How Irrigation Works

Sub-Surface Irrigation

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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