Editing Drainage (section)

==Reasons for artificial drainage==
[[Image:MagomeDrainage.jpg|thumb|upright|An agricultural drainage channel outside [[Magome-juku|Magome]], [[Japan]] after a heavy rain. Protuberances create turbulent water, preventing sediment from settling in the channel.]]

[[Wetland]] soils may need drainage to be used for [[agriculture]]. In the northern United States and Europe, [[glaciation]] created numerous small [[lake]]s, which gradually filled with [[humus]] to make [[marsh]]es. Some of these were drained using open [[ditch]]es and trenches to make [[muck (soil)|muckland]]s, which are primarily used for high-value crops such as [[vegetable]]s.

The world's largest project of this type has been in process for centuries in the [[Netherlands]]. The area between [[Amsterdam]], [[Haarlem]] and [[Leiden]] was, in [[prehistoric]] times, swampland and small lakes. Turf cutting ([[peat]] [[mining]]), [[subsidence]] and shoreline [[erosion]] gradually caused the formation of one large lake, the [[Haarlemmermeer]], or lake of Haarlem. The invention of wind-powered [[Windpump|pumping engines]] in the 15th century permitted some of the marginal land drainage. Still, the final drainage of the lake had to await the design of large [[steam-powered]] pumps and agreements between regional authorities. The lake was eliminated between 1849 and 1852, creating thousands of km<sup>2</sup> of new land.

[[Coastal plains]] and [[river deltas]] may have seasonally or permanently high [[water table]]s and must have drainage improvements if they are to be used for agriculture. An example is the [[flatwoods]] [[citrus]]-growing region of [[Florida]], United States. After periods of high rainfall, drainage pumps are employed to prevent damage to the citrus groves from overly wet soils. [[Rice]] production requires complete water control, as fields must be flooded or drained at different stages of the crop cycle. The Netherlands has also led the way in this type of drainage by draining lowlands along the shore and pushing back the sea until the original nation has been greatly enlarged.

In moist climates, soils may be adequate for cropping with the exception that they become [[Waterlogging (agriculture)|waterlogged]] for brief periods each year, from [[snow]] melt or from heavy [[rain]]s. Soils that are predominantly [[clay]] will pass water very slowly downward. Meanwhile, plant roots suffocate because the excessive water around the roots eliminates air movement through the soil.

Other soils may have an [[impervious]] layer of mineralized soil, called a [[hardpan]], or relatively impervious rock layers may underlie shallow soils. Drainage is especially important in [[Fruit tree|tree fruit]] production. Soils that are otherwise excellent may be waterlogged for a week of the year, which is sufficient to kill fruit trees and cost the productivity of the land until replacements can be established. In each of these cases, appropriate drainage carries off temporary flushes of water to prevent damage to [[annual plant|annual]] or [[perennial plant|perennial]] crops.

Drier areas are often farmed by [[irrigation]], and one would not consider drainage necessary. However, irrigation water always contains minerals and [[salt (chemistry)|salt]]s, which can be concentrated to toxic levels by [[evapotranspiration]]. Irrigated land may need periodic flushes with excessive irrigation water and drainage to control [[soil salinity]].
{{wide image|Drainbankstown.jpg|1800px|A typical drain in [[Bankstown, New South Wales]], Australia|alt=A typical drain in Bankstown, New South Wales}}