Water is essential to plant growth and for millenniums successful farmers have used different methods to apply water to their crops. This artificial addition of water is called irrigation. According to the Museum of Irrigation, the practice began around 6000 B.C. in Egypt and Mesopotamia, where water from the Nile, Tigris or Euphrates rivers were used to irrigate crops. Today, there are many different kinds of irrigation systems, but the most common are ditch irrigation, terracing, overhead irrigation, drip irrigation and subirrigation.
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Ditch irrigation is the simplest and oldest irrigation system and it is still common in many parts of the world. The only technology necessary is the manpower or machines to dig ditches or furrows between the rows of plants. Water is added to the ditches by a variety of means, including gravity flow, pumps and siphons. At one time, ditch irrigation was the most common system used in the United States. But most of these systems have been replaced by more efficient modern systems.
Terracing is another very old irrigation system. Sloping land on mountains and hillsides is divided into strips and the strips are levelled and kept from sliding or washing away by retaining walls. Crops are planted in the level areas in the terraces. As water is applied to the topmost terraces, it flows downward, watering each terrace as it goes. Because of the terrain and the nature of the work required, terracing is a very labour-intensive system.
Overhead irrigation is the artificial application of water to crops from above. Central pivot systems, which are in wide use in areas of flat terrain, have sprinklers spaced along very long aluminium or steel pipes that extend in two directions from a central supply point. These long pipes are supported by motorised wheels and the pipe-sprinkler assemblies pivot around the central supply point. They are capable of efficient and unattended watering of large areas and account for the distinctive green circles that can be seen when flying over parts of the western U.S.
Sprinkler systems are another very common overhead irrigation system. In these systems, water is piped to a point within the area to be irrigated and forced our over the area through sprinklers or spray nozzles. Sprinklers are sometimes attached to movable platforms connected by a flexible hose. Most lawn irrigation systems use overhead sprinkler systems.
Drip or trickle irrigation boasts the most efficient use of water of any of the systems described here. Water is delivered directly to the roots of the plants minimising evaporation and runoff. Pipes or hoses are run along the rows of plants with holes or nozzles that supply drips or trickles of water to the root zone of each plant.
Sub-irrigation is the watering of plants, usually potted plants, from piping beneath the roots. In some cases, nutrients are added to the water and excess liquid is collected and recycled. Sub-irrigation requires the use of expensive equipment and its most common use is in commercial greenhouses.
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