How Irrigation Works


Variable-flow irrigation sprinkler head.
Variable-flow irrigation sprinkler head.
Photo courtesy USDA ARS Photo credit David Nance

­Sta­rting an herb garden in a box on your windowsill or spending a Sunday afternoon planting flowers seems quite different from managing two hundred acres of watermelons, but in fact the underlying principle is the same. The art and science of growing plants - be they flowers, ornamentals, fruit or vegetables - is known as horticulture, and the goal of all horticulturists is to nurture their chosen plants from seeds to finished products they can be proud of. Putting it plainly, whether you're growing prize roses for a state fair or planting potatoes for the table, you want those plants to thrive. That means taking care of them and, just like us, plants have various needs:

  • A place to grow
  • The right temperature
  • Air and light
  • Water

­Irrigation helps take care of one of these needs by providing water. While the basic concept of irrigation is a simple one, there are many fascinating and remarkable systems in use today.

­In this article, we'll look at several different types of irrigation systems, starting with some of the oldest and simplest, which will lead us to an understanding of the more complex systems in operation today. But, first, let's take a closer look at what plants need in order to grow ­well.

A place to grow

Plants grow in many different places - from plain old dirt to richly fertilized soil. A branch of horticulture known as hydroponics even uses specially prepared solutions of mineral salts as a plant's growing space. Either way, as long as there's somewhere to put down roots and a plentiful amount of the right nutrients, a plant will do its best to grow.

The right temperature

After planting a seed, just the right temperature is needed for it to germinate and begin to grow. Some seeds need warmer temperatures than others to germinate. By relying on our knowledge of seasonal temperatures, it's easy to know when to plant what. And, if mother nature isn't agreeable, you can always make use of greenhouses and other artificial systems to get the temperature just right.

The plant absorbs carbon dioxide from the atmosphere, draws water up through its roots and uses light to photosynthesize sugars, which it uses as food. It excretes oxygen as a by-product of the process. Without water, photosynthesis cannot take place. Agronomist Larry Heatherly examines early maturing variety of soybean plants growing in a flood-irrigated field in Mississippi.
Photo courtesy USDA ARS Photo credit Keith Weller
The plant absorbs carbon dioxide from the atmosphere, draws water up through its roots and uses light to photosynthesize sugars, which it uses as food. It excretes oxygen as a by-product of the process. Without water, photosynthesis cannot take place. Agronomist Larry Heatherly examines early maturing variety of soybean plants growing in a flood-irrigated field in Mississippi.
Photo courtesy USDA ARS Photo credit Keith Weller

Air and light

Plants make their own food using a process called photosynthesis. During photosynthesis, the chlorophyll-containing green parts of the plant trap light energy and use it to perform a series of chemical reactions. The process involves carbon dioxide, and so plants also need plenty of air. We usually rely on the sun to provide light for our plants.

Water

Water is essential to plants. It carries important nutrients from the soil and is an important trigger for germination and the process of photosynthesis. Without water, plants simply won't grow.

Irrigation systems provide water. When it comes to watering plants in our yards or gardens, most of us don't always like to rely on the weather -- we may use watering cans or sprinkler systems. This is irrigation at its simplest level. And while this is fine for the home gardener, when you're a farmer trying to water an entire field these methods become impractical. So, how about flooding the field? It sounds drastic, but the ancient Egyptians made extensive use of the practice. Two thousand years ago, Herodotus wrote that Egypt was "the gift of the Nile." Diverted into large, flat-bottomed basins, the river Nile provided excellent irrigation for Egyptian crops, and Herodotus was well aware that without the Nile, the Egyptians wouldn't have enjoyed such productive farming methods.

Although the basin method is still popular, there are more refined methods of irrigation in use today:

  • Surface irrigation such as border irrigation, furrow irrigation and other forms of irrigation that use flooding
  • Overhead irrigation
  • Trickle or drip irrigation
  • Sub-surface irrigation and plastic mulch

First, let's take a look at surface irrigation systems.

Surface Irrigation

Because the water level of the river varies, farmers along the Missouri River use floating pumps like this one to collect irrigation water.
Because the water level of the river varies, farmers along the Missouri River use floating pumps like this one to collect irrigation water.
Photo courtesy USDA ARS

­­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.

Flood-tolerant soybeans from southeastern China
Photo courtesy USDA ARS

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:

  • A shaduf is an uncomplicated apparatus consisting of a bucket, raised and lowered by a simple lever.
  • A sakia consists of a geared wheel of a circular chain of buckets turned by an animal, such as a horse or mule.

­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.

Furrow irrigation
Photo courtesy USDA ERS

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.

Special furrow systems like these enhance water management. Wide-spaced furrows work like alternative-row irrigation, except that every row is irrigated and the rows are further apart.
Photo courtesy USDA ERS

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

Overhead Irrigation

Variable-flow irrigation sprinkler head
Variable-flow irrigation sprinkler head
Photo courtesy USDA ARS Photo credit David Nance

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.

Pivot irrigation system watering a field of cotton in Mississippi.
Photo courtesy USDA ARS Photo credit Tim McCabe

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.

Ryan Younkin and Dale Heermann download data about the movement of a center-pivot irrigation system to find out the amount of water and time it took to irrigate an area.
Photo courtesy USDA ARS Photo credit Scott Bauer

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.

"Hand Move" portable sprinkler system
Photo courtesy USDA ERS

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.

Trickle or Drip Irrigation

Drip Irrigation
Drip Irrigation
Photo courtesy USDA ERS

­

­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.

Drip irrigation provides water near the base of the plant, leaving the upper foliage dry and less susceptible to fungi.
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­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.

Sub-Surface Irrigation

Soil cut away to expose a drip irrigation line in a tomato field.
Soil cut away to expose a drip irrigation line in a tomato field.
Photo courtesy USDA ARS Photo credit Pete Mortimer

­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:

  • reduce evaporation
  • prevent the growth of weeds
  • help protect the roots from frost damage

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.

These strawberries probably benefited from proper irrigation and mulching.
Photo courtesy USDA ARS Photo credit Ken Hammond

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.

Plastic mulch has become an integral part of many drip irrigation systems.

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.

Severe soil erosion in a wheat field near Washington State University.
Photo courtesy USDA ARS Photo credit Jack Dykinga

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.

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