Controlled drainage systems to reduce contaminant losses and optimize productivity from New Zealand pastoral systems

Abstract

Drainage systems are essential for managing soil water levels, thereby ensuring optimal plant productivity while protecting soil quality. Although beneficial, drainage systems are also known to be a significant loss route for dissolved nutrients. A potential way of reducing nutrient loss through drainage systems is to use weirs to strategically control drainage of excess water from the soil profile. This review evaluates the scientific literature to ascertain whether controlled drainage could be a useful crop productivity and nutrient loss mitigation tool for New Zealand pastoral farming systems. While a range of risks and potential disadvantages have been identified, evidence from studies of cropped systems with controlled drainage in Europe, Canada and the US suggests that suitably managed controlled drainage offers significant benefits for water quality, agricultural productivity and nutrient- and water-use efficiency. The practical efficacy of controlled drainage requires field evaluation under New Zealand farming conditions.

Keywords:

• agriculture
• controlled drainage
• diffuse pollution
• mitigation
• nitrate
• pasture
• phosphorus
• water quality

Introduction

The issue

Soil moisture levels (i.e. volumetric content of water) are a major factor affecting the growth and utilization of pasture and crops in agricultural systems. Adequate supplies of water are required to maintain plant growth, but excess water retards growth by restricting soil oxygen availability, causing root hypoxia and build-up of toxic anaerobic metabolites (Armstrong 1982). Soils that are susceptible to waterlogging therefore require some form of artificial drainage for sustainable and profitable production of pasture and crops (Bowler 1980; Smedema et al. 2004). The drainage intensity required will not be the same in all years or at all times of the year and it is likely that achieving sufficient drainage to cope with the wettest periods of the year will result in significant periods when drainage is excessive. While artificial drainage of waterlogged soils offers the potential to both improve pasture productivity and reduce soil damage by vehicles and livestock in wetter seasons, it may result in reduced water availability during the main growing season, resulting in reduced production compared with non-drained pastures (Armstrong & Garwood 1991). At such times of the year it would be valuable from a crop productivity perspective to retain a greater proportion of water within the soil profile, while avoiding the risk of excessive saturation of the upper root zone. Retention of water in the soil profile would be expected to increase pasture and crop productivity between rainfall events when there was a moisture deficit. In irrigated systems, retention may allow for reduced flow-through losses, resulting in improved water use efficiency.

In this review, we focus on subsurface drainage systems (often known as ‘tile-drains’ after the clay tiles that were historically used) which are commonly used in New Zealand to increase the productivity of soils with high water tables and impeded infiltration. The major effect of subsurface artificial drainage is to alter the route of water movement from surface run-off to subsurface drain-flow. This alteration has major consequences for the transport of non-point source pollutants such as sediments, nutrients and pesticides, as these will be routed into surface receiving waters through subsurface drainage systems rather than by surface pathways. Passage through the soil may act to ‘filter out’ some contaminants during transport to subsurface drains (e.g. particulate associated material); however, practices such as mole-ploughing can reduce this filtering action by fracturing the soil profile and creating macropores which provide a rapid conduit for contaminants, including eroded soil particles. Subsurface drainage also tends to markedly increase leaching losses of mobile dissolved forms of contaminants such as nitrate, losses of which increase as agricultural intensity increases (e.g. increased stocking density).

Studies in New Zealand are in general agreement with the results from overseas studies. In an early study, under relatively low-intensity dairy grazing, Sharpley & Syers (1979) and Turner et al. (1979) reported subsurface drainage annual losses of 12–16 kg N ha⁻¹ per year and 0.9–1.1 kg P ha⁻¹ in grazed, non-irrigated pastures. In poorly drained Pallic soils in Southland, fertilized at rates of 0–400 kg N ha⁻¹, Monaghan et al. (2005) measured dissolved inorganic N losses of 26–66 kg ha⁻¹ and total P losses of 0.14–0.69 kg ha⁻¹. In studies of subsurface drainage losses in three New Zealand dairying areas (Northland, Waikato, Southland), prior to wetland treatment, Tanner & Sukias (2011) reported localized annual subsurface drainage losses of 14–66 kg total nitrogen (TN) ha⁻¹ and 0.12–1.38 kg total phosphorus (TP) ha⁻¹ for rain-fed dairy pastures and 72–109 kg TN ha⁻¹ and 1.7–2.9 kg TP ha⁻¹ for irrigated dairy pastures.

