How Irrigation Works

­­Surface irrigation is arguably the least complex form of irrigation. At its simpl­est, no attempt is made to stop fields from naturally flooding. In general, this is only suitable in situations where the crop is of little value, or where the field will be used only for grazing or even recreation. Of course, while this method is simple and easy, it is totally dependent upon a suitable water source.

A more refined variation, though still reliant on a plentiful supply of water, is basin irrigation. Closely-spaced crops with deep roots are particularly suited to this method, and the growing of rice in paddy fields is an example familiar to most of us. The basin referred to is simply a field, enclosed with a raised bank, or dike, to contain the water. The water is directed into the field by various channels and pipelines, or may even be brought in manually. Two extremely old methods use these devices:

­By whatever method, the water is transported to the channels where it floods the field, causing a deep layer of mud to form in the bottom of the basin. In the case of rice, fresh seedlings are planted in this rich and fertile mud, and in September or October the mature rice is harvested, dried and stored.

Border irrigation is similar, but in this case the field is not entirely enclosed by a dike. Instead, it is watered from one end and allowed to drain from the other. Border irrigation works well with sloping land, as does furrow irrigation, in which the water is further controlled by the use of channels within the field itself. Water is directed along these channels, and by controlling the flow of water into each channel, the farmer can control the amount of water in different portions of the field surface.

Next, we'll find out about overhead irrigation methods.

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.

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­While surface irrigation methods rely on watering the whole surface of the field and overhead irrigation leaves the plants wet and produces runoff, drip irrigation is far more controlled. Water is slowly provided to a very specific area, close to the roots of the plant, by a network of drip emitters.

Despite their name, these tiny nozzles - about the size of a quarter - don't hang above the plants and drip but are actually laid along the ground. Linked to an appropriate water source by a main feeder hose, they provide a slow and steady flow of water. An alternative to drip irrigation is trickle tape - essentially a length of hose with built in drip emitters.

­The advantage to using trickle or drip irrigation is, simply, control. This method of irrigation is precise an­d economical. A standard lawn sprinkler, for example, might measure the water flow in gallons per minute -- somewhere between one and five is normal. A drip emitter, on the other hand, is rated in gallons per hour. The flow of water is so slow that it is easily absorbed into the ground. In a well-tuned system there is little opportunity for excess water running off and being wasted.

And this approach can be even more fine-tuned by going underground.

­Though initially expensive -- between $500 and $1000 per acre -- and not suitable­ for many areas, the economical advantages of drip irrigation can be further enhanced by placing the irrigation tubing about 5 inches (about 12.7 centimeters) below the surface. Down there, the water really does get straight to where it's needed - the roots of the plant. Evaporation is greatly reduced, and there is no opportunity for surface runoff.

A similar effect can be gained far more cheaply by making use of plastic mulch. Traditionally, mulch is a protective covering of organic material placed around plants to:

It can also help keep fruit off the ground - anyone who's grown strawberries will appreciate the value of mulching with a layer of clean straw.

More recently, plastic mulch has become an integral part of many drip irrigation systems. By laying sheets of plastic across the fields, the horticulturalist can further improve conditions for their plants. However, there are concerns that extensive use of plastic mulch may have long-term detrimental effects on the environment, perhaps increasing the amount of rain and pesticides that runs off into nearby water.

It would seem that, with the right amount of money and time, you can set up a system that delivers exactly the right amount of water to your plants at exactly the right moment, and you can sit back and watch them grow. It almost sounds too easy, doesn't it? What's the catch?

In developed countries we have a ready supply of fresh, clean water. It merely needs filtering before it can be used for irrigation; the finely-tuned systems used in drip irrigation are easily clogged by dirt or deposits from unfiltered water. Developing countries, however, may have to rely on rivers or seasonal rainfall for their supplies of water. While this may not always be reliable, the alternative is to create dams or canals, each of which may cause unwanted changes to the local environment.

Even though no one wants to wash away the fertile soil from their field, soil erosion does occur. This is an unwanted and unfortunate side-effect of surface irrigation. In addition to this, the constant evaporation of water may also lead to a build up of salt in the upper layers of soil, particularly if the soil has a high saline content to begin with, rendering it unsuitable for farming.

So, we've seen that while methods of irrigation vary in complexity and efficiency, they are all just ways in which farmers or gardeners attempt to simplify the task of watering their crops. Each method has its own advantages and disadvantages, which is why there is still such a wide range of methods in use.

For more information on irrigation and related topics, check out the links on the following page.