Photo courtesy USDA ARS
Photo credit David Nance

Variable-flow irrigation sprinkler head
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 horticulturalists 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.

The Basics
We've already mentioned that plants need certain things if they are going to flourish. They need a place to grow, the right temperature, air, light and water.

Let's look a little more closely at each of these:

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.

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.

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.

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.

Common sense and experience tells us how important water is to plants. And, even if it didn't, you'd simply have to visit any local DIY superstore like Home Depot or Lowes to remind you. Just think about the sheer number of watering accessories the eager home gardener can buy, ranging from watering cans to hosepipes and oscillating sprinklers to traveling sprinklers. When it comes to watering plants in our yards or gardens, most of us don't always like to rely on the weather. This is irrigation at its simplest level.

While this all seems all right for the home gardener, what about people who have more sizeable horticulture concerns. There's something relaxing about meandering through the garden sprinkling with a watering can, but when you're a farmer trying to water an entire field it becomes impractical. 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.

Photo courtesy USDA ARS
Photo credit Keith Weller

Agronomist Larry Heatherly examines early maturing variety of soybean plants growing in a flood-irrigated field in Mississippi.

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
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.

Photo courtesy USDA ARS
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
Flood-tolerant soybeans from southeastern China
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.

Photo courtesy USDA ERS
Furrow irrigation

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.

Photo courtesy USDA ERS
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.

Overhead Irrigation
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.

Photo courtesy USDA ARS
Photo credit David Nance

Variable-flow irrigation sprinkler head

These may be mounted on an overhead network of aluminum pipes or even simply mounted on the top of a stake.

Photo courtesy USDA ARS
Photo credit Tim McCabe

Pivot irrigation system watering a field of cotton in Mississippi.

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.

Photo courtesy USDA ARS
Photo credit Scott Bauer

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.

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.

Photo courtesy USDA ERS
"Hand Move" portable sprinkler system

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 Web site.

Trickle or Drip Irrigation
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.

Photo courtesy USDA ERS
Drip Irrigation

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.

The advantage to using trickle or drip irragation 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.

Sub-Surface Irrigation
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.

Photo courtesy USDA ARS
Photo credit Pete Mortimer

Soil cut away to expose a drip irrigation line in a tomato field.

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.

Photo courtesy USDA ARS
Photo credit Ken Hammond

These strawberries probably benefited from proper irrigation and mulching.

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.

What's the Downside?
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.

Photo courtesy USDA ARS
Photo credit Jack Dykinga

Severe soil erosion in a wheat field near Washington State University.

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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