Management of soil water content by controlling drainage depth, discharge rate and timing is one of the practical options available to farmers to reduce the release of nutrient-rich drainage water into freshwater systems (e.g. surface drains, streams and rivers). Implementation of controlled drainage involves installing a water table control structure, either within subsurface drains or in surface drains to which they discharge. The primary aim of controlled drainage is generally to restrict drainage to only the excess water that will damage crops or limit grazing or farm equipment access to paddocks.

There are a range of ways to ‘control’ tile drainage systems. At one end of the spectrum, the simplest approach is to manually place weirs in the surface drains at the end of the wet period (late spring) to raise the water table, and then remove or lower them again before the onset of the main wet season (late autumn/early winter). With this system, there is a risk of increased surface runoff from late summer rainfall or thunderstorms. At the other extreme, automated management of weirs could enable dynamic adjustment in response to soil water content and, potentially, forecast rainfall events. Another option that has been suggested (McDowell et al. 2012) is to use simple outlet controls to merely slow drainage rates (e.g. peak run-off control, Marttila & Kløve 2009). This has the potential to allow physical settling of suspended sediments and associated contaminants, but is less likely to enable sufficient contact time to stimulate microbial processes such as denitrification.

An extension of the controlled drainage concept is sub-irrigation, where drainage water is stored in a reservoir and later pumped back into the soil profile via the subsurface drainage system to ‘sub-irrigate’ the root zone (Madramootoo et al. 1995; Borin et al. 2001; Ng et al. 2002). Drought stress, resulting in reduced crop growth, can potentially be mitigated using this combination.

In this contribution, we provide a review of controlled drainage systems and their performance, in particular their potential to reduce contaminant losses from pasture systems, based on international and New Zealand literature. We have included a consideration of criteria for optimal water table depth and drainage management of pastures and key crops under New Zealand conditions. Benefits and disadvantages of controlled drainage systems are discussed, and recommendations are given on environmental settings in which controlled drainage systems might be used to greatest effect.

Controlled drainage as a nutrient loss mitigation option

Controlled drainage has been proposed, and designated in the US, as a beneficial management practice (BMP), primarily aimed at decreasing loads and concentrations of nutrients in drainage ditches that feed into river networks by decreasing total drainage outflows (e.g. Evans et al. 1996; Borin et al. 2001) and promoting higher nutrient-use efficiency (Wesstrom & Messing 2007), increased N retention and NO₃-N attenuation (Borin et al. 2001). Other benefits accrue from using controlled drainage, e.g. improved water use efficiency (especially along with sub-irrigation) and increased crop productivity.

How does controlled drainage work?

Under controlled drainage, the drainage discharge can be restricted or prevented from leaving the system. This is done using a weir or other water flow control structure to raise the water level in the drainage outlet and hold water in the drain, which then reduces the hydraulic gradient to the drain, subsurface drainage rates and annual drainage volumes. Controlled drainage is a flexible management system that can be set to accommodate specific crops, topographic and soil characteristics, and the water flow control structures can be adjusted to allow drainage once a critically high water table is achieved in the system to reduce the potential of anoxia stress to crops.

Controlled drainage is an attractive option for producers because it allows soil to be drained during the wetter months, while retaining water within the soil during the growing season to prevent plant moisture stress. Raising the water table during the growing season has the potential to retain nitrate (NO₃-N) as well as other nutrients, thus allowing them to be more available to the crop root zone during the growth period. It may also promote the potential removal of NO₃-N via in situ processes such as denitrification (Smith & Kellman 2011).

There are questions that need to be answered, for example, at what depth should the water table be controlled, and how will water table management affect how we use the land? These are challenging questions as water table depth will differ for different soils, situations and climate types. Trafficability is an important consideration and is essential for efficient production. Having a higher water table will perhaps mean wetter fields, resulting in soil damage if farmers try to till the soil when it is too wet. Equally, wetter pastures might be less suitable for grazing animals, and may leave the soil even more vulnerable to pugging and compaction. This issue is largely undocumented and is a current gap in research.

Applications of controlled drainage in New Zealand

To date there have been few studies to test controlled drainage or water table management as a nutrient-loss mitigation tool for artificial drainage systems in New Zealand. Two known studies have been undertaken, both of which have focused on NO₃-N.

