Subsurface (Tile) Agricultural Drainage
Summary and Keywords
Agricultural (tile) drainage enables agricultural production on millions of hectares of arable lands worldwide. Lands where drainage or irrigation (and sometimes both) are implemented, generate a disproportionately large share of global agricultural production compared to dry land or rain-fed agricultural lands and thus, these water management tools are vital for meeting the food demands of today and the future. Future food demands will likely require irrigation and drainage to be practiced on an even greater share of the world’s agricultural lands. The practice of agricultural drainage finds its roots in ancient societies and has evolved greatly to incorporate modern technologies and materials, including the modern drainage plow, plastic drainage pipe and tubing, laser and GPS-guided installation equipment, and computer-aided design tools. Although drainage brings important agricultural production and environmental benefits to poorly drained and salt-affected arable lands, it can also give rise to the transport of nutrients and other constituents to downstream waters. Other unwanted ecological and hydrologic environmental effects may also be associated with the practice. The goal of this article is to familiarize the reader with the practice of subsurface agricultural drainage, the history and extent of its application, and the benefits commonly associated with it. In addition, environmental effects associated with subsurface drainage including hydrologic and water quality effects are presented, and conservation practices for mitigating these unwanted effects are described. These conservation practices are categorized by whether they are implemented in-field (such as controlled drainage) versus edge-of-field (such as bioreactors). The literature cited and reviewed herein is not meant to be exhaustive, but seminal and key literary works are identified where possible.
Keywords: agricultural production, crop production, waterlogging, soil salinity, environment, hypoxia, nutrient loss, hydrology, best management practices, controlled drainage, bioreactor, saturated buffer
Agricultural drainage (sometimes referred to as artificial drainage) is practiced throughout the world where agricultural production may be limited by excess water or soil salinity due to poor natural drainage. Poor natural drainage can be caused by one or more of the following factors: excess rainfall or snowmelt (i.e., too much water at the wrong time and the wrong place), soil with a limited or poor permeability rate (the ability to transmit water through the root zone), and poorly developed surface drainage topography. Surface drainage systems (e.g., grassed waterways, swales, and ditches) are used where the topography permits in order to convey excess surface water and runoff from the field in a timely and nondestructive manner. Subsurface drainage systems (sometimes referred to as tile drainage systems) are implemented when saturation, waterlogging, or soil salinity makes agricultural field operations difficult or impossible or threatens crop establishment and growth (Baker & Johnson, 1981; Ritzema, 2006; Strock, Sands, & Helmers, 2011; Abdel-Dayem et al., 2004).
Soil is a structured, porous medium with a matrix comprised of about 50% pore space and 50% solid minerals and organic material (specific percentages differ by soil texture and structure). The distribution of pore sizes within a soil enables it to retain water in smaller pores with adhesive and cohesive (matric) forces, while water drains from larger soil pores, where gravitational forces exceed matric forces. The result, for a well-drained soil, is that it retains a significant amount of water for plant uptake (in smaller pores) and provides sufficient air-filled pores for plant respiration processes to occur. Soils with adequate drainage (either natural or artificial) are able to drain water from their largest pores via gravity. If drainage occurs for a sufficient period of time, the soil moisture reaches the state of “field capacity,” where gravitational drainage ceases to occur and an air-water mixture in the soil profile exists, which is favorable for plant growth and development. The matric forces at field capacity are typically considered to be less than 1 bar, and a range of matric pressure values has been adopted throughout the world. An aerobic environment with available water is essential for many of the crops that are cultivated throughout the world.
A subsurface agricultural drainage system typically consists of a network of narrow trenches, and underground channels or pipes/tubing (see Fig. 1) that allows gravitational water to drain from the larger pores and create an aerobic soil environment. This removal of excess water also provides a pathway for accumulated salts to be leached from the soil profile in irrigated, sodic soils. Subsurface drainage techniques have been used to enhance crop production for millennia, with an evolution in the techniques and materials used to accomplish these objectives. There is evidence that forms of drainage systems were first used in Mesopotamia around 7000 BCE, with the addition of pipes in the underground channels implemented in the Indus River Valley around 2000 BCE (Stuyt, Dierickx, & Martinez Beltran, 2005).
Drainage systems were also employed by the ancient Greeks and Egyptians. In the oldest written description of subsurface drainage, recorded around 130 BCE, the Roman statesman and orator Marcus Porcius Cato gave instructions on materials and methods that should be used to facilitate the drainage of agricultural fields (Beauchamp, 1987). The Romans further developed subsurface drainage methods by placing roofing tiles in the bottom of the trench, partially backfilling it with small stones and bundles of brush, and finally using some of the excavated soil to fill in the top portion of it. The Romans facilitated the spread of this technology throughout Europe, with little change in methodology over the next 1,000 years (Beauchamp, 1987; Stuyt et al., 2005).
Cylindrical pipe entered the picture much later, and while there is some indication that clay pipe was used prior to 1620 in France and other areas, its use did not become widespread until after the clay pipe extruder was patented in England in 1843 (Beauchamp, 1987). The extruder greatly reduced the cost of manufacturing the pipe and led to an increase in the installation of subsurface drainage systems, with the technology spreading across Europe and North America (Stuyt et al., 2005). In addition to using pipes or other material to keep the subsurface channels open, it was observed that in highly cohesive soils, such as clays, an open channel could be maintained for a few years simply by forming an opening in the subsoil. This method, called mole drainage, was developed in the 19th century and installed using animal power to pull through the ground a cylindrical metal slug attached to a long shank (Ritzema, Nijland, & Croon, 2006). Mole drainage made it possible to drain without digging a trench.
The next major drainage advancement came in the late 19th and early 20th centuries, with the advent of mechanized trenching equipment, which eliminated the need to dig drainage trenches laboriously by hand. Mechanized trenchers were part of standard drainage installation equipment for decades, and many remain in use today. In the 1960s, the U.S. Department of Agriculture undertook a project with the initial goal of developing a plastic liner for mole drains that would extend their usable life. While this objective was eventually abandoned, it led directly to the development of what is now called the tile plow or drainage plow (Fouss, 1968). Similar to the mole drain, the tile plow creates an open channel in the subsoil without digging a trench and without the need to backfill a trench. While the opening is being formed, a flexible, corrugated plastic pipe (which had been developed a few years earlier in Germany) is fed into the bottom of the tile plow, where it is placed firmly in the bottom of the channel at the desired depth and grade (slope).
