and 8-row sand-slot pipes. The 8-row regular-perforated pipes allowed for a wider optimum drain spacing, thereby reducing the initial system cost in soil without a drain sedimentation issue compared to 4-row regular-perforated pipes. In conclusion, even though the sock-wrapped pipe reduced the total length of the lateral drain pipe, the 8-row sand-slot pipe had a lower initial system cost compared to the sock-wrapped pipe, when designed at the same design drainage rate and drain depth. Keywords: Drain spacing, Effective radius, Entrance resistance, Geotextile, Knitted sock, Perforation.

Accurate phosphorus (P) load estimation in subsurface drainage water is critical to assess the field-scale efficacy of conservation practices. The HydroCycle-PO4 instrument measures real-time total reactive P (TRP) concentration without the need for sample filtration, thereby enabling comparative evaluation of different sampling strategies. The main objective of this study was to evaluate the effects of water sampling strategies on the uncertainty of P load estimation. Hourly TRP concentration and hourly drainage discharge measurements formed the reference P load dataset. Four hypothetical water sampling strategies were evaluated: (a) time-proportional discrete sampling, (b) time-proportional composite sampling, (c) flow-proportional discrete sampling, and (d) flow-proportional composite sampling. All sampling strategies underestimated TRP load compared with the reference dataset. Total reactive P load underestimation changed from 0.2 to 51% as time-proportional discrete sampling intervals increased from 3 h to 14 d. Total reactive P load underestimation changed from 12 to 43% as the time-proportional compositing scenario increased from 1 to 7 d, each with one aliquot per day. In the case of flow-proportional discrete sampling scenario, the lowest (0.6%) and the highest (-5.1%) uncertainties were observed when 1- and 5-mm flow intervals were used. The relative error based on the results provided by the flow-proportional composite sampling ranged from 0.2% when using 1-mm flow interval to -6.7% when using 5-mm flow interval. In conclusion, the flow-proportional sampling strategies provided a more accurate estimate of cumulative P load with fewer number of samples because a greater portion of samples were taken at higher flow rates compared with time-proportional sampling strategies.

A knitted-sock geotextile envelope is commonly used in the USA, Canada, and parts of Europe to prevent sediment entry into agricultural subsurface drain pipes. Sand-slot pipes have a narrow width to prevent sediment from moving into the drain pipe, and they are used in the USA and Canada as a cheaper alternative to knitted -sock-wrapped pipes. The type of drain pipe (sand-slot or sock-wrapped) is important in subsurface drainage design because it affects the entrance head loss caused by the convergence of flowlines toward the drain per-forations, thereby influencing the effective radius of the pipe, which in turn affects drain inflow. Although the theoretical effective radius of a knitted-sock-wrapped pipe can be estimated based on analytical equations, no study has experimentally measured it to verify the theoretical equations. This study had two objectives: (1) Experimentally measure the effective radius of sock-wrapped and sand-slot pipes to verify the theoretical method, and (2) comparatively evaluate the drain inflow performance of a sock-wrapped pipe and a sand-slot pipe. We tested the pipe materials in a sand-tank experiment equipped with manometers. The results showed that the measured and theoretical effective radii were 5.5 and 5.6 cm for the sock-wrapped pipe, 0.7 and 0.7 cm for the 4-row sand-slot pipe, and 1.6 and 1.6 cm for the 8-row sand-slot pipe, respectively. This agreement confirmed that the theoretical method reliably estimated the effective radius of the pipes. The sock-wrapped pipe had a considerably lower entrance head loss (0.4 cm) than the 8-row (6.1 cm) and 4-row (10.4 cm) sand-slot pipes, thereby increasing the effective radius of the sock-wrapped pipe. The estimated drain inflow of the sock -wrapped pipe was 12% and 20% higher than that of the 8-row and 4-row sand-slot pipes, respectively. In conclusion, we verified that the theoretical method accurately estimated the effective radius of the pipes and demonstrated that the sock-wrapped pipe had considerably higher drain inflow than the sand-slot pipe.