Singleton et al. (2001) carried out a lysimeter study in which two levels of controlled drainage were compared with a conventionally drained system in soils irrigated with farm dairy effluent over a 2-year period. In the first year, effluent was applied at a rate of 511 kg N ha⁻¹, while in the second year, the application was increased to 1518 kg N ha⁻¹. Effluent irrigation totals for the 2 years were 586 and 697 mm, respectively. The lysimeters were planted with pasture, and managed under a cut-and-carry system. Reductions in NO₃-N losses of 57% and 86% were observed for the controlled drainage plots when compared with the conventionally drained land over a 3-year period. Singleton et al. (2001) also found that large amounts of organic nitrogen were being leached by bypass flow from the effluent irrigated systems. It should be noted that the effluent application rates were far in excess of current guidelines, and thus the relevance of this study for pasture systems should be carefully considered.

Fonterra Co-operative Group Ltd (John Russell, pers. comm.) tested controlled drainage to reduce NO₃-N drainage losses from land at Hautapu (Waikato), to which dairy factory effluent was applied. The water table was raised by placing weirs in the surface collector drains in the paddocks. There was no significant difference in the NO₃-N in the drainage water between the controlled and conventionally drained treatments; however, the soils on both treatments had high organic matter contents and naturally high soil water contents, suggesting that denitrification would have been taking place in both situations, and that there was little potential for further enhancing NO₃-N removal through water table management. Fonterra concluded that raising the water table through the year made farm management difficult and was less conducive for grazing dairy cows. Again, given the high application rates of effluent, these results probably have limited relevance for application of controlled drainage in rain-fed pastures.

Where are controlled drainage systems most effective?

Controlled drainage appears to be most beneficial on land that:

•	is mainly flat with a very gentle gradient, and
•	has an impermeable clay layer about 1–3 m below the surface.

To establish the suitability of land for controlled drainage in New Zealand, a feasibility study would be beneficial, following what has been done in Sweden. Joel et al. (2009) evaluated the suitability of land in coastal areas of southern Sweden for controlled drainage using digital data of land use, slope gradient and soil type to rate the suitability of land. This was followed by an examination of drainage information derived from local authorities, questionnaires to land owners and physical examination of soils. They found that the ideal conditions for controlled drainage were soils with relatively high water permeability and either a shallow natural groundwater table or a poorly permeable soil layer at a depth of 1–3 m (necessary to retain water in the soil profile). Topography was also considered important; they found that the slope gradient should not exceed 1%–2%, so land with slope gradients >2% was excluded from their study. Ayars et al. (2006) actually concluded that the gradient should not exceed 1%. A similar approach could be adopted to assess the suitability of New Zealand's agricultural land for controlled drainage.

Impact on nutrient and contaminant losses

Nitrogen

Several studies have demonstrated that controlled drainage systems can lead to reduced nitrate losses from agricultural land (Table 1).

Table 1  Summary of reductions in NO₃-N concentrations and loads, and the main mechanisms for loss reductions from a range of controlled drainage studies.

Study	Location	% reduction NO3-N concentrations	% reduction NO3-N loads	Soil type	Loss mechanism
Gilliam et al. (1979)	N. Carolina, US		50	Silty loam	Mainly due to reduced drain flow
Lalonde et al. (1996)	Canada	62.3–95.7	40.9–95.0	Silt loam	Mainly due to reduced drain flow, possibly some denitrification
Borin et al. (2001)	Italy		46–63	Sandy silt loam	Possibly enhanced evapotranspiration, denitrification
Ng et al. (2002)	Canada	41	36	Sandy loam
Elmi et al. (2004)	Canada	55–73	42.3–79.2	Fine sandy loam	Denitrification
Wesstrom & Messing (2007)	Sweden		76.5–85	Loamy sand	Reduced drain flow, possibly denitrification
Bonaiti & Borin (2010)	Italy		70 (CD), 72 (CD+SI)	Silty sand loam	Reduction of drainage (major), denitrification (minor)
Woli et al. (2010)	Illinois, US		73	Silty clay loam	Possibly denitrification
Bohlen & Villapando (2011)	Florida, US	13.6 (TN)	44.3 (TN)	Loamy sand (90% sand)	Possibly uptake by aquatic organisms or denitrification
Smith & Kellman (2011)	Canada	−14.4–8.7	85.6	Sandy loam	No evidence of denitrification
CD = controlled drainage.
CD +SI = controlled drainage and sub-irrigation.
TN=total nitrogen