Because the drainage plow was able to install pipes faster than the operator could monitor and control the exact pipe location manually, a laser-guided automatic grade control system was developed to ensure the accuracy of the pipe placement. At the start of the 21st century, this technology began to be superseded by the increasing availability of a high-accuracy, real-time kinematic (RTK) global navigation satellite system (GNSS) for controlling the depth, grade, and location of the pipe installation. In developed countries, both mechanized trenching machines and tile plows are commonly used to install corrugated plastic drainage tubing/pipe [made of polyvinyl chloride (PVC) or high-density polyethylene (HDPE)] using either laser or RTK-GNSS grade control. However, mole drains, hand-dug channels, traditional grade control techniques, and a variety of pipe materials, including bamboo, are still used in many developing countries and other areas where access to machinery and other materials is limited (Akinbile, Fakayode, & Sanusi, 2011).
Artificial subsurface drainage is a critical component of agricultural production in many regions of the world. Estimates by the International Commission on Irrigation and Drainage (ICID) suggest that 10%–15% of croplands globally have some degree of drainage improvement (ICID, 2015; Smedema, 2000; also see Fig. 2). The distribution of these improvements, however, is nonuniform across economic and regional groupings (Fig. 3). The ICID estimates that 25% of cropland in developed countries has some level of drainage improvement, compared to 9% in developing or emerging countries and 5% in the least-developed countries.
When viewed on a regional basis, the Americas and Europe have drainage improvements covering 15% of their cropland, compared to 12% in Asia and 5% in Africa. With drainage needs unmet in about one-third of rain-fed cropland and half of irrigated cropland, it has been suggested that an additional 450 million hectares of cropland worldwide would benefit from some level of drainage improvement (Ritzema, Nijland, & Croon, 2006; Smedema, 2000; Abdel-Dayem et al., 2004). These numbers are coarse estimates related to the extent of drainage works around the world. It is also reasonable to assume that additional drainage needs exist within areas currently listed as drained (namely, intensification of existing drainage infrastructure to improve soil water management and secure higher levels of agricultural production). At the present levels of drainage implementation, it is estimated that 20% of global agricultural production comes from land with drainage improvements, with an additional 11% coming from land with both irrigation and drainage improvements (Ritzema et al., 2006). Irrigated land and land with no irrigation or drainage improvements account for the remaining 39% and 30%, respectively.
Benefits of Subsurface Agricultural Drainage
Agricultural subsurface drainage is implemented on arable land where agricultural production is potentially negatively affected by waterlogging, high soil salinity levels, or both. In these areas, subsurface drainage improves conditions for crop establishment and development, helps prevent and mitigate damage to soil and improve soil quality, and may increase farm efficiency. Many benefits of subsurface drainage (such as increased crop growth and yield) accrue directly to farm income; other benefits (such as improved soil quality and farm efficiency) are more difficult to monetize, but nonetheless drainage practitioners consider them to be significant (Kanwar, Johnson, Schult, Fenton, & Hickman, 1983). Subsurface drainage is increasingly being viewed as a multiobjective practice (Schultz, Zimmer, & Vlotman, 2007).
The effects of subsurface drainage on crop production, in its many facets, have been vigorously researched and described in the published literature throughout the 20th century. This body of work has been aptly summarized in three American Society of Agronomy monographs on agricultural drainage: Luthin (1957), describing research from around 1900 to 1955; van Schilfgaarde (1974), presenting work from the mid-1950s to the early 1970s; and Skaggs and van Schilfgaarde (1999), summarizing work from the 1940s and 1950s through the mid-1990s. The summaries, presented by various authors in the three papers, are extensive and are based on hundreds of referenced articles. Such extensive summaries of the effects of subsurface drainage on crops and soils will not be attempted here. Rather, the benefits of agricultural drainage are broadly described in two primary categories: (a) benefits directly affecting crop growth and yield, and (b) benefits to agricultural productivity and efficiency that may not be realized directly in terms of crop yield and farm income.
Crop Growth and Yield
Subsurface drainage has the potential to increase crop growth and yield and decrease annual crop yield variability in poorly or inadequately drained soil. Increases in crop growth and yield from drainage occur through a number of mechanisms, the most prominent of which are better soil aeration (i.e., less waterlogging from shallow water tables); mitigation of planting delays from wet soil conditions; and decreased soil salinity in salt-affected soil. Mitigation of planting delays effectively lengthens the growing season for poorly drained soils; in northern climates where the growing season is relatively short, lengthening the growing season by an extra week or two can have quite an impact on crop growth and yield. The effects of delayed planting are illustrated in studies such as Bollero, Bullock, and Hollinger (1996). Adequate drainage also can help soil warm up more quickly in the spring (Jin, Sands, Kandel, Wiersma, & Hansen, 2008), which can have positive effects on seed germination and development (Bollero et al., 1996).
Evans and colleagues (1999) did an extensive review of the effects of drainage on crop growth and yield, which summarized field and large plot studies relating crop yield to drainage and water table depth, excess water, or both. This summary included 15 different crops as reported in 27 different studies between 1974 and 1999. Moreover, Evans et al. (1999) provided a tabular summary of 21 studies relating crop response to static water table depths for 27 grain, vegetable, and forage crops. High variability exists in crop response data summaries such as this because of the myriad physical, climatic, and biological factors that interact in unique ways for a given location. Nevertheless, a quick inspection of the data summarized by Evans et al. (1999) shows that crop yield precipitously declines for static water tables shallower than 60 cm, for most of the crop species described; the exceptions were the forage/perennial species, which generally displayed greater tolerance to shallow water tables.
Reduction of soil salinity represents another significant benefit brought about by subsurface drainage. Salts that occur naturally in soil as part of the soil formation processes are often leached by precipitation and irrigation to depths beyond the root zone of plants. Where poor natural drainage exists, the required root zone leaching is reduced or eliminated, allowing salts to accumulate at shallower depths and negatively affecting crop production. In addition, the application of large quantities of imported surface water has disturbed the hydrologic equilibrium of groundwater basins in many canal command areas in the world and caused waterlogging and secondary salt buildup (Ritzema, 2006; Ritzema, Satyanarayana, Raman, & Boonstra, 2008). Plant species exhibit different tolerances to soil salinity, and yield reductions and eventual crop death can occur if soil salinity levels exceed 5–10 deci-Siemens per meter (dS/m), a measure of electrical conductivity (Smedema, Vlotman, & Rycroft, 2004).
