Drainage is used to carry extra water off agricultural land and to deposit it elsewhere. Areas that do not have adequate drainage can become waterlogged, which inhibits plant growth. Before deciding which type of drainage method to use, it is important to understand the drainage needs for the land and where the water will be deposited. Proper planning and construction of the drains will minimize any damage to the environment and natural wildlife.
A land drain is also known as a French drain. It consists of perforated plastic drain pipes that are laid into trenches dug in the ground. Small stones are piled over the pipe and in the trench. This allows water to easily drain into the ground through the pipe. The pipe carries the water away to a different area. Land drains are used in an attempt to distribute water evenly over the land. Land drains used in gardens will drain the water to a central point.
A bedding system is used in fields where crops are grown. The width of the bed will depend on the crops as permanent pasture and hay, for example, use wider beds than other field crops. A bedding system can take several years to become effective as the bedding is formed using a plough. Ultimately the main goal is to have a bed with a high point in the center with sloping sides on each side of the high point. At the edge of each bed is a collector drain where excess water can collect and be diverted to another spot. The bed does not need to have a uniform slope on each side. The collector drain just needs to be at the lowest point of the entire bed to ensure complete water drainage.
Tubewell drainage works by controlling the salinity and water table of a specific area. Ground water is pumped out of specially dug wells. The amount of ground water that is removed is equal to the drained surplus water. This type of drainage is not used extensively because it is only successful when the aquifer's physical properties and the hydrogeological conditions meet very specific conditions. The hydraulic conductivity of the soil layers above the aquifer have to be specific so that the water table will respond and change. Additionally the individuals that maintain and run the drainage system must be highly trained to monitor the water tables and water quality as well as maintain and run the drainage system. The land in Russia and the U.S. does not suit this type of drainage, but it has been highly successful in Pakistan.
Mole drainage is particularly effective on very wet, heavy soils. This drainage is used extensively in England and New Zealand for dairy farms and fields. A mole drain consists of an unlined channel dug with a ripper blade. The channel is formed in the clay subsoil and the sides of the channel are compacted. This type of drainage is appropriate for heavy clay soils and areas that don't slope, which prevents any downward drainage. Ultimately soil that contain 35 percent or more clay and less than 30 percent sand benefit from mole drainage.