The reductions in NO₃-N losses may, in some instances, be due to reduced NO₃-N concentrations. For example, Ng et al. (2002) reported a 41% reduction in the flow weighted mean NO₃-N concentration in drainage water for a controlled drainage system when compared with a free draining system, while Lalonde et al. (1996) reported reductions in NO₃-N concentrations ranging from 68.9%–75.9% at two different controlled drainage depths compared with free drainage. Bohlen & Villapando (2011) reported lower TN concentrations in pastures with restricted flow (controlled drainage) compared with pastures with free drainage, possibly because the longer retention of water in pastures allowed more time for N forms to be taken up by plants or processed by aquatic organisms in the surface drains. They observed no differences in the NO₃-N or ammoniacal nitrogen (NH₄-N) concentrations between the two treatments. Reduced concentrations may not always be due to controlled drainage as Lalonde et al. (1996) reported reduced similar NO₃-N concentrations in both the controlled and free draining systems due to crop uptake through the growing season.

In many of the field studies reported, no reductions in NO₃-N concentrations were observed. Skaggs & Youssef (2008), in their review of results from a number of studies (e.g. Wesstrom & Messing 2007), concluded, along with Lalonde et al. (1996) and Bonaiti & Borin (2010), that reductions in NO₃-N loss were mainly due to reductions in drainage flows, as the effectiveness of controlled drainage in decreasing N losses in drainage water was nearly the same, on a percentage basis, as its effect in decreasing drain flows.

The question about what happens to the N (mostly in the form of NO₃-N) in the drainage water in controlled drainage systems remains unanswered. When controls are placed on traditional drainage systems, the higher water table in the soil results in anaerobic conditions that will promote microbial denitrification when soil carbon is high, which will reduce the likelihood of NO₃-N transport to surface waters (e.g. Gilliam et al. 1979). Elmi et al. (2000, 2002) found that drainage water from free draining plots had higher NO₃-N concentrations than that from controlled drained plots, with the difference assumed to be due to denitrification in the controlled drainage plots. Further, Elmi et al. (2002) measured NO₃-N concentrations in the soil profile and found that the NO₃-N concentrations in soil water in controlled drainage plots were lower than those in free draining plots, and that concentrations declined rapidly with depth in the soil profile, again suggesting denitrification promoted by anaerobic conditions, resulting in lower NO₃-N concentrations. Woli et al. (2010) compared controlled and conventional drainage systems and while they speculated that denitrification may have occurred in shallow groundwater flow, they had no supporting data. Smith & Kellman (2011), however, in their evaluation of the usefulness of different drainage systems in controlling nutrient losses, found that denitrification did not play a significant role in removing NO₃-N. If anaerobic conditions that promote denitrification are created, controlled drainage should be an effective practice for pollution reduction.

Decreases in NO₃-N concentrations may be accompanied by increases in N₂O emissions (Dalzell et al. 2007). Emissions of N₂O from soil have important implications for agricultural production and environmental quality, and can contribute to global warming and the destruction of the ozone layer (IPCC 2006).

Emissions of N₂O from controlled drainage systems are thought to be minimal. Kliewer & Gilliam (1995) estimated that N₂O accounted for only 2% of the measured denitrification potential for each water table treatment, and concluded that the soil water level had no impact on the percentage of total denitrification evolving as N₂O to the atmosphere. Results from trials by Elmi et al. (2005) suggested that N₂O was only a minor proportion of the total gaseous N emissions, and that N₂ formed a significant portion of the denitrification gaseous end products. The wet conditions under controlled drainage increase the residence time of N₂O in the soil by restricting diffusion, which consequently promotes the reduction of N₂O to N₂ (Elmi et al. 2005). It is thought that the percentage of N₂O in denitrification gaseous products decreases with increasing soil water content until N₂ becomes the major gas evolved (e.g. Maag & Vinther 1996).