Despite an abundance of research throughout the 1900s on the effects of waterlogging, delayed planting, salinity, and drainage on crop growth and yield, it remains challenging for contemporary practitioners of drainage to estimate the effects of drainage on specific crops at specific locations. For precisely the same reasons that the data summarized by Evans et al. (1999) display high variability, site-specific estimates of drainage benefits are difficult to make. Data presented by Schwab, Fausey, Desmond, and Holman (1985) summarized 10 years of crop response to drainage, in some of the most widely used crop yield data to date. Crop response data such as this are sometimes regarded as representative of the region and are used to make estimates of profitability of drainage, such as those presented in Eidman (1997). It was this very conundrum—estimating site-specific benefits of drainage without site-specific crop response data—that led to the development of drainage performance models such as DRAINMOD (Skaggs, 1982).
A second category of benefits associated with subsurface drainage are those effects that may not be directly expressed in crop yield, are not easily monetized, or both. Poorly drained soil is prone to increased surface runoff and erosion and increased degradation of soil structure due to soil compaction, elevated soil salinity, or both (Smedema et al., 2004). Mitigation of these negative soil quality effects by providing adequate subsurface drainage represents a significant benefit to the sustainable cultivation of poorly drained soil. Nevertheless, the quantification of these effects can be difficult and has similar site-specific challenges as the cropping effects described previously.
The capacity of a soil to support field machinery and livestock traffic without undue harm has been termed trafficability in the soils literature. Trafficability is typically reduced in poorly drained soil, with a reduced number of days where soil moisture conditions are favorable for mechanical field operations or use by livestock (i.e., planting, tillage, cultivation, grazing, etc.) (Aldabagh & Beer, 1975). The effects of poor trafficability on planting delay and growing season length were previously discussed. In addition to increasing the number of suitable days for field operations, drainage can increase the efficiency of these operations by creating more uniform soil moisture conditions throughout the field and farm (French, 1860). Farm fields that dry predictably and more uniformly allow agricultural producers to implement mechanical field operations more efficiently by reducing or eliminating the need to return to the field to work areas that were too wet for mechanical traffic, minimizing the number of passes in a given field. Thus, field operations can be managed more efficiently on the whole-field scale, rather than being done in piecemeal fashion (Aldabagh & Beer, 1975). The magnitude of these effects is site- and production system-specific and is annually variable, similar to other subsurface drainage benefits.
Environmental Effects of Subsurface Agricultural Drainage
Subsurface drainage systems can have both positive and negative environmental effects (Skaggs, Breve, & Gilliam, 1994; Blann, Anderson, Sands, & Vondracek, 2009; Ritzema, 2016). For example, drainage improvements on cropland typically decrease the potential for surface runoff and erosion, as well as losses of phosphorus and organic nitrogen to receiving waters. Conversely, subsurface drainage systems typically increase losses of nitrate-nitrogen, soluble salts, and other contaminants to surface waters through enhanced leaching of the soil profile (Gilliam, Baker, & Reddy, 1999) and rapid conveyance to downstream waters by subsurface drains. A better understanding of the hydrologic and water quality effects of subsurface drainage systems is important for designing and managing such systems in order to achieve long-term sustainability of agriculture on poorly drained and salt-affected lands.
Subsurface drainage systems affect the hydrology of cropland primarily by lowering the water table, increasing the air-filled pore volume, and increasing the opportunity for infiltration of water compared to undrained fields (Hill, 1976; Skaggs & Broadhead, 1982; Irwin & Whiteley, 1983; Fraser & Fleming, 2001; Ritzema, Satyanarayana, Raman, & Boonstra, 2008). The general impact on hydrology is a shift from a surface runoff–dominated hydrology (typical of poorly drained soil) to an infiltration-drainage hydrology. A number of intervening factors, however, complicate the relationship between artificial drainage and hydrology and have led to much debate about the topic (Robinson, 1990; Blann et al., 2009). The complex roles and interaction of temporal and spatial scales, precipitation and antecedent moisture characteristics, drainage methods and intensity, climate and soil variability, and other factors make the hydrological effects of drainage difficult to summarize. Several important relationships and key references are described next.
Infiltration and Surface Runoff
Subsurface drainage removes excess (gravitational) water from the soil profile continuously when the water table rises above the depth of the drain. The removal of this gravitational water allows air to fill some of the pore space in the soil profile and in turn creates an opportunity for more infiltration of precipitation/irrigation. Generally, increased infiltration leads to field-scale reductions in surface runoff peak flows and volumes in artificially drained soil. These phenomena have been reported in both modeling and field studies conducted in the United States (Skaggs & Nassehzadeh-Tabrizi, 1983; Istok & Kling, 1983; Konyha, Skaggs, & Gilliam, 1992), Canada (Irwin & Whiteley, 1983; Natho-Jina, Prasher, Madramootoo, & Broughton, 1987), and Germany (Baden & Eggelsman, 1968). These studies showed that improved subsurface drainage reduced not only surface runoff volume, but also the peak runoff rate and the lag time between the beginning of a precipitation event and the peak flow in poorly drained soil (Baker & Johnson, 1981; Skaggs, Breve, & Gilliam, 1994). For example, Istok and Kling (1983) found that the installation of subsurface drainage systems in a silt loam watershed in the state of Oregon decreased surface runoff by 65%. Similarly, through computer simulation, Konyha, Skaggs, and Gilliam (1992) found that the surface runoff volume and peak runoff rate from a subsurface-drained Wadasa muck soil field in North Carolina were reduced by 66% and 73%, respectively.
