Put simply, the purpose of drainage works is to remove excess water from the land to increase the yield of crops.
Effective drainage of land can have a number of benefits:
- Cultivation and sowing of crops and pastures can be carried out earlier in the spring and sooner after rain. Drainage if well designed, tends to remove excess water back to field capacity where optimal water is available for plant growth.
- The poaching (or pugging) of pastures is reduced or eliminated.
- Less heat is required to cause a given rise of soil temperature for plant germination.
- Deeper root penetration of plants gives greater drought resistance.
- Beneficial aerobic soil bacteria thrive and increase in numbers in well-drained soil.
- Better soil structure and improved soil tilth results from deeper plant penetration and increased capillary moisture-holding capacity.
However, if not properly planned, drainage can also have some harmful effects:
- Land can become too dry in summer if overdrained with no watertable controls.
- Very deep drains can lower the summer water table and adversely affect crop and pasture growth.
- Care must be taken not to overdrain peat swamps otherwise the top organic layers easily break down and are washed or blown away.
- Adjustable weirs are sometimes used to control water tables in peat swamps throughout the year to avoid too much shrinkage and also fire risk.
Surface drains are a common technique to remove excess rainfall water on relatively flat soils with slow subsoil drainage due to a tight clay subsoil.
Surface drains include open drains or ditches which often drain both surface and subsoil water. These drains are relatively easy to construct with hydraulic excavators but their location within paddocks is critical to stock access so they are usually dug along fencelines. Fenced off drain lines are important for stock safety but can be harbours for weed infestation of pastures.
Sub-surface drains are run beneath the soil of a paddock usually where the soil is either poorly structured or not very free-draining.
Sub-surface drains may be either constructed using ceramic tiles laid end to end with gaps to allow ingress of water from the soil or can also be constructed from broken stone, gravel, bundles of brushwood, boards nailed in a square or triangle or small tree trunks. The function of all these materials is to develop an easy exit passage for excess water within the soil profile.
A more specialised form of sub-soil drainage is the Mole drain, which is a tunnel formed in the subsoil by the passage of a torpedo-shaped Mole attached to a vertical blade. These can only be formed in certain soil moisture conditions in firm clay subsoils free from stones and other impediments. They are very effective, although require regular renewal as they tend to silt up over time.