Phosphorus

The role of tile drains in phosphorus (P) transport has been discussed widely (Skaggs et al. 1994; Sims et al. 1998); some regard it as unimportant (Skaggs et al. 1994), whereas others have measured significant P losses via tile drainage, and have identified drainage as a dominant P loss pathway to surface waters (Dils & Heathwaite 1999; Gelbrecht et al. 2005). Unlike surface P transport, practices to control P transport in tile drainage are not well characterized (Coelho et al. 2012), meaning that controlling subsurface drainage losses of P remains a significant challenge.

P removal processes are predominantly biogeochemical. Phosphorus sorption to sediments is one of the main P removal mechanisms (Kroger et al. 2011). Phosphorus can sorb to, or be desorbed from, soils and sediments, depending on the redox conditions and the presence of adequate iron and aluminium oxides (Dunne et al. 2007a,b; Olli et al. 2009; Zhuan-xi et al. 2009). As mentioned earlier, when controls are placed on traditional drainage systems, reducing conditions may be created, promoting denitrification of NO₃-N to gaseous N forms. Reducing conditions may however also increase the solubilization of iron phosphates in soils and sediments (e.g. Patrick & Khalid 1974). Depending on environmental conditions and the degree of soil P saturation (which will control sorption/desorption processes), there is the potential for adsorbed P to be released in bioavailable forms that are subsequently exported in drainage flows.

The over-application of P fertilizers results in supra-optimal soil P saturation levels, which can lead to a higher risk of P loss to drainage systems (Sims et al. 1998; Heathwaite & Dils 2000). Studies have shown that there can be significant losses of the dissolved reactive form total dissolved phosphorus (TDP) through tile drainage in sub-irrigated/controlled drainage systems compared with free draining systems (Guo et al. 2011; Kroger et al. 2012), caused by a reduction of Fe³⁺ to Fe²⁺ in the anoxic, waterlogged conditions, leading to increased P solubility. A study by Valero et al. (2007) showed that controlled drainage resulted in increased P loads in tile drainage when compared with free draining plots, most likely caused by an increase in P solubility due to the shallow water table. Of the TDP concentration, dissolved reactive phosphorus (DRP) represented 80% for the controlled drainage system compared with 67% for the free draining system. Bohlen & Villapando (2011) reported that while concentrations of TP were lower in pastures with ‘reduced flow’, available soil P concentrations were higher, most likely attributable to anaerobic, flooded conditions that stimulate the release of P.

Although experiences are varied, there is evidence that controlled drainage systems can help reduce P losses from tile drains. For example, the effects of controlled drainage on P losses from soil were examined in a 4-year field study in Southern Sweden by Wesstrom & Messing (2007). They found that the relative reduction in P loads in drain outflows were similar to the reduction in drainage outflow rates, suggesting that controlled drainage reduced particulate P rather than dissolved P. Similar to Wesstrom & Messing (2007), Tan & Zhang (2011) found that reductions in TP loss were mainly due to particulate phosphorus (PP) retention in the soil profile due to the reduced drainage. This may be the most important main mechanism for decreasing P loads in controlled drainage systems.

By examining P fractionation in drainage water, the complexity of P transformations and removal processes in controlled drainage systems becomes clearer. Tan & Zhang (2011) in a 5-year comparison of controlled and free draining systems, observed that controlled drainage resulted in increased concentrations of most forms of P in surface runoff, due to the increased soil moisture content which enhanced P release through desorption. However, due to reduced drainage flows, the controlled drainage system still provided overall reductions in losses of most forms of P in tile drainage compared with the free draining system. They observed that the controlled drainage system produced greater TP and TDP losses in surface runoff, but lower TP and TDP losses in tile drainage than the free draining system.

The overall picture for P is therefore somewhat mixed and there is little consistency between studies. Reductions in P losses from subsurface controlled drainage systems will be mainly due to reductions in loads of sediment, as particles are retained in the soil profile. Due to the anoxic conditions, P fractionation may change in the drainage system with P release from sediments, resulting in a higher proportion of dissolved P in drainage outflow.