Streamflow effects from agricultural subsurface drainage activity could be assumed to be translational from the field-scale effects. The nature and magnitude of downstream effects, however, have not been sufficiently resolved in the academic literature and have been the subject of much debate for over a century. See Robinson (1990) for an interesting summary of this historical debate, a synthesis of seminal works on the subject, and elucidation of the key factors to consider for these phenomena. Robinson (1990) asserted that lack of factual evidence and limited individual observation encourage and exacerbate emotive arguments regarding the cause-and-effect relationship between agricultural drainage and streamflow hydrology. The debate on this subject stems from people’s perceptions of change in their natural environment over time: for example, rivers appear to rise more quickly or fall more slowly after subsurface drainage implementation. Robinson (1990) asserted that differences in viewpoint (increasing or decreasing peak flows) essentially stem from the importance that one places on either (a) the increase in infiltration capacity due to reduced saturation (decrease in peak flow) (Skaggs & Broadhead, 1982; Irwin & Whiteley, 1983; Fraser & Fleming, 2001); or (b) the capacity of subsurface drains to carry water to the outlet faster than it would arrive if traveling through the soil matrix (increase in peak flow) (Robinson, 1990).
Scientific consensus regarding the downstream effects of subsurface drainage is also elusive. Robinson (1990) further posited that the scientific opinion on artificial drainage and hydrology could be categorized by five causal mechanisms: (a) the effect of increased drainage density, (b) the effect of increased soil water storage capacity, (c) the effects of precipitation characteristics and antecedent moisture conditions, (d) the effects of different types of drainage systems, and (e) the effects of drainage extent and location within a catchment. Indeed, the influence of artificial drainage on stream hydrology depends on all these factors (among others) and no single study has, or can, isolate or control all of them at meaningful spatial scales. Fraser and Fleming (2001) conducted a review of studies that showed reductions in streamflow volume and peak flow with subsurface drainage. They cited higher infiltration capacities and volumes as the mechanism for reducing surface runoff volumes. The soil moisture condition prior to antecedent precipitation affects how well a subsurface-drained soil can reduce peak flow. Skaggs and Broadhead (1982) monitored five different storm events in a field experiment in North Carolina and found that good subsurface drainage reduced peak flows by 20% under wet antecedent conditions, and by 87% under dry antecedent conditions.
Bailey and Bree (1981) conversely reported that arterial drainage improvements in southern Ireland shortened the time to peak and increased flood peaks, increasing the three-year flood value by about 60%. Although subsurface drainage may reduce surface runoff and increase infiltration once infiltrated water reaches the drains, it can be delivered to the outlet at a faster rate than subsurface movement through the soil matrix. This situation is especially true for regions with high depression storage and more permeable soil (Robinson & Rycroft, 1999; Robinson, Ryder, & Ward, 1985). The typical effect of subsurface drainage under these situations is to increase the speed of subsurface discharges, which tends to increase peak flows and reduce the time to peak. While studying the flashiness of 515 Midwestern streams over a 27-year period (1975–2001), Baker, Richards, Loftus, and Kramer (2004) found statistically significant increases in flashiness in 22% of the streams, primarily in the eastern portion of the Midwest, while decreases in flashiness were also present in 22% of the streams. Flashiness reflects the frequency and rapidity of short-term changes in streamflow, especially during runoff events. The authors attributed the increased flashiness of rivers in Indiana, Ohio, and Michigan to higher clay content soil, along with the continuing improvement in systematic subsurface drainage in the eastern portion of the study area relative to the western portion.
Evapotranspiration (ET) and Water Yield
Water yield from a field or watershed is the volume of water that leaves the system by the combined processes of surface runoff, subsurface drainage, and seepage. Water yield also can be expressed as Precipitation—ET, where evapotranspiration (ET) is the sum of evaporation and plant transpiration. Thus, any change in water yield from subsurface drainage requires a change in the quantity Precipitation—ET, or a change in ET, due to drainage. Skaggs et al. (1994) stated that subsurface drainage may increase water yields by 5%–10%, with the caveat that such increases would be difficult to measure in the field. A few researchers have examined the role of drainage and other cropping practices in ET (Brye, Norman, Bundy, & Gower, 2000; Rijal et al., 2012). In one of the first field-based studies of the impact of subsurface drainage on ET, Rijal et al. (2012) showed an overall increase in ET with subsurface drainage compared to undrained conditions for corn and soybeans. On an annual basis, ET in the drained field was 16% higher in 2009 (for corn) and 7% higher in 2010 (for soybeans) compared to the undrained field. These annual increases were due, in large part, to significantly better crop growth and higher mid-season ET rates for the drained field compared to the undrained field. These results illustrate that subsurface drainage also has the potential to decrease the overall water yield (to surface waters) from agricultural fields. Conflicting results in the published literature are evidence that more work is necessary is this area.
Water Quality Effects
Subsurface drainage has both positive and negative effects on water quality. Agricultural lands with surface drainage systems are typically less prone to surface runoff compared to lands without drainage improvements; therefore, they typically exhibit lower soil erosion rates and sediment loss. Moreover, because a portion of lost phosphorus moves with eroded soil, these losses can be mitigated under well-drained conditions. Conversely, subsurface drains may enhance the leaching of dissolved nutrients, pesticides, salts, and other contaminants through the soil profile and transport these substances to downstream surface waters. Gulf of Mexico hypoxia is an example of downstream nutrient loss impacts, wherein subsurface agricultural drainage in the midwestern United States plays a significant role (Alexander et al., 2008).
Erosion and Soil Loss
Subsurface drainage implementation on cropland has been shown to substantially reduce soil erosion and sediment loss due to reduction in surface runoff (Baker & Johnson, 1981; Skaggs et al., 1994). For example, field experiments conducted over multiple years in Indiana (Bottcher, Monke, & Huggins, 1981), Louisiana (Bengtson & Sabbagh, 1990), and Ohio (Logan, 1981) showed a 36%–97% reduction in average annual soil loss due to implementation of subsurface drainage. Reductions in soil erosion and sediment loss due to subsurface drainage depend on drainage intensity, soil properties, and other factors. For example, installing drains closer together or deeper increases drainage intensity and thereby may reduce surface runoff and soil erosion further. Subsurface drainage also improves soil structure, and as a result, the soil becomes more stable and less susceptible to erosion (Hundle, Schwab, & Taylor, 1976). A reduction in sediment loss also reduces the loss of nutrients (e.g., phosphorus), pesticides, and salts attached to sediments. In the United States, subsurface drainage has been considered a best management practice for erosion control and surface water quality improvement (Bengtson, Carter, Morris, & Kowalczuk, 1984; Loudon, Gold, Ferns, & Yokum, 1986; Skaggs et al., 1994), and for many years, it received federal government subsidies for implementation on poorly drained cropland.
