Drainage system (agriculture)

An agricultural drainage system is a system by which water is drained on or in the soil to enhance agricultural production of crops. It may involve any combination of stormwater control, erosion control, and watertable control. While there are more than two types of drainage systems employed in agriculture, there are two main types: (1) surface drainage and (2) sub-surface drainage. Figure 1 classifies the various types of drainage systems. It shows the field (or internal) and the main (or external) systems. The function of the field drainage system is to control the water table, whereas the function of the main drainage system is to collect, transport, and dispose of the water through an outfall or outlet. In some instances one makes an additional distinction between collector and main drainage systems. Field drainage systems are differentiated in surface and subsurface field drainage systems. Sometimes (e.g., in irrigated, submerged rice fields), a form of temporary drainage is required whereby the drainage system is allowed to function only on certain occasions (e.g., during the harvest period). If allowed to function continuously, excessive quantities of water would be lost. Such a system is therefore called a checked, or controlled, drainage system. More usually, however, drainage systems are meant to function as regularly as possible to prevent undue waterlogging at any given time and it is this regular drainage system that is most often employed. In agricultural literature, this is sometimes also called a "relief drainage system". The regular surface drainage systems, which start functioning as soon as there is an excess of rainfall or irrigation applied, operate entirely by gravity. They consist of reshaped or reformed land surfaces and can be divided into: Bedded systems, used in flat lands for crops other than rice; Graded systems, used in sloping land for crops other than rice. The bedded and graded systems may have ridges and furrows.