Other contaminants

Herbicide losses are highly dependent on, among other things, the incidence of rain after herbicide application and rainfall intensity (Wauchope 1978; Zhang et al. 1997; Gaynor et al. 2000), and are greatest when heavy rain is received soon after their application (Wauchope 1978; Gaynor et al. 1995). Controlled drainage systems can help reduce herbicide losses by holding surface runoff in the soil profile, which increases herbicide adsorption to soil, and subsequent degradation by soil micro-organisms or chemical reactions (Gaynor et al. 2002). In a study of drainage from paddy fields in Japan, Watanabe et al. (2007) found that herbicide losses in rain storms were lower from fields with controlled drainage compared with free draining fields, most likely because the water holding period was increased, with greatest loss reductions observed in the period immediately following herbicide applications. However, Gaynor et al. (2001, 2002) reported no overall reduction in total herbicide loss in a controlled drainage system. They observed increased herbicide loss due to increased surface runoff, and decreased herbicide loss in tile drains, meaning no significant change in overall herbicide loss. Further, the authors observed no difference (decrease) in the half-life of pesticides in soils under controlled drainage.

Crop productivity

Controlled drainage systems (and combined controlled drainage and sub-irrigation systems) have been widely used with positive results for crop yields in corn and soybean cropping systems in the US and Canada, but little used in pasture systems. For example, Ng et al. (2002) observed an increase of 64% in corn yield for a controlled drained system compared with a free draining system. Borin et al. (2001) observed that plant biomass and N uptake were 20% higher in crops under controlled rather than conventional drainage due to increased plant absorption. They also observed reduced NO₃-N losses and increased evapotranspiration. As a consequence of increased N uptake by plants, it could be expected that there would be a lower amount of NO₃-N in the soil at the end of the growing season in the controlled drainage system, which would then result in lower percolation losses during winter. There are very few studies that document pasture response to controlled drainage and we consider this is an area that deserves attention.

Climatic effects

Climatic effects can be important and are linked to crop productivity. In dry years, there are obvious benefits to having controlled drainage. For example, in DSSAT (Decision Support System for Agrotechnology Transfer) simulations, Liu et al. (2011) found that crop production in controlled drained plots was higher than in tile-drained plots in dry years because more water was held in the soil profile. Ramoska et al. (2011), in field studies in Lithuania, observed that impacts were minimal in a moderate growing season, but that an additional 5.6% yield was achieved in a dry growing season. Mejia et al. (2000) reported higher crop (corn and soy bean) yields with controlled drainage over a 2-year field experiment in eastern Ontario, which they attributed to greater crop water uptake in the controlled drainage plots with the higher water tables, than in conventionally drained plots. Their experiments took place in wetter than average years and yield differences between controlled and free draining plots could be even greater during drier years. Where there is a critical period of water deficit, or where peak growing months are drier than normal, crops would benefit from an elevated water table and increased water availability, resulting in more favourable growing conditions and enhanced nutrient uptake.

To ensure the benefits of additional water and nutrient storage in the soil profile are fully exploited, species need to be carefully selected. Deeper rooting species will benefit most from the higher water tables in spring and summer. In winter however, if a high water table is maintained, deeper rooting species may be adversely affected, and shallow rooting species would be preferable.

Soil moisture and water use efficiency

Soil moisture levels under controlled drainage systems are expected to be higher than in free draining systems, as indeed has been reported. Mejia et al. (2000) found that soil moisture levels were higher in controlled drained plots when compared with free draining plots in July (northern hemisphere summer) when soil moisture was crucial. Fisher et al. (1999) and Ng et al. (2002) both observed higher soil moisture in plots with sub-irrigation and controlled drainage, and Fisher et al. (1999) usefully reported that the sub-irrigation and controlled drainage treatments did not impede the use of field equipment. Woli et al. (2010), however, found that soil moisture levels were lower in a controlled drainage system than in a free draining system, the reverse of what they might have expected.

Implementation of controlled drainage will lead to reduced drainage volumes from agricultural land. Ng et al. (2002) reported reductions in drainage volumes of 11%, while, in their review of controlled drainage experiments, Skaggs & Youssef (2008) reported drainage volume reductions ranging from 17%–85%. Controlled drainage can mean more efficient water use, especially when used in conjunction with sub-irrigation, where drainage water is stored in a reservoir and later pumped back into the soil profile via the subsurface drainage system. For example, Bonaiti & Borin (2010) reported greater water use efficiency on controlled drained plots with sub-irrigation when compared with controlled drained plots without sub-irrigation over a period of 6 years.