Controlled drainage (CD), sometimes called drainage water management, is a practice whereby the drainage system outflow is managed during specific periods to retain more water in the field. Although CD has been shown to reduce downstream nitrate-N (NO3--N) load, seasonal patterns have been less consistent which can potentially impact the effectiveness of conservation practices. The main objective of this study was to assess the regional and seasonal impact of conventional free drainage (FD) and CD on drainage flow and nitrate-N load. Using experimental data from ongoing and historical CD experiments across the Corn Belt and in North Carolina, we evaluated subsurface drain flow, nitrate-N load, and performance of CD systems. Across the data set and regions, there was little difference in annual flow from FD conditions. Seasonally, more northern and western sites experienced a greater percentage of the annual flow occurring in the spring. There was no nitrate-N concentration reduction with CD. Flow and nitrate-N load reductions with CD did not vary by plant hardiness zone across the region, but the season with the greatest reduction did shift from winter to spring moving north and west in the study area. Absolute flow reductions (in mm) were similar regardless of precipitation category. Consequently, the percent reduction was lower as the amount of precipitation (category) increased. Overall, this analysis found CD to be an effective practice for reducing drain flow and nitrate-N loading directly delivered by the drains to downstream water bodies across the region.

The choice of drain depth in subsurface drainage design affects system performance. Previous studies have focused on investigating the hydrology of shallow drains at discrete locations. However, to our knowledge, no study has investigated the profitability of shallow drains across a wide range of soil and climatic conditions. The main objective of this study was to comparatively evaluate the effects of 75-cm shallow drains versus 125-cm deep drains on profitability and hydrology of subsurface drainage systems across the eastern USA. We conducted DRAINMOD simulations under continuous corn production (Zea mays L.) using 30 years of weather data (1990–2019) for combinations of three factors: two drain depths, four soils, and seven locations. Simulations were optimized for crop production that maximized annual economic return on investment. The results showed that corn yield for shallow drains went from 1.6% increase to –1.3% decrease with increasing growing-season rainfall from northeast to southeast USA. Shallow drains provided the benefit of reduced year-to-year corn yield variability across the region. Corn yield for coarse-textured soil benefited more from shallow drains than fine-textured soil across the region. Even though shallow drains increased annual benefit from higher corn yield under certain soil and weather conditions, they were less profitable than deep drains (lower benefit-cost ratio and higher payback period). The benefit of drainage discharge reduction under shallow drains generally intensified with increasing annual precipitation from northeast to southeast USA. Results were used to develop an empirical equation for estimating the benefit of drainage discharge reduction with shallow drains in the eastern USA. In conclusion, even though shallow drains were less profitable than deep drains, they provide benefits that may be of interest to farmers and policymakers.

HighlightsDRAINMOD-P has been developed to simulate phosphorus (P) dynamics in drained croplands.Key hydrological and biochemical processes affecting P cycling are represented in the model.The model predicts surface and subsurface P losses as affected by weather, soil, and management factors.Abstract. High phosphorus (P) loads to streams and lakes can promote harmful algae blooms and cause water quality deterioration. Recent research has identified subsurface drainage as an important pathway for the transport of dissolved P from drained croplands to receiving surface water bodies, particularly when macropore flow contributes a considerable portion of the subsurface drainage outflow. Currently, a few models are capable of simulating P dynamics in poorly drained soils with artificial drainage systems. The objective of this study was to develop DRAINMOD-P, a field-scale, process-based model that simulates P cycling and transport in drained croplands. Processes represented in the model include atmospheric deposition, organic and inorganic fertilizer applications, plant uptake, sediment-bound and dissolved P losses in both surface runoff and subsurface drainage, tillage practices, and P mineralization and immobilization. The model predicts P losses under different management practices, climatic conditions, drainage systems, and crop rotations. The model is an extension to the nitrogen model DRAINMOD-NII, with full integration of the nitrogen and P model components. DRAINMOD-P uses the recently modified hydrology component that simulates macropore flow. A soil erosion component, based on the RUSLE approach, has been incorporated into the model to estimate sediment loss and associated particulate P loss. Sediment deposition in tile drains is considered to quantify particulate P settling in the drainage system. In this article, we review the approaches used in DRAINMOD-P for simulating P-related processes. Model testing against field-measured data from a subsurface-drained field in northwest Ohio is presented in a companion article. Keywords: Best management practices, Phosphorus model, Phosphorus processes, Soil erosion, Water quality modeling.