The primary nutrients lost through subsurface drainage are nitrogen and phosphorus. Nitrogen in its nitrate form (NO3–N) is a highly soluble and mobile nutrient that can move rapidly through the soil profile and be intercepted by subsurface drains. Nitrate concentrations in drainage water from agricultural lands in the midwestern United States often exceed the U.S. Environmental Protection Agency’s maximum contaminant level of nitrate for drinking water of 10 mg/L (Kladivko, Van Scoyoc, Monke, Oates, & Pask, 1991; Kalita, Cooke, Anderson, Hirschi, & Mitchell, 2007) and can negatively affect municipal water supply utilities. Nitrate loading is also a primary causal factor in the occurrence of coastal hypoxic zones, over 400 of which have now been documented worldwide. Longstanding efforts in Europe, including the Water Framework Directive and Nitrates Directive, have aimed to reduce such nitrate pollution (Council of the European Union, 1991, 2000). In the United States, nitrate loading to the Mississippi River Basin has become a major concern in recent years, contributing to hypoxic conditions in the northern Gulf of Mexico, along the Louisiana-Texas coast (Goolsby, Battaglin, Aulebach, & Hooper, 2001; Rabalais, Turner, & Wiseman, 2001). The nutrient-rich water from the Mississippi River forms a surface lens in the northern parts of the Gulf of Mexico and fuels phytoplankton growth. As the phytoplankton die, they sink to the bottom, where they are decomposed by bacteria, which consume oxygen in the bottom water and create hypoxic conditions. The Gulf of Mexico hypoxic zone is the largest human-caused hypoxic zone in the United States and the second-largest such hypoxic zone worldwide, with an average size during 1985–2014 of about 13,650 km2 (LUMCON, 2016). Nitrate loading from the Mississippi River Basin to the Gulf of Mexico increased by 300% between 1970 and 2000, with nonpoint sources, including subsurface drainage systems, contributing most of the load (Goolsby, Battaglin, Aulebach, & Hooper, 2001).
Nitrate concentrations and mass loads transported through subsurface drains vary with drainage system design (drain spacing and depth), soil properties (physical, chemical, and biological), climatic conditions (precipitation and temperature patterns), crop management practices (fertilizer nitrogen rates and timing, tillage, crop rotation, and the presence of cover crops) and the presence of conservation practices (e.g., controlled drainage and bioreactors) (Jaynes, Colvin, Karlen, Cambardella, & Meek, 2001; Dinnes et al., 2002; Ale, Bowling, Youssef, & Brouder, 2012). These factors affect the hydrologic behavior of drainage systems and may increase or decrease subsurface drainage volumes, which directly affects subsequent nutrient loss. For example, a decreased drain spacing generates increased subsurface drainage volumes and thereby may increase nutrient loss (Hofmann, Brouder, & Turco, 2004; Kladivko et al., 2004; Sands, Song, Busman, & Hansen, 2008). Some of the oldest drainage water quality work performed in the late 1800s at the Rothamsted Research station in the United Kingdom documented the influence of the nitrogen fertilizer application rate on nitrate losses in subsurface drainage water (Lawes, Gilbert, & Warington, 1882). They also documented that plots that had not received nitrogen applications for many years still leached nitrate, a finding that continues to have important implications today (Lawlor, Helmers, Baker, Melvin, & Lemke, 2008).
Numerous field and modeling studies have been conducted to quantify the effects of drain spacing and depth, fertilizer application rates and timing (i.e., fall versus spring application), and climatic variability on the quantity and quality of subsurface drainage (Kladivko et al., 2004; Thorp, Malone, & Jaynes, 2007; Gentry, David, Below, Royer, & McIsaac, 2009; Ale, Bowling, Youssef, & Brouder, 2012). Some of the hydrologic and water quality models that are widely used by researchers to simulate subsurface drainage and associated nutrient losses from drained agricultural fields and watersheds include DRAINMOD-NII (Skaggs, Youssef, & Chescheir, 2012; Youssef, Skaggs, Chescheir, & Gilliam, 2005); RZWQM (Root Zone Water Quality Model; Ma et al., 2012); ADAPT (Agricultural Drainage and Pesticide Transport; Gowda, Mulla, Desmond, Ward, & Moriasi, 2012); and SWAT (Soil and Water Assessment Model; Arnold et al., 2012).
Relatively small quantities of dissolved phosphorus are lost through subsurface drainage systems compared to nitrate, and hence enough attention was not paid to phosphorus loss. For example, on average, about 0.04 kg/ha of soluble phosphorus was lost from a subsurface-drained agricultural field in Indiana annually, compared to about 18–70 kg/ha of nitrate loss (Kladivko et al., 1991). In another Indiana study, about half of the total phosphorus loss from drained agricultural land in the St. Joseph River watershed occurred via subsurface drains (Smith et al., 2015).
Loss of phosphorus through subsurface drainage systems has been the subject of much research worldwide, including in the United Kingdom (Heathwaite & Dils, 2000), Sweden (Djodjic, Ulén, & Bergström, 2000), and Denmark (Grant et al., 1996), and often with more of a focus on drained grassland or pasture in New Zealand (McDowell, Sharpley, & Bourke, 2008; Sharpley, Syers, & O’Connor, 1976), Northern Ireland (Smith, Lennox, Jordan, Foy, & McHale, 1995), and Switzerland (Stamm, Flühler, Gächter, Leuenberger, & Wunderli, 1998). Similar to nitrates, the proportion of phosphorus loss through subsurface drains varies with soil properties, weather, drainage system design, and crop, tillage, fertilizer, and water management practices. Preferential or macropore flow was found to be an efficient mechanism of phosphorus transport into subsurface drains (Stamm et al., 1998). Although phosphorus loss from subsurface-drained croplands is small in terms of mass loading, it can have a significant impact on downstream surface waters due to its powerful eutrophication effects.
