Soil drainage poses particular challenges for mineral soils in Ireland. The great variety of soils and drainage problems means that every drainage challenge is unique. Careful investigation of the site; professional diagnosis; prudent assessment of the costs/benefits; expert implementation of a planned project and due consideration of environmental factors are all essential.
The objective of any form of land drainage is to lower the water table providing suitable conditions for grass growth and utilization. A controlled water table promotes deeper rooting which improves productivity and improves load-bearing capacity of the soil. How best to achieve this will vary with soil type. There is a need therefore for a better understanding of the underlying causes of drainage problems and of the design and implementation of appropriate drainage systems to resolve these problems
Land drainage design varies with soil type and site conditions
When planning any drainage programme, the potential of the land to be drained needs to be first assessed to determine if the costs incurred will result in an economic return through additional yield and/or utilisation. Some thought is needed in deciding the most appropriate part of the farm to drain. From a management point of view it is better to drain that land which is nearer to the farmyard and work outwards, however it may be more beneficial to target areas with high potential for improvement. This ensures a better return on the investment. If you intend to undertake land drainage works that (a) exceed 15 hectares, (b) the works are to be carried out within (or may effect) an NHA, a proposed NHA, an SAC, an SPA or a nature reserve or (c) the proposed works may have a significant effect on the environment, screening by DAFM is required.
The drainage of mineral soils affects a reduction in greenhouse gas emissions both directly through lower nitrous oxide emissions, which tend to be higher in poorly drained soils, and indirectly through the benefits of extended grazing. However the drainage of organic & peat dominated soils will result in substantial CO2 emissions to the atmosphere, that would dwarf any non CO2 benefit.
In this context, further drainage of peat soils cannot be justified and a significant programme of water table management is required on those organic soils that were previously drained. Future iterations of the CAP could see measures targeted to rewetting of organic soils. This will involve proposing suitable locations and liaising with landowners to establish a workable programme of rewetting, and thereafter managing existing drainage features (open and field drains) on selected sites to manipulate the depth of the water table and reduce CO2 emissions to the atmosphere. This water table management regime will also help prevent the release of sediment, carbon and nutrients, and have benefits for biodiversity.
Performance analysis of drainage systems installed on Heavy Soils Programme farms is highlighting how drainage system type, soil type and seasonal variations in soil moisture affect drainage system performance. All systems reduce the overall period of waterlogging and improve the conditions for both the production and utilization of the grasslands they drain.
However, the implementation of land drainage works is known to affect the dynamics of water movement from drained sites and as such may act as a conduit transporting a percentage of surplus nutrients not being used by the crop system to waters. Recent research looks at methods which reduce potential for losses, leaving pipe drains and ditches. Ongoing research is focusing on intercepting the pathway of loss along ditch and pipe networks (see Figure 1) thereby breaking connectivity with waters and minimising such losses further in a more controlled manner (see www.smarterbufferz.ie ). Appropriately planning mitigation practices at the outset of drainage works, could help reduce the need for retro-fitting mitigation practices further down the line.
Examinations of potential environmental impact of drainage systems showed that both phosphorus and nitrogen attenuation capacity are dependent on surface and subsurface soil chemistry and drainage design specification. The potential for nutrient and sediment loss is related to soil type, chemistry and the level of interaction that drained water has with attenuating layers or elements of the soil body. Shallow drainage systems, for example are more likely to promote high intensity flows which have little interaction with the soil body relative to groundwater systems which promote water movement through the soil. Furthermore, soils with high levels of organic matter are known to have poor nutrient retention capacity and as such are vulnerable to nutrient loss. Land drainage system design needs to account for such variability and implement works that identify and negate against impacts on water quality.
Figure 1: Targeted buffer zone with actions to intercept subsurface artificial soil drains