Fertilizer use efficiency

Controlled drainage systems can promote increased fertilizer use efficiency. Wesstrom & Messing (2007) reported increased crop uptake of N of 3–14 kg ha⁻¹ for a controlled drainage system when compared with free drainage. Even greater N use efficiency can be attained using a combination of controlled drainage and sub-irrigation, as reported by Fisher et al. (1999).

Potential risks from controlled drainage

While there appear to be obvious benefits through reduced nutrient losses from using controlled drainage, there are also some potential disadvantages.

Losses to deep seepage

Controlled drainage has the effect of decreasing drainage outflows compared with conventional drainage systems, and changing drainage routes, which raises the question as to what happens to the water that is not removed by drainage (Bohlen & Villapando 2011). Modelling and experimental studies have shown that increases in deep and lateral seepage can account for a significant proportion of the reduction observed in the drainage. Indeed, Zhuan-xi et al. (2009) showed that controlled drainage decreased subsurface drainage volumes by increasing evapotranspiration, surface runoff and seepage (deep and lateral). Long-term simulations of controlled drainage experiments have shown that the decrease in N loss to tile drains was mainly associated with increased N loss to seepage and lateral flow, and also crop uptake, depending on soil and climate conditions (Fang et al. 2012).

Losses to deep seepage could result in contaminants still reaching surface waters, but by a different route or with delays. It would be useful to identify and estimate contaminant losses through any additional flow pathways activated in controlled drainage systems, which to date, few studies have attempted to do.

Increased surface runoff

Controlled drainage can lead to increased risk of surface runoff losses, especially during high rainfall, by decreasing the available subsurface storage volume. For example, using hydrological simulations from DRAINMOD to test the effectiveness of controlled drainage on agricultural land in Iowa, Singh et al. (2007) found that surface runoff increased when controlled drainage was used.

Increases in surface runoff are of concern since they may lead to increased soil erosion and losses of pollutants such as P, NH₄-N, pesticides and sediments, as well as increased hydrologic impairment to receiving streams. Increased surface runoff may be the result of inappropriate water table control, and may be able to be avoided by dynamic adjustment in response to near-surface soil moisture and measured or forecast rainfall. Bohlen & Villapando (2011) reported significantly lower total net runoff in pastures with control structures than in pastures with unobstructed flow, indicating that once stabilized, the control structures were effective in decreasing runoff from the pastures.

Damage to crops

Subsurface drainage systems are designed for a certain drainage intensity to provide a healthy crop growth environment. Changing the drainage regime impacts the rate of removing excess water from the soil profile and DRAINMOD simulations showed that controlled drainage could have a negative effect on crop productivity, and could lead to an increase in excess water stress and decreased crop yields (Singh et al. 2007), rather than the expected benefits to crop production from the improved water availability. Similarly Cicek et al. (2010) also considered that reduced field drainage and associated lower soil temperatures might be detrimental to crop establishment and growth early in the season.

Damage to mole drains

In some parts of New Zealand, tile drain systems (typically 70–80 cm deep) are fed by mole drains (typically 45–50 cm deep). Mole drains are simply subsurface tunnels (diameter 5–10 cm) through the soil, carved out by a mole plough, leaving a tunnel below the soil. The main function of mole drains is to promote shallow lateral flow through clay soils which have a minimum clay content of ~30% (Smedema et al. 2004). The lifetime of mole drains depends, to some extent, on the inherent stability of soil to repeated wetting, and it is generally considered important to prevent water stagnating in the mole channels since this will weaken their walls and lead to premature collapse (Smedema et al. 2004). It has been suggested that, with a higher water table, mole drains would be under water and so at greater risk of collapse, potentially contributing sediment to the tile drainage system and impeding water movement from the soil surface to the subsurface drainage network (Ross Monaghan, AgResearch, pers. comm.). The water table in mole drained fields should therefore be managed so that risk of damage to mole drains, and subsequent sedimentation in tile drains, is minimal.

Management considerations

The successful management of controlled drainage systems rests on the balance between decreasing water quality impacts while maintaining reasonable production efficiency. For effective management, various factors need to be considered, of which the most important is likely to be the depth at which the water table is controlled.