Pesticides from subsurface-drained agricultural lands may be transported to surface waters by both surface runoff and subsurface drainage. As the presence of subsurface drains generally decreases surface runoff and sediment losses when compared to undrained fields, pesticide losses via surface runoff (in soluble form) and sediment (in sorbed form) are also typically reduced. Subsurface losses of soluble pesticides, in contrast, may increase because soluble pesticides can move through the soil profile easily and reach subsurface drains through macropores such as surface cracks, worm burrows, and root channels. Summarizing findings from 30 North American studies of pesticide transport into subsurface drains, Kladivko, Brown, and Baker (2001) also reported that the dominant mechanism for pesticide transport to subsurface drains is preferential flow through macropores during rainfall/drainage events occurring soon after pesticide application. The total mass of pesticide loss through subsurface drainage is typically quite small. For example, Kladivko, Grochulska, Turco, Van Scoyoc, and Eigel (1999) conducted a field study in a silt loam soil in Indiana and detected trace amounts of pesticides (namely, carbofuran, atrazine, cyanazinc, and alachlor) in subsurface drain flow in the first large precipitation event after chemical application, which occurred between 3 and 14 days after pesticide application. Annual carbofuran losses in subsurface drain flow varied between 0.6 and 28.1 g/ha, or 0.04%–1.9% of the amount applied to the soil, depending on drain spacing and weather patterns. Losses of all other pesticides were less than or equal to 0.1% of the amount applied.
Other Substances (Pharmaceuticals; Escherichia coli, Salts, Selenium)
Cattle manure and liquid swine manure are commonly applied on cropland in the midwestern United States and Canada, and subsurface-drained agricultural lands are no different. Animal manure application supplies important nutrients for crops, allows safe and economic disposal of animal wastes, and is an important soil amendment. However, manure-derived nutrients and pathogens can also move through soils preferentially and reach surface waters via subsurface drainage systems. Substantial increases in nitrate and phosphate concentrations in drainage flow from the application of solid cattle manure (Stoddard, Grove, Coyne, & Thom, 2005; Olson, Bennett, McKenzie, Ormann, & Atkins, 2009; Hergert, Klausner, Bouldin, & Zwerman, 1981) and liquid swine manure (Bakhsh, Kanwar, & Karlen, 2005; Ball-Coelho, Murray, Lapen, Topp, & Bruin, 2012) were reported in the literature. In addition, Escherichia coli concentrations exceeding irrigation and recreation water quality guidelines were detected in subsurface drainage effluent in numerous field experiments conducted in Canada and the United States (e.g., VanderZaag, Campbell, Jamieson, Sinclair, & Hynes, 2010; Tomer et al., 2010).
Biosolids are also applied as fertilizer to subsurface-drained agricultural lands in some places, and in such cases, pharmaceuticals and personal care products (PPCPs) carried in biosolids can reach surface waters through subsurface flow and cause environmental problems (Lapen et al., 2008; Edwards et al., 2009; Larsbo et al., 2009). These PPCPs include compounds such as antibiotics, antidepressants, bactericides, antifungals, and various other products. Although soil characteristics, weather, the form of biosolids (liquid versus dewatered), and the method of land application (surface spread versus injected) can affect the degree of PPCP transport to receiving waters, macropores in the soil can facilitate rapid preferential flow of PPCPs to subsurface drains. For example, precipitation that occurred shortly after land application of about 8 Mg dry weight ha-1 of dewatered municipal biosolids facilitated the leaching of PPCPs to subsurface drains installed at an 80-cm depth below the surface in silt clay loam soil in eastern Ontario, Canada, and residues of PPCPs were detected in drainage water nearly nine months after application (Edwards et al., 2009).
Subsurface drainage systems installed for reclaiming waterlogged and/or salt-affected irrigated lands in the arid and semiarid areas of the world (e.g., San Joaquin Valley of California, Indus Basin of Pakistan, and the northwestern states of India) discharge large quantities of soluble salts and pollutants such as selenium into receiving waters, negatively affecting water quality. Selenium is a bioaccumulative trace element that is essential for animals, but ingestion of too much of it can cause adverse growth and reproductive effects in fish and birds.
Effects on Wetlands and Habitat
Subsurface drainage was used in the past to drain wetlands from agricultural lands and create a soil-water regime favorable for annual and perennial crop production. For example, in the midwestern United States, drainage implementation spanning from the 19th century through the 1980s has removed a significant percentage of historical wetlands (Blann et al., 2009). Wetlands provide hydrologic and ecological benefits, such as water flow regulation, water quality improvement, water supply, flood control, erosion control, wildlife habitat, and recreation. Although drainage of wetlands over the past 150 years provided many agronomic, hydrologic, economic, and health benefits, ecosystem services provided by the wetlands were negatively affected. Recognizing the valuable functions of wetlands, a provision was made in the 1985 U.S. Food Security Act to restrict further conversion of wetlands.
Conservation Practices Targeting Subsurface Agricultural Drainage
There is great interest in reducing the negative water quality impacts associated with subsurface drainage in many areas where this practice is necessary for agricultural production. The heavily subsurface-drained states of the upper Mississippi River Basin in the United States are a prime example, with their state nutrient loss reduction strategies emphasizing practices that reduce drainage-related nitrogen loadings (IDALS, 2014; IDOA, 2015; MN PCA, 2014). In the eastern United States, the Delmarva Peninsula, which is heavily drained by open ditches and subsurface drains, is under pressure to meet the 2010 Chesapeake Bay Total Maximum Daily Load, which calls for a 25% reduction in nitrogen loss by 2025 (USEPA, 2015). Similar efforts were described by Goulding (2000) to mitigate nitrogen loss from arable lands in the United Kingdom. Because the primary historic water quality concern for subsurface drainage has been dissolved nitrogen, generally nitrate (Randall & Goss, 2008), the water quality improvement practices discussed in the remainder of this article focus on reducing nitrogen loss rather than phosphorus loss. Phosphorus loss through subsurface drains is increasingly being seen as important, but it is not yet well understood (King et al., 2015).
There have been few comprehensive assessments of subsurface drainage water quality improvement practices (Dinnes et al., 2002; Christianson, Frankenberger, Hay, Helmers, & Sands, 2016). The focus here is on practices affecting the drainage system in particular, although in-field management practices (e.g., improved nitrogen management, winter cover cropping, and use of perennials within a rotation) are also effective for helping to reduce drainage nitrogen loads. The following strategies are practices that modify the drainage system and reduce delivery of nitrogen to the field’s edge (controlled drainage, reduced drainage intensity, and drainage water recycling) or are practices that remove nitrogen at the edge of a field before migrating downstream (saturated buffers, bioreactors, and wetlands).
