The need for drainage
If wet soils are to be used for agriculture or horticulture, some drainage is usually needed, because the roots of pasture or crops will not survive and grow in waterlogged soil. Even partly drained soils will slow pasture or crop growth, because wetter soils take longer to warm up, and so growth does not start until later in the spring. Grazing animals, especially cattle, damage the soil when walking over wet land, leaving deep hoof marks that trap rainwater. Farm or orchard machinery may become bogged in wet paddocks.
Until the late 20th century Department of Agriculture drainage advisory officers advised farmers on drainage techniques, and until the 1980s Massey University operated a drainage extension service in Manawatū. Today commercial drainage companies service the farming community.
Peat drains for irrigation
In the development of a large area of peat land in Waikato and the Hauraki Plains, deep drainage channels were excavated to allow the water table to be lowered. However, it was found that in summer these areas dried out too much for continued pasture growth. The remedy was to block the drains up to a certain level in spring, so plants still had access to groundwater.
The first aim of drainage is to lower the water table (the level at which groundwater sits in the soil) below the level of plant roots. Early settlers first tried digging deep drains at wide intervals, with a series of smaller shallow drains leading into them. On West Coast pakihi soils, the soil surface was reshaped into giant hump-and-hollow corrugations, so water ran into drainage channels.
Grassed waterways are wide, shallow channels constructed across almost-flat land to contain occasional floodwater, or to reshape a meandering stream. They are not normally cultivated, but can be used for grazing.
On hills, contour furrows or graded banks are sometimes used to lead water across the slope to prevent erosion, and stop water pooling on lower flat land.
Later, clay pipe drains began to be used. They were constructed at a depth of about 50 centimetres, and carried drainage water into the larger drains. More recently, clay pipes have been replaced with perforated, corrugated plastic tubing.
In clay soils, mole drains can be formed to lead water into clay tile or corrugated plastic drains. These tunnels are created by dragging a 75-millimetre-diameter torpedo-shaped device on the end of a blade through the soil. Mole drains are on a slight slope and at a shallower depth than tile drains.
Mole drains are created at about 5-metre intervals, forming an intensive drainage network under a paddock. Mole drains cannot be used in stony or sandy soils.
Mole draining is best done in late spring or summer, when the soil is moist enough to be shaped, but then can dry so the tunnel and the crack formed by the blade hold their shape.
The main role of subsurface drains is to remove excess water from the soil. However, any nutrients in the drainage water are also lost into main drains and waterways. This means that more careful fertiliser management may be needed on drained soils to avoid environmental problems, such as nitrate leaching.
Draining wetlands for farming had previously been seen as beneficial. In more recent years, the value of wetlands and habitats for wildlife has been recognised. Preserving wetlands may be achieved through a national conservation order, a legal ruling that prevents any changes to the existing condition of the area.