Depth of water table

Controlled drainage is essentially a tool to manage the groundwater table. In the scientific literature, various depths have been used as shown in Table 2. Clearly, different cropping systems will have different optimal water table depths to ensure productivity is maintained, and even enhanced. Finding this depth will require some testing before the optimal balance is achieved. In a pasture system, the water table will be able to be maintained nearer to the soil surface than for deeper rooting crops without any harm to grass production (e.g. 0.4–0.5 m; Kevin Earle, soil and land drainage consultant, pers. comm.). It is important, however, to ensure that mole drain systems are not damaged. It is also important to maintain access to pasture, but at the same time, minimize damage to soil, meaning that it may be necessary to lower the water table when access to the paddock is needed.

Table 2  Examples of water table depths for optimal crop growth.

		Water table depth (m)
Study	Land use	Highest yield	Lowest yield
Mejia et al. (2000)	Corn, soybean	0.5–0.75
Madramootoo et al. (1995); Kalita & Kanwar (1993)	Corn, soybean	0.6–0.9	0.2–0.3
Zhou et al. (2000)		0.7–1.0
Evans et al. (1996)		0.3–0.9

There are various options available for water level control. For example, it could be:

•	changed at different times of the year to maximize water availability to limit drought stress and to reduce risks of surface runoff, or
•	dynamically controlled to respond to imminent or actual rainfall and to hold soil water for only sufficient time to allow denitrification, then slowly released to increase soil water storage volume.

In temperate zones, controlled drainage permits two design groundwater depths: a deep depth to provide aeration and trafficability in periods with excess of water, and a shallower depth to facilitate sub-irrigation in dry periods (van der Molen et al. 2007). Changing the water table depth, rather than maintaining it at a fixed depth, throughout the year, can give better N loss reductions. For example, Borin et al. (2001) found that using controlled drainage and sub-irrigation with a variable water table depth reduced N losses by 63%, whereas control of the water table at a constant depth of 0.6 m, which is more difficult to achieve in field conditions, reduced N losses by 46%.

The primary benefit of using controlled drainage on agricultural land with connectivity to rivers, streams and drains is a reduction of total outflow and nutrient loading. These goals can be accomplished by setting the weir at a specified level and making minor adjustments to accommodate production requirements. Without attentive management, the potential production and water quality benefits of drainage control will not be realized.

Soil types and land use

Overseas published results showing the effectiveness of controlled drainage are generally for arable cropping systems under different climatic conditions and on different soil types than are found in New Zealand, which may mean that comparison is not realistic. Unfortunately there are few documented cases of controlled drainage in pasture systems and associated benefits for productivity and environmental protection except for the study by Bohlen & Villapando (2011). However, findings from cropped systems suggest that controlled drainage systems could be beneficial in decreasing NO₃-N losses from pastures and also, with careful management, dissolved P losses.

Conclusions

Controlled drainage systems have been proposed as a beneficial management practice, primarily aimed at decreasing loads and concentrations of nutrients in drainage ditches before being transported further to receiving waters. Controlled drainage is used in many places around the world, including Italy, Sweden, Canada and the US, where it is a designated beneficial management practice. Studies have focused on cropped systems and there are few reported applications of controlled drainage in grazed pastures. Examples of controlled drainage use in New Zealand were limited to pastoral systems that were heavily irrigated with effluent.

Potential benefits of controlled drainage systems include reduced drainage outflow volumes and velocity, reduced nutrient losses and increased water and fertilizer use efficiencies.

Disadvantages include the potential for increased surface runoff, increased P release, reduced trafficability in pastures, NO₃-N losses to deep seepage and damage to crops (reduced growth through waterlogging stress).

Controlled drainage systems require careful management to be successful, mainly through optimizing the water table depth, which ideally should be managed for both optimal production and maximum environmental protection. Ideal conditions for controlled drainage systems include flat land (preferably with a gradient of less than 1%), with an impermeable soil layer at 1–3 m below the soil surface.

Based on experiences elsewhere, we believe there could be environmental benefits from using controlled drainage systems in pasture systems in New Zealand without compromising pasture productivity. Studies in this area could be of key strategic benefit to the New Zealand pastoral sector.

Acknowledgements

We thank Mike Scarsbrook of Dairy NZ for providing funding to support the preparation of a review report which provided the genesis of this review paper. We thank Kevin Earle, soil and land drainage consultant, Te Puke for assisting our understanding of New Zealand agricultural drainage practices, and James Sukias (NIWA) and Drs Ross Monaghan and Richard McDowell (AgResearch) for their constructive comments on the original report.