In-field Drainage Modifications
Controlled drainage, also known as the practice of managed drainage or drainage water management, consists of the use of adjustable structures typically placed along drainage system mains that allow the outlet level of the subsurface drain to be adjusted. Because the water table must rise above the drain’s outlet level before drainage will occur, more water is retained in the soil profile than with free drainage systems. Raising the outlet level in this way during portions of the year when drainage is less critical for agricultural production reduces the overall amount of drainage and thereby drainage nitrogen loads (Frankenberger et al., 2006). The underlying idea behind controlled drainage for water quality improvement is the golden rule of drainage: “Drain only what is necessary for good trafficability and crop growth—and not a drop more.” This practice was pioneered as a nitrogen loss reduction practice in North Carolina in the early 1980s, and it now has gained attention in many surface- and subsurface-drained agricultural areas around the world (Gilliam, Skaggs, & Weed, 1979; Skaggs, Fausey, & Evans, 2012; Ritzema & Stuyt, 2015).
In a controlled drainage system, one control structure is recommended for every 30- to 60-cm change in field elevation. Therefore, this practice is most practical to use on relatively flat fields (slopes < 0.5%–1.0%). In addition to reducing nitrogen loss, crop yields may potentially be enhanced, particularly during years with wet and dry spells interspersed. While some studies show a moderate crop yield benefit (1%–20% increase), many studies show that controlled drainage has no impact on corn or soybean yield in many years (Skaggs et al., 2012). Management of the control structures is key to the agronomic and environmental success of this practice, typically involving the following:
• After harvest: Raising the outlet level (i.e., putting more stop logs in the control structures) to reduce drainage and nitrogen loss during the nongrowing season
• Prior to planting in the spring: Lowering the outlet (i.e., removing stop logs from the control structures) to improve trafficability and to allow field operations
• After-spring field operations: Raising the outlet (to within an acceptable limit) to potentially store water from early season rains for use later in the growing season
Controlled drainage is generally considered to reduce annual nitrogen loss from subsurface drainage systems by 20%–80% (averaging 30%–40%) predominantly by reducing annual drainage volumes. However, one of the biggest issues surrounding the effectiveness of controlled drainage is the final fate of the retained water and nitrate. The ultimate nitrogen loss reduction impact at a watershed scale may be reduced if the retained water simply migrates via lateral seepage to another subsurface drain, or if the practice increases surface runoff.
Reduced Drainage Intensity Through Either Wider Lateral Spacing or Shallow Drainage
Modification of the drainage system itself, through either wider lateral spacing or placing the laterals at shallower depths in the soil, can help reduce the volume of drainage water leaving a field, and thus the mass loading of nitrogen. Closer spacing of drainage laterals has become increasingly popular due to the increased cost efficiency of drainage over the past decades and the desire to reduce the possible risk of excess water in the soil profile. Drainage systems that remove excess water beyond what is required for good crop growth and trafficability are desirable from a reduced risk viewpoint, but they do not balance agronomic and environmental goals in a satisfactory way. The practice of shallow drainage involves the placement of drainage pipes closer to the surface than conventionally done (75 to 105 cm rather than 90 to 150 cm), which leaves more soil water remaining in the soil profile. Shallow drainage systems typically need narrower lateral spacing than conventional drainage systems to meet the same crop need requirements, although overall drainage flow from the field is reduced. There is an approximately 20% reduction in nitrogen loads when subsurface drains are placed at a 90-cm depth rather than 120 cm (Cooke, Nehmelman, & Kalita, 2002; Sands, Song, Busman, & Hansen, 2008), and increasing the spacing of subsurface drainage pipes from 9–12 m to 15–18 m at a given depth is also thought to reduce drainage nitrogen loads by at least 20% (Kladivko et al., 1999; Yuan, Bingner, Locke, Theurer, & Stafford, 2011).
Reducing the drainage intensity of a field may modify the field’s water balance, resulting in the potential for increased surface runoff, although surface runoff and associated pollutants would likely still be less than if subsurface drainage were not present. There are also concerns about potentially reduced crop yields when the drainage intensity is reduced. Research documenting the crop yield impacts of this practice in a variety of climates and precipitation scenarios is ongoing. Nevertheless, as subsurface drainage systems continue to be replaced and upgraded in many agricultural areas, designs that incorporate consideration for nutrient loss reduction are a necessary part of the future of agricultural drainage.
Drainage Water Storage/Recycling
Addressing drainage and water management needs is increasingly being done in a more holistic fashion, incorporating meeting seasonal needs, water storage, and recycling (Ayars & Evans, 2015; Vlotman, Wong, & Schultz, 2007). The practice of drainage water recycling involves removing excess water from fields when necessary via subsurface drainage, storing the water in an on-site pond or reservoir, and recycling the water later for crop irrigation. The practice of subirrigation, or using the drainage system to apply irrigation water below the soil surface, is often used as a part of drainage water recycling, but surface irrigation (e.g., sprinkler or center pivot irrigation) can also be used. Drainage water capture and recycling may reduce or even potentially eliminate the movement of drainage water and nutrients downstream in what could essentially be a zero-discharge system. However, the actual water quality improvement benefit of this practice is somewhat unknown, being a relatively new and less studied practice. Research is being driven particularly by practitioners and landowners interested in the crop yield benefits of supplemental irrigation during dry periods.
Despite the obvious potential crop yield benefits, the largest drawbacks of this practice are (a) the necessity of either an existing pond on site or the ability to build a reservoir that may require land to be removed from production; and (b) the capital expense of such a system (irrigation pump, pond building, etc.). When subirrigation is used as a part of drainage water recycling, designing an economically feasible drainage system that can provide adequate drainage during wet periods and supply crop irrigation needs during dry periods is the biggest challenge. Irrigation requirements of a given crop typically exceed the amount of drainage water released per acre. High-value crops tend to allow faster return on investment for the expense of these systems.
