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Drainage, or the removal of excess surface and groundwater from irrigated land, is as important as the application of water. Too much water in the soil caused by over irrigation and the lack of adequate drainage results in an increasing buildup of salts and in waterlogging. The accumulation of salts, called salination, occurs because plants absorb water but leave the salts dissolved in it behind. Also, unless an effective drainage system is constructed, the groundwater table under the irrigation field gradually rises and may eventually reach the root zone, thus inhibiting plant growth.
The concentration of salts in water is harmful to some plants if it exceeds .09 ounce per gallon, and it is injurious to almost all plants if it reaches .26 ounce per gallon. As the water table rises nearer the surface and evaporation increases salt content, severe problems may eventually develop even if the irrigation water has a low salt content.
Planning a system
In some areas the normal water level in the soil is high, in others low; this variable is always investigated before a drainage system is planned.
Types of drainage systems
Drainage systems may be divided into two categories, surface and subsurface. Each has several components with similar functions but different names. At the lower, or disposal, end of either system is an outlet. In order of decreasing size, the components of a surface system are the main collection ditch, field ditch, and field drain; and for a subsurface system, main, sub main, and lateral conduits from the sub main. The outlet is the point of disposal of water from the system; the main carries water to the outlet; the sub main or field ditch collects water from a number of smaller units and carries it to the main; and the lateral or field drain, the smallest unit of the system, removes the water from the soil.
The outlet for a drainage system may be a natural stream or river or a large constructed ditch. A constructed...