An overhead irrigation system is a lot like a lawn sprinkler -- the basic principle is the same. Water is pumped in under pressure and sprayed down onto the plants from flat spray nozzles.
These may be mounted on an overhead network of aluminum pipes or even simply mounted on the top of a stake.
Because it can be difficult to produce an even coverage, some more expensive systems may feature a moving overhead boom. This mechanism, which moves across the length of the whole crop, can then disperse the water in a much more even manner. Another overhead irrigation device is the water gun, which, as its name suggests, shoots water into the air and out over a field. A large water gun can cover several acres of land without needing to be moved.
Overhead systems are particularly useful when covering large areas of land, and some can even be dismantled and moved from field to field with little trouble.
Because overhead irrigation systems need a plentiful supply of water at a relatively high pressure, they vary greatly in complexity and cost depending on the acreage you're covering. Another important thing to keep in mind is that, with overhead irrigation, the foliage of a crop does get wet. If the leaves remain wet for an extended period of time, anywhere from as little as 10 and up to 24 hours, this can cause problems with fungi and bacterial disease. For more information regarding plant diseases, visit the North American Plant Disease Forecast Center and this article by the American Phytopathological Society.
In the next section, we'll take a look at a more controlled irrigation method called drip irrigation.