A saturated buffer is a modification of the drainage system between the field and the stream or drainage ditch that allows drainage water to percolate as groundwater through the buffer’s soil. In other words, a saturated buffer hydrologically reconnects subsurface drainage to the stream’s base flow as shallow groundwater. A saturated buffer combines the numerous benefits of a riparian buffer with the ability to remove nitrogen from subsurface drainage. Conventional riparian buffers have limited effectiveness for the treatment of nitrogen in subsurface-drained landscapes because subsurface drainage outlets typically convey subsurface waters in closed pipes, straight through the riparian zone. In a saturated buffer, a perforated drain pipe extends laterally between the field edge and the riparian zone and is connected to the drainage main via a control structure. This control structure forces drainage water from the field laterally to the perforated drainpipe so that it can seep through the buffer where the existing vegetation and native soil microbes can uptake and process both water and nitrogen.
This practice was first tested in Iowa in 2011 (Jaynes & Isenhart, 2014), and a recent Agricultural Drainage Management Coalition (ADMC) project documented the performance of 15 saturated buffers across the Midwest, with the majority of the buffers consistently providing promising nitrogen removal benefits. Across all sites and years, nitrate-nitrogen load reductions ranged from 0–85% and averaged 23% ±28% (mean ±standard deviation; n = 23 site-years; ADMC, 2015). The project also developed important siting criteria for this new practice. Buffers suitable to be saturated buffers have soils with at least 1% organic carbon present at a depth of 2.5 ft to ensure that there is enough carbon to fuel denitrification and make hydrology conducive to submerging the carbon-rich soil to create anaerobic conditions required for denitrification. Buffers that contain gravel or sandy layers may not be suitable. Research is ongoing to improve the efficacy of this practice.
A denitrifying bioreactor designed to treat subsurface drainage outflow consists of an excavation filled with woodchips that receives subsurface drainage through a manifold routing flow parallel to the bioreactor’s longitudinal axis (Christianson, Helmers, & Bhandari, 2012; Schipper, Robertson, Gold, Jaynes, & Cameron, 2010). Inside a bioreactor, provision of a solid organic carbon source in addition to the maintenance of anoxic conditions allows the natural process of heterotrophic denitrification to be enhanced, meaning waters leaving these treatment systems have significantly reduced nitrogen loads. Subsurface drainage bioreactors have annual nitrogen load removal efficiencies averaging 25%–43% (IDALS, 2014; IDOA, 2015) and annual nitrogen removal rates ranging from 2.9 to 7.3 g of nitrogen removed per cubic meter of bioreactor per day (Addy et al., 2016). Nitrate removal performance is affected most significantly by water temperature and hydraulic retention time, the latter of which is a function of bioreactor design and drainage flow rate (Addy et al., 2016). A bioreactor’s bypass flow line is an essential design component to prevent reduction of drainage capacity in the field under spring’s high-flow conditions, but this means that a portion of the total annual drainage flow bypasses the bioreactor untreated. Most sites would require an impractically large bioreactor to treat the entire annual drainage volume, and such a bioreactor would be vastly overdesigned for the low flow rates that are predominant much of the year.
Interest in bioreactors has grown exponentially from this practice’s early roots in New Zealand and Canada in the mid-1990s (Blowes, Robertson, Ptacek, & Merkley, 1994; Robertson & Cherry, 1995; Schipper & Vojvodic-Vukovic, 1998). Recent work has focused on advancing bioreactors to remove pollutants other than nitrogen (e.g., dissolved phosphorus, pathogens, viruses, and agricultural chemicals), expanding the technology into additional subsurface-drained areas (e.g., the Chesapeake Bay watershed and across Europe) and using bioreactors for new applications such as aquaculture wastewater treatment (Christianson & Schipper, 2016). Negative side effects, such as the leaching of organics during bioreactor start-up, production of nitrous oxide or methane, and reduction of sulfate and potential methylation of mercury under highly reducing conditions (i.e., under very long retention times), continue to be areas of scientific investigation. While bioreactor nitrous oxide emissions have been shown to be a fairly small percentage of the nitrogen balance across a bioreactor, and start-up strategies to mitigate the impact of high organic outflows have been suggested, much work remains to further optimize bioreactors to maximize nitrogen removal and minimize pollution swapping (Healy et al., 2015).
When constructed wetlands are placed to intercept subsurface drainage, substantial reduction in nitrogen loadings are possible (e.g., 50%). Wetlands remove nitrate from water primarily via the process of denitrification, where naturally accumulated organic matter in the wetland fuels heterotrophic denitrifying bacteria under anoxic (low-oxygen) conditions. Wetland plants also contribute to nutrient removal, and wetlands are extremely effective at storing water within the landscape (i.e., reduction of drainage volume). While wetlands and bioreactors use the same process of denitrification for nitrogen removal, wetlands provide many other benefits, such as wildlife habitat, carbon sequestration, and flood regulation. Recent multistate assessments of wetlands in the U.S. Midwest have assigned no long-term credit for phosphorus removal (IDALS, 2014; IDOA, 2015; MN PCA, 2014), although in the short term, there may be some benefit to it.
The removal of land from agricultural production continues to be a large barrier to the implementation of wetlands in drained landscapes. In addition to the actual cost of wetland design/construction and the opportunity cost of land removed from production, there are significant cultural barriers involving the perception of wetlands in areas where subsurface drainage has been practiced for more than a century. Nevertheless, the state of Iowa has had notable success with constructed wetlands via their Conservation Reserve Enhancement Program (CREP), a wetland program targeted for treatment of large subsurface drainage-sheds (e.g., > 200 ha). These CREP wetlands are designed with a water surface area of 0.5%–2.0% of the drainage area with an additional buffer generally of 3.5% of the drainage area.
Subsurface agricultural drainage is an ancient and vital water management practice that has converted soil where waterlogging and salinity limit agricultural production to some of the most productive soil in the world. Meeting the expected increase in global food demands in the coming years and decades will require the increased agricultural production experienced on irrigated and drained lands. In spite of an array of agronomic benefits, unwanted environmental effects associated with subsurface agricultural drainage are receiving much attention, both in research and practice. A suite of effective practices is used by local and national programs to mitigate negative water quality and quantity effects associated with agricultural drainage, while attempting to preserve, or even enhance, its benefits. Drainage of agricultural lands must meet the future challenges of a changing climate, environmental regulation, and an aging and failing drainage infrastructure. The stakes could not be higher, with both global food demands and environmental expectations on the rise.
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